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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.
In more specific terms, 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. |
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Low-speed injection molds
High-speed injection molds
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| Causes : |
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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. |
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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. |
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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: |
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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.
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Dies
- It is advantageous to increase the sectional area of the gates
so that the speed of the material passing through the gates becomes
slower. |
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