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Definition: Shrinkage 1: drawback 2: reduction in size
Shrinkage as one knows is based on wall thickness as previous stated, and this should not be ignored and is why one of the golden rules of design is to use uniform wall thickness. But a side note is that if you need to thin out parts, than this should be done progressively in the direction of flow with the thicker area closer to the gate and thinnest area furthest from the gate. Note that shrinkage shall have to be calculated on this.
The other area of shrinkage is a concern of steel temperature and in particular the use of semi-crystalline materials. As can be stated steel temperature affects the cooling rate of the material which in a semi-crystalline material affects the degree of crystallization and thus the shrinkage seen in the part. The most appropriate graph the author has come across is in the Ryton® PPS Design Guide from Chevron Phillips. It is located on page 7. It is a discussion of mold temperature and its effect on crystallinity, which is followed by a discussion on shrinkage. The take away here is what is the finished part or product going to experience in its use? Is it an elevated temperature or it shall never see a temperature other than room temperature? A great graph of this is in an out of print "Monsanto Plastics Design Manual" copyrighted 1994 by Monsanto Company page 94 DFIM-57. Basically it shows that an additional 0.004 to 0.005" /inch shrinkage is seen in parts after they are run through a paint oven and that greater post shrinkage is seen in the lower mold temperature part than the one molded at higher mold temperature, i.e. the degree of crystallinity changes.
So with a semi-crystalline material mold temperature and the resulting cooling rate shall play a big part in shrinkage, both immediate (1 hour) and post molded (24 to 48 hours). Thus the following can be factors for shrinkage, in semi-crystalline materials:
· Mold temperature
· Injection pressure
· Screw forward time
· Melt temperature
· Gate size
· Part thickness
· Material composition such as fillers and colorants.
One factor or resultant from some of the points above is gate freeze time, which as one would suspect can affect part size, but is also a data point one can use to insure different tools for the same part or multicavity tools for the same part are processing the same. The data point is gate freeze time. If we have excessive shrinkage, is the gate freezing off, too soon or maybe gate freeze was never achieved, relate this back to not enough material injected or that injected material is back flowing out of the cavity.
With amorphous materials we do not have this crystallinity issue and the shrinkage is based on what is stated in the various manufactures literature. The effect on shrinkage is minimal but can still be affected by the points in the above list for semi-crystalline materials. Generally speaking the amorphous materials shrink uniformly unless filled and while having a lower shrinkage valve due to their softening, issues still arise given to packing and gate freeze times though. When one uses mold temperature for an amorphous resins it is more in order to promote flow of materials, to thus prevent premature freeze off so as to properly fill the cavity and pack it out, and in certain materials to prevent over stressing while filling the parts.
Ryton is the registered Trademark of Chevron Phillips Chemical Company LP
http://www.cpchem.com/bl/rytonpps/en-us/Pages/DesignGuideRytonPPS.aspx
SL SIlvey
silveysplastics@hotmail.com
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