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When designing injection molds with cavities, the core issue to be addressed is how to make the "raised part" (i.e., the core) in the mold both precisely shape the cavity and facilitate easy demolding after injection.

From the perspective of product design:

Before designing the mold, it is necessary to review the product model. The inner and outer corners of the cavity must be rounded, even if it is only a millimeter of a round corner. Sharp corners can cause the product to crack under force, and plastic flows poorly at sharp corners. Additionally, the wall thickness of the entire product should be consistent. If it varies, an ugly shrinkage mark will appear on the surface after cooling.

Another point that is often overlooked is the ejection angle. Simply put, make the inner surface of the cavity slightly inclined, like a slightly open trumpet mouth. This angle is usually one to three degrees. Without it, the product may get stuck in the mold and not be removed.

The core is the soul

The cavity is formed by the core, which is the raised part on the mold. The design approach for the core varies depending on the complexity of the cavity.

If it is a simple shell or box, a standard core is sufficient. This type of core is solid or has a cooling water channel, and it is relatively simple to design. However, note that the heat dissipation conditions of the core are much worse than those of the cavity, so it is necessary to design the cooling water channels well, otherwise it will take a long time to cool down one product before it can be removed.

If there are inverted parts inside the cavity, such as snap-fit positions or threads, then a retractable core is needed. There is a type of foldable core that, after injection, it will shrink inward, allowing the product to be easily removed. If it is a thread, a rotational ejection structure is needed, and the core rotates while removing the product from the thread.

For deep and complex cavities, such as those in motor housings, a combined expansion core can be considered. It consists of an inner core and a segmented sleeve on the outer layer. During molding, the core expands outward to compress; before opening the mold, the core contracts inward, allowing the product to be easily removed. This solution is more costly, but it can solve many difficult problems.

System design should be coordinated

The mold is a complete system, and apart from the core, other parts also need to be coordinated.

1. The location of the gate should be selected correctly.

2. The ejecting mechanism is also crucial. The pin should be designed based on the depth of the cavity.

3. The design of the cooling system directly determines the production cycle.

Several common pitfalls

The first pit is not considering plastic shrinkage. The size of the mold cannot be directly based on the numbers on the product drawing; it must be enlarged according to the shrinkage rate of the used plastic. For example, the shrinkage rate of ABS plastic is approximately five parts per thousand, so the mold size should be five parts per thousand larger than the product size. The molten plastic will shrink after cooling and tightly hug the core, so this characteristic must be utilized.

The second pit is insufficient core strength. Long and slender cores are prone to bending or breaking under high-pressure injection. Generally, the length of the core should not exceed three to five times the diameter. If it needs to be very long, consider adding a guiding support at the other end.

The third pit is the selection of the gate location. Sometimes, the gate is deliberately placed at an eccentric position, creating a fragile connection between the solidified plastic at the gate and the finished product. This way, during ejection, the waste from the gate can easily break off without damaging the finished product. This technique can save the subsequent trimming of the gate.