Mold Technologies

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Mold Technologies

13.1 Introduction This book has sought to provide an engineering approach to mold design; the emphasis has been on the examination and modeling of fundamental mechanisms that govern the use and failure of injection molds. The examples have purposefully been made as simple and clear as possible, so that the practitioner can apply the design and analysis methods to more specific and advanced molding applications. There are many advanced molding process technologies and corresponding mold designs. A flow chart has been provided in Figure 13.1 to guide the selection of some of available mold technologies. Such mold technologies can be used to compete effectively by providing molded parts with higher quality in less time and at lower costs. Most of these technologies have been developed for specific purposes, such as to produce a molded part with unique properties, or to more economically produce large quantities of molded parts. Many molding technologies are interwoven. For instance, multi-shot molding (in which a molded part is made of two or more materials) has characteristics that are related to coinjection molding, insert molding, stack molding, and even injection blow molding. Regardless of the level of technology, the underlying physics and mold design fundamentals that have been previously provided still apply. As such, this chapter provides an overview of some available molding technologies, and discusses associated mold design issues. Examples of illustrative mold designs have been sourced from the U.S. patent literature. The objective here is not to provide an exhaustive survey of mold related technologies, or even to recommend specific mold designs. Rather, the intent is to show some interesting examples that will imbue the practitioner with specific insights into a range of mold technologies so that they may become better mold designers.

13.2 Coinjection Molds Coinjection molding is a process in which two materials are sequentially injected into a mold cavity, typically through the same gate. Since the first material forms a skin and the second material forms the core of the molded part, it is possible to use coinjection molding to produce plastic parts with unique aesthetic or structural properties with potentially lower costs than injection molding. Some typical coinjection molding applications include: •

The use of a first virgin material having preferred cosmetic properties followed by a second material having different structural properties and/or recycled content, as in the fascia of a car bumper;

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Compete effectively

Higher quality

Multiple materials

Hollow parts

Aesthetic surface

Complex geometry

Lower costs

Higher yields

Plastic over other

Insert mold

Plastic over plastic

Multi-shot mold

Plastic within plastic

Coinjection mold

Fluid within plastic

Gas/water assist mold

Inflated plastic

Injection blow mold

Complex interior

Lost core mold

Decorated surface

In mold labeling

Glossy/clear surface

Mold wall temperature

No witness marks

Reverse ejection

Complex exterior

Split cavity mold

Interior features

Rotating core mold

Tight tolerances

Injection compression

Better flow control

Dynamic FeedTM Melt FlipperTM

Higher productivity

Faster time to market

Higher cavitation

Hot runner mold

Lower clamp tonnage

Stack mold

Less material waste

Insulated runner mold

Lower tooling cost

Lower cavitation

Two plate mold

Faster mold tooling

High speed machining

Prototype mold

Figure 13.1: Mold technology selection flow chart

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• •

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The use of a first material followed by a second foaming material to produce a cosmetic part with lower density, as in structural foam applications; The use of a first material followed by a second fluid, such as air or water, to produce a hollow part like a door handle.

While this last example (commonly known as gas assist or water assist or fluid assist molding) may not seem a coinjection process, the molding process and mold designs are sufficiently similar to warrant a joint discussion.

13.2.1 Coinjection Process In coinjection molding, two materials are sequentially injected, often similar to the sequence provided in Figure 13.2 [42]. As shown, a first melt is partially injected into the mold through a sprue 6 or some other feed system. After a desired volume of the first material 7 has been injected, a second material 8 is injected at the same location. If the volume of the first material is too small, then the second material may “blow through” the first material. Conversely, too large an initial charge of the first material may leave too small a volume for the injection of the

Figure 13.2: Coinjection molding process

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second material. Since the first material is adjacent to the mold wall, and may have partially solidified, the second material will tend to flow through the core of the first material. After the second material has been injected, it is fairly common to then inject a small amount of the first material 9. This latter injection of the first material serves to purge the feed system of any undesired amount of the second material, which might otherwise contaminate the subsequent molding cycle. It is observed in Figure 13.2 that the mold core 2 is moving in and out of the mold cavity 1 during the injection of the materials into the mold to thereby adjust the wall thickness of the cavity 3. This injection compression serves at least two purposes. First, in foam molding, the compression and subsequent expansion of the mold cavity can be used to delay and subsequently encourage the nucleation of gas cells, thereby controlling the distribution and density of the injected foam. Second, in non-foam molding, the compression of the cavity can be used to control the pack pressure throughout the mold and thereby control the shrinkage characteristics of part features molded of the first material while injecting the second material. The control of the cavity wall thickness can be accomplished by profiling the displacement of the molding machine’s platen during the filling stage, or alternatively profiling the clamp tonnage profile. The mold uses a sliding fit (refer to Section 12.4.1) along the vertical sides where the mold core mates with the mold cavity. While not discussed in this reference [42], the sliding fit can be assisted through the use guide pins, interlocks, or keyways to mate the cavity and core inserts to avoid accelerated wear on the sliding surfaces.

13.2.2 Coinjection Mold Design A schematic for a coinjection mold and feed system is shown in Figure 13.3 [43]. As shown, material is delivered to the mold from the barrels of two injection units 8 and 9 to the mold 10 via corresponding flow channels 15 and 16. These two channels converge at a control valve 17 prior to the sprue 11. The control valve uses a valve pin 18 with two skewed flow channels. By rotating the pin, one of the two flow channels in the pin will register with the channels 15 or 16 to allow material to flow from the corresponding barrels 8 or 9 into the mold while also preventing the materials from flowing between the barrels. A control system is required to coordinate the actuation of the valve pin 18 with the injection of material from the barrels 8 and 9. Given this feed system design, a first and last injection of the first material is warranted to avoid contamination of any material residing between the valve pin 18 and the sprue 11 as previously discussed with respect to Figure 13.2. For the most part, design of coinjection molds is very similar to that of conventional molds; many conventional molds can be successfully used in a coinjection process since the mechanisms for coinjection are mostly integrated with the molding machine and not the mold itself. However, the mold designer should modify the analyses for coinjection. With regard to mold filling, the mold designer should ensure that the mold cavity is designed to achieve the desired filling patterns at reasonable pressures. Analytical solutions and simulations have been developed for the coinjection of two materials with dissimilar viscosities into a mold

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Figure 13.3: Coinjection mold and process

[44, 45]; however, in many coinjection applications, the mold will operate successfully if the mold is designed to fill completely with only the more viscous material. Analysis of cooling, shrinkage, and ejection should also be modified to consider the melt temperatures and thermal properties of the two materials. Given the multi-layered structure of the coinjected molding, a reasonable approach is to derive a “meta-material” that has material properties in proportion to the layer thickness of the two constitutive materials.

13.2.3 Gas Assist/Water Assist Molding Gas and water are both fluids, so both gas assist molding and water assist molding can be considered as types of fluid assisted molding processes. Since these assisting fluids are injected inside of a first material, all fluid assist molding processes can be considered a type of coinjection molding process. Compared to traditional coinjection with polymer melts, fluid assisted molding has two distinct differences. First, the second injected fluid (such as nitrogen or water) has a very low viscosity compared to the previously injected polymer melt. This low viscosity provides for a very low pressure drop along the flow path, and thus gives excellent pressure transmission for packing out the previously injected polymer melt. Second, the assisting fluid is later removed from the molded part so as to hollow out the inside of the molded part. With careful mold design, fluid assisted moldings can have increased strength, lighter weight, and reduced cycle times compared to conventional or coinjected molds.

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Fluid assisted molding is a fairly old process having served as an alternative to blow molding [46]. Two variations of a more modern gas assist process with injection decompression are shown in Figure 13.4 [47]. In the first method, two mold halves 10 and 11 form a mold cavity 12 into which the plastic melt will flow. The molding machine’s nozzle 13 has an internal core 16 with a sliding valve 19 that is actuated by compressed gas alternately introduced through gas lines 20 and 21 through three-way valves 22 and 23. At the beginning of the process, the plastic melt 24 is introduced into the mold cavity through the machine nozzle 13; at the same time, the sliding valve 19 is in a position which blocks the gas inlet tube 18 compressing the gas through line 21 decompressing the gas through line 20. After the mold cavity is partially filled, gas line 20 is pressurized while gas line 21 is depressurized. This causes the sliding valve to assume the position as shown so that gas inlet tube 18 is opens and delivers compressed gas to the mold cavity. Once the gas has been injected, the sliding valve is then actuated to prevent the undesired flow of the plastic melt. After the molded part cools, the opening of the mold causes the sprue to break and the release of any compressed gas to the ambient atmosphere. A second method is also shown in Figure 13.4 in which the reverse of injection compression, injection decompression, is used to form a hollow part with a very large cavity. In this design, the plastic melt flows into a cavity formed between two mold halves 30 and 32. While not shown, these mold halves can have fine details like bosses that are fully formed by the initial filling of the cavity with the polymer melt. The compressed gas is then injected into a thicker portion of the mold cavity. At the same time, the mold core 32 is retracted from its opposing mold half 30 to enlarge the cavity. In this manner, moldings with very large internal gaps (for example, 50 mm) can be formed while preserving fine features on the exterior surfaces of the molded part.

Figure 13.4: Gas assist with injection decompression

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The mold design of fluid assisted molds needs to vary considerable from that of injection and coinjection molds. In particular, the mold designer needs to consider the location for the injection of the gas or water. As demonstrated in Figure 13.4, both the nozzle and cavity are common locations. As importantly, the mold designer must carefully design the mold to have appropriate flow channels to strategically direct the gas or water through the mold cavity. In most mold designs, the cavity wall thickness is made as uniform as possible to avoid non-uniform cooling and shrinkage. However, such a mold design will not lead to an effective mold for fluid-assist. The reason is that the gas or water will permeate or “finger” in random directions through a uniformly thick mold cavity, thereby weakening the molding without significantly reducing the part weight. As such, thick flow channels as shown in Figure 13.5 are commonly added to the mold cavity to direct the gas or water through the mold cavity [48]. All the gas channels will exhibit some irregularity regardless of the magnitude of penetration. In general, it is desirable to develop a gas channel to provide as uniform a molded wall as possible while providing the necessary fluid flow and part stiffness. For this reason, the top right gas channel in Figure 13.5 is least preferred. Since the other flow channels are cored out by the fluid, the cooling and shrinkage is made relatively uniform without extended cycle times. Water assist molding seems to have received renewed interest lately [49, 50]. Compared to gas assist, water assist provides at least three key benefits. First, water has a very high specific heat and so can be injected to reduce the cycle time compared with gas assist molding application. In fact, in some water assist molding applications, the flow channels are designed with inlets and outlets, such that the water can be circulated within the molded part and thereby greatly reduce heat transfer via heat convection. Second, water is incompressible compared to gas, and so can be used to provide higher melt pressures in the cavity with less energy and risk than gas. Third, it has been shown that water assist provides more uniform and smooth surface in the inside of the molded parts. With these advantages, however, water assist does bring two significant disadvantages. First, the water must be removed from the interior of the molded parts; various schemes have been developed to remove the water internal to the molding prior to the mold opening [51]. Second, the use of water in the molding environment tends to increase humidity and corrosion, so a corrosion resistant mold material such as SS420 is recommended.

Figure 13.5: Flow channel sections for fluid assisted molding

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13.3 Insert Molds Insert molding refers to a process in which a discreet component is placed within a mold, and then at least partially encapsulated by the subsequently injected plastic melt. Some commonly inserted components include electrical devices, nuts or other fasteners, stiffening members, and other plastic components. After the insert molding process, the inserted component is usually permanently joined with the molded plastic.

13.3.1 Low Pressure Compression Molding One common method for encapsulation of delicate components is compression molding as shown in Figure 13.6 for the production of a tantalum capacitor [52]. In this process, the capacitor 19 is placed between two layers of plastic 17 and 18 prior to the mold closure. The mold design provides matching cavities 16 and 22 to receive the plastic as well as grooves 21 to receive the lead wires 20. In any molding process, the mold designer should explicitly consider the handling of the molded parts upon de-molding. In the design of Figure 13.6, the lower cavity 22 is deeper than the upper cavity 16. In addition, the lower mold half 11 is provided with a flash well 23 for the collection of any plastic that flows out of the cavity during the compression molding process. As a result of these design elements, the molded part will remain on the lower mold half when the mold opens. In this compression molding process, the plastic layers 17 and 18 were cut from sheet stock in a form to fit into their corresponding cavities while also supporting the capacitor 19 and lead wire, 20. It is desirable that the plastic fully contact the rear surface of the mold cavities to facilitate heat transfer and plastic forming. Prior to mold closure, cartridge heaters in the two mold halves 10 and 11 bring the temperature of the mold and plastic layers to above the glass transition temperature of the plastic. Once the plastic is softened, the mold is slowly closed with low force. As the mold slowly closes, the plastic slowly flows around the capacitor until it is fully encapsulated – any excess plastic in the cavity will flow out the flash surface, 24, and into the flash well, 23. Once the inserted component is fully encapsulated, full clamping force may be applied to the two mold halves to compensate for shrinkage and achieve the desired dimensions while the mold is cooled. This mold design has some unique features. First, the grooves secure the insert component in the mold to avoid undesired movement caused by the movement of the mold or the flow of the plastic melt during the molding process. Second, this process was specifically designed to impart low stress on the inserted component by the controlled heating of the mold and softening of the plastic followed by the clamping and cooling of the mold. Given this heating and cooling cycle, the mold should be carefully designed to minimize the size and thickness of the plates so that energy consumption and cycle time are minimized. Third, this process used a flash surface and reservoir to control flashing of excess plastic; it is clearly desirable to select an amount of plastic stock that minimizes the amount of flashing while ensuring a fully filled cavity. In this design, the mating flash surface, 24, on the lower mold requires a relatively large clearance with mating surface on the upper mold half. If this clearance is too small, then the rate of the

13.3 Insert Molds

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Figure 13.6: Compression molding with inserted component

compression molding process can be limited by the flow of the plastic melt out of the cavity. The filling and cooling analyses of Chapters 5 and 9 can provide useful design support.

13.3.2 Insert Mold with Wall Temperature Control Another example of insert molding is provided in Figure 13.7 [53], which is particularly directed to the control and improvement of weld lines around an inserted component for the production of a water faucet handle. The mold design consist of two separable mold halves 30 and 31 having recesses 32 and 33 that together form a mold cavity. The inserted component 35 is held in position by two opposing pins 36. After mold closure and prior to

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mold filling, a substantially uniform cavity thickness exists between the inserted component 35 and the mold halves 30 and 31. In this mold design, the mold wall temperature of the mold is locally controlled by the flow of a controlled fluid through channels 40 and 42. Different fluids such as water, oil, or steam can be provided to different portions of the mold at different temperatures.

Figure 13.7: Insert molding with mold temperature control

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In the molding process, plastic melt is fed through the gate 38 and will follow the path of least resistance through the mold cavity. Upon entering the cavity, the plastic melt will divide into two streams 44 and 45 flowing around the insert. In Figure 13.7, the recess 33 in the lower mold half is controlled at lower temperature compared to the recess 32 in the upper mold half. As a result, the upper melt stream 44 will advance more rapidly than the lower melt stream 45. Given the importance of aesthetics in this molding application, the melt front advancement and knit-line location 50 can then be adjusted by specifying the difference between the mold wall temperatures in each zone. Furthermore, a heating element 46 is used to locally heat the mold wall to a temperature above the plastic’s glass transition temperature to melt and fuse the area around the knit-line ensuring desirable aesthetic and structural properties. The design of the multiple temperature control channels seems quite advanced, especially for 1937 when this patent application was filed. To facilitate the implementation of the cooling channels, the recesses 32 and 33 are themselves provided as mold plates 48 and 49 that are placed into the cavities in the two mold halves 30 and 31. This design is quite similar to the bezel example of Figure 9.16 in which the cooling lines have been milled into the rear surface of the core insert. The cooling and structural analysis of Chapters 9 and 12 should be applied to determine the cooling channel’s hydraulic diameter and layout, as well as the required amount of plate stock require to avoid excessive stresses. The local heating 46 by resistive means will be discussed in more detail in Section 13.7.1.

13.3.3 Lost Core Molding Lost core molding refers to a process in which a mold core is inserted into a mold cavity to form the interior of a molded plastic part. After the molding with the core insert is ejected, the core is melted out of the molded part to leave a complex interior cavity. One lost core molding application is shown in Figure 13.8 [54]. This particular application molds a valve housing with internal threads and an internal cavity containing a spring and ball check. The lost core molding process requires two sets of molds. The first mold design consists of two mold halves 4 and 6 which meet at a parting line 8. The mold cavity 10 includes threaded ends 12, a central bore 14, and a valve seat 16. Prior to mold closure, a ball check 26 and a compression spring 28 are inserted into the cavity 10 from the parting plane. After these components are inserted into the cavity and the mold is closed, a pin 22, is lowered into the mold cavity in opposition to the spring to lift the ball check from its seat. A low melting point material 30 is then injected through an opening 20 to completely filly the mold cavity. Upon solidification, the low melting point material has locked the ball check and spring in position. The mold is then opened, and the molded part 32, is removed with the pin 22. The molded part 32 is then inserted into the second mold’s cavity 46 formed by mold halves 40 and 42 for use as a core piece to form the inside surfaces of the valve housing. From the previous molding process, the core includes externally threaded ends 34 and a central section 36 that encloses the ball check 26, spring 28, and a conical surface 38, which is used to form the contour of the valve seat of the valve housing. The sliding fit of the pin 22 with the hole 52 serves to center the core 32 properly within the mold cavity 46. Plastic 56 is then injected

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Figure 13.8: Lost core molding with internal components

through the sprue 50 to fill the mold cavity and surround the core 32. Upon removal of the solidified molding, the core 32 is melted away to leave the final structure 60. This housing 60 has internally threaded ends 62 and a central chamber 64, which contains the ball check 26 and the spring 28. It may seem unlikely that such a lost core molding process is feasible. To melt the core from the housing, after all, the first material 30 that makes up the core must have a lower melting point than the second material 56, which makes up the housing. Then, why doesn’t the core melt during the injection of the second material? The reason is that the first material 30 has sufficient mass to act as a heat sink and absorb heat from the second material 56 without

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melting. For example, the first material is suggested to be a metallic alloy of 58% bismuth and 42% tin. Such an alloy melts at about 138 °C. For the second material, various plastics with a wide range of melting points have been used, including acetal and polycarbonate. For molding with higher temperature plastics, both the mold and the lost core can be cooled to minimize melting of the core.

13.4 Injection Blow Molds Injection blow molding is a process by which complex, thin-walled parts can be made including large internal cavities. In this section, two different blow molding processes are presented. The first design utilizes a conventional injection molding machine with a rotary mold while the second design uses a four position indexing system with multiple molding stations.

13.4.1 Injection Blow Molding An injection blow mold design is shown in Figure 13.9 [55]. The design includes two injection molds 48 and 50 which are positioned at equal radial distances from the axially located main sprue. The injection molds each include gating 54 and 56, an injection molding cavity 58 and 60, and other common mold components. The design also includes two sets of split cavity blow molds 140 a/b and 142 a/b so that undercut parts may be readily ejected after inflation. The injection blow molds are located diametrically opposite each other on the same radius as the injection molds 48 and 50. All four molds in this design spaced at 90 degree angles. The mold cores are spaced on the same radius as the mold cavities so that the four cores can engage the four mold cavities simultaneously upon mold closure. A manifold delivers melt from the nozzle of the molding machine to the mold cavities. The manifold rotator 18 is designed to oscillate between two orthogonal positions through actuation of a hydraulic drive cylinder 124 pivoted at one end by a pin 126 to the bearing block 78. A piston 128 having a clevis end 130 pivotally engages a crank arm 132 which is secured by screws or other suitable means to the circumference of the manifold axle 15. Stops 134 and 136 are fastened to the bearing block to limit the travel of bellcrank arm to a 90 degree sweep. This design ensures that the mold cores 114 and 116 engage the injection mold cavities 58 and 60 at one position of the manifold unit, while the other two mold cores engage the injection blow molding cavities 65 and 67. During the molding process, the machine clamps the mold cores against the mold cavities. The injection unit of the molding machine provides plastic through the main sprue 42 and runners 44 and 46 of the manifold 14 to the injection mold cavities 58 and 60 where the blow molding parisons or pre-forms are molded. Afterwards, the cores carrying the hot parisons, are reciprocated out of the cavities by action of the clamp. The manifold assembly then rotates 90 degrees to align the injection blow mold cavities with the molded parisons. The

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machine then clamps the mold and injects compressed air through the bores of the parisons to inflate the parisons and form the blow molded products while the injection unit fills the two injection mold cavities to form the next set of parisons. Upon mold opening, the split blow molds open and the finished parts are ejected. This design utilized a rotating set of mold cavities with a reciprocating set of mold cores. A clear alternative would be to utilize a stationary set of cavities with a rotating and reciprocating set of mold cores. Either design strategy provides a method for compactly and economically performing injection-blow molding through the modification of a conventional molding machine. The design may be guided by filling analysis to ensure appropriate runner system and cavity design, cooling analysis to control the temperature of the hot parisons, and structural analysis to minimize the size and stress of the injection and blow molds.

Figure 13.9: Injection blow mold with rotating cavities and reciprocating core

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13.4.2 Multilayer Injection Blow Molding A different machine configuration for injection blow molding is shown in Figure 13.10 for the molding of a two layered product [56]. The inner and outer layers are chosen for particular reasons related to the use of the product. For example, the inner layer 44 may be made of material which resists reaction with the contents of the container while the outer layer 44′ may be made of a material of substantially greater strength than the inner layer. In this design, the injection molding system includes a first injection station 10, a second injection station 12, a blowing station 14, and a stripper station 16. The system has an indexing head 18 with four faces. A set of core rods 22 extends from each of the four faces. The indexing head rotates intermittently about a center shaft 20 to sequentially index each set of cores with the different stations. Due to its configuration, this design is known as a “four position machine”. At the first injection station 10, the injection mold 26 is supplied with plastic melt from an injection unit 28 via a runner system 30. The injection mold is a split cavity design with a stationary lower section 32 and a movable upper section 34. When these mold sections are closed together, each of the core rods 22 extends into a cavity. The neck portion 38 of each core rod is firmly gripped by the wall of the opening 36. The plastic is injected into the cavity at an opening opposite the top of the core rod, and flows around and down the length of the core rod to form a parison 44. As the plastic flows down the length of the cavity, the melt loses some of its heat to the mold 26 and the core rod 22. Because the core is slender and has a large thermal resistance as analyzed in Section 9.3.5.5, the highest temperature of the parison will occur at the end of the core rod 40 near the gate and runner 30. After the molding of the parison in the first station 10, the mold 26 opens and the core rods 22 are lifted clear of the mold cavity in the lower section 32 by rising movement of the indexing

Figure 13.10: Two layer injection blow molding

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head. The indexing head turns 90 degrees in the counterclockwise direction and brings the core rods with the parisons 44 on them over the lower section of a second injection mold 48 at the second injection station 12. The second injection mold closes on the neck portion 38 to form mold cavities 42a which are larger in cross-section than the corresponding cavity 42 if the first molding station. As previously noted, the tip of the first parison 40 is at the highest temperature and so most easily deformed. If the second layer of the parison were gated from the same location, then the direct impingement and high flow rates of the second layer could wash some or all of the first parison off the core rod and thereby lessen the value of the molded product. For this reason, the plastic for the second layer 44′ is introduced into the cavity 42 from a manifold 50 and individual runners to the neck end of the cavity. The plastic flows around the first layer to form a parison with two layers. The second mold is then opened, and the indexing head moves the core rods to a blow mold, 58, at the blowing station, 14. The blow mold holds the neck portion of the laminated parison. The mold cavity, 42b, is in the form of the desired article which is to be blown from the laminated parison. The blowing operation then fully inflates the two layer parison 60. The blown products 60 next advance to the stripper station 16 where a stripper plate 64 pushes the molded products 60 off the core rods, 22. With the next rotation of the indexing head, the bare core rods are presented to the first forming station to begin the next round of moldings. There are three suggested benefits of this mold design. First, the witness mark formed by the gating at the tip of the core is wiped away by the flow of the second layer. Second, the design reduces the cycle time associated with the molding of the first layer by gating the second layer into a location away from the hottest portion of the first layer. Third, the improved consistency of the first layer facilitates the molding of a thinner first layer and with it a lower cost product.

13.5 Multi-Shot Molds Multi-shot molding sequentially injects different types of plastic, to mold a part with distinct regions. There are several potential advantages for the use of a multi-shot molding process. These include the use of multiple shots with: • • • •

different shot capacities to sequentially mold very large parts; different colors to mold multi-color parts, such as automotive taillights; different structural properties to mold parts with improved living hinges, tactile feel, etc.; and others.

There are many different methods to accomplish multi-shot molding. Perhaps the oldest and simplest is overmolding, which can be considered a variant of insert molding. Another

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approach is the core back method, which lends itself to a relatively simple mold design. However, multi-station mold designs are the most common method in use today due to the capability of this process to economically produce more complex part geometries. Each of these mold design strategies will be discussed. Regardless of the type of multi-shot mold design, the provided mold design and analysis methods generally apply with a few special considerations. First, multi-shot molds may require extended cooling times. The reason is that the first layer will not be at the mold coolant temperature when the second layer is injected. Furthermore, the first layer will largely prohibit the transfer of heat from the second layer to the mold. For these reasons, the mold designer should consider multi-shot molding using the analysis for one-sided heat flow as discussed in Section 9.3.5.6; the second layer should be 40% thinner than the first layer to avoid extending the cycle time. Second, multi-shot molds provide the mold designer the opportunity to utilize the second injection of plastic to melt and wipe out small imperfections or witness lines on the first layer of plastic. Because of this effect, however, the mold designer should avoid the placement of fine details on the some surfaces of the first molding that may be degraded by the second injection of the plastic melt.

13.5.1 Overmolding A multi-shot mold design using an overmolding approach is shown in Figure 13.11 [57]. In this design, a separate mold has used a branching runner 4 to fill two lateral runners 2 and a series of mold cavities for the production of key caps, B. Each key cap has a window 7 molded into its top surface 6 in the form of that key’s desired character, such as the number “5”. This particular design is directed to the bonding of incompatible plastic materials through the use of a solvent such as acetone applied to the key caps’ rear faces 5. Accordingly, the branching runner 4 is intended to be used as a handle by the operator during the application of the solvent; more modern designs may use bosses or shoulders to assist automated part handling systems. Once the solvent has been applied to the key caps’ rear faces 5 the key caps are placed in the cavities 1 of the lower half of a mold, A. The upper half of the mold, C, provides cores 12 that mate with the cavities in the upper half of the mold for the molding of the keys. After the mold is closed, the second plastic material is injected through the primary runner 13 and secondary runner 14 into the mold cavities. A portion of the material will fill the back 9 and window 7 of the key cap as well as the key’s boss 17 for later assembly with other components. This type of mold design is quite common for production of keys and signs to avoid noticeable wear. Specifically, the number “5” is formed by two materials, each with the same thickness as the window 7. The key cap’s entire top face 6 will have to be worn away before the character disappears. On a side note, this mold design has two features that may be useful in other multishot molding applications. First, the projections 11 increase the surface area and therefore the bond strength between the two materials; these same projections will also tend to increase the lateral strength of the molded parts. Second, a rib 10 is placed below the window 7 to undercut the second molded material and ensure that the two pieces are not separable.

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Figure 13.11: Two layer injection blow molding

13.5.2 Core-Back Molding Core-back molding refers to a multi-shot molding process in which a portion of the mold, typically a core, is moved to create or reveal a mold cavity into which a second plastic melt can be injected adjacent to a previously molded first plastic melt. A design for a mold with core-back capabilities used to make front or tail indicator lights for vehicles is shown in Figure 13.12 and Figure 13.13 [58]; the molded piece in this application may consist of three different colors such as red, clear, and amber. The mold consists of an upper mold half 4 and a lower mold half 5 that together form a cavity. The cavity is split into three different portions 11, 12, and 13 through the use of four blades 14, 15, 16, and 17 that are independently actuated by pistons 18, 19, 20, and 21 integral with cylinders 22, 23, 24, and 25. Each cavity is fed polymer melt through runners 7, 8, and 9.

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Figure 13.12: Plane view of mold with core-back

b

Figure 13.13: Section view of mold with core-back

In the molding operation, the mold and the blades are closed to isolate the different portions of the mold cavity so that the different plastic materials can be sequentially injected. To reduce cycle time, it is preferable to concurrently inject polymer melts from nozzles 1 and 2 through the runners 7 and 8 into the areas 11 and 12. Once these materials are sufficiently solidified, the cylinders are actuated to retract the blades 14, 15, 16, and 17. The third plastic may then be ejected through runner 9 into the third area 13 of the mold cavity. In this manner, a molded part can be produced consisting of multiple materials without ever opening the mold.

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There are two items of note regarding this core-back design. First, it is possible to have designed a mold utilizing a single set of blades 16 and 17 to reduce the size and complexity of the mold. Indeed, for strength reasons a preferred design would use a single set of blades that interlock with a slot on the opposing face of the mold cavity. However, one possible reason for the design of Figure 13.12 and Figure 13.13 is that the retraction of the blades into both sides of the mold provides a means for the molding of protruding ribs into the upper and lower sides of the mold 4 and 5. In any case, structural analysis should be used to ensure that the blades are of sufficient thickness to avoid excessive shear stress and bending in the blades given the thickness of the cavity and the operating melt pressure. The second item of note concerns the use of two sets of blades as opposed to a single central core that could be withdrawn. An alternative mold design could avoid the use of blades altogether by making the entire center section 13b of Figure 13.13 a single actuated member. In this alternative design, the center section 13b would be in a forward position upon mold closure, closing the cavity area 13 of Figure 13.12 from the melt and providing the same cavity side walls effectively provided by the blades 14, 15, 16, and 17 in the previous design. After the left and right areas 11 and 12 were filled with plastic, the center section 13n could be retracted and the cavity area 13 filled with a third plastic via runner 9. It may seem that this alternative design would require extremely high actuation forces for the center section 13b given its large projected area subjected to melt pressure. However, this is not the case since the center section will not see significant pressure when molding areas 11 and 12, and can be supported by a shoulder or other mold components when retracted and exposed to high melt pressures.

13.5.3 Multi-Station Mold Parts consisting of multiple materials are also often molded in multi-station molds. One such design is shown in Figure 13.14 to produce a replica of the Canadian national flag [59]. In this design, a first mold is composed of a transfer plate 47, a cavity plate 48, and a runner plate 49. The cavity plate 48 defines a cavity 51 including lateral panel segments 52, a central maple leaf image segment 53, and bridges 54 that connects all of the segments together. In operation, the first plastic melt is injected from a runner 58, into a sprue 56, through a gate 57, and into the cavity 51. Once this plastic solidifies, the mold is opened and the transfer plate is removed from the first mold. Because of the mold design, the solidified runner 68 and molding 61 remain with the transfer plate. The transfer plate 47 with the solidified runner 68 and molding 61 is then transferred to a second mold. In this design, the transfer plate inserts the solidified molding 61 into another cavity 70 in a second mold cavity plate 69. The transfer plate is then moved laterally to strip the feed system from the molding 68. Additional plates 71 and 75 are then positioned with the mold cavity plate 69 to completely form a second mold. The second material is then injected adjacent and over the first material to form a single part integrating the two plastic materials.

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Figure 13.14: Multi-station mold design

Compared to a core back mold design, the multi-station mold provides for greater flexibility in the molded part design. Specifically, multi-station molds allow for complex parts to be molded, and then inserted into other arbitrarily complex cavities for injection of additional plastic materials adjacent to, above, and around the prior molding(s). Accordingly, several different mold and machine designs have been developed to support multi-station molds. These include the transfer of parts via rotating mold sections similar to the designs shown in Figure 13.9 and Figure 13.10. More recently, dedicated two-shot molding machines have been developed as shown in Figure 13.15 [60]. In this type of design, the injection units provide material to two sets of mold cavities mounted on two opposing platens. Because the platens oppose each other, a single clamping mechanism can be used to provide the mold closure force for both sets of cavities, very similar to stack molds as discussed in Section 13.6.2. Different drive mechanisms have been developed to index the cores including turret drives as shown in Figure 13.15, rack and pinion arrangements [61], and others.

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Figure 13.15: Turret style molding machine and mold design

13.6 Feed Systems 13.6.1 Insulated Runner As presented in Chapter 6, the most common types of feed systems are cold runners and hot runners. Both types of feed systems have disadvantages. With cold runners, there is considerable material waste associated with the formation of the feed system as well as the potential for extended cycle times.With hot runners, there is the additional cost and complexity associated with the temperature control systems as well as the potential for temperature variations and color change issues. As an alternative to both cold runner and hot runner designs, the insulated runner was designed in an attempt to eliminate these disadvantages. An insulated runner design is shown in Figure 13.16 [62]. The design layout is very similar to a three plate mold with a runner section 15, a cavity section 16, and core sections 17. The runner layout is also similar with a sprue 19 conveying the melt through the plate thickness to primary and secondary runners 18 that convey the melt across the parting plane to a second set of sprues, 22 and 23, which convey the melt down to the mold cavities. Compared to a traditional three plate mold,

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Figure 13.16: Insulated runner design

however, all segments of the feed system are purposefully designed to have large diameters. In addition, the runner section 15 is secured to the cavity section 16, and does not open at all during normal molding. During the molding process, the melt is injected from the nozzle of the molding machine and completely fills the feed system. A skin, 18a and 18b, immediately forms on the surface of the runners. However, the solidified skin does not fully propagate throughout the runner since • •

the thermal conductivity of plastic is very low, and each molding cycle conveys heated polymer melt from the molding machine throughout the feed system.

As a result, the diameter of the molten core remains nearly consistent during the molding cycle. In this manner, the insulated runner can be operated as a hot runner albeit without any heaters, thermocouples, or temperature controllers. The color change issue is resolved by removing the fully solidified feed system with the release of runner section 15 from the cavity section 16. The design of Figure 13.16 [62] was specifically intended for the molding of semi-crystalline polymers such as polyethylene and polystyrene. Experiments were conducted with runner diameters of approximately 25 mm and cycle times in the vicinity of 60 s; the thickness of the skin was approximately 6 mm. Of course, the optimal specification of runner diameters will depend on the material properties, the melt and mold temperatures, and the flow rates and cycle times. The use of internal heaters and insulating layers (such as the airs gaps 40a and 40b around the sprue inserts 39a and 39b as shown in Figure 13.16) can provide greater process robustness albeit with increased design complexity. Perhaps because of these processing uncertainties, the use of insulated runner systems has decreased with the commoditization of hot runners. Even so, insulated runners can provide good performance at low cost.

13.6.2 Stack Molds When the plastic melt is injected into the mold cavity at high pressure, significant clamp force is required to keep the mold closed so that the melt does not escape the mold cavity. Because

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Figure 13.17: Early stack mold design

the clamp force is proportional to the projected area of the mold cavities, the clamp force increases proportionally with the number of mold cavities across the parting plane. However, if the cavities are “stacked” one on top of another, then the clamp force used to close one set of cavities can also be used to close any sets of cavities that are in the stack. One such stack mold design is shown in Figure 13.17, which was to designed to mold two vinyl records with the clamp force and cycle time normally used to mold one vinyl record [63]. In this design, two sets of stampers are mounted between an inner plate 12 and two outer plates 14 and 16; the inner plate 12 is guided by bearings 20. The melt flow from the nozzle 54 of the molding machine through extended sprue 40 to two sets of cavities where the records are formed. After the plastic solidifies, the melt shut-off rod 65 is actuated to seal the sprue inlet 51 with the shut-off 66. This action also connects the sprue 40 to the chute 64, such that the sprue may be stripped from the moldings with actuation of the sprue knock-out rod 75. The molded records are then ejected after retracting the sprue knock-out rod and opening the mold. There are two deficiencies in the mold design shown in Figure 13.17. First, the stack mold requires the formation of a sprue, which is scrap. Second, the melt flow to the two cavities is not balanced due to the additional length of the sprue to the left cavity. Both these deficiencies are resolved in modern stack mold designs that utilize hot runner systems; one such stack mold design is shown in Figure 13.18 [64]. In this design, a central moving plate 56 houses two sets of cavities 60 on opposing parting planes 62 and 64. A hot manifold 65 delivers the polymer melt to the cavities through the runner 70 and subsequent drops. The design uses two single axis valve gates to deliver the melt from the molding machine nozzle 17, across the parting plane 62, and to the manifold 65. During filling and packing stages of the molding process, the actuators 50 and 54 retract the valve pins 24 to deliver the melt from the nozzle to the manifold. Otherwise, the valve pins seal the feed system during the plastication, cooling, and mold reset stages.

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Figure 13.18: Hot runner stack mold design

While the design increases the size and complexity of the mold, it enables the molding of two sets of cavities with the same cycle time and clamp force as a single set of cavities. Furthermore, the flow to both sets of cavities is completely balanced and there is no material waste associated with the hot runner feed system. Given the significant part cost reductions afforded by this type of stack mold design, stack molds are now quite common with two, three, and four levels of cavities. Clearly, the stack mold design requires carefully balancing of potential processing cost savings with issues related to investment, maintenance, color change, stack height, and injection volume.

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13.6.3 Branched Runners A potential issue in “naturally balanced” branched runners, such as shown in Figure 13.19 [65], is flow imbalances due to thermal variations caused by the flow and related shearing of the melt [27]. Despite the geometrical balance of the feed system, it has been observed that parts formed in cavities may be larger and heavier depending on their location in the branched feed system. The flow imbalance is created by a non-symmetrical shear distribution within the laminar plastic melt as it flows through the runner system. Specifically, in the feed system there is a distribution of shear rates and temperatures across the radius of the runner: a hot polymer melt at the center of the runner is surrounded by a layer of higher sheared, hotter, and lower viscosity plastic melt. When the laminar melt flow reaches a branch in the runner system, the lower viscosity melt remains in its outer position while the more viscous melt at the core is split and flows to the opposite side of the branch 14. This lateral variation in viscosity will cause a non-uniform flow distribution at the next downstream branch, 16 and 22. To resolve the flow imbalance, it is necessary to eliminate the lateral viscosity variation in the polymer melt. One approach shown in Figure 13.20 [65] imposes a level change just prior to the branch. Specifically, the upstream section 100 of Figure 13.20 corresponds to the primary runner 12 of Figure 13.19 while the downstream section 104 corresponds to the secondary runner 14. Prior to the branch, a flow diverter 106 forces the melt upwards into the runner extension 102. When the melt subsequently flows down into the runner 104, the more viscous inner core is directed to the side of the runner that is opposite the level change. Since the viscosity variation is now distributed vertically through the runner, the melt flow is balanced when the downstream runners branch laterally. Figure 13.20 provides a design for a set of inserts to accomplish the level change. The cavity insert 150 and the core insert 156 are placed at any necessary junction between the upstream and downstream runners. An indented cavity 164 and a protruding core 162 accomplish the level change. Because the viscosity variation is only reoriented and not eliminated, the use of multiple level changing inserts at consecutive runner branches will re-establish the

Figure 13.19: Branched runner system

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Figure 13.20: Level change mold inserts, also known as Melt Flipper

flow imbalances. More recently, research has shown that the flow imbalance and the ability to control the melt flow is related to the melt rheology and the processing conditions [28]. For this reason, additional designs have been developed to adjust the viscosity distribution in the feed system [66].

13.6.4 Dynamic Melt Control The goal of injection molding process control is to specify the pressure and temperature distribution across the entire cavity. There are many possible concepts for adding the necessary degrees of freedom but one generic approach is to provide a means for instantaneously modifying the flow resistance in each branch of a runner system. As shown in Figure 13.21 [67], one design uses a set of strategically located, variable impedance melt valves each individually controlled with a rapid-response hydraulic actuator. The valves are designed with an adjustable annular clearance 81 between a tapered valve stem 45 and a tapered surface 47 of a bore 19. Since the resistance to flow is determined by the annular gap between the valve stem and the mold wall, axial displacement of the valve stem can be used to selectively vary the flow rate and pressure drop through each valve. When used in a closed loop control system, this method can provide simultaneous control of multiple cavity pressures. This system implementation introduces three new characteristics into the molding process [68]. First, the independent control of each valve allows the pressure and flow in multiple regions of the cavity to be decoupled. Previously, changes aimed at improving one area of the part could result in detrimental effects elsewhere in the cavity since process changes could not be controlled independently. With this process, the flow through each valve can be controlled independently, bringing extra degrees of freedom to the molding process. Second, the capabilities of this system can be leveraged by dynamic re-positioning of the valve within

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Figure 13.21: Dynamic feed control

the molding cycle. This strategy can be used, for instance, to specify one set of valve positions to profile flow rates in the filling stage followed by a completely different set of valve positions to profile pack pressures. Third, the dynamic capabilities of this process allow the valves to be quickly controlled in response to feedback from process sensors in the mold cavity, thus providing closed loop control of the cavity state variables which directly determine the product quality. Variation in molding machine input parameters, machine behavior, or material properties can be dynamically compensated to produce consistent parts. Moreover, the control of cavity variables directly enables the use of pressure measurements as a process control technique for automated detection of quality problems. This could eliminate the need for manual inspection of part quality in many circumstances. Since the dynamics of the molding machine are decoupled from the cavity, details of molding machine performance are made less significant. The size, complexity, and cost of a closed loop melt control system can raise significant barriers to implementation in many molding applications. To reduce the cost and complexity of the system, a self-regulating valve design was developed as shown in Figure 13.22 [69] to work with an open loop control system design and not require any melt pressure transducers. The melt entering the flow channel 14 will flow into the aperture 20b and around the head of the valve pin to apply a dynamic force 24 against the projected area of the valve pin, which will tend to shut-off the melt flow and reduce the melt pressure. At the same time, an opposing control

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26

24

14b Figure 13.22: Self-regulating valve design

force 26 is applied to the stem 16 of the valve pin, which will tend to increase the melt flow and the melt pressure. As a result, the pin will move until equilibrium is established between the dynamic force 24 and the control force 26. In other words, any difference between the control force 26 and the dynamic force 24 will cause movement of the control member 16 until the control force and the dynamic force equilibrate, thereby regulating the melt pressure. The outlet melt pressure will be approximately equal in magnitude to the control force divided by the projected area of the valve. Previous research [70, 71] has shown that shear stresses and pressure drops along the length of the valve pin 16 may contribute to errors in the output melt pressure of a few percent. If the control force is provided via a hydraulic or pneumatic cylinder, then the output melt pressure is equal to the pressure supplied to the actuator times an intensification factor, typically on the order of 100 : 1 as determined by the ratio of the push area of the actuator to the area of the head of the valve pin. By controlling the actuation pressure to each valve cylinder during the molding process, the molding process can be made more consistent and more flexible compared to conventional injection molding while not requiring cavity pressure transducers or a closed loop control system.

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13.7 Mold Wall Temperature Control The analyses and designs presented in Chapter 9 for cooling systems are adequate for most injection molding applications. However, there are some applications in which the use of conventional cooling designs is unacceptable. Normally, the development of a solidified skin occurs when the hot polymer melt contacts the cold mold wall. In some molding applications, the solidified skin may lead to premature freeze-off of the melt in the cavity, excessive birefringence in the molded part, or inadequate levels of gloss or surface replication. In other applications, mold wall temperature fluctuations across the surface of the mold cavity may lead to a lack of dimensional control. As such, some molding applications involving lenses, airplane cockpit canopies, optical storage media, and fiber reinforced materials may seek to improve the quality of the moldings through dynamic control of the mold wall temperature. Several different strategies are next discussed.

13.7.1 Pulsed Cooling One approach to controlling the mold wall temperature is to use one or more sets of cooling channels to actively heat and then actively cool the mold. One such mold design is shown in Figure 13.23, which was developed to provide tight tolerances when molding highly sensitive plastic materials or very thin walled moldings [72]. In this design, a mold cavity 7 is formed by a cavity insert 10 and a core insert 9. The core insert is purposefully designed to be as thin as possible, and surrounds an internal core 12 so as to provide a channel 14 for circulation for temperature controlled fluids. The cavity insert 10 is similarly designed to mate with the cavity plate 28 and the outer insert 29 to form channels 24 and 25. In operation, two fluids are separately temperature-controlled with a heating device 35 and a cooling device 34; two separate fluids are recommended to reduce the cost and time associated with sequentially heating and cooling a single fluid. Prior to the injection of the polymer melt, the control valves 36 and 37 will direct the heated fluid to the inlet 18 and through the mold core via channels 14 and 15 before returning via the outlet 16; a similar heating circuit is formed for the mold cavity via elements 26, 22, 25, and 27. Once the inserts 9 and 10 are at a temperature above the freezing point of the plastic melt, the plastic melt is injected into the cavity 7. The control valves can then be actuated to direct the cooling fluids from the cooling device 34 through the same channels previously used for heating. The success of this mold design is highly dependent on minimizing the mass of the mold steel and coolant required to form and cool the walls of the mold cavity. It is clearly desirable to minimize the thickness of the mold inserts, the length of the cooling channels and lines, and the heat transfer to adjacent mold components. In this design, air gaps 20, 29, and 38 are used to reduce the amount of heat transfer and so improve the thermal efficiency and dynamic performance of the mold; insulating sheets (not numbered) are also provided adjacent the top and rear clamp plate to minimize heat transfer to the platens. Unfortunately, the size of the cavity and the structural requirements on the mold components necessitates the use of fairly large mold components that need to be heated and subsequently cooled. The dynamic

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Figure 13.23: Mold design for pulsed cooling

thermal response is limited. For example, a 100 kg section of P20 that needs to be heated 100 °C and subsequently cooled will require a minimum of 10 MJ (2 times the specific heat of 500 J/kg°C times the mass of 100 kg times the temperature change of 100 °C) or 3 kW h of energy. At a cost of $0.10 per kW h, the energy cost for heating and cooling alone is on the order of $0.30 per molding cycle. For this reason, pulsed cooling is not commonly used except in very demanding applications.

13.7.2 Conduction Heating Given the large thermal mass of the mold and the cooling system, another strategy to control the mold wall is to use conduction heaters at or near the surface of the mold. One design is shown in Figure 13.24 which was developed to provide a smooth surface finish to one side of a foamed plastic product [73]. The mold consists of a cavity insert 12 and a core insert 10, both including a network of cooling lines 34 and 36 as per conventional mold design. A thin metallic sheet 38 conforms to the surface of the mold cavity 12, with a thin insulating layer of oxide deposited between the sheet and the cavity insert. The thin metallic sheet 38 includes an

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Figure 13.24: Mold design with conduction heating

opening 40 to deliver the plastic melt from the sprue 32 to the mold cavity 14. Electrical cable attachments 46 and 48 attach the sheet 38 to low voltage, high current electric cables 50 and 52. Because a very high current load is applied for a short period of time during each cycle and then disconnected by switch 54, a bypass load in the form of heating pipe 62 is provided to minimize arcing in the switch with current surges applied to the current source 56. The electrical resistance of the pipe 62 is high relative to the resistance of the sheet 38 so closure of the switch 54 effectively creates a short circuit path through the sheet 38. The bulk of the current from source 56 passes through the sheet 38 when the switch 54 is closed. Just prior to mold closure, the switch 54 is closed to pass a high current through the sheet. In this design, a 0.2 cm thick steel plate was used with a length and width of 30 cm and 10 cm, respectively. Given an electrical resistivity of 30 μΩ cm for the steel plate, the electrical resistance between the cables 50 and 52 would be 450 μΩ. The specification of the patent indicates that experiments were conducted which yielded a temperature rise of 200 °C in 0.4 s with

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a current of 500 A. With this electrical current and resistance, the driving voltage would be 0.23 V leading to a power dissipation in the sheet of 113 W. Given the 0.4 s heating time, the energy consumption per molding cycle would be 0.013 kW h. For comparison, the mass of the steel sheet would be 4.68 kg and would require a minimum of 0.5 MJ or 0.13 kW h of energy to heat. There is a large discrepancy between the supplied and the required heater power. To further analyze the heating requirements, consider a typical molded part with a heat capacity of 2000 J/kg °C, a 3 mm thickness, a melt temperature of 240 °C, an ejection temperature of 100 °C, and a cycle time of 30 s. In this case, the heat load imposed on the mold by the ABS melt is 28 kW/m2; given that the cooling lines are placed on two sides of the mold, the cooling power is approximately 1.4 W/cm2. As such, a 30 cm by 10 cm heating plate must deliver at least 420 W simply to overcome the heat transfer to the cooling lines before the temperature of the heating plate begins to increase significantly. It is noted that conduction heaters are widely available with power densities exceeding 250 W/cm2. Such a heater, if placed on the surface of a mold cavity, could increase a 0.2 cm by 30 cm by 10 cm steel plate’s surface temperature by 200 °C in 6 s. Attempts have been made to incorporate higher power, thin film heaters directly into the mold surface [74]. However, such efforts to incorporate conduction heaters into molds have not been widely successful for at least three reasons. First, the large, cyclic pressure imposed on the heater(s) by the polymer melt tends to fatigue the heaters. Second, it is difficult to configure the heater(s), mold cavity, and cooling channels to provide the uniform wall temperature required to deliver aesthetic surfaces with tight dimensional controls. Third, the heaters are located between the mold cavity and the cooling channels, tend to reduce the rate of heat transfer during cooling, and so extend the cooling time.

13.7.3 Induction Heating Induction heating is another approach to increasing the mold wall temperature prior to mold filling. One design is shown in Figure 13.25 [75], which was developed to injection mold reinforced thermoplastic composites with superior surface gloss and substantially no surface defects. To reduce energy consumption and heating time, only a small portion of the mold’s surface is selectively heated by high-frequency induction heating. As shown in Figure 13.25, a conventional injection molding machine 3 delivers polymer melt to a mold consisting of a stationary mold half 4 and a movable mold half 5. Prior to mold closure and filling, a high-frequency oscillator 1 drives alternating current through an inductance coil (inductor) 2 temporarily placed near the surface(s) of the mold. When a high frequency alternating current is passed through the inductor 2, an electromagnetic field is developed around the inductor which subsequently generates eddy currents within the metal. The resistance of the mold metal subsequently leads to Joule heating of the mold surface. Traces A and B in Figure 13.25 demonstrate the increased mold surface temperature at locations A and B caused by induction heating; traces C and D show no initial effect at location C and D away from the induction heating but later increase with the heat transfer from the injected polymer melt into the mold cavity.

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Figure 13.25: Mold design with induction heating

As with all the previously described schemes for mold wall temperature control, it is desirable to elevate the surface temperature of the mold as quickly as possible. The heating power through a high-frequency induction heating is proportional to the square of the alternating frequency, the square of the current, and the square of the coil density, among other factors. As such, the inductors must be carefully designed to locally heat the mold surface in a controlled manner to avoid an undesirable temperature distribution. For example, an inductor was made from copper tube of 5 mm diameter and wound as a spiral with a pitch of 5 mm. The distance between the surface of the metal mold and the inductor was set to 1 cm. Experiments indicated that a driving frequency of 400 kHz yielded a heating power at the mold surface on the order of 1000 W/cm2, which required approximately 10 s to increase the surface of the mold by 50 °C. Compared to conduction heating, induction heating provides for increased heating rates and the potential for very little additional mold complexity. The primary issue in implementation is the design of the inductor, and in particular the spacing of its coil windings and their relation to the mold surfaces. If the design is improper, then the heating may be limited to low power levels. Experiments [75] indicated that a heating power less than 100 W/cm2 did not significantly increase the mold surface temperature and eventually caused the overload breaker to actuate. On the other hand, when the power output exceeded 10,000 W/cm2, the rate of the surface temperature increase became too steep to control such that uniform heating was no longer possible; defects such as gloss irregularities, sink marks, etc. were observed with temperature differences of more than 50 °C across the surface of the mold.

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13.7.4 Managed Heat Transfer Given the difficulties associated with active mold wall temperature control, a “passive” cooling design has been developed; the term “passive” is used to imply that the mold does not utilize any external power to control the mold wall temperature. The design shown in Figure 13.26 was specifically developed to control the mold wall temperature during the molding of optical media [76]. The mold includes two halves 12 to form a mold cavity 14. Cooling lines 20 are provided per conventional design to remove the heat from the polymer melt. However, a thermal insulating member 22 is placed between the mold halves 12 and the stampers 31 and 33. The thermal insulating member 22 is made from a low thermally conductive material, preferably a high temperature polymer, such as polyimides, polyamideimides, polyamides, polysulfone, polyethersulfone, polytetrafluoroethylene, and polyetherketone. The insulating polymer is typically spin coated in an uncured form to provide a layer with a thickness on the order of 0.25 mm and subsequently heat cured. The stamper 33 is typically fabricated from nickel, and provides the surface details for replication while also protecting and providing the insulator with a uniform, highly polished surface during molding. During molding, the insulating layer 22 behind the stamper 33 slows the initial cooling of the resin during the molding operation. Because of this insulation, the stamper’s temperature increases and so the skin layer retains heat longer during the mold filling stage, thereby

Figure 13.26: Mold design with managed heat transfer

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avoiding the surface irregularities created by rapid surface cooling. The temperature of the stamper:melt interface can be controlled by specification of the process conditions as well as the layers’ thicknesses and material properties; one-dimensional cooling analysis can be used to understand the physics and assist in the design optimization. In this example, it was found that the centerline temperature 51 of the disc dictates the minimum cooling time for the part to cool below the glass transition temperature of the polymer melt. The temperature 52 at the stamper:melt interface impacts the thermal stress and pit replication on the disc’s surface and is measured. The temperature 53 in the mold behind the insulator suggests that the mold acts as a heat sink and is maintained at a substantially constant temperature. The mold designer and process engineer should intuitively understand that the addition of an insulating layer will tend to reduce the rate of heat transfer from the melt to the mold, and therefore require extended cooling times. To alleviate this issue, the cooling lines can be operated at a lower temperature to provide for higher rates of heat transfer after the initial heating of the stamper. Accordingly, this design strategy provides a reasonable level of mold wall temperature control without any additional energy consumption or control systems. However, the level of temperature control is limited compared to the other active heating designs. In addition, this approach may be difficult to apply to complex three dimensional geometries.

13.8 In-Mold Labeling Injection molding generally provides highly functional and economical parts. In many applications, however, there is a requirement for decorating, detailing, or otherwise labeling the molded parts according to the consumer’s needs. In some cases, the decorating may be provided by post-molding processes such as hot stamping, pad printing, painting, screen printing, and others. In other cases, these techniques are not feasible since some plastic resins such as polypropylene, polystyrene, polyethylene, and others are inherently resistant to dies, inks, and paints. One approach might be to utilize two-shot molding with different types of plastics to achieve the necessary detail. However, this approach is relatively expensive to implement. In addition, two shot molding does not provide for the level of drawing detail or the number of colors that may be desired. As such, molds can be designed to incorporate printed labels that are secured to the molded part during the molding process. There are several advantages related to such in-mold labeling. First, the labels themselves can have very fine graphic details with multiple colors produced by screen printing or offset lithography. Second, the labels can provide for wear resistance through the use of an acrylic or polycarbonate layer laminated over the printed surface. Third, the cost of in-mold labeling is relatively small and little additional tooling cost, only slight extensions of the molding process, and requires no post-molding processes.

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13.8.1 Statically Charged Film In this design, labels are typically printed on a film with a thickness on the order of 0.15 mm (5 mils), and of a polymer (such as polypropylene or polystyrene) that is compatible with the plastic being molded. Since a thin film is made of flexible plastic, the thin film can be placed onto curved surfaces. Different approaches may be used to secure the film in place during the molding process including adhesives, compressed air, vacuum, and static charge. Figure 13.27 shows a method for in mold labeling using a film that is statically charged prior to its placement in the mold [77]. In this design, two mold halves 17 and 19 form a cavity 23. Prior to molding, a film 11 is placed in the mold cavity and secured by static charge applied to either the film or the mold block; most often, the film is charged by ionizing the air around the film with a high voltage from a nearby electrode. The film 11 is made of the same material as the molded part 25, and has a printed design 13 facing the mold cavity. Once the film is placed in the mold, the molding process continues as normal. The heat of the polymer melt causes the film 11 to melt and fuse with the part 25, such that the printed design 13 appears without any evidence of the film 11. Although the printed areas 13 may not fuse into the plastic, these areas can be adequately bonded by the fusion of the surrounding non-printed areas. If necessary, the printing may be imperceptibly dithered to facilitate fusion between the molded part and the film. A few comments are warranted about the film thickness and the processing conditions. The mold designer should recognize that the film must withstand both thermal and structural loadings. The structural loading is driven by the shear stress applied by rate of the polymer melt flowing across the film and not the magnitude of the melt pressure; the analysis of Section 5.3.1 can be used to estimate the shear stress as a function of the polymer properties, part geometry, and processing conditions. The thermal loading is related to the heating and melting of the film by the polymer melt. If the film is too thin, then the printed design may be destroyed by the complete melting and flow of the film during the molding process. Analysis and experimentation may be required to optimize the film and process.

Figure 13.27: In mold labeling with static charge

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13.8.2 Indexed Film Statically charged films are quite common with in-mold labeling. One potential issue, however, is the placement of the films into the mold by either human or robotic operation. If a human operator is used, then issues may arise pertaining to processing delays, safety, or repeatability. If a robot is used, then issues may arise related to processing delays and cost. For molding applications with higher production volumes, it may be better to resolve these issues through the use of a mold design that automatically indexes the printed film through the mold with each molding cycle. One such design is shown in Figure 13.28 [78] for the production of bottle cap with a retaining ring. In this design, the mold 25 includes a top clamp plate 26, a movable cavity plate 28, and a conventional moving half of the mold, 30. The cavity plate 28 is retained to the top clamp plate 16 via fasteners 37. However, a counterbore is provided in the top clamp plate to allow the fasteners to slide such that the cavity plate is moved away from the top clamp plate by compression springs 39 when the mold opens. The resulting gap is approximately 0.5 mm in height, and provides clearance for the carrier ribbon 55 or 90 to slide between the cavity plate and the top clamp plate. The carrier ribbon is supplied from roll 58, around guide rolls 59, through the gap 41, to the mold cavities 50.

Figure 13.28: In mold labeling with indexed film

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When the printed film 60 is properly positioned, the mold closes and the film is secured by the clamping of the carrier ribbon between the mold cavity plate and the top clamp plate. With the mold filling, the printed design is transferred from the carrier ribbon to the molded part 72 which subsequently solidifies. After the opening of the mold and the removal of the molded part, the used carrier ribbon is indexed by the drive motor 64 and the feed rolls 62 and 66, then directed to a suction tube 68 where it is recycled or discarded. As such, this mold is designed to operate very rapidly. More recently, advanced mold and process designs have been developed to allow for very complex in-mold labels that may be shaped and contain cut-outs. These designs may utilize an indexing mechanism to consecutively handle thicker printed sheets. In such an operation, each sheet is thermoformed to conform to the surface of a complex mold cavity. Then, one or more punches may provide holes or otherwise size the formed and printed sheet to the shape of the mold cavity. Finally, each completed label is placed into the mold where it is bonded to the molded part. As such, in-mold labeling can be used to provide nearly any appearance (such as marble, national flags, etc.) to nearly any part (such as cell phones, etc.)

13.9 Ejection There are many types of ejection systems, and Chapter 11 provided guidelines for analysis and design of the most common types. In addition to these previously discussed designs, many specialized ejection system designs have been developed to provide molded parts with very complex exterior details, very complex interior details, an aesthetic surface completely free of defects, and other purposes. Some of the relatively common ejection systems are next discussed.

13.9.1 Split Cavity Molds As discussed in Sections 11.3.6 and 11.3.7, core pulls and sliding inserts are commonly used when there is one or more external undercuts. If the section of the cavity with undercuts is very large, or if the exterior of the molded part necessitates a parting plane that is transverse to the mold opening direction, then a split cavity mold is often designed. As the term “split cavity” implies, a split cavity mold is a mold design in which the cavity insert is split into two or more pieces, such that the walls of the cavity can be moved away from the molded part during the ejection stage of the molding cycle. One split cavity mold design is shown in Figure 13.29 for the molding of bowling pins [79]. The mold includes a top clamp plate 14, a cavity retainer plate 16, and a support plate 12, among others. The split cavity is formed by two moving cavity inserts 23 and 24 that mate with the conical bore 21 in the cavity retainer plate 16 when the mold is closed. Four elongated angle pins 30 are fastened in the top clamp plate and extend through the cavity inserts 23 and 24. Each cavity insert is fastened to two gibs 28 that can traverse in slideways 26.

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Figure 13.29: Split cavity mold design

When the mold opens, the support plate 12 is moved away from the cavity retainer plate 16. Since the angle pins 30 are stationary and inclined relative to the mold opening direction, the cavity inserts 23 and 24 are forced to move away from each other through a cam action. Ultimately, a sufficient clearance is produced between the molded part 66 and the cavity side walls 44 so that the molded product may be removed. There are a few interesting items to note regarding this particular split cavity mold design. First, there is a significant amount of mold cheek provided in the cavity retainer plate 16. The thickness of the cheek is required to avoid excessive shear stress and deflection of the cavity side walls 44. It is observed that the thickness of the cheek is approximately the same as the depth of the mold cavity, as suggested by the analysis of Section 12.2.4. Second, wear can be an issue in this mold design due to the large mass of the inserts, the length of travel, and the high number of molding cycles. For this reason, the gibs should be specified to include lubricity and be easily replaced when necessary. In addition, wear plates should be incorporated between the support plate 12 and the cavity inserts 23 and 24. Third, internal cooling of the core is provided through the use of a large bubbler 75 with coolant inlet 74 and outlet 76.

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Split cavity molds have been designed for quite some time, and this design was not selected solely due to its incorporation of a split cavity design. Another interesting feature of the design is the forward actuation of the core pin 50 during the filling and packing stages to provide injection compression molding. This actuation is needed to compensate for the very high volumetric shrinkage during the solidification of the thick side walls 54 of the molded part. As such, the mold design includes a bearing 46 that supports the shoulder 65 of the core pin. Since the molded part 66 will tend to shrink onto the core pin 50, the core pin must be retracted after the mold is opened as shown in Figure 13.29 to release the molded part.

13.9.2 Collapsible Cores Split cavity molds are often used when the part design includes complex and undercutting external surfaces. Collapsible cores are often used when the part design includes complex and undercutting surfaces on the interior of the part. The design of a mold which includes a collapsible core is shown in Figure 13.30, which was developed to mold the head of a doll with a nearly uniform wall thickness [80]. The mold cavity (14 and 15 together) is formed by two cavity inserts 12 and 13, which are hollowed out by a collapsible core 17. In this design, the collapsible core is comprised of eight segments 18, 19, 20, 21, 22, 23, 24, and 25. Four of the segments, 18, 19, 20, and 21 are mostly triangular in section and fitted at the corners with a contoured outer surface in the desired form of the core. The other four segments, 22, 23, 24, and 25 are mostly planar in section and fitted between the corner segments with a contoured outer surface to complete the desired form of the core. A core rod 37 is located at the center of the core, and prevents the radial displacement of the eight segments when the collapsible core is assembled. To prevent the axial displacement of the collapsible core, all eight segments have a stem 35 with external threads 35a that engage the internal threads 39 in a sleeve 38. The operation of the collapsible core relies upon the threads 37b of the core rod 37, and their engagement with the threaded passageway 41 of the sleeve 38. Specifically, prior to molding the core rod is rotated within the sleeve so that it fully extends until its distal (far) end is flush with the ends of the eight segments to form a rigid core 17. The sleeve with the rigid core is then placed in the mold cavity and the part is molded according to conventional practice. Once the part is solidified, the mold is opened and the molded part is removed along with the core and sleeve. The core rod 37 is then unscrewed from sleeve 38 and removed from the inside of the core 17. Without any support, the eight segments can collapse and be removed from the inside of the molded part. The segments, core rod, and sleeve are then reassembled for the next molding cycle. The collapsible core design of Figure 13.30 allows very complex and undercutting features to be formed internal to the molded part. Because of its design, however, a significant amount of time is required to assemble and disassemble the moving core. To facilitate the design and manufacture of molds with collapsible cores, standard collapsible core designs have been developed and are available from a number of mold base and component suppliers. In typical designs, the actuation of the ejector plate slides the segments along a retaining sleeve, which provides a cam action to collapse the core segments during the ejection of the molded part.

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Figure 13.30: Mold design with collapsible core

The diameter of commercially available collapsible cores ranges from 13 to 90 mm, with a collapse of approximately 6% of the core diameter. While their collapse is not nearly as much as the design of Figure 13.30, these standard components support fully automatic molding of small features such as internal threads for molded closures.

13.9.3 Rotating Cores Collapsible cores are relatively simple to incorporate into mold designs when using a purchased assembly, and can be used for the forming of threads, dimples, windows, and other internal features. However, one issue with collapsible cores is the formation of witness lines on the interior of the molded part where the core segments interface. Depending on the application requirements, these witness lines may prohibit the use of the collapsible cores. As such,

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Figure 13.31: Mold design with coarsely threaded rotating core

many different mold designs have been developed with rotating cores for the formation and demolding of internal threads. One design is shown in Figure 13.31 for a 64 cavity mold for the production of threaded caps [81]. The mold design includes cavities 16 that are formed by matching sets of cavity inserts 10 and core inserts 15. The back of each core includes an integral support 17, which is mounted upon a shaft 18 that extends from a coarsely threaded helix 19. The helix is axially located between the rear clamp plate 21 and the support plate 23, and radially supported by bearings 20 and 26. Follower pins 30 have been fitted to the actuated plate 29 that engage the threads of the helix 19. Since these pins can not rotate, the actuation of the plate 29 will cause the rotation of the helix 19, and the subsequent rotation of the threaded cores 15. Regarding the design, a coarsely threaded helix is necessary since the torque and wear will increase substantially as the pitch decreases. As such, the required length of the helix is related to the friction between the helix and the follower, as well as the number of rotations in the molding application. Another mold design for rotating cores is shown in the plan view of Figure 13.32 [82]. In this mold design, a central sun gear 84 simultaneously actuates multiple planetary gears 86, which in turn drive shafts 88. The core inserts are not shown in Figure 13.32, but are keyed to the drive shafts through slots 66. There are many possible designs to rotate the sub gear 84. In this design, a pinion 42 is attached to a shaft 74 which ends at a bevel gear 76. This bevel gear 76 meshes with another bevel gear 78 that is locked to the central sun gear 84. In operation, the mold opening stroke causes

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the rack 44 to engage the pinion 42, the pinion 42 to rotate the shaft 74, the shaft 74 to rotate the bevel gear 76, the bevel gear 76 to rotate the bevel gear 78, the bevel gear 78 to rotate the sun gear 84, the sun gear to rotate the planetary gears 86, the planetary gears 86 to rotate the shaft 88, and the shaft 88 to rotate the cores keyed to slot 66.

There are advantages and disadvantages of this mold design compared to the previous design of Figure 13.31. The primary advantage is the use of multiple gearing stages to decouple the actuation of the rack and pinion from the rotation of the cores. As such, it is possible to delay and otherwise program the rotation of the cores during the mold opening while avoiding the very large stack height associated with the coarse helix of the previous design. The primary disadvantage is the large number, complex layout, and large volume of the gearing stages. In addition, the planetary layout suggests a radial layout of cavities and so may require very large molds for a high number of cavities. Accordingly, the planetary gear design may be preferable in a mold with a relatively low number of cavities requiring high actuation torques.

Figure 13.32: Mold design with planetary gearing of rotating cores

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With either design strategy, the mold designer should ensure that the part geometry is designed to prevent the rotation of the molded part with the rotating core. In some cases, the runner and gate may provide sufficient strength to prevent the molded part’s rotation. In other cases, however, this approach is inadequate since the ejection forces will tend to vary with the material properties, processing conditions, and surface finish as analyzed in Chapter 11. For this reason, the mold design may use some small undercuts or other non-asymmetric features to prevent the part rotation.

13.9.4 Reverse Ejection The cavity inserts in most molds are located within the stationary side of the mold and the core inserts are located on the moving side of the mold. Since the molded part shrinks onto the cores as the plastic cools, the molded parts will tend to remain with the cores on the moving side of the mold when the mold is opened. Accordingly, molds are usually designed with an ejector housing and ejector plate on the moving side of the mold such that ejector pins can remove the part from the core. However, this conventional design is problematic in that it does not provide for a purely aesthetic surface, completely free of defects, on either side of the molded part. Witness marks will typically be left on the core side of the molded part from the ejector pins while witness marks will typically be left on the cavity side of the molded part from the feed system.

Figure 13.33: Mold design with reverse ejection

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To provide a completely aesthetic surface, molds can be designed with “reverse ejection”. One such design is shown in Figure 13.33 [83], which includes a mold cavity plate 68 on the moving side of the mold and a mold core plate 38 on the stationary side of the mold 36. The sprue 76 conveys the plastic melt from the machine nozzle 14 through the mold core plate 38 to the mold cavity 40 and 74. Because the molded part will tend to remain on the mold core, the stationary side of the mold 36 also includes ejector pins 48 and other components that operate with an ejector plate 30 located between rails 18. Since the molding machine’s ejector rod is located on the moving side of the molding machine and is useless with this mold design, the mold design also includes hydraulic cylinders 32 for actuation of the ejector plate. As a result of this mold design, the entire surface of the molded part opposite the core is free of cosmetic defects.

13.10 Review There are many technologies that can be incorporated into a mold’s design to: • Enable extremely complex molded part geometries, • Make molded parts containing multiple materials, • Produce a hollow part with simple or complex internal features, • Provide a molded part with aesthetic or decorative surfaces, • • • • • •

Control the flow of the melt in the molded part, Increase the consistency of the molded parts, Increase the molder’s productivity by increasing the number of cavities in a mold, Increase the molder’s productivity by decreasing the required clamp tonnage, Decrease the cost of the plastic consumed in molding the product, And other objectives.

There seems to be almost no limit to what the injection molding process can accomplish with advanced mold designs. For many molding applications, however, the issue to be deliberated is not what can be done but rather what should be done for a specific application. The decision as to how to develop a mold design is for the mold designer, who must strive to serve the needs of the molder and end-user. For this reason, critical decisions about the mold design and related technologies should be approved and documented between all the involved parties with a common understanding of the costs, benefits, and risks.