As it was said, due to the compatibility of PVC with various additives, the products of this polymer are available in various flexible, hard, composite and foam types. In this section, we would like to describe PVC engineering. Due to its dimensional stability, weather resistance, and flame retardancy properties, hard PVC formulations are used to produce products such as construction parts, parts of electrical products, and household appliances such as televisions.
Basic points for producing a PVC engineering product
To produce a PVC product with good quality and long useful life, all aspects of formulation design, process, material properties and product usage conditions should be considered. For example, it is very important to know the stress distribution in products and to recognize the high areas during use. Because by knowing the areas with high stress and the maximum amount of stress, the piece can be designed so that it can withstand the maximum stress in these areas.
The geometric shape of the products is also effective on the stress distribution. In general, the corners and sharp areas of the piece have more tension. Among other useful information that must be known to produce a suitable product, is the temperature range of the product and its lifespan.
The effect of process conditions on the quality of PVC products
Process conditions have an important effect on melting degree, crystallization percentage, residual strain and in general the final properties of PVC products. For example, by adjusting the temperature, time and amount of thermal and mechanical work applied to the mixture, the desired degree of melting can be achieved.
Normally, PVC does not melt uniformly due to the presence of fine crystalline particles. Therefore, it is necessary to accurately determine the melting rate of PVC mixtures by tests such as solvent immersion test (ASTM D2152) or mechanical tests. The melting rate of PVC depends on several factors such as the percentage of crystallization. Commercial grades of PVC have a small percentage of crystallization, but this small percentage of crystallization also plays an important role in the properties of products. For this reason, crystalline particles in the irregular matrix of PVC are often referred to as physical networks.
The most common process problem regarding PVC is destruction and as a result the loss of physical and mechanical properties because, as mentioned, PVC is quickly destroyed due to its special molecular structure.
The formation of bubbles also reduces the physical and mechanical properties, because the presence of these bubbles creates places for stress concentration. For example, this problem can be partially solved by reducing the residence time, reducing the injection back pressure, reducing the injection speed, and thus reducing the heat generated from cutting, enlarging the dimensions of the gate and mold ducts, and reducing the speed of the screw.
Properties of molded PVC products
Considering that PVC is a viscoelastic material, the properties of its mixtures depend on temperature and time. Therefore, with the passage of time and temperature change, PVC molded products gain tension and strains and undergo expansion or contraction. The speed of release of stress and residual strains depends on several factors such as the components of the mixture and its usage conditions. What is more important among these is that it is possible to predict the useful life of the products by using the amount of stress and strains remaining in the products.
Various mathematical models have been presented by researchers that accurately predict the probability of defects occurring in products under several types of successive stress and strain, which is considered a branch of PVC engineering. The passage of time, in addition to helping to relieve the stress and strains of PVC products, also has another important effect, called stiffness or aging. Aging is actually a change in the properties of PVC over time.