Plastic products shrinking is a very common issue encountered in various industries. It is often caused by the rapid cooling of molten plastic or improper processing conditions. The shrinkage rate of plastic products is generally expressed in percentage, usually between 1-3%, but sometimes much higher depending on the specific material. In severe cases, the plastic part will warp or deform, making it unsuitable for final use. This article discusses the causes and solutions of product shrinkage in plastic products and provides some general guidance to help product makers better understand and address the problem.
1. Causes of Shrinkage
1.1 Cooling rate
The rate at which molded parts cool down is an important factor influencing the dimensional accuracy of plastic parts. If the cooling rate is too high, the material will tend to shrink more than the specified dimensions. Generally speaking, the slower the cooling rate, the better the products dimensional accuracy. Rapid cooling can be caused by improper product design, using a cooling channel that is too small or inadequate molding temperature.
1.2 Process Temperature
Injection molding process temperature plays an important role in product accuracy and shrinkage. If the molding temperature is too high, the plastic will cool down more quickly and cause more shrinkage. On the other hand, there needs to be enough heat to ensure proper molten flow for the desired results. It is important to set the molding process temperature correctly and make sure that the plastic is heated up evenly to ensure proper circulation and elimination of dead pockets.
1.3 Plastic Resin Sensitivity
The type of plastic used in the product making process can be one of the major causes of shrinkage. Different plastic resins have different sensitivity to temperature, pressure and cooling rate. In addition, some plastic resin chemistries are more prone to warpage and shrinkage than others due to the different amount of energy released when they start to solidify. By using a resin with greater dimensional stability, such as an impact modified grade, this issue can be minimized or even eliminated.
1.4 Gate Location
Gate location plays an important role in controlling the flow rate of molten plastic into the mold cavity. If the gate is located too far away from the core and the gate size is small in relation to the flow rate, then the plastic will tend to shrink more in order to compensate for the velocity of the plastic facing the back end of the cavity, resulting in increased shrinkage.
1.5 Wall Thickness
Wall thickness of plastic parts is another factor that can affect product shrinkage. Generally, thicker walls will shrink more in an injection molded product than thinner walls. Therefore, thicker walls should be offset by increasing the cooling time or by redefining the flow pattern of the plastics.
2. Solutions for Shrinkage
2.1 Proper Product Design
Product design is a key factor influencing the accuracy of the molded part. The cooling system should be designed to provide adequate cooling of molded parts with uniform cooling rates. Designers can take advantage of the cooling channel shape, size, and arrangement to help reduce the problem. The wall thickness should be balanced and optimally varied to minimize the shrinkage effect.
2.2 Adjust Process Parameters
The molding process parameters must be adjusted correctly in order to minimize shrinkage and ensure uniformity in the molded parts. The mold temperature, molding pressure, filling time and cooling time should all be adjusted to ensure that the plastic material is adequately heated and uniformly cools.
2.3 Use the Right Resin
There are a wide variety of plastic materials available in the market, each of them with different shrinkage rates. It is important to choose a plastic material with high dimensional accuracy and low shrinkage. Additionally, resin modifiers and additives may be used to reduce the shrinkage rate and improve the materials dimensional stability.
2.4 Adjust Gate and Runner Systems
Correct gate and runner sizing and location can help reduce shrinkage. Gate and runner sizes should be optimized to match the flow rate of the plastic material. Ideally, the gate should be located as close as possible to the core and the runner should be sized to properly fill up the cavity.
3. Conclusion
Shrinkage in plastic products is a common issue and can be caused by various reasons. By following the right design, process, and material selection guidelines, product makers can better ensure that their plastic parts are dimensionally accurate and fit for purpose. Proper product and process design is essential to ensure that any product shrinkage is minimized or eliminated.
