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Mold shrinkage and warpage in injection molding
Contents
1. Shrinkage rate of the plastic injection mold
2. Shrinkage and warpage of plastic injection mold
3. Remedy
Most non-plastic materials show relatively small shrinkage rate of the plastic injection mold under the influence of temperature changes. On the other hand, most plastics cause shrinkage and warpage of plastic injection molds due to environmental changes.
Shrinkage rate of the plastic injection mold
The most significant change in shrinkage and warpage of plastic injection molds may occur during phase change, which has caused the greatest concern of plastic injection mold raw material suppliers. It also represents a major problem for suppliers of raw materials for plastic injection molds. When undergoing a normal cycle from a solid state to a plastic melt and then back to a solid state, the plastic will exhibit a dimensional change, which varies greatly from one resin to another. There is no practical method to accurately predict the exact shrinkage margin of any plastic material. In the past few years, especially plastic injection mold raw material suppliers have worked together to develop meaningful and useful data on the subject of shrinkage. But like most laboratory measurements, the development date is of little significance to the molder, because the parts he must produce rarely match the size of the test rod or disc. In addition, other variables, such as gates, part geometry, wall thickness variations, and the efficiency of the machine itself, make these data not only meaningless, but also misleading.
Most plastic injection mold raw material suppliers must rely on their past experience in the materials under consideration and the types of parts they design in order to get very close to the shrinkage allowance for a specific mold design. In any process involving cooling materials from high temperatures, a certain amount of shrinkage is inevitable. When designing the mold, the shrinkage rate of the plastic injection mold must be considered, allowing the shrinkage rate to be calculated according to the characteristics of the resin. Injection molding machines can prevent excessive shrinkage by controlling operating conditions, such as molding at lower injection temperatures, running cold molds, or packaging molds, which will make close tolerances impossible. The holding pressure can be achieved by molding at medium melt temperature and high melt pressure. However, excessive temperature or pressure may cause shrinkage and warpage of plastic injection molds. Another way to reduce mold shrinkage is to use high melt pressure and extend the injection time. When the material in the mold cools and shrinks, allowing extra resin to flow into the mold and refill the mold as much as possible without causing mold flash. Or the part is stuck in the cavity.
Shrinkage and warpage of plastic injection mold
Basically, shrinkage and warpage of plastic injection molds are caused by thermal changes when the plastic is in the mold. The shrinkage rate of the plastic injection mold refers to the shrinkage of the cured and demolded molded part mainly due to the relaxation of the stress in the mold, which is usually completed within one day after the part is demolded. Although the role of shrinkage is not important in many injection molded parts, in some applications it can cause trouble for moldmakers. In closures and parts that will become components, too much or too little shrinkage may be an effective cause of part rejection. In addition, uneven shrinkage can cause the molded product to warp. The uneven shrinkage, bending or distortion of the molded part after ejecting from the mold will not only change its size, but also its contour and angle. Warpage mainly occurs in large, flat molded products, although it is unpopular in any molded product, but it is especially true in items such as container lids, closures, or drain plates. When the part is ejected from the mold, it assumes its natural shape by releasing the stress applied to it when it is molded in a viscous state in the mold. The problem faced by plastic injection mold raw material suppliers is also a difficult problem is to minimize the internal stress that the part may remember in the future, and release it when it is cooled to room temperature. The locking stress in the mold is generated in the mold due to operating conditions such as excessive molding pressure, uneven mold cooling, or low melt temperature.
Remedy
There is no single and definite remedy for shrinkage and warpage of plastic injection molds. You can adjust mold conditions, redesign mold parts, switch to another resin or some combination to reduce the in-mold cooling process in molded parts. Set the internal stress to these steps. Generally, the best warpage resistance comes from the highest melt temperature, high mold temperature, lowest injection pressure and short injection time. Molding at high melt temperatures tends to reduce the elastic memory of the resin, thereby reducing the tendency to generate stresses that can cause warpage. Running a warm mold can release pressure before the melt solidifies or freezes, and it can also reduce the tendency to warp. In addition, a uniform mold temperature is a necessary condition for the production of warpage-free parts.
Part design and mold design have a lot to do with shrinkage and warpage of plastic injection molds. The shrinkage rate of the plastic injection mold may increase, for example, if the wall section of the part is very different, if the gate is located in a thinner part of the part, if the gate is placed improperly, or if the mold is built up with uneven heat dissipation inner surface .
At Pubo, we are a plastic injection molding manufacturer with many years of business experience, helping customers complete all the work from prototype development to production. If you need an experienced partner, please contact us immediately.
Tags: shrinkage and warpage of plastic injection mold, plastic injection mold raw material suppliers, shrinkage rate of the plastic injection mold
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