Molded-in stresses

To be successful, molded optical elements of plastics must be produced with careful control of the fabricating process. In the case of these optical products it is particularly important that the molding conditions be carefully controlled to minimize molded-in stress. In addition to these stresses reducing the dimensional stability of the products leading to distorted images, the stresses themselves affect the quality of the image. This is a result of the fact that the stresses/strained areas have a different refractive index from that of the... 

Distortion Another problem with extrusions is caused by distortion of the section by the effect of heat and other environmental conditions such as exposure to water or chemical agents that tend to soften the plastic. These distortions are generally reversals of the profile back to the shape that it had exiting in the die. This action indicates that the post die forming operations were done at a lower than desirable temperature which results in a molded-in stress. When the stress is relieved the product distorts. In some instances these stresses cannot be eliminated by process changes so that the product is inherently deficient in performance. 

The problem-solving approach that ties the processing variables to products properties includes considering melt orientation, polymer degradation, free volume/molecular packing and relaxation, cooling stresses, and other such factors. The most influential of these four conditions is melt orientation, which can be related to molded-in stress or strain. 

Rotational molding creates a wide variety of plastic products that cannot be made effectively, efficiently, or economically by other means. What sets this method apart from others is that it can create thin-walled, hollow parts that exhibit no weld lines or scarring from ejector pins and from the process itself. It also has the advantage of having little scrap and minimal molded-in stresses, due to the low pressure and low shear rate characteristics of the process. Finally, it can be used to make parts that are very large which would be impossible to manufacture by other methods. 

Processing Stability. As with elastomers or other rubber modified polymers, the presence of double bonds in the elastomeric phase increases sensitivity to thermal oxidation either during processing or end use. Antioxidants are generally added at the compounding step to ensure retention of physical properties. Physical effects can also have marked effects on mechanical properties due to orientation, molded-in stress, and the agglomeration of dispersed rubber particles under very severe conditions. Proper drying conditions are essential to prevent... 

Part of the polyolefin family are PBs. They are similar to PPs and HDPEs but exhibit a more crystalline structure. This crystallinity produces unusual high strength and extreme resistance to deformation over a temperature range of-10 to 190F. Its structure results in a rubberlike, elastomeric material with low molded-in stress. Tensile stress that does not plateau after reaching its yield point makes possible films that look like PE but act more like polyester (TP) films. Compared to other polyolefins, they have superior resistance to creep... 

As the size of the part increases, the anisotropic nature of LCP shrinkage becomes more critical because the absolute magnitude of the shrinkage increases. Here gating geometry also becomes important to minimize molded-in stresses and attendant warpage. 

Uniform heat removal from the mold is critical for securing consistent dimensional tolerances and the best set of properties. Higher mold temperatures are required for thin walls, large parts, and parts that require dimensional stabUity and low level of molded-in stresses. Mold temperature has minimal effect on gloss level. 

PC/ABS blends have better flow properties than PC, which means that the parts have fewer knit lines, sink marks as well as less molded-in stress. Molded parts offer excellent surface appearance and good paint adhesion. Up to 20% regrind may be used for all grades. 

Because of the low melt viscosity of LCPs, thin sections can he molded with highly filled (70%) LCPs. The addition of hulk lubricants reduces the melt viscosity. Injection molded parts are produced with a minimum of molded-in stress, little or no flash, and high dimensional stability. Fast cycle times, reduced mold wear, elimination of secondary operations, and other processing and performance gains of LCPs often lead to lower overall finished part costs even below those of less-expensive resins. 

The CDs are manufactured either by injection compression molding or injection molding. Injection compression molding minimizes molded-in stress but requires longer cycle times than injection molding and produces flash. For both processes very clean room conditions, up to 1,000 times cleaner than medical molding, are essential. 

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