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| Shrinkage & Warpage(An extract from Rotational Molding: A Practical Guide) 8.12 Shrinkage
and Warpage Many of the variables used to control rotational molding are interrelated. It can be difficult to separate the effects of one from another when tuning the process. At the center of many of the interactions are shrinkage and warpage. Since the shape, size and appearance are usually critical aspects of a part, almost every factor affects these final requirements in some way. Crystalline materials (such as polyethylene) are the most difficult to control as the development of the crystal structure is affected by many variables; amorphous materials have no crystal structure and are less affected by some variables. Some of the parameters discussed so far can have a direct effect on final part shrinkage and warpage, including:
Rotational molding is a
single sided process, i.e. the mold controls only the outer surface of the part
while the inner surface forms under the influence of variables that affect heat
transfer, powder contact time and material flow. Other processes such as blow
molding and pressure forming (thermoforming) are also single sided processes but
these have the advantage of being able to use higher internal pressures to hold
and control parts during cooling. The low pressure used by rotational molding is
a benefit in terms of the residual stresses produced, but limits the ability to
control final part size. Rotational molding has advantages over blow molding and
thermoforming (pressure forming) in that they are affected by material
stretching, but in terms of distortion and warpage they can have more control. The
problems of shrinkage have generated enough interest that the Process, Equipment
and Tooling Committee of ARM developed a shrinkage test mold to examine shrink
in materials by polling molders who use it for evaluation. The objective is to
collate records of material type and pigments with processing conditions vs.
final part size. The mold is available for member molders through the ARM
office.
Typical
Shrinkage Ranges Material shrinkage values will have a range that is affected by the variables listed above. Table 8.13 lists reported shrinkage values for a range of materials used in rotational molding.
Table 8.13: Reported Shrinkage Ranges for Rotational Molding Materials Polyethylene is the most commonly molded material and is therefore the most studied. Figure 8.51 shows the general trend for shrinkage vs. density. The curve shown lies along the middle of a range, as shrinkage at each density will vary due to molding variables........... More details in Rotational Molding: A Practical Guide. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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