Design Principles For Plastic Molds

The plastic mold imparts the desired shape, quality, cooling and ejection of the plastic to the plasticized plastic. The mold determines the performance, shape, size and precision of the final part. Therefore, the design of advanced and reasonable injection mold structure is the key to obtaining quality requirements, stable product quality and achieving the best economic benefits. To do a good job in plastic mold design, we must grasp the following principles:

1. The selection of the parting surface is reasonable.

The parting surface allows the product and the gating system to be removed from the mold. It is the structure that must be set in the mold. It determines the mold structure and also determines the processing technology of the mold. And affect the flow of the melt and demolding of the plastic parts. The overall selection principle of the parting surface is to ensure the quality of the plastic part, simplify the mold structure, and facilitate the demoulding. The choice of the location of the parting surface follows the “three no-principles”: it is not in the obvious position and affects the shape, and it is not allowed to form a depression or a position that is not easy to process.

2. The draft angle is properly selected.

Draft angle is a necessary condition for the product to be smoothly removed from the mold. The size of the draft is different depending on the shape of the molded article, the type of plastic, the structure of the mold, the surface precision, and the processing direction. There is no precise calculation formula for the draft angle. In most cases, depending on the empirical value, a large draft angle is taken without hindering production.

3. The wall thickness is even.

The wall thickness of plastic parts is an important structural factor, which is determined by the use requirements and process requirements, and has a great influence on the process. Therefore, it is very important to choose the wall thickness of the plastic parts reasonably. As far as the process is concerned, the wall thickness is too small, the flow resistance of the plastic in the cavity is large, and the molding is difficult.

Especially for complex shapes and large-sized plastic parts, it is easy to have a defect of filling or to increase the injection pressure; if the wall thickness is too large, it not only increases the cost, but also prolongs the cooling time, lengthens the molding cycle, and reduces the production efficiency. It is easy to produce defects such as shrinkage cavities and bubbles. The wall thickness should be based on the principle of uniformity everywhere, but it must be changed due to the structure of the plastic part or the thickness of the formed upper wall. And for economic reasons, it is necessary to make appropriate adjustments to the wall thickness.

Therefore, the following points must be considered when determining the wall thickness:

(1) Construction strength.

(2) Demoulding strength.

(3) It can evenly distribute the impact.

(4) The fitting metal part is partially prevented from cracking (the thermal expansion coefficient of the molding material and the metal material is different, and cracks are likely to occur at the time of shrinkage).

(5) The structure hinders the flow and prevents insufficient filling. The wall thickness is selected according to the type of plastic and the size of the molded part.

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