Jetting is a phenomenon that occurs in injection molding when molten plastic material flows
through the gate and forms a wormlike formation without laying down a skinned surface. The formation becomes cold, and as molten material is injected around it and bonds poorly with the cold
material, a weakness develops in the part. This cold, solidified formation can also trap airand
create a void in the part, also reducing the parts structural integrity. The surface appearance can
vary as a result of differences in color or gloss. The lists causes and solutions for jetting
Causes:
1. At high injection speeds, the molten material will shoot rapidly into and across the cavity, forming a"worm
2. Slower injection speeds can cause the melt to cool too quickly, leading to jetting
3. Low melt temperatures can cause jetting because at lower melt temperatures, more pressure is needed to force the material to flow. The higher pressure will again force the material to shoot across the cavity
4. Gate diameters that are too small form a restriction for the melt that causes the injection pressure in the runner to increase. This higher pressure will force the melt to shoot across the cavity
5. Gates located in an area of a part where the melt is hitting a hollow cavity or core opening will cause the melt to worm
6. Low mold temperatures contribute to the jetting phenomenon by causing the melt too cool too quickly as it hits the cavity, forming poor bonding between this colder material and the hotter material flowing behind
Solutions
1. Reducing injection speed and using injection speed profiling will eliminate jetting
2. Increasing melt temperatures will reduce the chance of jetting
3. Raising the melt temperature will allow the material to flow easily and at reduced pressures
4. Increasing the gate diameter will reduce the runner Injection pressure
5. Relocating the gate into a barrier, such as a cavity wall, will prevent worm formation and eliminate jetting
6. Increasing mold temperature will keep the melt warmer and provide better bonding to the hotter flowing material
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