The term "jetting" is used to describe the phenomenon where plastic passing through a gate does not adhere and where flow patterns are formed on the surface of the molded product.
| Characteristics |
Plastic at a relatively low temperature is injected from the nozzle during the initial stage of molding, upon coming into contact with the mold wall, this plastic becomes highly viscous and swirling takes place; furthermore,as hotter plastic is continually injected into the die, the original material is pushed deeper into the die and leaves flow marks. |
| Cause |
- When the plastic temperature is low, the viscosity of the molten material is high, and this become higher in the case of plastic which has been injected into the die; consequently, the resistance to flow is large and jetting occurs.
- When the die temperature is low, the material injected into it will be rapidly cooled, and the corresponding increase in viscosity leads to the occurrence of this problem.
- When gates are small, the speed of plastic injected into the cavity will be relatively fast, and this leads to the occurrence of jetting in many cases.
(Gate sectional area x flow speed = Fixed injection amount )
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| Countermeasures |
Molding conditions
- - Increase the temperature of the plastic to lower its viscosity.
- - In the case of amorphous plastics, the ideal temperature of the die is between 20deg.C and 30deg.C lower than the plastic's thermal deformation temperature.
- - It is also beneficial to reduce the injection speed.
- Die
- - It is advantageous to increase the sectional area of the gates so that the speed of the material passing through the gates becomes slower.
[Reference] Product Design Guide
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