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Solutions to the problems of air streaks, exposed glass fibers and charring in molds

• 2025-08-21

Solutions to the problems of air streaks, exposed glass fibers and charring in molds

The air streaks of the mold

Phenomenon: During the production process of the front shell slider of communication products, a thin layer of air streaks will appear near the water outlet position.

Analysis: The raw material melt is prone to decomposition at high temperatures during shearing, generating gas. The gas is drawn in through the water inlet due to the pressure drop.

Characteristics of injection molding machines

Grade: HT-80T, clamping force: 80t, plasticizing capacity: 90g.

(2) Mold characteristics

Mold ejection number :1×1, injection method: lap gate, ejection method: Ejection by ejector pin, mold temperature: 120°C (constant temperature machine)

(3) Product characteristics

Material: POM M90, Color: black, Product weight (single piece) : 2.5g, nozzle weight: 3.2g.

(4) Analysis of causes of defects

The main channel of the mold is quite large, and the injection method is lap injection. When the molten material flows near the injection port, due to the excessive speed, high shear occurs, causing the molten material to heat up rapidly instantly. This leads to the decomposition of the raw material, generating gas and pressure drop, and the gas is drawn in through the water inlet, forming gas streaks.

(5) Countermeasures

1.Adopt a high mold temperature.

2. Use multi-stage injection and position switching.

3. In the first section, fill the runner opening just to the glue inlet at a moderate speed and find the corresponding switching position. Then, in the second section, fill the area near the glue inlet at a slow speed and a very small position. In the third section, fill at a moderate speed to prevent cold glue and facilitate gas discharge. Use a high holding pressure.

Fan mold Manufacturer in China (jfmoulds.com)

The glass fiber of the mold is exposed

Phenomenon: The front shell of the casing product shows obvious exposure of glass fiber on the product surface.

Analysis: The raw materials include glass fiber, which is prone to form streaks on the product surface. The material flow orientation freezes rapidly.

Characteristics of injection molding machines

Grade: HT-DEMAG, clamping force: 100t, plasticizing capacity: 150g

(2) Mold characteristics

Mold ejection number: 1×1, injection method: point gate, ejection method: Ejection by ejector pin, mold temperature: 125°C (constant temperature machine)

(3) Product characteristics

Material: PC+GF20%, Color: Gray, product weight (single piece) : 7.9g, nozzle weight: 5.6g

(4) Analysis of causes of defects

The product uses glass fiber reinforced materials, which can easily make the material appear rough and dull on the product surface. Glass fibers form metallic reflective stripes on the material surface. During injection, the glass fibers form an orientation in the flow direction. When it comes into contact with the mold wall, the melt freezes rapidly, and the glass fibers have not been fully surrounded by the melt. In addition, due to the significant difference in shrinkage, the surface becomes rough, resulting in obvious exposure of the glass fiber.

(5) Countermeasures

Use multi-stage injection; Increase the temperature of the barrel; Increase the mold temperature; Increase the injection speed; Increase the holding pressure and extend the holding time.

Auto Mould_Taizhou Jiefeng Mould Co.,Ltd. (jfmoulds.com)

Black spots on the surface of the mold

Phenomenon: The product surface is transparent, but there are black spots on the product surface during the molding process.

Analysis: Black spots appear on the product surface due to wear, thermal decomposition or dirt.

Characteristics of injection molding machines

Grade: FNC125T, clamping force: 125t, plasticizing capacity: 96g

(2) Mold characteristics

Mold ejection number: 1×2, glue insertion method: lap joint, ejection method: Ejection with ejector pins, mold temperature: 115°C (constant temperature machine)

(3) Product characteristics

Material: PC IR2200N, color: transparent, Product weight (single piece) : 8.6g, nozzle weight: 7.4g

(4) Analysis of causes of defects

The transparent raw materials used in the design of mold products have obvious black spots on the surface due to the shearing and thermal decomposition of the products, wear and contamination.

(5) Countermeasures

1.Optimize plasticizing capacity, reduce screw plasticizing and lower back pressure; Increase plasticizing delay and reduce residual amount; Keep the drying process clean and avoid contamination.

2. Clean the flow channel from time to time to avoid residual glue and dirt in it.

3. Keep the environment clean and add static air dust removal and cooling after injection molding to avoid dust contamination.