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Prototype Molds and Mold Filling Analysis
Chapter 17
Prototype Molds and Mold Filling Analysis In the previous chapter we described various errors that depend on either an incorrect part or mold design. When designing new parts or starting the molding of a new product, you will get a number of new questions and challenges: ■■
Will the part get the correct dimensions?
■■
Will it warp?
■■
Are the runners too long? Will the part be completely filled?
■■
Where should the gate be placed in order to make the part as strong as possible?
■■
Are the temperature-control channels correctly dimensioned and located?
17.1 Prototype Molds In order to avoid unpleasant surprises when starting the production in a new mold, you can use a prototype mold to see what the part would look like once it has been molded. Another option is to complete only one of several cavities in a production mold. These procedures could save both money and time. But they are not always completely reliable as runners, and mold temperature systems seldom correspond to the final production mold. Producers are using prototype molds when the plastic part is very complex or when the production mold is very expensive. Within the automotive industry these kinds of molds sometimes are named “soft molds” as they often are made in aluminum or in soft steel. When it comes to more simple part or mold design most prototypes have been replaced by a mold filling analysis.
Figure 17.1 Prototype mold (highlighted in red) in aluminum for the cams shown in Figure 17.2. Here only one part is made in each shot. Below is the production mold in steel with 16 cavities. This mold is about 30 times more expensive to produce compared to the prototype mold in aluminum.
145
Chapter 17 — Prototype Molds and Mold Filling Analysis
Figure 17.2 Here we can see a common component used in assembly of furniture. AD-Plast in Sweden developed this component in PPA with glass. They won the prestigious price “Plastovationer 2009” by successfully replacing metal with plastic. The cam is stronger than the former zinc one without needing to change the outer dimensions. [Source: AD-Plast AB]
17.2 Mold Filling Analysis Mold filling analysis is a computer-based tool that facilitates the ability to get accurate plastic parts in less time when producing a new part or modifying an existing mold.
Figure 17.3 On the image you can see a mold designer in front of his PC working with Moldflow, a mold filling software. Such software is able to run on standard PCs but requires a lot of computing power. In order for the calculations to run as fast as possible it is necessary to have a large internal memory as well as a fast processor.
With mold filling analysis you get:
146
■■
The ability to reach the right result in less time
■■
A powerful tool for successful “lean production” with focus on continuous improvement
■■
The correct process parameters with a major influence on the property profile of the part
■■
Knowledge that leads to an increased process window and more robust production
■■
In general, lower development costs compared to a prototype mold
17.3 Workflow
Figure 17.4 Here we can see a product in the shape according to the design that the designer had in mind. In Figure 17.5 we can see what it actually looked like when it left the mold.
Figure 17.5 Here we can see that the product is heavily bent. The reason is internal stresses probably dependent on an uneven wall thickness or unfavorable temperature control. By using the shrinkage & warpage modulus in the Moldflow software this could have been predicted before production and could have been corrected so that the defect never occurred.
17.3 Workflow 17.3.1 Mesh Model Mold filling simulation should be a natural step in the development process of new plastic products. Many designers are using various CAD software packages such as Pro-E, Catia, or Solid Works when creating a 3D model of new products. The STL or Igesfile STL or just IGES file that is generated is then used to create a so-called mesh model.
Triangular shape
Figure 17.6 This is a 3D model in STL format of a headlight housing used for cars. The model consists of a large number of small triangles.
Figure 17.7 Here we see the same headlight housing as shown in Figure 17.6. By using Moldflow analysis you can create a mesh model that is used as “input” into the continuing simulation process.
147
Chapter 17 — Prototype Molds and Mold Filling Analysis
17.3.2 Material Selection Once the mesh model is created, the next step is to choose which plastic resin to work with. In Moldflow there is a database with a large number of materials to choose from. If the material of choice is not found among those listed in the database, it is possible to add some needed (rheological) parameters yourself.
17.3.3 Process Parameters As the melt viscosity of the material is affected by melt and mold temperature and shear rate you must enter these parameters into the database if the material is not found among the preselected materials listed in the database.
Figure 17.8 The picture here shows the window in which the process parameters are to be entered in Moldflow. If the values for injection speed, hold pressure switch, hold pressure, or cooling time are missing, you will be able to select the “automatic” choice for these parameters.
17.3.4 Selection of Gate Location Before any calculations in the simulation program you must select a possible gate location. If you have no idea where it should be, Moldflow is able to suggest a suitable location.
Gate locaon
Figure 17.9 The image shows the selected gate location by the yellow cone. It is easy to move the gate location and then recalculate if you are not satisfied with the filling or if weld lines occur in places with high stress level.
148
17.3 Workflow
17.3.5 Simulations When starting the calculation process, which may take many hours to complete, depending on how complex the part is (how dense the mesh model is) or how powerful your computer is, the calculations can provide answers to the following questions: ■■
Is this the best possible solution (optimization)?
■■
Where is the best gate location when considering weld lines and sink marks?
■■
What will the process window look like?
■■
How to balance a multi-cavity mold?
■■
How does my choice of material affect the final product?
■■
Why are there quality problems (when analysis is performed later in the process)?
17.3.6 Results Generated by Simulations The results of the various simulation calculations are usually presented as graphs. Below are some that you can analyze: ■■
Filling sequence
■■
Pressure distribution
■■
Cooling time
■■
Temperature distribution
■■
Levels of shear stress
■■
Necessary clamping force
■■
Location of weld lines
■■
Air traps
■■
Process parameters
■■
Glass fiber orientation
149
Chapter 17 — Prototype Molds and Mold Filling Analysis
17.3.7 Filling Sequence In Figure 17.10 you can see the filling time for the headlight housing shown as a function of different colors. Blue is the shortest time and red the longest time. It is also possible to see where the part will be filled last (gray color). Normally the filling process is presented by animations. It is also possible to view the process in Flash format instead by using Moldflow. The Flash format can be used on any computer that has a Flash-enabled browser (available free of charge) installed.
Figure 17.10 Here you can see the mold filling time displayed as a color spectrum. The axis on the right shows what the different colors represent in seconds. Within the gray area, that has not yet been filled, you will most likely get air entrapments and weld lines. Besides being able to determine this by yourself there are specific graphs for this generated with risk analysis.
17.3.8 Pressure Distribution
Figure 17.11 This image shows the pressure distribution as a color spectrum. The axis on the right displays the meaning of the different colors in MPa. You can see that a pressure of 17.71 MPa (which is the maximum value on the scale) is not sufficient to fill the headlight completely as you still have a gray area at the bottom.
17.3.9 Clamping Force
Figure 17.12 By using Moldflow you are able to analyze how much mold clamping force is needed. The image shows necessary clamping force over the entire injection-molding cycle. In the case of the headlight housing a clamping force of at least 110 tons is needed.
150
17.3 Workflow
17.3.10 Cooling Time
Figure 17.13 This image shows necessary cooling time before ejection of the product. The green area corresponds to approximately 25 seconds. However, there are some red areas that require 50 seconds. By optimizing the temperature control in these areas from the beginning you will avoid costly surprises.
17.3.11 Temperature Control
Figure 17.14 If you are able to conclude that the temperature control is not sufficient, it is possible to modify the cooling channels using Moldflow in order to study the temperature distribution. The image shows the cooling channels in a two-cavity mold used for production of jars.
17.3.12 Shrinkage and Warpage Most plastics are anisotropic, meaning that properties such as strength and shrinkage vary with the material’s flow direction compared to the cross direction. If you are not aware of this you can run into major surprises concerning shrinkage and warpage when a new mold is used initially. The mold shrinkage that is common in injection molding of thermoplastics depends on several factors, such as: ■■
The shrinkage properties of the material in various directions (the gate location is a major factor of importance here)
■■
Molecular chain orientation and fiber orientation in the cavity
■■
Variations of wall thickness of the part
■■
Hold pressure and hold pressure time in the molding process (shrinkage compensation)
■■
The temperature distribution in the cavities during the cooling process
If the shrinkage varies within the part depending on any of the factors above, you will get internal stresses. These stresses cause the part to warp when being released.
151
Chapter 17 — Prototype Molds and Mold Filling Analysis
17.3.13 Glass Fiber Orientation
Figure 17.15 By using Moldflow it is possible to see how the glass fibers are oriented within the cavity during the filling process.
17.3.14 Warpage Analysis
Gate
Figure 17.16 One of the most advanced modules of Moldflow is the shrinkage and warpage analysis. Here you can see the results for the headlight housing. If you place the gate in the center of the part as shown in the image, you will get a deviation of the dimensions that is greater than 2.2 mm. This will be on the left short side and a deviation of approximately 1 mm on the right short side.
17.3.15 Gate Location If the gate is moved from the middle of the headlight housing to the left-hand side shown in Figure 17.16, you get a considerably lower value for warpage, as shown in Figure 17.17.
Gate
Figure 17.17 By moving the gate to the left short side, a smaller deviation of approximately 0.4 mm on each of the short sides is obtained. This is a major improvement. To make this kind of change in a mold that has already been produced is very costly. This is therefore something that is well justified to do early in the simulation process whenever there is a possible risk of warpage.
152
17.3 Workflow
17.3.16 Material Replacement An alternative to moving the gate sometimes can be switching to a different material. In Figure 17.18 the gate has been located at the original location, in the center of the headlight, and by replacing the semi-crystalline polypropylene with glass fiber with the unreinforced amorphous material PC/ABS, you can see a significant reduction in warpage.
Figure 17.18 By keeping the original location of the gate but making a material replacement you get a considerably smaller deviation than that in Figure 17.16.
17.3.17 Simulation Software Below are a few links to the simulation software producers: ■■
Moldflow (Autodesk, USA): www.moldflow.com
■■
CadMould (SimCon, Germany): www.simcon-worldwide.com
■■
Moldex3D (CoreTech, Taiwan): www.moldex3d.com
153
Prototype Molds and Mold Filling Analysis In the previous chapter we described various errors that depend on either an incorrect part or mold design. When designing new parts or starting the molding of a new product, you will get a number of new questions and challenges: ■■
Will the part get the correct dimensions?
■■
Will it warp?
■■
Are the runners too long? Will the part be completely filled?
■■
Where should the gate be placed in order to make the part as strong as possible?
■■
Are the temperature-control channels correctly dimensioned and located?
17.1 Prototype Molds In order to avoid unpleasant surprises when starting the production in a new mold, you can use a prototype mold to see what the part would look like once it has been molded. Another option is to complete only one of several cavities in a production mold. These procedures could save both money and time. But they are not always completely reliable as runners, and mold temperature systems seldom correspond to the final production mold. Producers are using prototype molds when the plastic part is very complex or when the production mold is very expensive. Within the automotive industry these kinds of molds sometimes are named “soft molds” as they often are made in aluminum or in soft steel. When it comes to more simple part or mold design most prototypes have been replaced by a mold filling analysis.
Figure 17.1 Prototype mold (highlighted in red) in aluminum for the cams shown in Figure 17.2. Here only one part is made in each shot. Below is the production mold in steel with 16 cavities. This mold is about 30 times more expensive to produce compared to the prototype mold in aluminum.
145
Chapter 17 — Prototype Molds and Mold Filling Analysis
Figure 17.2 Here we can see a common component used in assembly of furniture. AD-Plast in Sweden developed this component in PPA with glass. They won the prestigious price “Plastovationer 2009” by successfully replacing metal with plastic. The cam is stronger than the former zinc one without needing to change the outer dimensions. [Source: AD-Plast AB]
17.2 Mold Filling Analysis Mold filling analysis is a computer-based tool that facilitates the ability to get accurate plastic parts in less time when producing a new part or modifying an existing mold.
Figure 17.3 On the image you can see a mold designer in front of his PC working with Moldflow, a mold filling software. Such software is able to run on standard PCs but requires a lot of computing power. In order for the calculations to run as fast as possible it is necessary to have a large internal memory as well as a fast processor.
With mold filling analysis you get:
146
■■
The ability to reach the right result in less time
■■
A powerful tool for successful “lean production” with focus on continuous improvement
■■
The correct process parameters with a major influence on the property profile of the part
■■
Knowledge that leads to an increased process window and more robust production
■■
In general, lower development costs compared to a prototype mold
17.3 Workflow
Figure 17.4 Here we can see a product in the shape according to the design that the designer had in mind. In Figure 17.5 we can see what it actually looked like when it left the mold.
Figure 17.5 Here we can see that the product is heavily bent. The reason is internal stresses probably dependent on an uneven wall thickness or unfavorable temperature control. By using the shrinkage & warpage modulus in the Moldflow software this could have been predicted before production and could have been corrected so that the defect never occurred.
17.3 Workflow 17.3.1 Mesh Model Mold filling simulation should be a natural step in the development process of new plastic products. Many designers are using various CAD software packages such as Pro-E, Catia, or Solid Works when creating a 3D model of new products. The STL or Igesfile STL or just IGES file that is generated is then used to create a so-called mesh model.
Triangular shape
Figure 17.6 This is a 3D model in STL format of a headlight housing used for cars. The model consists of a large number of small triangles.
Figure 17.7 Here we see the same headlight housing as shown in Figure 17.6. By using Moldflow analysis you can create a mesh model that is used as “input” into the continuing simulation process.
147
Chapter 17 — Prototype Molds and Mold Filling Analysis
17.3.2 Material Selection Once the mesh model is created, the next step is to choose which plastic resin to work with. In Moldflow there is a database with a large number of materials to choose from. If the material of choice is not found among those listed in the database, it is possible to add some needed (rheological) parameters yourself.
17.3.3 Process Parameters As the melt viscosity of the material is affected by melt and mold temperature and shear rate you must enter these parameters into the database if the material is not found among the preselected materials listed in the database.
Figure 17.8 The picture here shows the window in which the process parameters are to be entered in Moldflow. If the values for injection speed, hold pressure switch, hold pressure, or cooling time are missing, you will be able to select the “automatic” choice for these parameters.
17.3.4 Selection of Gate Location Before any calculations in the simulation program you must select a possible gate location. If you have no idea where it should be, Moldflow is able to suggest a suitable location.
Gate locaon
Figure 17.9 The image shows the selected gate location by the yellow cone. It is easy to move the gate location and then recalculate if you are not satisfied with the filling or if weld lines occur in places with high stress level.
148
17.3 Workflow
17.3.5 Simulations When starting the calculation process, which may take many hours to complete, depending on how complex the part is (how dense the mesh model is) or how powerful your computer is, the calculations can provide answers to the following questions: ■■
Is this the best possible solution (optimization)?
■■
Where is the best gate location when considering weld lines and sink marks?
■■
What will the process window look like?
■■
How to balance a multi-cavity mold?
■■
How does my choice of material affect the final product?
■■
Why are there quality problems (when analysis is performed later in the process)?
17.3.6 Results Generated by Simulations The results of the various simulation calculations are usually presented as graphs. Below are some that you can analyze: ■■
Filling sequence
■■
Pressure distribution
■■
Cooling time
■■
Temperature distribution
■■
Levels of shear stress
■■
Necessary clamping force
■■
Location of weld lines
■■
Air traps
■■
Process parameters
■■
Glass fiber orientation
149
Chapter 17 — Prototype Molds and Mold Filling Analysis
17.3.7 Filling Sequence In Figure 17.10 you can see the filling time for the headlight housing shown as a function of different colors. Blue is the shortest time and red the longest time. It is also possible to see where the part will be filled last (gray color). Normally the filling process is presented by animations. It is also possible to view the process in Flash format instead by using Moldflow. The Flash format can be used on any computer that has a Flash-enabled browser (available free of charge) installed.
Figure 17.10 Here you can see the mold filling time displayed as a color spectrum. The axis on the right shows what the different colors represent in seconds. Within the gray area, that has not yet been filled, you will most likely get air entrapments and weld lines. Besides being able to determine this by yourself there are specific graphs for this generated with risk analysis.
17.3.8 Pressure Distribution
Figure 17.11 This image shows the pressure distribution as a color spectrum. The axis on the right displays the meaning of the different colors in MPa. You can see that a pressure of 17.71 MPa (which is the maximum value on the scale) is not sufficient to fill the headlight completely as you still have a gray area at the bottom.
17.3.9 Clamping Force
Figure 17.12 By using Moldflow you are able to analyze how much mold clamping force is needed. The image shows necessary clamping force over the entire injection-molding cycle. In the case of the headlight housing a clamping force of at least 110 tons is needed.
150
17.3 Workflow
17.3.10 Cooling Time
Figure 17.13 This image shows necessary cooling time before ejection of the product. The green area corresponds to approximately 25 seconds. However, there are some red areas that require 50 seconds. By optimizing the temperature control in these areas from the beginning you will avoid costly surprises.
17.3.11 Temperature Control
Figure 17.14 If you are able to conclude that the temperature control is not sufficient, it is possible to modify the cooling channels using Moldflow in order to study the temperature distribution. The image shows the cooling channels in a two-cavity mold used for production of jars.
17.3.12 Shrinkage and Warpage Most plastics are anisotropic, meaning that properties such as strength and shrinkage vary with the material’s flow direction compared to the cross direction. If you are not aware of this you can run into major surprises concerning shrinkage and warpage when a new mold is used initially. The mold shrinkage that is common in injection molding of thermoplastics depends on several factors, such as: ■■
The shrinkage properties of the material in various directions (the gate location is a major factor of importance here)
■■
Molecular chain orientation and fiber orientation in the cavity
■■
Variations of wall thickness of the part
■■
Hold pressure and hold pressure time in the molding process (shrinkage compensation)
■■
The temperature distribution in the cavities during the cooling process
If the shrinkage varies within the part depending on any of the factors above, you will get internal stresses. These stresses cause the part to warp when being released.
151
Chapter 17 — Prototype Molds and Mold Filling Analysis
17.3.13 Glass Fiber Orientation
Figure 17.15 By using Moldflow it is possible to see how the glass fibers are oriented within the cavity during the filling process.
17.3.14 Warpage Analysis
Gate
Figure 17.16 One of the most advanced modules of Moldflow is the shrinkage and warpage analysis. Here you can see the results for the headlight housing. If you place the gate in the center of the part as shown in the image, you will get a deviation of the dimensions that is greater than 2.2 mm. This will be on the left short side and a deviation of approximately 1 mm on the right short side.
17.3.15 Gate Location If the gate is moved from the middle of the headlight housing to the left-hand side shown in Figure 17.16, you get a considerably lower value for warpage, as shown in Figure 17.17.
Gate
Figure 17.17 By moving the gate to the left short side, a smaller deviation of approximately 0.4 mm on each of the short sides is obtained. This is a major improvement. To make this kind of change in a mold that has already been produced is very costly. This is therefore something that is well justified to do early in the simulation process whenever there is a possible risk of warpage.
152
17.3 Workflow
17.3.16 Material Replacement An alternative to moving the gate sometimes can be switching to a different material. In Figure 17.18 the gate has been located at the original location, in the center of the headlight, and by replacing the semi-crystalline polypropylene with glass fiber with the unreinforced amorphous material PC/ABS, you can see a significant reduction in warpage.
Figure 17.18 By keeping the original location of the gate but making a material replacement you get a considerably smaller deviation than that in Figure 17.16.
17.3.17 Simulation Software Below are a few links to the simulation software producers: ■■
Moldflow (Autodesk, USA): www.moldflow.com
■■
CadMould (SimCon, Germany): www.simcon-worldwide.com
■■
Moldex3D (CoreTech, Taiwan): www.moldex3d.com
153