Design

Proper material selection alone will not prevent a product from failing. While designing a plastic product, the designer must use the basic rules and guidelines provided by the material supplier for designing a particular part in that material. One must remember that with the exception of a few basic rules in designing plastic parts, the design criteria change from material to material as well as from application to application. The most common mistakes made by designers when working in plastics are related to wall thickness, sharp corners, creep, draft, environmental compatibility, and placement of ribs. Failure arising from designing parts with sharp corners (insufficient radius) by far exceeds all other reasons for part failures. Stresses build rapidly in internal sharp corners of the part, as shown in Fig. 3, which illustrates the influence of fillet radius on stress concentration. Maintaining uniform wall thickness is essential in keeping sink marks, voids, warpage, and more importantly areas of molded-in stresses to a minimum. The viscoelastic nature of plastic materials as opposed to metals requires designers to pay special attention to creep and stress relaxation data. Plastic parts will deform under load over time depending upon the type of material, amount of load, length of time, and temperature. Design guides for proper plastic part design are readily available from material suppliers. Table 1 shows a typical part design checklist. Figure 4 illustrates a typical part failure arising from improper design. 774 Failure Analysis of Plastics Table 1 Part Design Checklist 1 Introduction 775 Figure 4 Brittle failure due to sharp corner (lack of radius) on part.