General design principles for vulcanized polyolefin elastomer TPV injection molded parts
Vulcanized Polyolefin Elastomer TPV is a blend of dynamically vulcanized PP and EPDM. It has uniform properties in the temperature range of -60 ° C to 130 ° C, is not easy to crack and stick, and has excellent heat aging resistance and grease resistance. It has excellent finishing effect with PP and is suitable for all kinds of handles.
In some cases, a component is required to have multiple characteristics, so multiple materials may be required. These parts can be produced by two-color injection molding, two-shot molding or co-injection molding. In addition, it can also be produced by insert molding or overmolding, that is, a two-step injection molding process. The reason why the injection molding process is widely used is the ability to produce multifunctional, multi-material complex parts.
1, wall thickness
Design the part as a product with a uniform wall thickness as a whole, so that the entire part has a uniform shrinkage, which reduces wing and dent.
In some cases it may be necessary to change the wall thickness of the product. In this case, it is recommended to adopt a design method with gradually increasing wall thickness to reduce the number of parts. Wings and dents. The maximum wall thickness varies by approximately 25%.
2, stripping bevel
Setting the draft angle is good for product release. The size of the draft angle depends on the demolding depth of the part and the final conditions of use should be considered. The draft angle is generally set at the mold cavity side and the center pin to facilitate demolding of the product during the mold opening phase of the injection molding cycle.
The groove is a recessed part of the product that makes it difficult or impossible to simply remove the product from the rigid two mold halves. Due to the low modulus of the TPV, no special cores and slides are required and the grooved product may be removed. Of course this depends on the hardness of the TPV and the shape or size of the groove.
The role of the ribs is to increase the stiffness and strength of the part without increasing the wall thickness. The thick section should be designed as a hollow, using ribs to achieve the same stiffness, saving material while shortening the processing cycle. The thickness of the ribs, flanges and reinforcements shall be 50% of the thickness of the section they support. Higher ribs may require a draft of about 0.5° to facilitate demolding. Figure 3 shows a typical rib design.
5, rounding and chamfering
Concentration of stress at sharp corners can cause damage to the product during use. Proper rounding and chamfering help to reduce stress concentrations and provide a smooth flow path for the polymer melt, making the part easy to release. For TPV, the recommended minimum radius for rounding and chamfering is 0.5mm. Moreover, the wall thickness between the inner and outer circles should be uniform.
6, live articulation
With live articulation, a single molded part can be flexibly opened or closed. It is a dynamic structure that requires good flexural resistance. With the right design, the live hinge can be easily bent and flexed millions of times.
7, surface texture and appearance
The surface texture of TPV is generally rough, but as the hardness increases, the gloss will increase. In order to give the TPV surface a certain degree of gloss, some treatment may be required, such as applying a slip coating or painting, but doing so will change the surface properties of the TPV. In short, it is best to design the surface to a rough appearance.
Add some surface texture to the TPV injection molded parts, such as chemical etching, EDM, sand blasting or general machining methods. Process it. Textures that are too deep may interfere with the flow of polymer within the mold cavity. If the depth is too shallow, the texture may not be revealed due to the high viscosity of the TPV melt. So the depth is preferably between 0.075 and 0.125 mm. Note that a softer grade requires a deeper texture to achieve the best results; a harder TPV can have a smaller draft angle, depending on the depth of the surface texture used. Moreover, the addition of a pattern structure on the surface of the vertical core or cavity can also prevent the mold.
In most cases, the cavity surface of the injection molded TPV is SPE-SPI#B3, and the corresponding surface roughness is 0.0001 to 0.0002 mm.
8. Bonding in the overmolding process
Elastomer parts are often molded into other materials using insert molding or overmolding. The best case is the formation of a chemical bond between the substrate and the overmolded material to form a cohesive bond between the two.
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