IMPAS 4 1 Shrinkage
The depression or empty on the Plastic products surface are called "shrink", it will affect the appearance of the product, reduce the product quality and strength. Shrinkage related to molding technology, mold design and molding material.
Usually, the materials which are easy to get shrinkage belong to crystalline, such as nylon, PP, etc. In the injection process, the heated plastic melt into the liquid crystalline state, molecular arrangement was irregular; when injected into the cooler mold cavity, the plastic Slowly lined up to form molecular crystals, the shrink are more than standard size, the so-called "shrink."
Table II: Normal plastic shrinkage rate
|
Sign
|
Shrinkage %
|
|
GPPS
|
0.4
|
|
HIPS
|
0.4
|
|
SAN
|
0.2
|
|
ABS
|
0.6
|
|
LDPE
|
1.5~5
|
|
HDPE
|
2~5
|
|
PP
|
1~4.7
|
|
PPO
|
0.6
|
|
PA6
|
1.0
|
|
PA66
|
0.8~1.5
|
|
ACETAL
COPOHYMER
|
2
|
|
CAB
|
0.5
|
|
PET
|
2.25
|
|
PBT
|
1.5~2.0
|
|
PC
|
0.6
|
|
PMMA
|
0.5
|
|
PVC Hard
|
0.1~0.5
|
|
PVC Soft
|
1~5
|
|
PU
|
0.1~3
|
|
EVA
|
1.0
|
|
PSF
|
0.7
|
Injection technology:
In injection technology control, what cause of shrinkage: insufficient pressure, injection speed too slow, the gating is too small, runner is too long and so on. So in the use of injection machine, must pay attention to the forming condition and the packing pressure is enough or not, to prevent shrinkage.
Regard mould and product design:
Mould flow design and cooling device effect products quality greatly because of heat transfer of plastic is lower, the farther the distance from mold wall, the solidification and cooling slowly. Should have enough plastic to fill mould cavity, preventing injection machine screw in injection or holding stage, the melting plastic won't refluent and reduce pressure.
On the other hand, the cooling shall not too fast, or the semi-solid plastic obstruction the filling ways caused pressure drop and shrinkage. Different mold flow process have different shrinkage rate, melting temperature be controlled well can prevent overheating; Extend the cycle time, ensure products has sufficient time to cool.
To solve the problems such as the shrink appropriately, improve product quality, reduce waste products and improve the production efficiency. The table below that is possible for shrink the reasons and processing methods.
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Fault
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Processing methods
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Plastic shortage
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Increase the material
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Plastic is not enough
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Increase material, moderate Raise back pressure
|
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Injection pressure is too low
|
Increase the plastic measurement trip
|
|
Back pressure too low
|
Improve shoot pressure, holding pressure
|
|
Injection time too short
|
Increase injection time
|
|
Injection speed too slow
|
Increase rate of fire
|
|
Gate asymmetry
|
Adjusting the gate size or location.
|
|
Blocked shot material gate
|
Dismantling and clean up
|
|
Material too high temperature
|
Low down the temperature
|
|
Mould temperature is not stable
|
Adjust the cooling system
|
|
Cooling time not enough
|
To pour out the cooling time
|
|
Poor exhaust
|
Venting in a shrinking area
|
|
Feed tube too big
|
Make feed tube smaller
|
|
Screw inverse ring damage
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Dismantling and overhaul
|
|
Plastic product uneven thickness
|
Shot pressure increase
|
|
