Warpage

1. Moisture in the granules
2. Feed system too small
3. Wrong gate position
4. Hold time too short
5. Wrong melt temperature
6. Wrong tool temperature
7. Poor surface finish
8. Problems with hot runners
9. Warpage
10. Mould deposit

Partially crystalline substances such as POM (acetal), PA (nylon), PBT and PET (polyesters) tend to warp far
more than amorphous ones. This point should be taken into consideration already when designing moulds and
mouldings. If this is not done, it is almost impossible to rectify at a later stage. This article discusses the
causes of warpage and steps that can be taken to prevent and reduce it.

What are the main causes of warpage?

Shrinkage is relatively high in partially crystalline materials and is influenced by a number of factors. In the
case of unreinforced materials, warpage is greatly influenced by wall thickness and mould surface temperature.
It follows that major differences in wall thickness and unsuitable mould temperatures will cause the moulding
to warp. Totally different shrinkage characteristics will be evident in the case of glass fibre reinforced
materials, due to orientation of the glass fibres. The effect of wall thickness differences on shrinkage is
relatively slight. Here, the main cause of warping is the difference between fibre orientation longitudinally and
at right angles to the direction of flow. Warpage is essentially due to wall thickness distribution, gate location,
flow restrictions and by-passes, as well as the inherent rigidity of the moulded part.
These different causes of warping, depending on whether the material is fibre-reinforced or not, frequently
result in contrary warping phenomena in the same part.

How can warpage be prevented?

Unreinforced materials require uniform wall thicknesses. Melt accumulations should be avoided as far as
possible. Multi-point gating can be used to achieve a high pressure gradient and thus reduce shrinkage
differences to a minimum. The mould heating system should be designed so that heat is dissipated as evenly as
possible
With glass fibre reinforced materials, the symmetry of the moulded part is as important as uniform wall
thickness. Asymmetrical parts hinder melt flow and thus orientation, and eventually cause warpage. In the case
of asymmetrical parts it is therefore necessary to achieve a balance by incorporating blind cores at the mould
planning and design stage. The position of the gate is also important – every by-pass and every weld line is a
potential cause of warping.

What possibilities are open to the moulder?

Assuming that the moulded part, the gate and the mould have all been correctly designed, the moulder can
control warpage up to a point via the holding pressure and mould temperature. The use of several heating
circuits to optimise heat dissipation is normal practice.
In the case of reinforced materials, changing the injection rate or lowering the mould temperature is a slight
help. If the possibility of subsequent warpage has not been foreseen at the mould and moulded part design
stage, this cannot be subsequently rectified by modifying moulding conditions.

Cooling channels caculations

Thermal conductivity for Different materials

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Know more recyle symbols

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INJECTION MOULDING TROUBLE SHOOTING GUIDE FOR PP

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Tips for reducing brittle parts of PP

Degradation of PP Material
l Reduce Barrel Temperatures
l Reduce Material Residence Time in Barrel
l Check Barrel-To-Shot Ratio
High Level of Molded-In Stresses
l Increase Barrel Temperatures
l Adjust Injection Speed (Moderate to Slower Speed)
Material Contamination
l Check Color Concentrates Used, as well as Base Resin
l Check Material Handling System, such as Material Feed Lines and Hopper for
Contaminant Materials
Poor Part Design
l Remove Sharp Corners and Use Radiused Corners
l Move Weld Lines Away from Areas Where Strength is Critical

Tips to Design runner system.

This system receives molten plastic
from the barrel and guides it into the mold cavity. In consists
of a sprue, runners, and a gate.
a. Sprue bushing/machine contact is usually curved. Radius
of nozzle tip + 1mm is less than or equal to the radius of the
indentation of sprue bushing. And, the diameter of orifice of
nozzle + 1mm is less than or equal to the diameter of the
orifice of sprue bushing.
b. A radius of 1 - 2mm is recommended at the root of the
sprue.
c. 5 - 10mm diameter runners are machined into mold plates.
A polished circular cross section across the parting line is
best due to small surface area, low friction loss, etc.
Machining into both mold halves is difficult and expensive.
A parabolic cross section is simpler machining, but leads to
higher heat loss and more scrap material. A trapezoidal is
another alternative, but causes even more heat loss and
scrap. Alternatively, hot runners can be used which involve
separate, heated manifolds.
d. Runners should be avoided as they complicate issues. Use
a runner only if necessary to fill from thick to thin sections.
If runner must be used, fan gates work the best.
e. Fill cavity with minimum knit lines, multiple gates means
multiple knit lines.
f. Minimize restrictions to flow.
g. Design runners for ease of demolding.
h. Place gate at the thickest section of the part.
i. Place gate at an unseen location on the part.
j. Gates are faced level to parting line to simplify machining.
Start with small gates and slowly increase size. If the gate is
too large the material going through the gate may freeze
prematurely.
k. A radius at the gate/part interface may result in a laminar
flow which will prevent jetting. However, it can also make
gate removal more difficult.
l. Best tensile and impact strength are achieved in direction
of flow.

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