Silver grain due to poor injection molding

1、 What is silver grain (appearance)?
The craze refers to the appearance of gas or air on the surface of molded products. Gas is caused by the decomposition of resins or additives, or the mixing of different resins, while air is involved in metering. In addition, there is the influence of water.
2、 Cause of craze formation
1. Resin decomposition products produce gas
Resin belongs to chemical substances, so it will decompose gradually with the increase of temperature. The higher the temperature of the resin, the more it decomposes, and the easier the craze appears.
2. Entrainment of air
If the screw speed is too fast and the back pressure is too low, the amount of air involved in the resin being plasticized will increase. The result is that the strip-like bubbles appear on the surface of the molded product, and the craze is easy to form.
3. Insufficient predrying
If the pre-drying of the material is insufficient, the moisture and the original gas components in the resin will be brought into the molding product intact, which is easy to form craze.
4. The exhaust port is too small
When the gas is not completely discharged, bubbles will remain on the surface of the molded product, which is prone to craze.
5. Mixing of different materials
If different components from the original resin are mixed due to insufficient cleaning and other reasons, and the temperature of the resin is low, sometimes gas will be generated and craze will be induced.
3、 Countermeasures for crazing
1. Check metering status
First, check the metering status. If the screw speed is too fast or the back pressure is insufficient, the craze is easy to appear. Please gradually adjust and observe whether there is any change. If yes, please adjust to the best value.
2. Check resin temperature: resin decomposition
Each resin has its recommended use temperature (indicated in the product bag and product catalog). Check whether the actual temperature is within this range. If it is out of range, it must be adjusted to within the range.
Recommended service temperature of various materials
3. Intensive drying
Drying should be strengthened. For specific drying conditions, see also the following:
◆ Drying temperature of various materials
The general recommended drying temperature of resin should be the temperature of craze
POM（80-90）℃ × （2-4）h
PS（75-80）℃ × （1-1.5）h
ABS（80-100）℃ × （2-4）h
PA6 （80-100）℃ × （2-10）h
PA66 （80-110）℃ × （2-10）h
PP0 （80-120）℃ × （2-4）h
TPV （80-100）℃ × （2-4）h
TPE （60-80）℃ × （2-4）h
Moisture is an important factor, so it must reach a minimum of 100 ℃. In addition, dehumidifier dryer also has effect.
4. Is there any pollutant?
Re-clean. Not only different resins should be cleaned, but also some time after the last molding. This is because the retained resin has been decomposing.
5. Check the exhaust port
Occasionally, craze may occur due to poor exhaust, so check whether the size of the exhaust port is sufficient.