Plastic parts design material selection

A successful plastic product depends on the optimization of each of the following four principal components part design, material selection and handhng, tool design and construction, and processing/machine capabilities. 

This chapter has covered some of the stress loading situations other than simple tensile, compressive and bending loads encountered by plastics parts. The unique characteristics of plastic materials of certain types make them especially suited to resist the specific stress conditions. With proper part design, material selection and modification the designer can make parts that will perform well under unique stress conditions. 

Polymeric materials are used in a vast array of products. In the automotive area, they are used for interior parts and in under-the-hood applications. Packaging applications are a large area for thermoplastics, from carbonated beverage bottles to plastic wrap. Application requirements vary widely but, luckily, plastic materials can be synthesized to meet these varied service conditions. It remains the job of the part designer to select from the array of thermoplastic materials available to meet the required demands. 

Risk is an inherent part of plastic product development, and the level of risk can actually increase as plastic part designs become more efficient. The reason is that as the plastic part designer improves the design, more constraints on the design become obvious. For example, consider the selection of wall thickness for the internal chassis depicted in Fig. 27.15. A large wall thickness of 5 mm can be used to provide stiffness but will tend to increase the cost of the plastic part due to excessive material costs and processing times. The plastic part designer will likely... 

Belter, A., Cardinal, J. M., and Ishii, K., Preliminary Plastic Part Design Focus on Material Selection and Basic Geometry While Balancing Mechanical Performance and Manufacturing Cost , Journal of Reinforced Plastics and Composites, Vol. 16, No. 14/97, pp. 1293-1302. 

This book focuses on the relationships between the chemical structure and the related physical characteristics of plastics, which determine appropriate material selection, design, and processing of plastic parts. The book also contains an in-depth presentation of the structure-property relationships of a wide range of plastics, including thermoplastics, thermosets, elastomers, and blends. 

Contents Introduction to Materials. Manufacturing Considerations for Injection Molded Parts. The Design Process and Material Selection. Structural Design Considerations. Prototyping and Experimental Stress Analysis. Assembly of Injection Molded Plastic Parts. Conversion Constants. 

In the design of parts for a particular device, density can be an important factor in the selection of a material, especially when deciding whether to use metal or plastic. For example, some specially designed plastics may cost 5/lb compared to l/lb for a steel part. If the plastic has a density of 0.04 lb/in.3 and the steel has a density of 0.28 lb/in., the difference in price will be small if the part has a small volume. Using the figures above, the cost of 1 in. would be 0.20 for the plastic part and 0.28 for the steel part. 

In the injection molding process, setting the temperature involves optimization of the temperature profile of the plasticating unit (extruder barrel), temperatures of the mnners and gates, (aU these determine the molten polymer temperature) as well as the mold temperature. The temperature setpoints depend on the material type (viscosity profile, thermal and shear stability, thermal properties) as well as machine or process considerations (machine capacity to shot size ratio, screw design, mold and part design, cycle time, etc.). Temperatures of the two basic units, the injection system and the mold, should be discussed separately since their selection stems from very different considerations. 

Physical dimensions of many processed parts must be held to fairly close tolerances to ensure proper assembly of parts into a complete structure, as, for example, molded fender panels bolted to steel chassis cars, plastic screw caps for glass jars, etc. In general, the final dimensions of the processed part will differ from the dimensions of the mold cavity or the pultrusion die. Such differences are somewhat predictable, but are usually unique to the specific material and to the specific process. The dimensions of a mold cavity for a phenolic part requiring close tolerances will often be different from dimensions of a cavity for an identical polyester part. Both the part designer and the mold or die designer must have a full understanding of the factors affecting final dimensions of the finished product, and often need to make compromises in tolerances of both part and cavity dimensions (or even in plastic material selection) in order to achieve satisfactory results with the finished product. 

It is important that the designer realize the unique opportunities and problans posed by each method of plastic part assembly. To do this well, he or she must have an understanding of materials science, chemistry, surface sdence, physics, mechanics, and industrial engineering. All of these disciplines wiU come into play. Even with this background, final selection of the most desirable assembly method involves some trial and error that can become costly and time consuming. 

This chapter will outline the procedure for the design of plastics parts and give the methods of sizing, material selection, cost consideration, design life, performance evaluation, and testing. The monitoring of quality will be indicated to insure performance. All the basic information and approaches have been covered, thus this chapter is a synthesis of previous discussions. 

Table 6-27 provides a basic outline that identifies the D 4000 line callouts (specifications). The classification system and its subsequent line callouts is intended to be a means of identifying plastic materials used to fabricate end items or parts. It is not intended for the selection of materials. Material selection should be made after careful consideration of the design and performance required of the part, the environment to which it will be... 

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