Common plastic mold problems -- mold cracks

Bubbles are caused by the fact that air cannot escape from the mold and remains in the finished product

Formed. The gas is wrapped by the front cold material and cannot pass through the mold gap,

Discharge from the inlet hole, exhaust hole and other places. Bubbles are usually located in the filled area. The formation of bubbles is usually caused by

There is no vent in the filling area or the vent is not large enough. Another is usually

The reason is that the product has rapid changes in meat thickness (the place with thick meat is preferred to be filled).

Causes of bubbles

The gas that has not escaped will form a vacuum or bubble in the product. If it is short shot (not full), the gas that has not been discharged will form bubbles in the product, or stain the surface of the finished product due to combustion. In order to remove bubbles, we can improve the filling mode by reducing the injection speed, increasing the exhaust, or setting the exhaust hole at the appropriate position.

In the following figure, the bubble is due to the excessive ratio of flow length to wall thickness. In this case, the exhaust can be increased by changing the thickness ratio or setting an exhaust hole (for example, adding a thimble) in the bubble forming area.

Solutions

1. In product design

Change the product design to reduce the thickness proportion. This can reduce the influence of flow length.

2. In mold design

Pay attention to the setting of exhaust holes.

Add vent in the full area.

Exhaust holes are specially set between parts, such as parting surface, sub hole, thimble hole and slider.

3. Redesign the sprue system

Changing the sprue system can change the filling mode by placing the post filling area at the appropriate exhaust position.

4. The exhaust hole is large enough to ensure that the gas can escape in time during injection molding

However, it should be noted that too large an exhaust hole will produce burrs. The recommended vent size is 0.025 mm (0.001 inch) for crystalline plastics and 0.038 mm (0.0015 inch) for amorphous plastics.

5. Adjust forming conditions

Reduce the injection speed.

High injection speed will lead to the formation of bubbles. Reduce the injection speed to allow enough time for air to escape from the vent.

Two black specks / black streaks

What are black spots and stripes?

Black spots and stripes are black spots or stripes on the surface or part of the finished product. Brown spots or stripes are the same type of defects except for those with more serious burning or discoloration.

Figure 1. Black spots (left) and black stripes (right).

Black spots and stripes are caused by contaminated plastic or over heated materials (cracking, burning).

Cracking of plastics

The material is cracked due to excessive heating, resulting in black stripes in the finished product. In the heating process, because there are scratches or rough surfaces on the surface of the barrel and the screw, which prevent the flow of plastic, the material is scorched or cracked after a long heating time, causing black spots and black stripes.

Figure 2. Improper forming conditions lead to the cracking of the material and the generation of black stripes in the finished product.

Air or material pollution, such as dirty waste, additive materials, materials with different colors or materials with low melting point, usually leads to black spots and black stripes. Dust in the air will also produce black spots on the surface of the finished product.

The same reason can also lead to other defects, such as:

Product embrittlement, scorch marks, discoloration.

Solutions

1. Use materials carefully

1). Make sure that the materials are not contaminated, such as dirty recyclables mixed with raw materials.

2). Cover the funnel and the box containing raw materials. Air and dust will pollute raw materials, resulting in black spots.

2. Change the mold design

1). Clean the ejector pin and slider. Grease or lubricant on the slider and thimble will cause black stripes on the product.

2). Improve the exhaust system. If black spots are found in the filling area, they are likely to be caused by the blocked exhaust system. The unexhausted gas is compressed and burned, resulting in black spots.

3) Clean the runner and ensure that the runner is free of scratches. The residual dirt in the runner will cause these defects.

4) Clean the mold before forming.

3. Choose the right molding machine