Warpage is surface distortion of plastic part that is not formed according to designed shape. Warpage of plastic part is caused by uneven shrinkage of formed plastic part. If the entire plastic part has a uniform shrinkage rate, plastic part will not warp when deformed, but will only reduce its size;
However, due to interaction of many factors such as molecular chain/fiber orientation, mold cooling, plastic part design, mold design and molding conditions, it is a very complicated task to achieve low shrinkage or uniform shrinkage.
Plastic parts warp due to uneven shrinkage. Reasons for change in shrinkage rate include:
Uneven temperature inside plastic part.
When plastic part solidifies, pressure difference and cooling rate difference along thickness direction.
Plastic parts are ejected before they are completely cooled, or ejector pins are deformed, undercut is too deep, ejection method is improper, and demolding angle is improper. These factors may cause plastic parts to warp.
Plastic parts have different thicknesses, resulting in different cooling rates.
Plastic parts have curved or asymmetric geometries.
Plastic parts have different materials with or without fillers.
Difference in molecular chain/fiber orientation in flow direction and perpendicular to flow direction causes different shrinkage rates.
Differences in holding pressure (e.g., excessive holding pressure at gate, but insufficient holding pressure away from gate).
Whether or not fillers are added to plastic part material will cause shrinkage differences, as shown in Figure 9. When a plastic part has shrinkage differences, its thickness direction and flow direction produce unequal shrinkage, and resulting internal stress may cause plastic part to warp.
Since reinforced fibers reduce thermal shrinkage of plastic parts and increase modulus, thermoplastics with added fibers can suppress shrinkage. Its shrinkage along arrangement direction of added fibers (usually flow direction) is smaller than shrinkage in transverse direction. Similarly, thermoplastics with particulate fillers shrink much less than plastics without fillers.
On the other hand, if plastic part without fillers has a high degree of molecular chain orientation, it is anisotropic shrinkage, with a relatively large shrinkage in direction of molecular chain arrangement. Liquid crystal polymers have a tightly arranged self-reinforcing structure, and their shrinkage tends to be anisotropic.

 

Figure 9 Difference in shrinkage in different directions caused by whether plastic part is filled or not.
Uneven cooling and asymmetric cooling of plastic part in thickness direction between male and female molds can cause shrinkage differences, as shown in Figure 10. Material cools and shrinks unevenly from mold wall to center layer, resulting in warpage after ejection.

 

Figure 10 Warpage of plastic parts due to (a) uneven cooling; and (b) asymmetric cooling.
Shrinkage of plastic part increases with thickness. Shrinkage difference caused by uneven thickness is main reason for warpage of thermoplastic plastic parts without reinforcing fillers. More specifically, changes in thickness of plastic part section usually cause differences in cooling rate and crystallinity, resulting in shrinkage differences and plastic part warpage, as shown in Figure 11.

 

Figure 11 High crystallinity in low cooling rate area causes plastic part to shrink more
Asymmetric geometry can lead to uneven cooling and shrinkage differences, causing plastic part warpage. For example, as shown in Figure 12, a row of reinforcing ribs is added to one side of flat part, which is an asymmetric geometry.

 

Figure 12 Ribbed side of plastic part cools poorly, causing warpage.
Residual stress can also cause warpage, which can be improved by extending cooling time of molded part in mold. Uneven cooling can also cause warpage. Molded part temperature is too high during ejection, and ejector pin causes molded part to warp.
In addition, when hot molded part falls into collection box, it can also cause warpage. Uneven temperature distribution of plastic part can cause plastic part warpage. Complex components can also cause uneven cooling, especially molds without a cooling system.

Shrinkage and warpage can be reduced or controlled through proper part design, mold design, molding conditions and material selection. Following design rules take into account factors that can help develop low shrinkage and warpage-free parts.

(1) Wall thickness

Avoid uneven wall thickness, or design length of wall thickness change zone to be three times wall thickness of thin wall thickness, as shown in Figure 13.

 

Figure 13 Design of wall thickness change zone
When a plastic part has obvious shrinkage, dents or pores, change design of these areas to a combination of uniform thin wall thickness and ribs to provide uniform shrinkage, good (strength/weight) ratio, and good cost efficiency, as suggested in Figure 14.

 

Figure 14 For most applications, design of thin wall thickness and ribs is better than thick parts.
(2) Balanced filling
Try to design a melt delivery system that can produce a balanced filling pattern with a fixed melt front speed.
(3) Holding pressure
Although high holding pressure helps reduce shrinkage, it may increase residual stress in plastic part and clamping force of injection molding machine.
A better design is to use appropriate holding pressure and sufficient holding time, and release holding pressure after gate solidifies. In addition, holding pressure used must be able to transfer additional plastic to compensate for volume shrinkage of plastic part.

(4) Cooling system

Design cooling system to achieve uniform and balanced cooling effect throughout plastic part and in cross-sectional direction of plastic part.
(5) Residual stress
Increasing melt temperature, mold wall temperature, filling time, and mold cavity thickness, or reducing holding pressure and flow length, etc., can help reduce residual stress and molecular chain/fiber orientation.