Precision injection molding is affected by several factors and environmental conditions, which include basic factors such as plastic materials, injection molds, injection processes and injection equipment. When designing plastic products, the first step is to select a suitable plastic based on the application scenario of the product. Secondly, select the appropriate injection molding machine for production based on the selected plastic material, finished product dimensional accuracy, part weight, quality requirements and expected mold structure.
Mold is a key factor affecting the quality of precision plastic products. The rationality of mold design directly affects the quality of plastic products.
The mold cavity size is usually derived from the required size of the plastic product plus the shrinkage rate of the material used. Shrinkage is often recommended by the plastic manufacturer or engineering plastics manuals within a range of values. It is related to the form of the gate of the mold, the location and distribution of the gate, the crystalline orientation (anisotropy) of the engineering plastic, and the shape and size of the plastic product to the distance and location of the gate.
The main factors affecting plastic shrinkage include thermal shrinkage, phase change shrinkage, orientation shrinkage, compression shrinkage and elastic recovery, etc., which are related to the molding conditions or operating conditions of precision injection molded products.
Therefore, mold designers must have rich design and injection molding experience, must consider the relationship between these factors and injection conditions, as well as injection pressure, cavity pressure, filling speed, injection melt temperature, mold temperature, mold structure, gate form, distribution, gate cross-sectional area, product wall thickness, the content of reinforced filler in the plastic material, the plastic material’s crystallinity and orientation and other factors.
The influence of the above factors also varies depending on the plastic material, or other molding conditions such as temperature, humidity, continued crystallization, and changes in the injection molding machine after molding.
The injection molding process transforms plastics from a solid state (powder or pellet) to a liquid state (melt) and then from a melt to a solid state (product). From pellet to melt, and from melt to product, it is necessary to go through the action of temperature field, flow field, and density field. Under the joint action of these fields, different plastics (thermosetting or thermoplastic, crystalline or non-crystalline, reinforced or non-reinforced, etc.) have different polymer structural forms and rheological properties.
Therefore, the analysis of these intrinsic links for the reasonable formulation of injection molding process, reasonable design and manufacture of molds according to the drawings, and even a reasonable choice of injection molding equipment are of great significance.
Precision injection molding and ordinary injection molding in the injection pressure and injection rate there are differences, precision injection molding is usually used in high-pressure or ultra-high-pressure injection, high-speed injection in order to obtain a smaller molding shrinkage.
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