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In-depth Understanding of Bubbles Problems in Plastic Injection Molding

Molded product surface often appear bubbles in plastic injection mold.

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When the plastic is removed from the mold after injection molding, the surface of the product gradually begins to protrude, and due to thermal expansion, the surface of the molded product protrudes.

In the other case, when the surface of the molded product is softened by high temperatures, the internal air is heated to cause the surface of the molded product to rise, resulting in bubbles.

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What causes bubbles in plastic injection molding?

Injection molding process involves a lot of air, it is easy to produce bubbles. Specifically, when the screw speed is fast, the back is low and the injection volume is large, it is easy to produce bubbles. In addition, during the cavity filling process, some flow patterns can sometimes cause air to be trapped, thus forming bubbles.

The large amount of gas produced in the resin is also prone to cause bubbles. When the temperature of the cylinder is too high and the residence time is too long, the gas produced will increase, so it is easy to form bubbles. In addition, bubbles can also form when there is insufficient drying and the material contains too much water.

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How do you fix bubbles in injection molding?

1- Reduce residual air. In order to reduce residual air in the metering process, it is necessary to "reduce the screw speed".

"Increase the back pressure" and "Do not set the injection volume too high".

In addition, due to air retention during cavity filling, the shape, gate position and injection rate need to be adjusted. Specific treatment is required according to the conditions of the molded product.

2- Sometimes changing the holding pressure will not improve the bubbles. The best option is to reduce the shear force during the filling process so that the material can fill the cavity smoothly, thus eliminating bubbles more effectively. Five conditions need to be changed during the operation: increase the mold temperature, reduce the injection speed, enlarge the door, increase the thickness (apply to too thin part only), and avoid spraying.

3- To suppress gas generation as much as possible, such as: improve drying, remove the low molecular polymer in the resin, and remove water. It can also lower the temperature of the cylinder and operate within the recommended temperature range, not too high.



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