Processing Limitations on Plastics Product Design

Plastics parts are made by a variety of manufacturing processes. A detailed discussion of these is outside the scope of this book on plastics product design. However, it is essential to know some of the process-imposed limitations or an unmanufacturable product will be designed. In this chapter we will examine some of the general limitations imposed on design by the processes used to make the part. 

The gate or point of entry of the plastics into a closed mold such as is used in injection and transfer molding is an important point for 

The product designer s principal concern with location of the gate is that it may affect the appearance or environmental properties of the product. It is advisable to confer with the mold designer before the 

For a complete study of gating considerations and gate designs, consult DuBois and Fribble.  

Gating for reaction injection molding requires laminar flow and a built in after mixer as part of the gate design. Structural foam and RIM molds require nonconventional gating and venting considerations and the mold designer and the molder should be consulted. 

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Figure 9-15B. Typical metal hold-down plate for use with ceramic or carbon packing. Note It rests directly on top of the packing bed-limiters are similar in design in metal or plastic, bolted to column wall above packing. Used by permission Norton Chemical Process Products Corp., Bull. TA-80.

Material and process interaction and their effects on the performance of plastic products are important factors for the designer to understand. It can result in design limitations such as a selected material meeting performance requirements but not pro-cessible by the desired method of fabrication. Most of the limitations that are reviewed in this book can be corrected and do not effect the product performances when qualified people handle the limitations. However they are presented to reduce or eliminate potential problems. 

In addition to what has been reviewed in meeting structural shapes, analyses of product shapes also includes factors such as the size of available processing equipment. The ability to achieve specific shapes and design details is dependent on the way the process operates and plastics to be processed. Generally the lower the process pressure, the larger the product that can be produced. With most labor-intensive fabricating methods, such as RP hand lay-up there is virtually no limit on size (Chapter 5). 

The constraints on design imposed by the processes for manufacture have been covered in this chapter for a number of the widely used plastics manufacturing processes. The first step in product design should be the selection of the material and manufacturing process. Not all processes were covered, just those that are most frequently met in design practice. The basic process limitations were discussed... 

The process effects on parts and the limitations they impose on part design and performance have been discussed for several of the more important production processes for plastics products. In many cases design modifications can substantially improve the producibil-ity of the parts and reduce part cost with improved product quality. 

The results of processing the plastic materials affect their performance profoundly. The basic information of plastics structures is given to enable the designer to predict the performance of his product within the limitations of the processing. The unique effects of the environment on plastics are examined so that the designer can design a product for a lifetime exposed to the use environment. 

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