Blush at the Gate of Plastic Injection Parts

Blush is hazy surface imperfection which is found at the gate of the workpiece. It appears as a visual difference in color and gloss surrounding the gate and is usually in the direction of flow. Here are some causes and solutions below.

The high pressures and velocity going through a gate, a phenomenon called melt fracture occurs. The material is squeezed through the small gate opening, and as this takes place, the polymer chain is extended. The material expands rapidly as it hits the cavity wall, the polymer material will cool, and the polymer chains will contact. Due to this cooling and contraction, a haze or discoloration will appear at the gate area, referred to as gate blush. The higher the injection speed, the more pronounced the gate blush. At the same time, lack of a cold slug well can force this colder material into the gate and into the part showing up as gate blush.

Reducing the injection speed so that the polymer chain do not extend as much as with higher speeds. This will lay down a clean skin of material without gate blush. Velocity profiling of the process can be used to start with a slow injection speed, building to a higher injection speed as the cavity is filled. Increasing the melt temperature to provide easier flow to the material. An increase in nozzle temperature can produce a higher melt temperature as it enters through the gate.

Setting a cold slug well at the ends of runner. The purpose of a cold slug well is to receive the first slug of material from the nozzle and keep it from interfering with the flow path of the hotter material behind. Increasing the mold surface temperature so that the cooling rate at the gate will be slowed and will be closer to the cooling rate of the rest of the part. Gate size can be increased to provide less resistance from the material in entering the cavity.

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