Product-Plastic-Process Performance

In order to understand potential problems and solutions of design, it is helpful to consider the relationships of machine capabilities, plastics processing variables, and product performance (Fig. 1-10). A distinction has to be made here between machine conditions and processing variables. For example, machine conditions include the operating temperature and pressure, mold and die temperature, machine output rate, and so on. Processing variables are more specific, such as the melt condition in the mold or die, the flow rate vs. temperature, and so on (Chapter 8). 

Those familiar with processing can detect and correct visible problems or readily measure factors such as color, surface condition, and dimension. However, less-apparent property changes are another matter. These may not show up until the products are in service, unless extensive testing and quality control are used. 

As there are many different plastics, a number of techniques for defining and quantifying their characteristics exist. As an example molecular weight distribution (MWD) is an indication of the relative proportions of molecules of different weights and lengths. In turn MWD relates to processing characteristics that directly relate to product performances (Chapter 8). 

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Plasticizers seem to have an importance equal to plasticized resins and should be considered as a spouse rather than as servant. This couple, resin plus plasticizer, is equally responsible for the physical properties of the plasticized product, its processing performance, and its cost. In selecting a plasticizer, one must consider compatibility, efficiency, permanence, and economy. Compatibility depends upon polarity, structural configuration, and size of molecule. Efficiency depends upon the solvating effect. Permanence depends upon volatility and susceptibility to extraction. And economy depends upon raw materials and conversion costs. 

A hard-and-fast rule to be followed by all intending to use plastics is to design for plastics. As an example, for the same-size cross-section the strength of conventional plastics (not the high-performance reinforced types) is considerably less than that of most metals. The designer will thus find it necessary to increase thickness, introduce stiffening webs, and/or possibly use design inserts of various types of threads to secure the proposed product. The process will in some instances also require modification to the shape of the equipment used to produce the product. 

Plastic membrane This is done by the use of a water permeable plastic membrane held deep enough under the sea so that the hydrostatic pressure is greater than the osmotic pressure of the seawater. The water distills out of the solution through the membrane and is pumped to the surface. Large areas of the membranes, mechanically supported to withstand the very high pressures are essential to make the process perform rapidly for the most economical production. 

MW is the sum of the atomic weights of all the atoms in a molecule. It represents a measure of the chain length for the molecules that make up the polymer and in turn the plastic that influences processing performances to meet product performance (2). The MWD is basically the amount of component polymers that go to make up a polymer. Component polymers, in contrast, are a convenient term that recognizes the fact that all plastic materials comprise a mixture of different polymers of differing molecular weights. 

With all types of plastic processes, troubleshooting guides are set up to take fast, corrective action when products do not meet their performance requirements. This problem-solving approach fits into the overall fabricating-design interface. One brief example of troubleshooting an RP/composite is in Table 8-44. 

For example, TCM can be used to determine the plastic process that is best for production without extensive expenditures of capital and time. Not only can TCM be used to establish direct comparisons between processes, but it can also determine the ultimate performance of a particular process, as well as identifying the limiting process steps and parameters. 

Designing with plastics based on material process behaviors , Donald V. Rosato, Marlene G. Rosato, and Dominick V. Rosato, Kluwer Academic Publishers (2000). This book provides a simplified and practical approach to designing plastic products that fundamentally relates to the load, temperature, time, and environment subjected to a product. It will provide the basic behaviors in what to consider when designing plastic products to meet performance and cost requirements. Important aspects are presented such as understanding the advantages of different shapes and how they influence designs. 

The process of injection molding (IM) is used principally for processing unreinforced or glass fiber reinforced thermoplastics (TPs) and thermosets (TSs) (Figure 4.1). Up to at least 90wt% of all plastics processed are TPs. There are many different types or designs of IM machines (IMMs) that permit molding many different products based on factors such as quantities, sizes (such as auto bumpers to medical micro products), shapes (simple to complex), product performances, and/or economics.1,150>157,173 176 476... 

Technologies, such as coextrusion and coinjection, allow PET and other plastics to package foods and other products.225 226> 227 Care must be taken to control the process so that the melt when blown will not have micro-voids in the container walls or will delaminate. Coextrusion and coinjection (or multilayer processes) are essential technique in the production of high performance BM products (Chapters 4 and 5). The parison or preform is coextruded with a number of different layers, each of which contributes an important property to the finished product. Increasingly, a mid layer may consist of recycled material which is encapsulated between inner and outer layers of virgin plastics. 

The molding of two or more different types of plastics in a single product may be accomplished to combine their specific properties and/or a better performing or lower-cost product. This process, called corotation, is similar to coinjection or coextrusion in terms of the performance of the designed product (Chapters 4 and 5). 

Zinc Sulfide and Zinc Oxides. Both materials are white but do not approach titanium dioxide for use as a tinting pigment or opacifier in plastics. Both materials can have nonpigmentary utility in plastics, such as providing whitening power at much lower abrasion levels than titanium dioxide. Zinc oxide, for instance, not only brings whitening to rubber products, but also performs as an accelerator in the vulcanization process. These products cannot compete directly with titanium dioxide when whiteness and opacity are the only criteria. However, they can play an important role when they contribute to chemical reactions and/or physical properties. 

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