Warping (deformation) and solutions

Warpage refers to a discrepancy between the shape of a plastic part and the requirements of the drawing, as shown in the image below; it is also called deformation.Warpage is usually caused by uneven shrinkage of the plastic part, but does not include deformation caused during demolding.

The following are some of the reasons that cause warping of plastic parts after molding and the corresponding solutions.

① Uneven molecular orientation, as shown in Figure 3-36. To minimize warping caused by differences in molecular orientation, conditions should be created to reduce flow orientation or orientation stress. An effective method is to lower the melt temperature and mold temperature.

When using this method, it is best to combine it with heat treatment of the plastic part; otherwise, the effect of reducing molecular orientation differences is often temporary. The heat treatment method is: after demolding, place the plastic part at a relatively high temperature for a certain period of time, then slowly cool it to room temperature. This can largely eliminate orientation stress within the plastic part.

Figure 3-36 shows how uneven molecular orientation leads to warping of plastic parts.

② Improper cooling. Improper cooling during the molding process can easily lead to deformation of the plastic part, as shown in Figure 3-37. When designing the structure of the plastic part, the cross-sectional thickness of each part should be as uniform as possible. The plastic part must have sufficient cooling and setting time within the mold.

For the design of the mold cooling system, attention should be paid to placing the cooling pipes in areas where the temperature is easily raised and heat is concentrated. For areas that are relatively cool, slow cooling should be implemented as much as possible to ensure even cooling of all parts of the plastic part.

Figure 3-37 Example of plastic part deformation caused by improper cooling

③ Inadequate mold gating system design. When determining the gate location, the melt should not directly impact the core; the force on both sides of the core should be even. For large rectangular or flat plastic parts, when using molecularly oriented plastic materials with high shrinkage, thin-film gates or multi-point gates should be used, and side gates should be avoided as much as possible. For ring-shaped plastic parts, disc gates or spoke gates should be used, and side gates or point gates should be avoided as much as possible. For shell-shaped plastic parts, direct gates should be used, and side gates should be avoided as much as possible.

④ Inadequate mold demolding and venting system design. In mold design, the draft angle, ejector pin position and number should be designed reasonably to improve the mold's strength and positioning accuracy. For small and medium-sized molds, anti-warping molds can be designed and manufactured according to warping rules. In mold operation, the ejection speed or ejection stroke should be appropriately slowed down.

⑤ Improper process settings. Specific manifestations include: excessively high mold and barrel temperatures; excessively high injection pressure or injection speed; excessively long holding time or insufficient cooling time. The corresponding process parameters should be adjusted according to the specific situation.

Unreasonable part structure, such as: uneven wall thickness, sudden changes, or excessively thin walls; improper product design without reinforcing structures to constrain deformation.

Regarding raw materials: Phthalocyanine pigments can affect the crystallinity of polyethylene, leading to product deformation; using a combination of reinforcement and powder fillers can effectively reduce the degree of deformation in plastic parts.

The causes and solutions for warping of plastic parts are shown in the table below.