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Mold Design and Product Quality
Chapter 16 – Mold Design and Product Quality In Chapter 11 problems that may occur on injection-molded parts due to defects in the plastic raw material are described. In this chapter we will look at problems due either to badly designed molds or improper part design. In Chapter 28 we will describe process-related problems.
Mold-Related Problems These types of problems are not always as easy to detect by visual inspection as material- or process-related problems are. Many of these problems are only discovered when the parts are mechanically tested or when the parts break under normal stress loads. Below are some common problems due to: x x x x x x x
Too-weak mold plates Incorrect sprue/nozzle design Incorrect runner design Incorrectly designed, located, or missing cold slug pocket Incorrect gate design Incorrect venting Incorrect mold temperature management
Too-Weak Mold Plates If you get a flash around the sprue or runners during the injection phase, it may indicate toohigh injection speed, too-low lock pressure on the machine, or too-weak mold plates.
Fig 252. In the picture to the left a fan shroud in acetal is shown. The gate is in the middle, and it is clear that the mold plate has become deformed so that a flash around the gate has been formed even though the cavity is not completely filled. Moving the mold to a larger machine with higher clamping force did not help in this case.
In the case above, the first attempt to solve the problem was choosing an acetal grade with a lower viscosity. However, this did not solve the problem entirely (see the figure below to the left). Another option to solve the problem would have been to increase the wall thickness of the shroud or change the grid thickness in the round hole. Neither of these solutions were chosen; instead a solution using flow directors was used. The wall thickness was increased by using a honeycomb pattern (as shown on the figure below to the right).
Fig 253. A less viscous grade of acetal with slightly less impact resistance was chosen to fill the fan shroud entirely, but still flash in the middle could not be avoided.
Fig 254. By applying a pattern of flow directors one succeeded to fill the shroud without getting the flash in the middle, nor did the cycle time need to be extended.
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Incorrect Sprue and Nozzle Design With incorrect dimensions of the sprue we generally mean that the sprue is either too small relative to the wall thickness of the part or that the diameter of the nozzle is too small. This is especially true for semi-crystalline plastics where correct dimensions are critical. Below is an example of a product that was exposed to a high impact and did not pass the mechanical tests because the setter had forgotten to change to a larger nozzle diameter when the mold was set. Problems may also arise if the nozzle size is too large, causing leakage between the mold and the cylinder. Fig 255. The picture shows a safety pin in a high-viscosity polyamide. The wall thickness at the gate is about 15 mm, and the runner to the six different cavities is correctly dimensioned while the sprue and the nozzle are both too small. The sprue should be at least 1 mm larger in diameter at the top in comparison to the runner. The nozzle should be 1 mm less than the smallest diameter of the sprue. In the small picture in the upper left corner the incorrect sprue is shown to the left and the correct one is shown to the right.
Incorrect Runner Design The most common problem is that the runners are too small in relation to the wall thickness of the part. This leads to problems as the semi-crystalline plastic freezes in the runner before the parts have been sufficiently packed. Another common problem occurs when there are unbalanced channels causing uneven filling and packing. Below is an example of a poorly designed runner.
Fig 256. In the figure to the left an example of a runner with disadvantageous design is shown. The six cavities that have the same shape and size are filled very unevenly. Photo: DuPont
Incorrectly Designed, Located, or Missing Cold Slug Pocket In the previous chapter, we mentioned that a cold slug pocket has two different functions in the mold. It is supposed to capture material that has frozen in the nozzle, and it should be designed so it facilitates the pulling of the sprue. Cold slug pockets are of greatest importance in molds used for semi-crystalline materials as the risk for cold slugs is very high. If the part design does not allow a cold slug pocket it is possible to use a special procedure instead to eliminate the risk that material is freezing in the nozzle. In such cases the cylinder must be moved back after dosing. Then you must have sufficient decompression (sucking material back into the cylinder) so potential cold slugs may re-melt. Another alternative is to use a hot runner system.
Fig 257. The picture shows the center ring with the gate on a 15" wheel cap made of mineral-reinforced PA66. The sprue is located at the back of the wheel cap because a sticker with the car brand will be placed on the front. This will prevent the need to make a cold slug pocket.
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Incorrect Gate Design A too-small gate is the most common gate design problem. In such cases semi-crystalline material freezes in the gate before it has been packed and the shrinkage compensation has been completed. This in turn causes voids, sink marks, or incorrect dimensions to occur. If you also have a large volume to fill, the amount of shearing can become too high (especially if you have a fast injection speed). This causes the material to degrade in the gate. This can also occur if you have too-small radii in the gate.
Fig 258. The picture to the left shows a sprue (cut) sitting on a cover made in impact-modified PA66. Due to too-small radii in the transition between the sprue and the cover as well as too-high injection speed, the material has been sheared and delamination has formed around the gate.
In some cases the gate can also be badly designed. In the figure below you can see a conical shaped gate to the right. This design, which is very common, works well for amorphous plastics but is inappropriate for semi-crystalline plastics as they freeze too early. The gate to the left in the picture shows how the gate is supposed to be designed when semi-crystalline materials will be used.
Fig 259. To the left is an example of a gate that is designed for semi-crystalline materials. The size of gate d should be at least half the wall thickness T, and the distance between the runner and part should be less than 0.8 mm. The diameter D should be at least 1.2 × T. The figure also shows that the gate should be located on the thickest wall of the part. Source: DuPont
A good way to check if a semi-crystalline material has been packed sufficiently is to saw the parts in pieces at several locations to determine if they are free from voids or pores. Fig 260. In the picture shown to the right is a railway insulator made of glass fiber reinforced polyamide. When a glass fiber reinforced material is not sufficiently packed, micro-pores will occur, which appear as lighter areas in the sawn surface. The right part is the starting material for a worm gear made in acetal. In the first cut no pores were detected, as shown on the upper wheel half. The next cut showed a big void in the wall. It is therefore important that you make several cuts when you want to check if a part is free of voids or pores. If voids are present they cause lower mechanical strength. The weight of the void-free parts should be recorded and used as a quality control tool.
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Incorrect Venting If venting channels are missing, have been blocked by mold deposit (degraded polymer), or have too-small dimensions, the air in the mold can be trapped during the filling phase. The air will then be compressed and heated to above 1000°C, and this causes degradation of the polymer. We call this a “diesel effect.” Normally this problem is discovered by a discoloration on the surface in combination with the part not being completely filled. On black materials this can be hard to see. The burn marks that occur are, however, slightly lighter than the black plastic.
Fig 261. The picture shows a part that is filled from two different directions, causing a weld line. It was intended that the weld line should be located on the ejector pin because slots for venting have been ground on it. One can also see that the plastic around the weld line has been exposed to high temperatures because it has become degraded and discolored.
Incorrect Mold Temperature Management In the previous chapter we looked at the mold temperature-control system and concluded that it is important to have a correct temperature in the cavity to achieve high-quality properties such as surface finish, mechanical strength, dimensions, and avoid the risk of warping. A common problem that you sometimes will see is that the temperature-control capacity may not be sufficient even if the channels have the correct dimensions. This may be due to the channels being blocked due to corrosion. Sometimes the setup is not done properly as only one temperature-control unit is used and connected in series to both the fixed and the moving halves. It is always recommended to use one temperature-control unit for each half. It may also be of importance in which order the pipes with the temperature-management fluid are connected.
Fig 262. The picture above shows the cross section of a T-profile in a semi-crystalline plastic injection molded with or without temperature-management control. Where the cooling is lacking, an increase in temperature occurs in the angle, which in turn causes the solidification process to slow down and crystal formation and shrinkage to increase. A good rule of comparison is: Plastics are very similar to cats: both are drawn toward the heat.
96
Mold-Related Problems These types of problems are not always as easy to detect by visual inspection as material- or process-related problems are. Many of these problems are only discovered when the parts are mechanically tested or when the parts break under normal stress loads. Below are some common problems due to: x x x x x x x
Too-weak mold plates Incorrect sprue/nozzle design Incorrect runner design Incorrectly designed, located, or missing cold slug pocket Incorrect gate design Incorrect venting Incorrect mold temperature management
Too-Weak Mold Plates If you get a flash around the sprue or runners during the injection phase, it may indicate toohigh injection speed, too-low lock pressure on the machine, or too-weak mold plates.
Fig 252. In the picture to the left a fan shroud in acetal is shown. The gate is in the middle, and it is clear that the mold plate has become deformed so that a flash around the gate has been formed even though the cavity is not completely filled. Moving the mold to a larger machine with higher clamping force did not help in this case.
In the case above, the first attempt to solve the problem was choosing an acetal grade with a lower viscosity. However, this did not solve the problem entirely (see the figure below to the left). Another option to solve the problem would have been to increase the wall thickness of the shroud or change the grid thickness in the round hole. Neither of these solutions were chosen; instead a solution using flow directors was used. The wall thickness was increased by using a honeycomb pattern (as shown on the figure below to the right).
Fig 253. A less viscous grade of acetal with slightly less impact resistance was chosen to fill the fan shroud entirely, but still flash in the middle could not be avoided.
Fig 254. By applying a pattern of flow directors one succeeded to fill the shroud without getting the flash in the middle, nor did the cycle time need to be extended.
93
Incorrect Sprue and Nozzle Design With incorrect dimensions of the sprue we generally mean that the sprue is either too small relative to the wall thickness of the part or that the diameter of the nozzle is too small. This is especially true for semi-crystalline plastics where correct dimensions are critical. Below is an example of a product that was exposed to a high impact and did not pass the mechanical tests because the setter had forgotten to change to a larger nozzle diameter when the mold was set. Problems may also arise if the nozzle size is too large, causing leakage between the mold and the cylinder. Fig 255. The picture shows a safety pin in a high-viscosity polyamide. The wall thickness at the gate is about 15 mm, and the runner to the six different cavities is correctly dimensioned while the sprue and the nozzle are both too small. The sprue should be at least 1 mm larger in diameter at the top in comparison to the runner. The nozzle should be 1 mm less than the smallest diameter of the sprue. In the small picture in the upper left corner the incorrect sprue is shown to the left and the correct one is shown to the right.
Incorrect Runner Design The most common problem is that the runners are too small in relation to the wall thickness of the part. This leads to problems as the semi-crystalline plastic freezes in the runner before the parts have been sufficiently packed. Another common problem occurs when there are unbalanced channels causing uneven filling and packing. Below is an example of a poorly designed runner.
Fig 256. In the figure to the left an example of a runner with disadvantageous design is shown. The six cavities that have the same shape and size are filled very unevenly. Photo: DuPont
Incorrectly Designed, Located, or Missing Cold Slug Pocket In the previous chapter, we mentioned that a cold slug pocket has two different functions in the mold. It is supposed to capture material that has frozen in the nozzle, and it should be designed so it facilitates the pulling of the sprue. Cold slug pockets are of greatest importance in molds used for semi-crystalline materials as the risk for cold slugs is very high. If the part design does not allow a cold slug pocket it is possible to use a special procedure instead to eliminate the risk that material is freezing in the nozzle. In such cases the cylinder must be moved back after dosing. Then you must have sufficient decompression (sucking material back into the cylinder) so potential cold slugs may re-melt. Another alternative is to use a hot runner system.
Fig 257. The picture shows the center ring with the gate on a 15" wheel cap made of mineral-reinforced PA66. The sprue is located at the back of the wheel cap because a sticker with the car brand will be placed on the front. This will prevent the need to make a cold slug pocket.
94
Incorrect Gate Design A too-small gate is the most common gate design problem. In such cases semi-crystalline material freezes in the gate before it has been packed and the shrinkage compensation has been completed. This in turn causes voids, sink marks, or incorrect dimensions to occur. If you also have a large volume to fill, the amount of shearing can become too high (especially if you have a fast injection speed). This causes the material to degrade in the gate. This can also occur if you have too-small radii in the gate.
Fig 258. The picture to the left shows a sprue (cut) sitting on a cover made in impact-modified PA66. Due to too-small radii in the transition between the sprue and the cover as well as too-high injection speed, the material has been sheared and delamination has formed around the gate.
In some cases the gate can also be badly designed. In the figure below you can see a conical shaped gate to the right. This design, which is very common, works well for amorphous plastics but is inappropriate for semi-crystalline plastics as they freeze too early. The gate to the left in the picture shows how the gate is supposed to be designed when semi-crystalline materials will be used.
Fig 259. To the left is an example of a gate that is designed for semi-crystalline materials. The size of gate d should be at least half the wall thickness T, and the distance between the runner and part should be less than 0.8 mm. The diameter D should be at least 1.2 × T. The figure also shows that the gate should be located on the thickest wall of the part. Source: DuPont
A good way to check if a semi-crystalline material has been packed sufficiently is to saw the parts in pieces at several locations to determine if they are free from voids or pores. Fig 260. In the picture shown to the right is a railway insulator made of glass fiber reinforced polyamide. When a glass fiber reinforced material is not sufficiently packed, micro-pores will occur, which appear as lighter areas in the sawn surface. The right part is the starting material for a worm gear made in acetal. In the first cut no pores were detected, as shown on the upper wheel half. The next cut showed a big void in the wall. It is therefore important that you make several cuts when you want to check if a part is free of voids or pores. If voids are present they cause lower mechanical strength. The weight of the void-free parts should be recorded and used as a quality control tool.
95
Incorrect Venting If venting channels are missing, have been blocked by mold deposit (degraded polymer), or have too-small dimensions, the air in the mold can be trapped during the filling phase. The air will then be compressed and heated to above 1000°C, and this causes degradation of the polymer. We call this a “diesel effect.” Normally this problem is discovered by a discoloration on the surface in combination with the part not being completely filled. On black materials this can be hard to see. The burn marks that occur are, however, slightly lighter than the black plastic.
Fig 261. The picture shows a part that is filled from two different directions, causing a weld line. It was intended that the weld line should be located on the ejector pin because slots for venting have been ground on it. One can also see that the plastic around the weld line has been exposed to high temperatures because it has become degraded and discolored.
Incorrect Mold Temperature Management In the previous chapter we looked at the mold temperature-control system and concluded that it is important to have a correct temperature in the cavity to achieve high-quality properties such as surface finish, mechanical strength, dimensions, and avoid the risk of warping. A common problem that you sometimes will see is that the temperature-control capacity may not be sufficient even if the channels have the correct dimensions. This may be due to the channels being blocked due to corrosion. Sometimes the setup is not done properly as only one temperature-control unit is used and connected in series to both the fixed and the moving halves. It is always recommended to use one temperature-control unit for each half. It may also be of importance in which order the pipes with the temperature-management fluid are connected.
Fig 262. The picture above shows the cross section of a T-profile in a semi-crystalline plastic injection molded with or without temperature-management control. Where the cooling is lacking, an increase in temperature occurs in the angle, which in turn causes the solidification process to slow down and crystal formation and shrinkage to increase. A good rule of comparison is: Plastics are very similar to cats: both are drawn toward the heat.
96