How to eliminate 3 surface defects of molded products?

How to eliminate 3 surface defects of molded products?

Visible imperfections on surface of injection molded parts include dark spots, differences in gloss or foggy areas, and surface wrinkles or a phenomenon known as orange peel. Usually these defects occur near gate or around sharp corners away from gate. Based on two aspects of shape and molding process, it is possible to find out causes of these defects.

1. Dark spots on finished product

How to eliminate 3 surface defects of molded products?

A dark spot appears near gate, similar to dim halo of sun. This is especially evident in production of products from materials with high viscosity and low flow, such as PC, PMMA or ABS. This visible defect can occur on surface of part when cooled resin of surface layer is carried away by resin flowing in center.

It is generally accepted that this defect often occurs during filling and packaging stages. In fact, dark spots appear near gate, usually at beginning of injection cycle. Tests have shown that occurrence of surface slip is actually due to injection speed, more precisely flow velocity of melt front.

Even if injection rate is constant, when melt enters mold, its flow rate will change. When entering gate zone of mold, melt flow rate is very high, but after entering cavity, that is, filling stage, melt flow rate begins to decrease. This speed change at front of melt flow can cause surface defects in product.

Reducing injection speed is one way to solve this problem. To reduce speed of front end of melt flow at gate, injection can be divided into several stages, and injection speed can be gradually increased, purpose of which is to obtain a uniform melt flow throughout filling stage.

The low temperature of melt is another cause of dark spots on part. Increasing barrel temperature and increasing auger backpressure reduces likelihood of this phenomenon. In addition, if mold temperature is too low, surface defects will also occur, so raising mold temperature is another possible way to eliminate product surface defects.

Flaws in design of mold can also lead to dark spots near sprue. The sharp corner at gate can be avoided by changing radius. When designing, pay attention to position and diameter of gate to ensure that gate design is suitable.

Dark spots appear not only at location of gate, but also often appear after formation of sharp corners of product. For example, surface of a sharp corner of a product is usually very smooth, and behind it is very matte and rough. This is also due to high flow rate and injection speed, due to which cooling surface layer is replaced by internal fluid and slips.

Again, it is recommended to use incremental injections and gradually increase your input speed. The best method is to allow melt velocity to increase only after passing through sharp edge.

In area far from gate, sudden changes in angle of product can also lead to this defect. Therefore, create artifacts with smoother transitions of rounded corners in these areas.

2. Improve gloss difference

For injection molded products, difference in gloss onmost noticeable on textured product surfaces. Even if surface of mold is very uniform, uneven sheen may appear on part. That is, surface effect of mold in some parts of product is not reproduced very well.

As distance of melt from gate gradually increases, melt injection pressure gradually decreases. If part is not filled at far end of sprue, then pressure is lowest there, so surface texture of mold cannot be correctly reproduced on surface of part. Therefore, region of greatest pressure in cavity (half path of fluid from sprue) is region where smallest gloss differences occur.

To change this situation, you can increase temperature of melt and mold, or increase pressure, and increasing holding time can also reduce difference in gloss.

Good detail design can also reduce likelihood of gloss differences. For example, abrupt changes in part wall thickness can cause uneven melt flow, making it difficult to reproduce surface texture of mold on surface of part. Thus, creating a uniform wall thickness can reduce occurrence of this situation, while excessive wall thickness or excessive ribs increase likelihood of a gloss difference. In addition, cause of this defect is insufficient degassing of melt.

Strength brother's note: I once came across a product used in a Shanghai Volkswagen Passat threshold plug. Fog phenomena, and then by improving fluidity of material and replacing emollient oil, finally managed to avoid difference in gloss.

3. Origin of orange peel

Orange peel defects or surface wrinkling usually occur at gate end when forming thick-walled parts from high-viscosity materials. During injection process, if melt flow rate is too low, surface of product will harden quickly. As resistance to flow increases, frontal melt flow becomes non-uniform so that first solidified outer material cannot fully contact cavity wall, resulting in wrinkling. These wrinkles become irreversible defects after curing and holding pressure. To eliminate this defect, it is necessary to increase temperature of melt and increase injection rate.

Source: Internet, thanks to author!

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