Design points of blow moulds and main accessories
The mold usually only has a cavity part without a punch. Generally, the surface of the mold does not need to be hardened. The inflation pressure of the cavity is much lower than that of injection molding, generally 0.2~1.0MPG, and the cost is lower.
1. Mold material
It is usually made of aluminum alloy, and for corrosive compounds such as PVC and POM, beryllium copper or copper-based alloys are also used; for molds with high life requirements, such as blow-molded engineering plastics ABS, PC, POM, PS, PMMA etc. need to use stainless steel to make molds.
2. Key points of mold design
1) Parting surface Generally, it should be placed on the symmetry plane to reduce the blow-up ratio, such as oval-shaped products, the parting surface is on the long axis, and the distance-shaped products pass through the center line.
2) Cavity surface
The PE material should be slightly rough, and the sand surface is conducive to exhaust; while other plastics (such as: ABS, PS, POM, PMMA, NYLON, etc.) blow molding, the mold cavity generally cannot be sandblasted, and it can be used in the mold cavity. Make an exhaust groove on the parting surface or make an exhaust hole on the cavity. Generally, the vent hole on the cavity has a diameter of φ0.1~φ0.3 and a length of 0.5~1.5mm.
3) Cavity size
The design of the cavity size should consider the shrinkage rate of the plastic. For details, please refer to the shrinkage rate of common plastics. You can click on the blue link to view: →Compendium of Shrinkage Rates of Common Plastic Materials
4) Cutting off the cutting edge and tailing trough
Generally, for blow-molded engineering plastics and harder plastics, the cutting edge should be made of materials with good wear resistance, such as beryllium copper, stainless steel, etc.; for soft plastic products such as LDPE and EVA, generally Aluminum alloy is fine;
The cutting edge should choose a reasonable size. If it is too small, it will reduce the strength of the joint. If it is too large, it will not be able to cut and the jaws at the parting surface will be large. A tailing groove is opened under the cutting edge, and the tailing groove is designed as a clamp. Angle, a small amount of melt can be squeezed into the joint when cutting, thereby improving the strength of the joint.
5) Injection blow mould
The design is different from the extrusion blow molding. The main difference is that the injection blow mold does not need a cutting edge and a tailing trough. The parison design of the injection blown part is very important, which is directly related to the quality of the finished product.
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