The design of precision injection molds is a complex and crucial process, and its design quality directly impacts the precision, quality, and production efficiency of injection-molded products. The following will elaborate in detail on the key design points of precision injection molds from aspects such as mold structure design, precision control, material selection, and hot runner systems.
(A) Parting Surface Design
The selection of the parting surface is of utmost importance for precision injection molding. A reasonable parting surface should ensure that the plastic product can be smoothly ejected while minimizing its impact on the product's appearance and precision. For example, for plastic products with high appearance requirements, the parting surface should be avoided on visible surfaces. In addition, the design of the parting surface also needs to consider the mold's machining process and injection molding process to ensure the sealing of the parting surface and prevent the overflow of plastic melt.
(B) Ejection Mechanism Design
The ejection mechanism of a precision injection mold must be designed precisely and reliably. The calculation of the ejection force should be accurate to avoid damaging the product during the ejection process. Common ejection mechanisms include pin ejection and plate ejection. For some products with complex shapes or high precision requirements, a combined ejection mechanism may be required. For instance, in some plastic products with undercuts, a combined ejection method using a slider and ejection pins can be adopted.
(C) Core-Pulling Mechanism Design
When a plastic product has side holes, side recesses, or other structures, a core-pulling mechanism needs to be designed. The design of the core-pulling mechanism should ensure smooth and accurate core-pulling actions. For example, a hydraulic core-pulling mechanism is suitable for situations with large core-pulling forces, while an inclined guide post core-pulling mechanism has a simple structure and is suitable for occasions with short core-pulling distances. When designing the core-pulling mechanism, it is also necessary to consider the sequence and timing of core-pulling to avoid damaging the product.
(A) Machining Precision of Mold Parts
The machining precision of mold parts directly affects the overall precision of the mold. During the design process, strict regulations should be made on the dimensional tolerances and geometric tolerances of each part. For example, the dimensional tolerances of the cavity and core should be controlled within an extremely small range to ensure the dimensional precision of the plastic product. At the same time, high-precision machining equipment and advanced machining processes, such as CNC machining and electrical discharge machining, should be selected to ensure the machining precision of the parts.
(B) Assembly Precision of the Mold
The assembly precision of the mold is also an important factor affecting precision injection molding. During the assembly process, it is necessary to ensure that each part is installed in the correct position and that the fitting clearances are reasonable. For example, the fitting clearance between the guide post and the guide bush should be controlled within a certain range to ensure the opening and closing precision of the mold. In addition, the mold should be debugged and inspected to promptly identify and solve problems that occur during the assembly process.
(C) Accurate Calculation of Shrinkage Rate
Plastics will shrink during the injection molding process, so accurately calculating the shrinkage rate is the key to ensuring the dimensional precision of plastic products. Different plastic materials have different shrinkage rates, and the shrinkage rate is also affected by injection molding process parameters and mold temperature. When designing the mold, the shrinkage rate should be accurately calculated through experiments and empirical formulas based on the specific plastic material and injection molding process, and corresponding compensation should be made in the mold design.
(A) Hardness and Wear Resistance of the Material
Precision injection molds need to withstand high pressure and friction, so the mold material should have sufficient hardness and wear resistance. For example, for some precision injection molds for mass production, alloy steels can be selected. These materials have high hardness and wear resistance after heat treatment, which can ensure the service life of the mold.
(B) Thermal Stability of the Material
During the injection molding process, the mold is subjected to high temperatures, so the thermal stability of the material is very important. Materials with good thermal stability can maintain dimensional stability and are not prone to deformation at high temperatures. For example, some hot work die steels have good thermal stability and are suitable for the manufacture of precision injection molds.
(C) Corrosion Resistance of the Material
If the plastic material used for injection molding contains corrosive substances, the mold material should have good corrosion resistance. For example, for some plastics containing flame retardants or plasticizers, stainless steel and other corrosion-resistant materials can be selected to manufacture the mold to prevent the mold from being corroded and affecting the quality of the plastic product.
(A) Layout of the Hot Runner
The layout of the hot runner should ensure that the plastic melt can flow evenly into each cavity. A reasonable hot runner layout can reduce the pressure loss and temperature difference of the plastic melt, improving the quality and dimensional precision of the plastic product. For example, using a balanced hot runner system can make the filling conditions of the plastic melt in each cavity basically the same.
(B) Temperature Control
Temperature control of the hot runner system is crucial for precision injection molding. Accurate temperature control can ensure the fluidity and molding quality of the plastic melt. For example, using high-precision temperature controllers and heating elements can control the temperature of the hot runner within a small range, reducing the impact of temperature fluctuations on the plastic product.
(C) Maintenance of the Hot Runner
The maintenance of the hot runner system cannot be ignored. Regularly cleaning and maintaining the hot runner, checking the working conditions of the heating elements and temperature controllers, and promptly replacing damaged parts can ensure the normal operation of the hot runner system and extend the service life of the mold.
The design of precision injection molds requires a comprehensive consideration of multiple factors such as mold structure, precision control, material selection, and hot runner systems. Only by fully paying attention to these key points during the design process can high-quality precision injection molds be designed, and precision plastic products that meet the requirements be produced.
