Shrinkage and Dimensional Stability

The injection pressure in the mould cavity decreases considerably with distance from the gate. The location, shape and number of gates influence melt orientation. This explains the very large variation in the shrinkage which sometimes occurs between different parts of a moulding, particularly complex mouldings. 

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When polymers are deformed and as the crystal axes tilt, the chains in the amorphous domains also become oriented. This orientation results in concentration of the intensity of the amorphous halo into an arc centered on the equator, as shown in Figure 2.lid from a drawn PET fiber [67]. The oriented chains in the amorphous domains become more densely packed than when they are not oriented. These domains are sometimes considered as intermediate phases or rigid amorphous phases [38,70]. The nature of the oriented amorphous phase influences numerous polymer properties such as diffusion, strength and modulus, and shrinkage and dimensional stability. XRD patterns can be used to determine the fraction of this phase, the degree of lateral and orientational order that is present in these ordered but noncrystalline domains [41]. 

Fibers have been used by humans for thousands of years, but only in the twentieth century has there been such an explosion in fiber types available to the textile manufacturer. The advent of synthetic fibers possessing improved resiliency and dimensional stability has placed natural fibers, particularly cotton (qv), at an ostensible disadvantage. Before synthetics, various means to control the shrinkage, dimensional stability, and smooth-dry performance of cotton had been investigated, but the appearance of synthetics such as polyester has placed a greater sense of urgency on cotton interests to focus on the perceived deficiencies of natural fibers. 

Glass-reinforced grades of SAN exhibit a modulus several times that of the unfilled polymer and, as with other glass-filled polymers, a reduced coefficient of thermal expansion and lower moulding shrinkage. The materials are thus of interest on account of their high stiffness and dimensional stability. 

Amorphous nylons are transparent. Heat-deflection temperatures are lower than those of filled crystalline nylon resins, and melt flow is stiffer hence, they are more difficult to process. Mold shrinkage is lower and they absorb less water. Warpage is reduced and dimensional stability less of a problem than with crystalline products. Chemical and hydrolytic stability are excellent. Amorphous nylons can be made by using monomer combinations that result in highly asymmetric structures which crystallize with difficulty or by adding crystallization inhibitors to crystalline resins such as nylon-6 (61). 

The dry process eliminated the problems of shrinkage or expansion inherent in wet processes, and dimensional stability of the proof was considered very important. 

The ductility of phenoxy resins resembles that of metals. They are transparent and also characterized by low mold shrinkage, good dimensional stability, and moderately good resistance to temperature and corrosion. Phenoxy resins are soluble in methyl ethyl ketone and have been used for coatings and adhesives. A typical injection-molded specimen has a tensile strength of 9000 psi, heat distortion point 86.6C at 264 psi load, and d 1.18. 

This section will cover physical properties, chemical properties, and flammability. Included in physical properties are tensile properties and dimensional stability at various temperatures in dry and moist environments, shrinkage properties, moisture uptake, and durability or wear resistance. Chemical properties cover resistance to sunlight, chemical and biological agents, and heat. The subsection on modification of properties addresses these property deficiencies and discusses progress made toward their improvement. 

In injection molding, the pol5uner is cooled from the melt temperature to a temperature below the solidification, which is accompanied by a reduction of pressure in a mold before the molded item is ejected from the mold. Since the ejected molded part is still hot, its cooling to room temperature occurs under atmospheric pressure. As a result of temperature and pressure changes, the cooled polymer shrinks according to the PVT (pressure-specific volume-temperature) relationship or the thermal expansion coefficient of the polymer. The linear mold shrinkage is 0.1-0.8% for amorphous polymers and 1.0-2.0% for crystalline polymers. For polypropylene, it is about 1.7%. Shrinkage affects the dimensional accuracy and dimensional stability of molded items. 

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