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Gas-assisted injection molding: the effects of process variables and gas channel geometry
Journal of Materials Processing Technology 121 (2002) 27±35
Gas-assisted injection molding: the effects of process variables and gas channel geometry M.A. Parvez, N.S. Ong*, Y.C. Lam, S.B. Tor School of Mechanical and Production Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore Received 24 December 2000
Abstract Polystyrene parts with different rib geometries but having the same aspect ratio were molded using gas-assisted injection molding (GAIM). The process variables that have an in¯uence on the gas bubble distribution, residual wall thickness (RWT), ®ngering formation and mechanical properties were explored. The test results revealed that there is an inherent relationship between gas ®ngering and gas bubble penetration that has consequences on part strength. Shot size and delay time are the most dominant factors affecting the gas penetration, ®ngering formation, RWT and mechanical properties of the GAIM parts. The effects of rib geometries are also discussed. Computer simulation of the GAIM process was carried out using Mold¯ow, a commercial software. The outcomes predicted by simulation are compared with the experimental results. Based on the results, some useful design guidelines are suggested. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Gas-assisted injection molding; Process variables; Rib geometry; Process simulation
1. Introduction Gas-assisted injection molding (GAIM) is an innovative technology for producing plastic parts and has received extensive attention in the plastic manufacturing industries [1±8]. In this process, gas is injected through a nozzle or directly into the core section of a part being molded. The gas ¯ows preferentially through local thick sections with hot interiors and pushes the plastic ahead to occupy the mold. The gas can be injected simultaneously and/or subsequently with the melt, either after some delay time or after the end of the ®lling phase. Despite some pitfalls in the practical application of the GAIM process, it offers a number of advantages over conventional injection molding (CIM). GAIM can substantially reduce material cost, clamp tonnage, cycle time and residual stress, and allows molders to mold parts with larger projected areas or cross-sectional geometry [9±12]. Part quality can be improved by reducing residual the stress, warpage, sink marks and shrinkage that are normally encountered in CIM. However, it is inherently more complex than CIM, as it introduces several additional parameters that are associated with gas injection and gas channel design. Process variables, part geometry and mate* Corresponding author. Tel.: 65-799-5537; fax: 65-791-1859. E-mail address: [email protected] (N.S. Ong).
rial properties are the three major aspects that affect the GAIM process and part quality [6]. Despite being commercially exploited for more than a decade, the understanding of the process characteristics, due to the relatively complicated two-phase ¯ow phenomena inherent in the process, is still lagging. Structural ribs are used for strengthening large and relatively thin parts. However, in CIM, the introduction of a sink mark on the back wall of the ribs restricted the design limits [13,14]. In this study, the rib geometry for the gas channel is of prime concern. Knowledge on the gas-assisted molding of thin, large parts using structural ribs is still inadequate. The design of a gas channel that acts as a conduit for gas penetration is a key factor. The cross-sectional shape of the gas channel and its dimensions are crucial in the GAIM process [15]. The gas-¯ow phenomenon is also affected by the process conditions, especially the shot size, delay time, melt temperature and gas pressure. Simulation will be a useful tool to obtain a better understanding of the GAIM process. Computer simulation predicts the gas distribution in a part as well as the resulting part properties, thereby contributing to molding experience [16,17]. In GAIM, the process variables determine the gas bubble formation. The residual wall thickness (RWT) is also sensitive to the process variables and gas channel geometries. The process variables as well as the gas channel geometry affect
0924-0136/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 1 ) 0 1 1 8 4 - 0
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M.A. Parvez et al. / Journal of Materials Processing Technology 121 (2002) 27±35
®ngering formation [18], which is a common defect associated with GAIM parts, and which reduces the structural strength of the part. In this study, crucial process variables governing ®ngering formation have been identi®ed. The effects of geometry on gas ®ngering are also investigated. Earlier studies [13] indicated that the mechanical properties of GAIM parts can be improved, as compared with those obtained from the CIM process. A number of guidelines for the rib geometry for GAIM molds had been recommended. Nevertheless, these were based only on empirical knowledge. Chien et al. [13] and Chen et al. [19] have evaluated the mechanical properties of ®ve different types of rib structure via tensile and bending tests. It was found that tensile stress showed only a slight in¯uence from gas channel design and part thickness. In this investigation, parts with four different types of gas channel geometry, but having the same aspect ratio were molded using polystyrene (PS). A comprehensive study integrating the effects of process variables and rib geometries on the gas bubble distribution, RWT, ®ngering formation and structural performance were conducted. Mold¯ow's MF/GAS software was utilized for a better understanding of the process. The outcomes predicted by Mold¯ow were compared with experimental results. 2. Experiments and simulation 2.1. Material specifications The material used in this study was general purpose polystyrene (GPPS). The grade, supplier, optical property, tensile strength, relative viscosity and speci®c gravity of the material are listed in Table 1. Transparent PS was chosen, as this allowed the easy observation of gas channels and ®ngerings. 2.2. Gas channel configurations This study examined four different types of gas channel geometries (Parts A±D) having the same aspect ratio and different cross-sectional area (see Fig. 1). The length of the part is 200 mm. The ratio of the rib width (w) and the part thickness (t), known as the aspect ratio, was 1.5:1. A singlecavity test mold was used to mold each part. There was one set of cavity and core for each part.
2.3. Machine and gas injection units The experiment was conducted using a 75 t JSW (Japan) injection molding machine equipped with a Cinpress II gas injection unit. The gas injection unit is volume controlled. Gas (N2) and polymer were injected into the mold at different locations. The machine has a maximum injection pressure of 154 MPa. The screw diameter was 40 mm and the maximum stroke was 300 mm. 2.4. Process parameters The process parameters chosen were listed in Table 2. They were chosen based on material and equipment supplier's recommended processing ranges. When one parameter was varied the other parameters were kept constant. Transparent PS parts of 3 mm thickness were molded for this study. After the molding process had stabilized, three parts were examined for each processing condition and the test results were based on the average of three samples. 2.5. Gas bubble penetration It is essential to study gas bubble penetration to achieve a successful gas injected component. It also de®nes the limits Table 2 Process parameters
Table 1 Properties of material used Material type Grade Supplier Visual optical property Tensile strength Relative viscosity Specific gravity
Fig. 1. Parts with four different types of gas channel geometries (mm).
PS GP-1 standard DENKA Transparent 44 MPa 1.94 1.05
Shot size (%) Delay time (s) Gas pressure (bar) Melt temperature (8C) a
90±96a 1.0±2.5a 40±70a 210±225a
In In In In
steps of 2% steps of 0.5 s steps of 10 bar steps of 5 8C
Indicates the base parameters which were kept constant (e.g. for varying shot size, the delay time, pressure and temperature were kept constant at 1.0 s, 40 bar and 210 8C, respectively).
M.A. Parvez et al. / Journal of Materials Processing Technology 121 (2002) 27±35
of the molding conditions. The limits of the molding conditions of a speci®c production process are determined by the material, the mold and the process parameters. In this study, the effect of four process variables as well as the effect of gas channel geometries on gas bubble penetration were examined. 2.6. Residual wall thickness RWT is the thickness of the polymer layer that is left behind at the mold walls after the gas front has passed. The thickness determines the ratio of polymer and gas to be injected and it has an effect on the ®nal part properties. RWT in GAIM is determined by two phenomena: the penetration of gas into a viscous liquid and the growth of a solid layer [20]. A study was conducted to measure the RWT at a constant interval along the length of the part to establish a criterion for wall thickness uniformity. Subsequently, the effects of process variables and rib geometries on the RWT were obtained. The molded parts were sectioned in the direction of polymer ¯ow at 30 mm intervals for a length of 120 mm. Needle ®les and scribbler were used to deburr the chips surrounding the sectioned parts. An Omis II Optic Video Probe machine was used to determine the RWT at three locations on the gas channel: the right (tr), left (tl) and bottom (tb) thicknesses for each location interval (see Fig. 2). 2.7. Fingering effects Fingering or gas permeation, is a common problem in GAIM. In ®ngering, gas escapes from the gas channel and migrates into undesired areas of the part. Fig. 2 shows a typical ®ngering defect encountered in GAIM. Fingering reduces the structural strength and causes premature failure of the molded part, as thinner sections will now have to carry the design loads. Gas ®ngering behavior is affected by both the gas channel design and the process conditions [18]. It is, therefore, essential to study the effects of process parameters and gas channel geometries on ®ngering to prevent it from occurring. The ®ngering formation was inspected visually on the transparent part in the current investigation.
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2.8. Mechanical properties Stiffness and strength are two of the most important measures of the structural performance of parts. These properties depend on the part geometry, material properties, loading conditions and constraint conditions [21]. Different types of testing modes can be used to measure the mechanical properties of GAIM parts. For the present investigation, bending tests were performed on an Instron machine. This test is important for predicting the overall structural performance of the plastic parts. A three-point loading system was used for this test. The specimen broke around the center region. Bending load versus de¯ection curves were obtained for all the parts and the maximum load was found at the point of breakage. The non-uniformity of the hollowed core distribution affects the second moment of area plate. Huang and coworkers [19] found that this non-uniformity has a negligible effect on the evaluation of the second moment of area. In this study, the second moment of area was obtained from the original hollowed core geometry. The second moment of area and the strength were calculated using the following equations [22]: s
MC I
(1)
where s is the bending stress of the plate, M the bending moment at the center point of the plate along span direction, C the largest perpendicular distance from the neutral axis to the free surface of the part, and I the second moment of area of the cross-section after gas is injected. sf
Mmax C 1=4Fmax LC 1=4Fmax L I I I=C
(2)
where sf is the flexural strength, Mmax the maximum bending moment, Fmax the maximum bending load of the plate, and L the distance between the two extreme supports. The ¯exural strength is the maximum stress in the exterior surface of the gas channel at breakage [14]. It is a measure of the load-carrying capacity of a part. The second moment of area was calculated about the X±X-axis of the part passing through the center of area of the section. The de¯ection at the mid-point can be found from the following equation: db
Fb L3 48Eb I
or Fb 48Eb I K L3 db
(3)
where K is the stiffness, the part's resistance to deflection under an applied load for the testing configuration. For constant values of Young's modulus (Eb) and span distance (L), Eq. (3) can be rewritten as follows: Fig. 2. RWT and typical fingering defects.
K/I
(4)
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M.A. Parvez et al. / Journal of Materials Processing Technology 121 (2002) 27±35
This equation shows that the stiffness increases with the increase in the second moment of area. Eqs. (2) and (4) are used to determine the flexural strength and the stiffness, respectively. 2.9. Simulation The design of an injection molded part and its tooling is an art rather than an exact science [4]. The design of molds and the production of parts involve trial and errors, which is expensive and time-consuming. To improve on this, a simulation tool has been developed. Mold¯ow, a commercial package, was employed to better understand the process. 2.5D approximation was utilized, is based on the Hele±Shaw approximation. However, it is likely that GAIM is characterized by 3D ¯ow phenomena. At the present time, a complete CAE package based on 3D theory for simulation of GAIM process is not available. It will be of interest, therefore, to investigate the agreement between simulated and experimental results. In this approximation, injectionmolded parts are assumed to be thin-walled. For the present investigation, a beam element was employed to represent the gas channel because of the aspect ratio of the part (less than 4). 3. Results and discussion
Fig. 4. Gas penetration length versus delay time.
Fig. 5. Gas penetration length versus melt temperature.
In GAIM, it is desired that the gas should penetrate into the extreme regions of a part. To avoid sink marks on crucial areas, a penetration length extending over 70% of the part length is required [8]. The effects of four process variables, i.e. shot size, delay time, melt temperature and gas pressure, were examined with respect to gas penetration length. Figs. 3±6 show the effects of varying the process variables on gas bubble penetration. In most cases, the gas penetration length varies linearly with the process variables. Fig. 3 gives the effect of varying the shot size on gas bubble penetration. The gas took up the volume that remained in the cavity after the polymer had been injected. A larger shot size will increase the melt deposition, which
eventually decreases the gas penetration length. From the experiment, it was observed that a relatively smaller shot size will cause the melt front to rupture. Fig. 4 shows that an increase in the gas delay time leads to an increase in the penetration length. A longer delay time will result in a thicker frozen plastic layer hence a smaller cross-sectional area but a longer gas bubble length is expected for the same volume of gas injected. It can be observed in Fig. 5 that plastics of high melt temperature produced a longer gas penetration length. An increase in melt temperature reduces the viscosity of the polymer and it is easier for the gas to core out the gas channels. However, the plastic's melt temperature had less in¯uence on the penetration length. In this study, the gas pressure showed little in¯uence over the gas penetration length. A minor decrease in gas bubble length was observed when the gas pressure was increased within the experimental limits (see Fig. 6). All parts (Parts A±D) exhibited the same trend for the variations of process vari-
Fig. 3. Gas penetration length versus shot size.
Fig. 6. Gas penetration length versus gas pressure.
3.1. Effects of process variables and gas channel geometries on gas bubble penetration
M.A. Parvez et al. / Journal of Materials Processing Technology 121 (2002) 27±35
ables on gas bubble penetration. The more crucial factors affecting gas bubble penetration, in the order of importance, were shot size, delay time, gas pressure and melt temperature. It was noted that the effects of shot size on gas penetration agreed with most of the reported studies. The gas channel will pre-determine the gas ¯ow as well as its in¯uence on the melt ¯ow. It also provides a reinforcement for the part structure. The penetration length becomes shorter as the cross-sectional area in a part increases, the change being approximately linear. Part D had less gas penetration than the other parts (see Fig. 3). This was due to the increased gas channel area which made the penetration length shorter. Jong et al. [12] also found a similar trend. The limits of the molding conditions can be extended for Part D as compared with Part A, since Part A had longer penetration for the same shot size than Part D. If less shot size was used for Part A (with no ®llet) then the entire part would be cored out by gas. This result agreed well with the experimental results obtained by Yang and Huang [8]. 3.2. Effects of process variables and gas channel geometries on residual wall thickness It was observed that the RWT increased at each incremental location (see Fig. 7) for the same shot size. This increasing trend was found to be similar for the other part types and the variation of other processing parameters. RWT (left) and RWT (bottom) exhibited the same trends. As shown in Fig. 7, an increase in the RWT was observed as the location was moved further away from the gate. In addition, the RWT was not uniform throughout the gas bubble path. Figs. 7±10 show the RWT measured at various sections along the length of the part (Part A). It was found that an increase in the shot size leads to an increase in RWT. As more plastic is injected into the mold cavity, there will be less space for gas to occupy the channel area. Fig. 8 shows that the RWT decreased with an increase in delay time for cross-sections further away from the gate. At 30 mm location, the RWT increased with an increase in delay time. As the delay time increases, plastic melt will have enough time
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Fig. 7. RWT (right) versus sectioned location for different shot size for Part A.
Fig. 8. RWT (right) versus sectioned location for different delay time for part A.
to solidify and the gas, rather than going in the lateral direction, was pushed forward longitudinally into the channel. The melt temperature of the plastic and the gas pressure had only minor in¯uences on the RWT (Figs. 9 and 10). From the results, it can be observed that the process variables had a different effect on the RWT at different locations. The percentage deviations of the RWT at a speci®ed location varied from 0 to 18%, 19 to 43 and 18 to 29% for gas pressure, shot size and delay time, respectively. The percentage deviation for melt temperature was found to be from 0 to 18%. From the above results, the shot size and delay time were the most signi®cant processing parameters in¯uencing the RWT. All the other part types (Parts B±D) exhibited the same trend. Fig. 11 shows the RWT versus shot size at 60 mm away from the gate. It was found that Part D had a thicker RWT than the other part types, due to a larger cross-sectional area. It was found that adding ®llets near the corner, as transition from rib to base, increased the RWT.
Fig. 9. RWT (right) versus sectioned location for melt temperature for Part A.
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M.A. Parvez et al. / Journal of Materials Processing Technology 121 (2002) 27±35
Fig. 10. RWT (right) versus sectioned location for gas pressure for Part A.
3.3. Effect of process variables and gas channel geometries on gas fingering Table 3 shows the in¯uence of the process variables on the ®ngering of the GAIM processed parts. Images from the parts were taken for various process variables. Fig. 12 shows a photograph of Part A molded with different shot size. It was found that an increase in shot size reduced the ®ngering formation, as well as the gas penetration length. When the shot size was increased, extra melt that was deposited onto the thin wall region, after the gas channel section had been was completely ®lled, prevented ®ngering from occurring. The same trend was observed for the other part types. Variation of
the gas delay time had a contrasting effect on ®ngering formation and gas penetration. An increase in delay time led to a reduction in the ®ngering effect but increased the gas penetration length. This was because the available plastic melt was predetermined by the shot size so that the internal penetration volume which the gas cored out was ®xed. Parts B±D showed the same responses for the variation of gas delay time. An increase in gas pressure had only minor effects on ®ngering. As the gas pressure was increased, the ®ngering area decreased slightly. The ®ngering formation due to gas pressure variation showed the same trend for the other part types. The effects of melt temperature on the ®ngering phenomena were not clear in this study as there was little difference in the Table 3 Qualitative indication of significance Increase in processing variable
Fingering phenomena
Shot size Delay time Gas pressure Melt temperature
#a #a &b Ðc
a
Fig. 11. Shot size versus RWT at 60 mm location.
Decrease. Slight decrease. c Insignificant. b
Fig. 12. Image of Part A for different shot size showing fingering formation.
M.A. Parvez et al. / Journal of Materials Processing Technology 121 (2002) 27±35
®ngering formation when the melt temperature was increased from 210 to 225 8C. The melt temperature variations showed a similar trend for the other part types. The ®ndings show good agreement with those Chen and coworkers [18]. From the above observations, it can be concluded that the occurrence of ®ngering was dominated by the shot size and the delay time. A shorter delay time and a smaller shot size were the main causes of gas ®ngering. An increase in the delay time leads to an increase in melt cooling. Gas penetration into thin-wall areas will then become more dif®cult. This will reduce the ®ngering effect and at the same time increase the gas penetration length. Focusing on the effects of gas channel geometries, it was found that Part D (with a circular ®llet) had a lesser ®ngering tendency. This was due to an increase in gas channel area in the lateral direction which consequently increased the RWT. The gas channel provided evenly distributed plastics and gas ®lling, as the transition corner was round. Part A (without a ®llet) had a slightly higher ®ngering tendency. Hence, it can be concluded that the addition of ®llets reduces ®ngering formation. 3.4. Effect of process variables and gas channel geometries on mechanical properties of GAIM parts Ribs are normally designed to strengthen large, thin and plate-shaped parts. The dependence of the mechanical properties on the process variables and the associated gas channel geometries was investigated. It was observed that if shot size was increased, the part would be able to withstand a higher bending load (see Table 4). This was due to the extra melt being injected into the cavity, thereby reducing the crosssectional area of the hollowed core, as well as the ®ngering effects. However, an increase in the delay time produced parts that could withstand a lower bending load. Contrarily, an increase in gas pressure had resulted in a slightly higher bending load at failure. There was very little effect of melt temperature on the part's bending failure load. From the above discussion, it appears that shot size was the most signi®cant factor affecting the mechanical properties. It was noted that Part D (with a quarter circular ®llet) can withstand a higher bending load. The minimum bending load was found for Part A. This was because Part D had a larger cross-sectional area due to the addition of a ®llet at the transition corner. Part D also exhibited less ®ngering. The RWT at the transition corner was uniform for Part D. Hence,
the weakening effect of the void at the transition corner in Part D was expected to be minimum. On the effects of gas channel geometries on the ¯exural strength of a part, the measurement of the load carrying capability of a part was measured. Part D had a slightly higher ¯exural strength than the others (The ¯exural strength for parts A, B, C and D were 53.7, 54.3, 55.4 and 56.0 N/ mm2 respectively). The part's second moment inertia was calculated on the basis of gas channel shape, associated geometries and hollowed core distribution. The second moment of area for the four part types was determined. The addition of ®llets near the corner increased the moment of area of the part (The moment of area for parts A, B, C and D were 1704, 1709, 1712 and 1741 mm4 respectively). Part D had a higher second moment of area than the other parts, thus an increase in the stiffness of the part (see Eq. (9)). 3.5. Simulated and experimental results Mold¯ow software was used to simulate the GAIM process. Mold ®lling and packing analysis were conducted with the same experimental inputs which produced a complete part during the experiment. In this simulation, the gas channel was modeled with a beam element since the aspect ratio was less than 4. This model was not exactly physically the same as the original part as appropriate simpli®cations were required for simulation. A thermoplastic multilaminate ®lling analysis was run to obtain the initial processing conditions for a constant-volume process gas ®lling analysis. The multi-laminate ®lling analysis used a ®nite difference scheme for performing thermal calculations, and used PVT data to account for the effect of material compression. With these, and along with other processing conditions as used in the physical testing, gas ®lling analysis was conducted. Packing analysis was performed from the end of ®lling by using the restart ®le. Fig. 13 shows the gas bubble penetration at various shot sizes, for a delay time of 1.5 s. An increase in shot size led to a decrease in gas bubble penetration. At delay time of 1 s, the part could not be simulated with Mold¯ow since, the polymer and gas could not be injected at the same time. The simulated results show similar trends with the experimental results. Fig. 14 shows the effect of varying the delay time on the gas bubble penetration. An increase in delay time increased
Table 4 Comparative significance of each variable on bending load Increase in processing variable
Bending load
Shot size Delay time Gas pressure Melt temperature
"a #b "a Ðc
a
Increase. Decrease. c Insignificant.
33
b
Fig. 13. Shot size versus gas penetration length (simulated result).
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M.A. Parvez et al. / Journal of Materials Processing Technology 121 (2002) 27±35
Fig. 14. Delay time versus gas penetration length (simulated result). Fig. 15. Penetration length at different delay time for Part A.
Fig. 16. Comparisons of fingering formation between experimental results and simulation.
the penetration length. The trend was found to be similar to that for the experimental results. With an increase in the melt temperature and gas pressure, the gas bubble penetration did not show any signi®cant changes. Fig. 15 shows a comparison of gas bubble penetration at different delay times for a part molded in the experiment and simulation. There was an estimated 16% deviation the penetration length of simulation from that of the experimental result. Fig. 16 compares the ®ngering formation between simulation and experimental observations. There was general agreement. It was observed, however, that in the case of simulation, the ®ngering position was also located at the end of gas channel instead of being only near to the injection front. The ®ngering position was changed from broad to narrow as delay time was increased. The RWT could not be determined since the gas cored out the entire gas channel. Similar simulation results were also obtained from all the other part types. Fig. 13 shows that Part D had less gas penetration for the same shot size. Part D also had less gas penetration for the same delay time (see Fig. 15). This agreed well with the experimental results. However, the simulation process was not exactly able to predict the effects of part geometry on the GAIM process. This is possibly because the geometrical conditions and ¯ow phenomena around the gas front were 3D.
4. Conclusions From the present investigation, the following conclusions were drawn: 1. Shot size and delay time were the most dominant factors for gas bubble penetration. Comparison of parts with various gas channel geometries indicated that the addition of a fillet improved the limits of the molding conditions. 2. Shot size and delay time were the most important factors for the RWT. The gas channel geometry also affects RWT. The wall thickness was not uniform throughout the gas channel. The addition of fillets smoothens the transition corner. 3. Gas fingering behavior was mostly affected by shot size and delay time. Smaller shot size and shorter delay time promoted gas fingering. However, to control the fingering phenomenon, a longer delay time with the same shot size is suggested. Among the parts, Part D had the least fingering tendency. 4. An increase in shot size gave a higher bending load on the part, whereas an increase in delay time gave a smaller bending load on the part. Part D had the maximum flexural strength and it showed higher stiffness since, it had a greater value of second moment of area because of the addition of the fillet.
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5. The simulated results using Moldflow agreed well with experimental results for the effects of shot size and delay time. However, over-prediction in penetration length was observed for most cases. Simulation could be a useful tool in understanding the GAIM process. This study provides a better understanding of the GAIM process and hopefully will assist part designers and molders to achieve a better GAIM component. Acknowledgements This research is supported by an Academic Research Fund from the Ministry of Education, Singapore. The authors are grateful to Mold¯ow Pty. Ltd., for its generous donation of the Mold¯ow software to Nanyang Technological University. References [1] S. Shah, Gas assisted injection molding: a technology overview, J. Injection Molding Technol. 1 (2) (1997) 96±103. [2] K.S. Barton, L.S. Turng, General design guidelines for gas-assisted injection molding using a CAE tool, ANTEC Technical Papers, 1994, pp. 421±425. [3] V. Kapila, N.R. Schott, S. Shah, An experimental study to investigate, the influence of processing conditions in the gas-assisted injection molding process, ANTEC Technical Papers, 1996, pp. 649± 654. [4] K.H. Lau, T.M. Tse, Enhancement of plastic injection molding quality through the use of the ABPLC nozzle, J. Mater. Process. Technol. 69 (1997) 55±57. [5] J.F. Stevenson, One-shot manufacturing: what is possible with new molding technologies, ANTEC Technical Papers, 1994, pp. 655± 662. [6] M.A. Parvez, N.S. Ong, Gas assisted injection molding technology, for the next millennium: an overview, in: Proceedings of the Sixth Annual Paper Meeting and International Conference, Vol. 11, IEB, Bangladesh, 2000, pp. 77±85.
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Gas-assisted injection molding: the effects of process variables and gas channel geometry M.A. Parvez, N.S. Ong*, Y.C. Lam, S.B. Tor School of Mechanical and Production Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore Received 24 December 2000
Abstract Polystyrene parts with different rib geometries but having the same aspect ratio were molded using gas-assisted injection molding (GAIM). The process variables that have an in¯uence on the gas bubble distribution, residual wall thickness (RWT), ®ngering formation and mechanical properties were explored. The test results revealed that there is an inherent relationship between gas ®ngering and gas bubble penetration that has consequences on part strength. Shot size and delay time are the most dominant factors affecting the gas penetration, ®ngering formation, RWT and mechanical properties of the GAIM parts. The effects of rib geometries are also discussed. Computer simulation of the GAIM process was carried out using Mold¯ow, a commercial software. The outcomes predicted by simulation are compared with the experimental results. Based on the results, some useful design guidelines are suggested. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Gas-assisted injection molding; Process variables; Rib geometry; Process simulation
1. Introduction Gas-assisted injection molding (GAIM) is an innovative technology for producing plastic parts and has received extensive attention in the plastic manufacturing industries [1±8]. In this process, gas is injected through a nozzle or directly into the core section of a part being molded. The gas ¯ows preferentially through local thick sections with hot interiors and pushes the plastic ahead to occupy the mold. The gas can be injected simultaneously and/or subsequently with the melt, either after some delay time or after the end of the ®lling phase. Despite some pitfalls in the practical application of the GAIM process, it offers a number of advantages over conventional injection molding (CIM). GAIM can substantially reduce material cost, clamp tonnage, cycle time and residual stress, and allows molders to mold parts with larger projected areas or cross-sectional geometry [9±12]. Part quality can be improved by reducing residual the stress, warpage, sink marks and shrinkage that are normally encountered in CIM. However, it is inherently more complex than CIM, as it introduces several additional parameters that are associated with gas injection and gas channel design. Process variables, part geometry and mate* Corresponding author. Tel.: 65-799-5537; fax: 65-791-1859. E-mail address: [email protected] (N.S. Ong).
rial properties are the three major aspects that affect the GAIM process and part quality [6]. Despite being commercially exploited for more than a decade, the understanding of the process characteristics, due to the relatively complicated two-phase ¯ow phenomena inherent in the process, is still lagging. Structural ribs are used for strengthening large and relatively thin parts. However, in CIM, the introduction of a sink mark on the back wall of the ribs restricted the design limits [13,14]. In this study, the rib geometry for the gas channel is of prime concern. Knowledge on the gas-assisted molding of thin, large parts using structural ribs is still inadequate. The design of a gas channel that acts as a conduit for gas penetration is a key factor. The cross-sectional shape of the gas channel and its dimensions are crucial in the GAIM process [15]. The gas-¯ow phenomenon is also affected by the process conditions, especially the shot size, delay time, melt temperature and gas pressure. Simulation will be a useful tool to obtain a better understanding of the GAIM process. Computer simulation predicts the gas distribution in a part as well as the resulting part properties, thereby contributing to molding experience [16,17]. In GAIM, the process variables determine the gas bubble formation. The residual wall thickness (RWT) is also sensitive to the process variables and gas channel geometries. The process variables as well as the gas channel geometry affect
0924-0136/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 1 ) 0 1 1 8 4 - 0
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®ngering formation [18], which is a common defect associated with GAIM parts, and which reduces the structural strength of the part. In this study, crucial process variables governing ®ngering formation have been identi®ed. The effects of geometry on gas ®ngering are also investigated. Earlier studies [13] indicated that the mechanical properties of GAIM parts can be improved, as compared with those obtained from the CIM process. A number of guidelines for the rib geometry for GAIM molds had been recommended. Nevertheless, these were based only on empirical knowledge. Chien et al. [13] and Chen et al. [19] have evaluated the mechanical properties of ®ve different types of rib structure via tensile and bending tests. It was found that tensile stress showed only a slight in¯uence from gas channel design and part thickness. In this investigation, parts with four different types of gas channel geometry, but having the same aspect ratio were molded using polystyrene (PS). A comprehensive study integrating the effects of process variables and rib geometries on the gas bubble distribution, RWT, ®ngering formation and structural performance were conducted. Mold¯ow's MF/GAS software was utilized for a better understanding of the process. The outcomes predicted by Mold¯ow were compared with experimental results. 2. Experiments and simulation 2.1. Material specifications The material used in this study was general purpose polystyrene (GPPS). The grade, supplier, optical property, tensile strength, relative viscosity and speci®c gravity of the material are listed in Table 1. Transparent PS was chosen, as this allowed the easy observation of gas channels and ®ngerings. 2.2. Gas channel configurations This study examined four different types of gas channel geometries (Parts A±D) having the same aspect ratio and different cross-sectional area (see Fig. 1). The length of the part is 200 mm. The ratio of the rib width (w) and the part thickness (t), known as the aspect ratio, was 1.5:1. A singlecavity test mold was used to mold each part. There was one set of cavity and core for each part.
2.3. Machine and gas injection units The experiment was conducted using a 75 t JSW (Japan) injection molding machine equipped with a Cinpress II gas injection unit. The gas injection unit is volume controlled. Gas (N2) and polymer were injected into the mold at different locations. The machine has a maximum injection pressure of 154 MPa. The screw diameter was 40 mm and the maximum stroke was 300 mm. 2.4. Process parameters The process parameters chosen were listed in Table 2. They were chosen based on material and equipment supplier's recommended processing ranges. When one parameter was varied the other parameters were kept constant. Transparent PS parts of 3 mm thickness were molded for this study. After the molding process had stabilized, three parts were examined for each processing condition and the test results were based on the average of three samples. 2.5. Gas bubble penetration It is essential to study gas bubble penetration to achieve a successful gas injected component. It also de®nes the limits Table 2 Process parameters
Table 1 Properties of material used Material type Grade Supplier Visual optical property Tensile strength Relative viscosity Specific gravity
Fig. 1. Parts with four different types of gas channel geometries (mm).
PS GP-1 standard DENKA Transparent 44 MPa 1.94 1.05
Shot size (%) Delay time (s) Gas pressure (bar) Melt temperature (8C) a
90±96a 1.0±2.5a 40±70a 210±225a
In In In In
steps of 2% steps of 0.5 s steps of 10 bar steps of 5 8C
Indicates the base parameters which were kept constant (e.g. for varying shot size, the delay time, pressure and temperature were kept constant at 1.0 s, 40 bar and 210 8C, respectively).
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of the molding conditions. The limits of the molding conditions of a speci®c production process are determined by the material, the mold and the process parameters. In this study, the effect of four process variables as well as the effect of gas channel geometries on gas bubble penetration were examined. 2.6. Residual wall thickness RWT is the thickness of the polymer layer that is left behind at the mold walls after the gas front has passed. The thickness determines the ratio of polymer and gas to be injected and it has an effect on the ®nal part properties. RWT in GAIM is determined by two phenomena: the penetration of gas into a viscous liquid and the growth of a solid layer [20]. A study was conducted to measure the RWT at a constant interval along the length of the part to establish a criterion for wall thickness uniformity. Subsequently, the effects of process variables and rib geometries on the RWT were obtained. The molded parts were sectioned in the direction of polymer ¯ow at 30 mm intervals for a length of 120 mm. Needle ®les and scribbler were used to deburr the chips surrounding the sectioned parts. An Omis II Optic Video Probe machine was used to determine the RWT at three locations on the gas channel: the right (tr), left (tl) and bottom (tb) thicknesses for each location interval (see Fig. 2). 2.7. Fingering effects Fingering or gas permeation, is a common problem in GAIM. In ®ngering, gas escapes from the gas channel and migrates into undesired areas of the part. Fig. 2 shows a typical ®ngering defect encountered in GAIM. Fingering reduces the structural strength and causes premature failure of the molded part, as thinner sections will now have to carry the design loads. Gas ®ngering behavior is affected by both the gas channel design and the process conditions [18]. It is, therefore, essential to study the effects of process parameters and gas channel geometries on ®ngering to prevent it from occurring. The ®ngering formation was inspected visually on the transparent part in the current investigation.
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2.8. Mechanical properties Stiffness and strength are two of the most important measures of the structural performance of parts. These properties depend on the part geometry, material properties, loading conditions and constraint conditions [21]. Different types of testing modes can be used to measure the mechanical properties of GAIM parts. For the present investigation, bending tests were performed on an Instron machine. This test is important for predicting the overall structural performance of the plastic parts. A three-point loading system was used for this test. The specimen broke around the center region. Bending load versus de¯ection curves were obtained for all the parts and the maximum load was found at the point of breakage. The non-uniformity of the hollowed core distribution affects the second moment of area plate. Huang and coworkers [19] found that this non-uniformity has a negligible effect on the evaluation of the second moment of area. In this study, the second moment of area was obtained from the original hollowed core geometry. The second moment of area and the strength were calculated using the following equations [22]: s
MC I
(1)
where s is the bending stress of the plate, M the bending moment at the center point of the plate along span direction, C the largest perpendicular distance from the neutral axis to the free surface of the part, and I the second moment of area of the cross-section after gas is injected. sf
Mmax C 1=4Fmax LC 1=4Fmax L I I I=C
(2)
where sf is the flexural strength, Mmax the maximum bending moment, Fmax the maximum bending load of the plate, and L the distance between the two extreme supports. The ¯exural strength is the maximum stress in the exterior surface of the gas channel at breakage [14]. It is a measure of the load-carrying capacity of a part. The second moment of area was calculated about the X±X-axis of the part passing through the center of area of the section. The de¯ection at the mid-point can be found from the following equation: db
Fb L3 48Eb I
or Fb 48Eb I K L3 db
(3)
where K is the stiffness, the part's resistance to deflection under an applied load for the testing configuration. For constant values of Young's modulus (Eb) and span distance (L), Eq. (3) can be rewritten as follows: Fig. 2. RWT and typical fingering defects.
K/I
(4)
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This equation shows that the stiffness increases with the increase in the second moment of area. Eqs. (2) and (4) are used to determine the flexural strength and the stiffness, respectively. 2.9. Simulation The design of an injection molded part and its tooling is an art rather than an exact science [4]. The design of molds and the production of parts involve trial and errors, which is expensive and time-consuming. To improve on this, a simulation tool has been developed. Mold¯ow, a commercial package, was employed to better understand the process. 2.5D approximation was utilized, is based on the Hele±Shaw approximation. However, it is likely that GAIM is characterized by 3D ¯ow phenomena. At the present time, a complete CAE package based on 3D theory for simulation of GAIM process is not available. It will be of interest, therefore, to investigate the agreement between simulated and experimental results. In this approximation, injectionmolded parts are assumed to be thin-walled. For the present investigation, a beam element was employed to represent the gas channel because of the aspect ratio of the part (less than 4). 3. Results and discussion
Fig. 4. Gas penetration length versus delay time.
Fig. 5. Gas penetration length versus melt temperature.
In GAIM, it is desired that the gas should penetrate into the extreme regions of a part. To avoid sink marks on crucial areas, a penetration length extending over 70% of the part length is required [8]. The effects of four process variables, i.e. shot size, delay time, melt temperature and gas pressure, were examined with respect to gas penetration length. Figs. 3±6 show the effects of varying the process variables on gas bubble penetration. In most cases, the gas penetration length varies linearly with the process variables. Fig. 3 gives the effect of varying the shot size on gas bubble penetration. The gas took up the volume that remained in the cavity after the polymer had been injected. A larger shot size will increase the melt deposition, which
eventually decreases the gas penetration length. From the experiment, it was observed that a relatively smaller shot size will cause the melt front to rupture. Fig. 4 shows that an increase in the gas delay time leads to an increase in the penetration length. A longer delay time will result in a thicker frozen plastic layer hence a smaller cross-sectional area but a longer gas bubble length is expected for the same volume of gas injected. It can be observed in Fig. 5 that plastics of high melt temperature produced a longer gas penetration length. An increase in melt temperature reduces the viscosity of the polymer and it is easier for the gas to core out the gas channels. However, the plastic's melt temperature had less in¯uence on the penetration length. In this study, the gas pressure showed little in¯uence over the gas penetration length. A minor decrease in gas bubble length was observed when the gas pressure was increased within the experimental limits (see Fig. 6). All parts (Parts A±D) exhibited the same trend for the variations of process vari-
Fig. 3. Gas penetration length versus shot size.
Fig. 6. Gas penetration length versus gas pressure.
3.1. Effects of process variables and gas channel geometries on gas bubble penetration
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ables on gas bubble penetration. The more crucial factors affecting gas bubble penetration, in the order of importance, were shot size, delay time, gas pressure and melt temperature. It was noted that the effects of shot size on gas penetration agreed with most of the reported studies. The gas channel will pre-determine the gas ¯ow as well as its in¯uence on the melt ¯ow. It also provides a reinforcement for the part structure. The penetration length becomes shorter as the cross-sectional area in a part increases, the change being approximately linear. Part D had less gas penetration than the other parts (see Fig. 3). This was due to the increased gas channel area which made the penetration length shorter. Jong et al. [12] also found a similar trend. The limits of the molding conditions can be extended for Part D as compared with Part A, since Part A had longer penetration for the same shot size than Part D. If less shot size was used for Part A (with no ®llet) then the entire part would be cored out by gas. This result agreed well with the experimental results obtained by Yang and Huang [8]. 3.2. Effects of process variables and gas channel geometries on residual wall thickness It was observed that the RWT increased at each incremental location (see Fig. 7) for the same shot size. This increasing trend was found to be similar for the other part types and the variation of other processing parameters. RWT (left) and RWT (bottom) exhibited the same trends. As shown in Fig. 7, an increase in the RWT was observed as the location was moved further away from the gate. In addition, the RWT was not uniform throughout the gas bubble path. Figs. 7±10 show the RWT measured at various sections along the length of the part (Part A). It was found that an increase in the shot size leads to an increase in RWT. As more plastic is injected into the mold cavity, there will be less space for gas to occupy the channel area. Fig. 8 shows that the RWT decreased with an increase in delay time for cross-sections further away from the gate. At 30 mm location, the RWT increased with an increase in delay time. As the delay time increases, plastic melt will have enough time
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Fig. 7. RWT (right) versus sectioned location for different shot size for Part A.
Fig. 8. RWT (right) versus sectioned location for different delay time for part A.
to solidify and the gas, rather than going in the lateral direction, was pushed forward longitudinally into the channel. The melt temperature of the plastic and the gas pressure had only minor in¯uences on the RWT (Figs. 9 and 10). From the results, it can be observed that the process variables had a different effect on the RWT at different locations. The percentage deviations of the RWT at a speci®ed location varied from 0 to 18%, 19 to 43 and 18 to 29% for gas pressure, shot size and delay time, respectively. The percentage deviation for melt temperature was found to be from 0 to 18%. From the above results, the shot size and delay time were the most signi®cant processing parameters in¯uencing the RWT. All the other part types (Parts B±D) exhibited the same trend. Fig. 11 shows the RWT versus shot size at 60 mm away from the gate. It was found that Part D had a thicker RWT than the other part types, due to a larger cross-sectional area. It was found that adding ®llets near the corner, as transition from rib to base, increased the RWT.
Fig. 9. RWT (right) versus sectioned location for melt temperature for Part A.
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M.A. Parvez et al. / Journal of Materials Processing Technology 121 (2002) 27±35
Fig. 10. RWT (right) versus sectioned location for gas pressure for Part A.
3.3. Effect of process variables and gas channel geometries on gas fingering Table 3 shows the in¯uence of the process variables on the ®ngering of the GAIM processed parts. Images from the parts were taken for various process variables. Fig. 12 shows a photograph of Part A molded with different shot size. It was found that an increase in shot size reduced the ®ngering formation, as well as the gas penetration length. When the shot size was increased, extra melt that was deposited onto the thin wall region, after the gas channel section had been was completely ®lled, prevented ®ngering from occurring. The same trend was observed for the other part types. Variation of
the gas delay time had a contrasting effect on ®ngering formation and gas penetration. An increase in delay time led to a reduction in the ®ngering effect but increased the gas penetration length. This was because the available plastic melt was predetermined by the shot size so that the internal penetration volume which the gas cored out was ®xed. Parts B±D showed the same responses for the variation of gas delay time. An increase in gas pressure had only minor effects on ®ngering. As the gas pressure was increased, the ®ngering area decreased slightly. The ®ngering formation due to gas pressure variation showed the same trend for the other part types. The effects of melt temperature on the ®ngering phenomena were not clear in this study as there was little difference in the Table 3 Qualitative indication of significance Increase in processing variable
Fingering phenomena
Shot size Delay time Gas pressure Melt temperature
#a #a &b Ðc
a
Fig. 11. Shot size versus RWT at 60 mm location.
Decrease. Slight decrease. c Insignificant. b
Fig. 12. Image of Part A for different shot size showing fingering formation.
M.A. Parvez et al. / Journal of Materials Processing Technology 121 (2002) 27±35
®ngering formation when the melt temperature was increased from 210 to 225 8C. The melt temperature variations showed a similar trend for the other part types. The ®ndings show good agreement with those Chen and coworkers [18]. From the above observations, it can be concluded that the occurrence of ®ngering was dominated by the shot size and the delay time. A shorter delay time and a smaller shot size were the main causes of gas ®ngering. An increase in the delay time leads to an increase in melt cooling. Gas penetration into thin-wall areas will then become more dif®cult. This will reduce the ®ngering effect and at the same time increase the gas penetration length. Focusing on the effects of gas channel geometries, it was found that Part D (with a circular ®llet) had a lesser ®ngering tendency. This was due to an increase in gas channel area in the lateral direction which consequently increased the RWT. The gas channel provided evenly distributed plastics and gas ®lling, as the transition corner was round. Part A (without a ®llet) had a slightly higher ®ngering tendency. Hence, it can be concluded that the addition of ®llets reduces ®ngering formation. 3.4. Effect of process variables and gas channel geometries on mechanical properties of GAIM parts Ribs are normally designed to strengthen large, thin and plate-shaped parts. The dependence of the mechanical properties on the process variables and the associated gas channel geometries was investigated. It was observed that if shot size was increased, the part would be able to withstand a higher bending load (see Table 4). This was due to the extra melt being injected into the cavity, thereby reducing the crosssectional area of the hollowed core, as well as the ®ngering effects. However, an increase in the delay time produced parts that could withstand a lower bending load. Contrarily, an increase in gas pressure had resulted in a slightly higher bending load at failure. There was very little effect of melt temperature on the part's bending failure load. From the above discussion, it appears that shot size was the most signi®cant factor affecting the mechanical properties. It was noted that Part D (with a quarter circular ®llet) can withstand a higher bending load. The minimum bending load was found for Part A. This was because Part D had a larger cross-sectional area due to the addition of a ®llet at the transition corner. Part D also exhibited less ®ngering. The RWT at the transition corner was uniform for Part D. Hence,
the weakening effect of the void at the transition corner in Part D was expected to be minimum. On the effects of gas channel geometries on the ¯exural strength of a part, the measurement of the load carrying capability of a part was measured. Part D had a slightly higher ¯exural strength than the others (The ¯exural strength for parts A, B, C and D were 53.7, 54.3, 55.4 and 56.0 N/ mm2 respectively). The part's second moment inertia was calculated on the basis of gas channel shape, associated geometries and hollowed core distribution. The second moment of area for the four part types was determined. The addition of ®llets near the corner increased the moment of area of the part (The moment of area for parts A, B, C and D were 1704, 1709, 1712 and 1741 mm4 respectively). Part D had a higher second moment of area than the other parts, thus an increase in the stiffness of the part (see Eq. (9)). 3.5. Simulated and experimental results Mold¯ow software was used to simulate the GAIM process. Mold ®lling and packing analysis were conducted with the same experimental inputs which produced a complete part during the experiment. In this simulation, the gas channel was modeled with a beam element since the aspect ratio was less than 4. This model was not exactly physically the same as the original part as appropriate simpli®cations were required for simulation. A thermoplastic multilaminate ®lling analysis was run to obtain the initial processing conditions for a constant-volume process gas ®lling analysis. The multi-laminate ®lling analysis used a ®nite difference scheme for performing thermal calculations, and used PVT data to account for the effect of material compression. With these, and along with other processing conditions as used in the physical testing, gas ®lling analysis was conducted. Packing analysis was performed from the end of ®lling by using the restart ®le. Fig. 13 shows the gas bubble penetration at various shot sizes, for a delay time of 1.5 s. An increase in shot size led to a decrease in gas bubble penetration. At delay time of 1 s, the part could not be simulated with Mold¯ow since, the polymer and gas could not be injected at the same time. The simulated results show similar trends with the experimental results. Fig. 14 shows the effect of varying the delay time on the gas bubble penetration. An increase in delay time increased
Table 4 Comparative significance of each variable on bending load Increase in processing variable
Bending load
Shot size Delay time Gas pressure Melt temperature
"a #b "a Ðc
a
Increase. Decrease. c Insignificant.
33
b
Fig. 13. Shot size versus gas penetration length (simulated result).
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M.A. Parvez et al. / Journal of Materials Processing Technology 121 (2002) 27±35
Fig. 14. Delay time versus gas penetration length (simulated result). Fig. 15. Penetration length at different delay time for Part A.
Fig. 16. Comparisons of fingering formation between experimental results and simulation.
the penetration length. The trend was found to be similar to that for the experimental results. With an increase in the melt temperature and gas pressure, the gas bubble penetration did not show any signi®cant changes. Fig. 15 shows a comparison of gas bubble penetration at different delay times for a part molded in the experiment and simulation. There was an estimated 16% deviation the penetration length of simulation from that of the experimental result. Fig. 16 compares the ®ngering formation between simulation and experimental observations. There was general agreement. It was observed, however, that in the case of simulation, the ®ngering position was also located at the end of gas channel instead of being only near to the injection front. The ®ngering position was changed from broad to narrow as delay time was increased. The RWT could not be determined since the gas cored out the entire gas channel. Similar simulation results were also obtained from all the other part types. Fig. 13 shows that Part D had less gas penetration for the same shot size. Part D also had less gas penetration for the same delay time (see Fig. 15). This agreed well with the experimental results. However, the simulation process was not exactly able to predict the effects of part geometry on the GAIM process. This is possibly because the geometrical conditions and ¯ow phenomena around the gas front were 3D.
4. Conclusions From the present investigation, the following conclusions were drawn: 1. Shot size and delay time were the most dominant factors for gas bubble penetration. Comparison of parts with various gas channel geometries indicated that the addition of a fillet improved the limits of the molding conditions. 2. Shot size and delay time were the most important factors for the RWT. The gas channel geometry also affects RWT. The wall thickness was not uniform throughout the gas channel. The addition of fillets smoothens the transition corner. 3. Gas fingering behavior was mostly affected by shot size and delay time. Smaller shot size and shorter delay time promoted gas fingering. However, to control the fingering phenomenon, a longer delay time with the same shot size is suggested. Among the parts, Part D had the least fingering tendency. 4. An increase in shot size gave a higher bending load on the part, whereas an increase in delay time gave a smaller bending load on the part. Part D had the maximum flexural strength and it showed higher stiffness since, it had a greater value of second moment of area because of the addition of the fillet.
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5. The simulated results using Moldflow agreed well with experimental results for the effects of shot size and delay time. However, over-prediction in penetration length was observed for most cases. Simulation could be a useful tool in understanding the GAIM process. This study provides a better understanding of the GAIM process and hopefully will assist part designers and molders to achieve a better GAIM component. Acknowledgements This research is supported by an Academic Research Fund from the Ministry of Education, Singapore. The authors are grateful to Mold¯ow Pty. Ltd., for its generous donation of the Mold¯ow software to Nanyang Technological University. References [1] S. Shah, Gas assisted injection molding: a technology overview, J. Injection Molding Technol. 1 (2) (1997) 96±103. [2] K.S. Barton, L.S. Turng, General design guidelines for gas-assisted injection molding using a CAE tool, ANTEC Technical Papers, 1994, pp. 421±425. [3] V. Kapila, N.R. Schott, S. Shah, An experimental study to investigate, the influence of processing conditions in the gas-assisted injection molding process, ANTEC Technical Papers, 1996, pp. 649± 654. [4] K.H. Lau, T.M. Tse, Enhancement of plastic injection molding quality through the use of the ABPLC nozzle, J. Mater. Process. Technol. 69 (1997) 55±57. [5] J.F. Stevenson, One-shot manufacturing: what is possible with new molding technologies, ANTEC Technical Papers, 1994, pp. 655± 662. [6] M.A. Parvez, N.S. Ong, Gas assisted injection molding technology, for the next millennium: an overview, in: Proceedings of the Sixth Annual Paper Meeting and International Conference, Vol. 11, IEB, Bangladesh, 2000, pp. 77±85.
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