Direction of shrinkage

As reviewed in the other chapters, shrinkage with TPs can be rather substantial, but with TSs it is less so, principally because of mixing with inorganic mineral additives that are not affected by heat changes. The degree and direction of shrinkage will depend on the molded part density, part configuration, ratio of resin to additives/reinforcements, and/or orientation of fibers (Chapter 2). 

Shrinkage of about 5-10% occurs at right angles to the direction of the preforming force. The amount of shrinkage is mainly dependent on the rate of cooling, but also to a minor extent, on the preforming pressure. 

In order to get the flow direction at the end of the filling phase and the beginning of the holding phase (representing the onset of shrinkage), an analogous model can... 

The direction of gas flow through the pellet bed could be important. A pulsating high speed flow of exhaust gases can cause rapid attrition of catalysts, especially if the converter has empty spaces due to catalyst loss or shrinkage, which would promote the internal circulation of catalysts in the converter. The design of a sideflow or an upflow bed must include provisions to avoid empty spaces. A downflow design would minimize these attrition losses. 

An account of the mechanism for creep in solids placed under a compressive hydrostatic stress which involves atom-vacancy diffusion only is considered in Nabarro and Herring s (1950) volume diffusion model. The counter-movement of atoms and vacancies tends to relieve the effects of applied pressure, causing extension normal to the applied stress, and shrinkage in the direction of the applied stress, as might be anticipated from Le Chatelier s principle. The opposite movement occurs in the case of a tensile stress. The analysis yields the relationship... 

A number of organic pigments cause distortion in certain types of polyolefins, especially in HDPE. Pigments act as nucleating agents in such partially crystalline plastics i.e., they promote crystallization, which creates stress within the plastic product (Sec. 1.6.4.3). These pigments also enhance the shrinkage of polyolefins, particularly in the direction of the flow. 

SD HD PE colorations containing 1 % TiOz are heat stable up to 290°C. The shrinkage of injection-molded articles is noticeably affected in the direction of flow if the processing temperature is low (220°C). This influence, however, is already very slight at 260°C. 

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