Honey I shrank the Molding, Injection Molding is tricky Part 2

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.