Analysis of Injection Molding Product Defects

Warpage, Bending and Twisting of Injection Molded Products

The occurrence of warpage, bending and twisting in injection molded products is mainly due to the fact that the shrinkage rate in the flow direction during plastic molding is higher than that in the vertical direction, resulting in warpage caused by different anisotropic shrinkage rates of the product. In addition, it is inevitable that large internal stress remains inside the product during injection filling, which also causes warpage. These are all manifestations of deformation caused by high-stress orientation. Fundamentally speaking, mold design determines the warpage tendency of the product. It is very difficult to suppress this tendency by changing molding conditions, and the ultimate solution must start from mold design and improvement.

This phenomenon is mainly caused by the following aspects:

1. Mold-related Factors

• a. The thickness and weight of the product should be uniform.

• b. The design of the cooling system should ensure uniform temperature in all parts of the mold cavity. The gating system should make the material flow symmetrical to avoid warpage caused by different flow directions and shrinkage rates. Appropriately thicken the runner and main runner in parts that are difficult to mold, and try to eliminate differences in density, pressure and temperature in the cavity.

• c. The transition areas and corners between thick and thin parts of the product should be sufficiently smooth, and the mold should have good demoldability. For example, increase the demolding allowance, improve the polishing of the mold surface, and ensure the balance of the ejection system.

• d. The exhaust performance should be good.

• e. Increase the wall thickness of the product or add anti-warpage directions, and use reinforcing ribs to enhance the product’s anti-warpage ability.

• f. The strength of the material used for the mold is insufficient.

What Causes Warpage and Deformation of Injection (Plastic) Molded Products?

2. Plastic-related Factors

• Crystalline plastics are more likely to have warpage deformation than amorphous plastics. Moreover, crystalline plastics can use the crystallization process—whereby crystallinity decreases and shrinkage rate becomes smaller as the cooling rate increases—to correct warpage deformation.

3. Processing-related Factors

• a. Excessively high injection pressure, too long holding time, too low melt temperature, or too fast injection speed will increase internal stress and cause warpage deformation.

• b. Excessively high mold temperature and too short cooling time will cause the product to be overheated during demolding, resulting in ejection deformation.

• c. Under the condition of maintaining the minimum filling amount, reduce the screw speed and back pressure to lower the density, thereby limiting the generation of internal stress.

• d. If necessary, perform mold soft shaping on products that are prone to warpage deformation or carry out annealing treatment after demolding.

Color Streaks, Color Lines and Color Spots in Injection Molded Products

This defect mainly occurs in plastic products colored with color masterbatches. Although color masterbatch coloring is superior to dry powder coloring and dye paste coloring in terms of color stability, color purity and color migration resistance, its dispersibility—i.e., the uniformity of mixing between color masterbatches and diluted plastics—is relatively poor, which naturally leads to regional color differences in the finished products.

Main Solutions

• a. Increase the temperature of the feeding section, especially the temperature at the rear of the feeding section, to make it close to or slightly higher than the temperature of the melting section. This allows the color masterbatch to melt as soon as it enters the melting section, promotes uniform mixing with the diluted plastic, and increases the chance of liquid mixing.

• b. Under a fixed screw speed, increase the back pressure to improve the melt temperature and shear effect in the barrel.

• c. Modify the mold, especially the gating system. For example, if the gate is too wide, the turbulent flow effect of the melt during passage will be poor, and the temperature rise will be insufficient, resulting in uneven mixing and color streaks in the cavity. In such cases, the gate should be narrowed