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In the field of mold design, there is a role that perfectly embodies the title of "invisible killer" - it is the ejection angle. Saying it is "invisible" is because it is almost impossible to distinguish it with the naked eye on the product drawing; calling it a "killer" is because once it goes wrong, the consequences are often devastating, and it always erupts at the most unexpected time.

Why is it "invisible"?

Designing a new product is actually a very difficult process. When a product designer is drawing a 3D diagram, their attention is often focused on the appearance design, function realization, and assembly relationship. The ejection angle is usually the last parameter to be considered - often it is not even considered at all. On product renderings or engineering drawings, a 0.5° inclination angle is completely undetectable with the naked eye, and it looks like a straight "straight wall".

When the design is given to our factory, we need to design the mold based on the product, and often some details of the product are difficult to achieve because if the design parameters are incorrect, various problems will occur during the ejection of the product. The ejection angle belongs to the "basic parameter" type of design elements. Unlike the "visible" features such as the parting surface, gate, and sprue, it is easily overlooked. Many mold designers assume that the product drawing already includes the ejection angle design, but the result is that the "zero-ejection-angle" straight wall parts are received.

The most fatal thing is that the problem will not be discovered during the design review, will not be exposed during mold processing, and may even manage to eject a few qualified products at the T0 trial molding stage. The real disaster often breaks out fully during mass production - at this time, the mold has already been processed, and the modification cost is astonishingly high.

Why is it called a "killer"?

It damages the appearance and causes scratches on the product. Products with straight walls without sufficient slope will have their surfaces rub against the mold cavity walls during demolding, resulting in obvious scratches, abrasions and whitening. For high-gloss surfaces, transparent parts or appearance components, this is equivalent to waste-out.

Causes deformation during ejection. When the ejection resistance is too high， the ejector pin needs to exert greater force to push the product out. As a result， the product gets whitened by the ejector pin or the entire product is "popped" out and deformed. This defect is particularly fatal in thin-walled parts and deep-cavity parts.

Shortens the mold life. Insufficient ejection slope means that every time the mold is opened， there is "hard rubbing" between the product and the mold. Over time， the mold cavity surface will wear， the polishing layer will be damaged， and even "scratching" may occur. A mold designed with a 1000，000-cycle service life may be prematurely scrapped after 300，000 cycles.

Limits production efficiency. To reduce ejection resistance， operators may lower the injection pressure， extend the cooling time， and frequently apply mold release agents. These "compromising measures" will directly prolong the molding cycle and increase the unit cost.

Repairs are extremely difficult. Once the mold has been fabricated， it is very difficult to remedy the insufficient ejection slope - it requires re-electroforming， re-polishing， or even replacing the entire mold core. This cost and time are often beyond the willingness of both the customer and the mold manufacturer to bear.

How much slope is required for different materials?

The requirements for demolding slope vary greatly depending on the type of plastic. For soft materials like PP and PE, the minimum slope is only 0.3° - 0.5°; while for PC and fiber-reinforced materials, it needs to be 1° - 1.5°. If there are burrs on the surface of the mold cavity, the slope should be doubled based on the polished surface - the deeper the texture, the greater the slope. The demolding resistance of deep cavity parts is 5 - 10 times that of shallow cavity parts, so the slope also needs to be increased accordingly. 

Conclusion

The reason why draft angle is called the "invisible killer" is that it is easily overlooked during the design stage, can be successfully removed during the initial trial mold stage, but suddenly emerges during mass production (pulling injuries, white spots on the mold surface, and comprehensive mold wear). And once discovered, the damage has already been done.

The best "antivirus" method is to force the inclusion of draft angle inspection in the product design stage - without relying on memory or experience, but by solidifying it as a mandatory inspection item in the design process. This is the lowest-cost and most effective solution. After all, the best way to deal with the "invisible killer" is to make sure it has nowhere to hide before it starts.