Shrinkage
Shrinkage is an
unescapable truth in plastic injection molding that engineers are faced with. With shrinkage comes warpage and therefore
once again twisted parts. Shrinkage is
determined by the part shape, thinkness and the fibre direction, which in
itself is determined by the flow direction of the injection molding plastic.
Plastic structure types
There are
two different plastic molecule types that affect how uniform the shrinkage of
the part will be. These are Amorphous and
Semi-crystalline structures. The table
below shows a list of which plastics have which structures. With Amorphous structures we can expect to
have uniform shrinkage of the part. This
does not mean that it ill just be a scaled down part of what you have designed,
as you must consider the shape of the part too.
If it is longer one way than the other it will shrink more where it is
longer and less where it is not. But the
shrinkage will still be uniform. Plastics
with a semi-crystalline structure have non uniform shrinkage, this makes these especially
difficult to estimate the shrinkage with.
This is why you need a computer aided analysis package, such as Moldflow
to help predict the distortion.
Amorphous
|
Semi-crystalline
|
Polyamideimide (PAI)
|
Polyetheretherketone (PEEK)
|
Polyethersulphone (PES)
|
Polytetrafluoroethylene (PTFE)
|
Polyetherimide (PEI)
|
Polyamide 6,6 (PA 6,6)
|
Polyarylate
|
Polyamide 11 (PA 11)
|
Polysulphone (PSU)
|
Polyphenylene sulphide (PPS)
|
Polyamide (amorphous)
|
Polyethylene terephthalate (PET)
|
Polymethylmethacrylate (PMMA)
|
Polyoxymethylene (POM)
|
Polyvinylchloride (PVC)
|
Polypropylene (PP)
|
Acrylonitrile butadiene styrene (ABS)
|
High Density Polyethylene (HDPE)
|
Polystyrene (PS)
|
Low Density Polyethylene (LDPE)
|
Acrylonitrile Styrene Acrylate (ASA)
|
Polybutylene terephthalate
(PBT)
|
Polycarbonate (PC)
|
Glass filled plastics (GF)
|
http://en.wikipedia.org/w/index.php?title=File:Polymerketten_-_amorph_und_kristallinEN.svg&page=1
In Practice
Moldflow
software is used to predict the distortion (deflection), volume metric shrinkage,
fibre orientation and cooling. Moldflow can
then work out from the volume metric shrinkage if the shrinkage is uniform or
not. Moldflow does most of the hard work
and thus allows for the shrinkage to be compensated for by the “packing stage”. The packing stage of injection molding is
where pressure is applied to the plastic to compress and force more material
into the mold. 5 to 25 percent more
material is used to add more material in the packing stage. The injection molding gate is frozen during the
packing stage to prevent material escaping the mold. Also a bigger cavity in the mold (tool) will
also compensate for shrinkage of the part.
As
discussed in part 1, thicker wall sections take longer to cool and
therefore can sometimes be more difficult to fill as the thinner section will
have already cooled. To combat this a
rib can be used to help get material to these problem areas by promoting the
material flow in a direction, this is called a Flow Leader. Flow leaders are also used to help with weld
lines, which will covered in a another part of the series. The opposite can be done to restrict the flow
direction by having less material in an area.
This is called a Flow Restrictor.
As
mentioned in the intro, fibre direction is determined by the flow
direction. This is controlled by the
placement of the gate, the point at which the material enters the mold at. This is very important when you have many
features in your part, such as holes or loops that the plastic must go around
as these affect the flow direction, making the shrinkage and warpage non
uniform.
http://www.feaanalysisservices.co.uk/images/moldflow-img.jpg
Real World
As with
everything in the real world you have to compromise and nothing is really
perfect. Manufacturing is governed by
cost and capacity, which heavily effects cycle time of injection molding machines. To cut cycle times down you can use Conformal cooling and help with distortion. This
however can effect aesthetics of the part, and to what extent is hard to predict
even with the help of Moldflow. Conformal cooling also adds extra cost, although because of the costs saved my quicker
cycle times, some of those costs can be got back. For more on Conformal Cooling click here.
Summary
- Design parts with even wall thicknesses.
- Design where possible uniform parts.
- Use Flow leaders and restrictors where necessary.
- Moldflow will tell you any issues with your part, however it is a good idea to know what to look for and think about when designing parts for manufacture.
