Tolerances and Shrinkages

To meet tolerances or shrinkages (as with other materials), more is needed to be applied than simple arithmetic. An important requirement is that someone such as the product moldmaker be familiar with plastics behavior and, particularly, its fabrication method. Of course, with experience in a product equal or similar, as with other materials, setting tolerances and shrinkages is automatic. 

Melt temperature (Tm) refers to a plastic when it melts or softens and begins to have flow tendency. It provides recommendation for processing temperature to be used and related to tolerance and shrinkage behavior of molded products. 

TSs has a much lower shrinkage than TPs. The TSs is also easier to control shrinkage however, TPs are controllable. When comparing the shrinkage behaviors of URPs with RPs, there is much less shrinkage with RPs. Thus with RPs, tolerances and shrinkages are significantly reduced or eliminated and provide more reliability in ease of repeatability than URPs. 

DESIGNING WITH PLASTICS AND COMPOSITES A HANDBOOK TOLERANCES AND SHRINKAGES... 

Warping, difficulty of moulding to close tolerances and wavy or fibre-patterned surfaces or faults arising from the high shrinkage during cure. 

As reviewed throughout this book and particularly in Chapter 3, Design Concept, there are many design features that keep expanding the use of plastics in different products. These features include shapes, sandwich constructions, shrinkages, tolerances, and processes. 

One factor associated with tolerance is shrinkage. Generally, shrinkage is the difference between the dimensions of a fabricated product at room temperature and after cooling,checked usually twelve to twenty-four hours after fabrication. Having an elapsed... 

Filling the Mold Cavity to Form the Product. The mold cavity is designed and machined to form the shape of the finished product. This is itself a complete art and science, based partly on experience, and increasingly on computerized engineering principles. Some major considerations are fast uniform flow, avoidance of degradation, minimization of orientation/anisotropy, fast cooling/ solidification, shrinkage and dimensional tolerances, and of course final properties of the product. 

Because PES is amorphous, mold shrinkage is low and it is suitable for applications requiring close tolerances and little dimensional change over a wide temperature range. Its properties include ... 

Shrinkage and warpage analysis to satisfy tolerances and predict dimensional stability of the manufactured part. 

---