Requirements and primary influencing factors of high-precision medical injection molding processing and various issues
Date: October 15, 2024
The basis for high-precision injection molding processing is the accuracy of injection molded products, namely the dimensional tolerance, geometric tolerance, and surface roughness of the products. There are many related conditions necessary for high-precision injection molding, and the most essential ones are plastic raw materials, injection molds, injection processes, and injection equipment. When planning injection molding of plastic products, engineering plastic materials should be selected first, and it is necessary to choose materials with high mechanical properties, stable scale, good creep resistance, and environmental stress cracking resistance for engineering plastics that can be finely injected. Secondly, suitable injection molding machines should be selected based on the selected plastic materials, finished product scale accuracy, part weight, quality requirements, and expected mold structure. In the processing, the factors that affect fine injection molded products mainly come from the accuracy of the mold, injection molding shortening, and the magnitude of changes in the environmental temperature and humidity of the product.
In high-precision injection molding, plastic molds are one of the key factors used to obtain fine plastic products that meet quality requirements. The molds used in high-precision injection molding should truly meet the requirements of product size, accuracy, and shape. But even if the precision and scale of the mold are the same, the practical scale of the molded plastic products will also vary due to differences in shortening. Therefore, effectively controlling the shrinkage rate of plastic products is crucial in fine injection molding technology.
Requirements and primary influencing factors of high-precision medical injection molding processing and various issues
The rationality of plastic mold planning directly affects the shrinkage rate of plastic products, because the mold cavity size is calculated by adding the estimated shrinkage rate to the plastic product size, and the shrinkage rate is a value recommended by the plastic manufacturer or engineering plastic manual within a range. It is not only related to the gating method, gating orientation, and dispersion of the mold, but also to the crystal orientation (anisotropy) of the engineering plastic, the shape and scale of the plastic product, and the distance and orientation to the gating. The primary factors that affect the shrinkage rate of plastics include thermal shrinkage, phase transition shortening, orientation shortening, compression shortening, and elastic recovery, which are related to the molding or operating conditions of fine injection molded products. Therefore, when planning the mold, it is necessary to consider the relationship between these influencing factors and injection conditions, as well as their apparent factors, such as injection pressure and mold cavity pressure and filling speed, injection melt temperature and mold temperature, mold structure and gate method and dispersion, as well as the influence of gate cross-sectional area, product wall thickness, content of reinforcing fillers in plastic materials, crystallinity and orientation of materials. The influence of the above factors also varies due to different plastic raw materials, other molding conditions such as temperature, humidity, continued crystallization, internal stress after molding, and changes in injection molding machines.
Because the injection molding process is the process of changing plastic from a solid state (powder or pellet) to a liquid state (melt) and then to a solid state (product). From pellets to melt, and then from melt to product, the center undergoes the effects of temperature field, stress field, flow field, and density field. Under the combined action of these fields, different plastics (thermosetting or thermoplastic, crystalline or amorphous, reinforced or non reinforced, etc.) have different polymer structure shapes and rheological properties. Any factors that affect the above-mentioned “field” will inevitably affect the physical and mechanical properties, scale, shape, accuracy, and appearance quality of plastic products.
In this way, the intrinsic connection between process elements and the properties, structural shape, and plastic products of polymers will be manifested through plastic products. Analyzing the connections between these connotations is of great significance for formulating injection molding processes, planning and manufacturing molds according to drawings, and even selecting injection molding equipment reasonably. There are also differences in injection pressure and injection rate between fine injection molding and ordinary injection molding. Fine injection molding often uses high-pressure or ultra-high pressure injection, high-speed injection to achieve a smaller molding shrinkage rate.
Taking into account the various reasons mentioned above, in addition to considering the planning elements of general plastic molds, Shenzhen Donglai Precision Mold Company also needs to consider the following 7 points when processing high-precision injection molds:
① Adopt appropriate plastic mold dimensional tolerances;
② To avoid errors in molding shrinkage rate;
③ Avoid injection molding deformation;
④ Avoid demolding deformation;
⑤ Minimize mold manufacturing errors;
⑥ Avoid errors in mold accuracy;
⑦ Adhere to mold accuracy.