The importance of understanding plastic shrinkage
In injection molding precision mold design, it is crucial to consider plastic shrinkage. Plastic shrinkage refers to the ratio of the size reduction of plastic products after they are taken out of the mold and cooled. This characteristic directly affects the final dimensional accuracy and quality of the product. If the shrinkage is ignored when designing the mold, the plastic products produced are likely to fail to meet the design requirements, and problems such as excessive dimensional deviation and uneven surface will occur. For example, when manufacturing high-precision electronic component molds, even a small dimensional error may cause the parts to be unable to be assembled normally, so accurately considering the plastic shrinkage is a key step in ensuring product quality.
Pre-adjustment of mold size
In order to cope with the shrinkage of plastic, the size of the cavity and core needs to be pre-adjusted during the mold design stage. This requires enlarging the relevant dimensions of the mold according to the shrinkage rate of the plastic. The specific calculation method is to divide the design size of the plastic product by (1-shrinkage rate) to obtain the size of the mold cavity or core. For example, if a certain dimension of the plastic product is 100mm and the plastic shrinkage rate is 0.02, then the corresponding cavity size of the mold should be designed as 100/(1-0.02)≈102.04mm. At the same time, for products of different shapes and structures, such as those with ribs or thin-walled parts, it is also necessary to consider the non-uniformity of shrinkage in each part and optimize the mold size locally.
Coordination design of cooling system
The design of cooling system is closely related to the shrinkage of plastic. Reasonable cooling can control the cooling speed and shrinkage process of plastic. Uneven cooling will lead to inconsistent shrinkage of plastic, resulting in quality problems such as warping and deformation. When designing the cooling system, it is necessary to ensure that the layout of cooling water channels is uniform so that the cooling speed of each part of the mold is similar. For example, for large flat plastic products, the cooling water channels should adopt a uniformly distributed grid layout to avoid excessive differences in cooling speed between the edge and the center. In addition, the selection of cooling medium and flow control are also important. By adjusting the cooling parameters, the shrinkage of plastic can be adjusted to a certain extent and the dimensional accuracy of the product can be improved.
Optimization of gate and runner system
The gate and runner system also affect the shrinkage of plastic. The location and number of gates will affect the flow direction and filling order of the plastic melt in the cavity, thereby affecting the shrinkage. Reasonable gate design can make the plastic melt fill the cavity evenly and reduce the shrinkage difference caused by uneven flow. For example, for products with complex shapes, using multiple gates can make the melt fill from different directions at the same time, reducing shrinkage stress. The size and shape of the runner system will also affect the pressure and temperature of the melt, indirectly affecting the shrinkage rate of the plastic. Designing a suitable runner can ensure that the melt has the right pressure and temperature when entering the cavity, which helps control the shrinkage process and improve the quality of the product.
