Design of Plastics Structural Parts for Static Loads

Design of Plastics Structural Parts for Static Loads 

This chapter is concerned with the detailed procedure for the design of plastics parts to take static loads. In order to make the analysis significant to real problems, we will work with a situation that involves significant loads and important structural requirements. In order to generalize the approach we will work with a structural problem common to a number of different structures and which will show how the different structural requirements will affect the engineering choices to be made. The particular structural section that will illustrate the sample design problem will be a roof section which may be used for anything from a work shed, to a house, to a vehicle, or even to a simple lean-to weather shelter. 

The roof section was designed to meet the static load requirements. However, it is necessary to consider transient loads such as people walking on the roof and fluctuating wind loads. The localized loads represented by people walking on the roof can be solved as indicated in Table 14-2 by assuming concentrated loads at various locations and by doing a short time solution to the bending problem and the 

With this basic approach in mind, let us do a design on a typical molded chair seat. The load will be assumed to be a 250 pound person and the typical load cycle is shown in Fig. 14-5, which includes loading times of 4 to 6 hours two or three times in 24 hours and a relaxation period of 1 to 2 hours during the day and of 10 hours during the night. The curve of loading is a random collection of these cycles over a 2-week period. The first step in the design is to select a section for 

Within the limits set above the design can vary widely. The seat can be attached to the rest of the chair frame by leg supports at the four corners, or it can be cantilevered from the back with a floor pad support, or, in another version, from the front (Fig. 14-6). The seat construction can range from a formed sheet in two or three dimensions to one with rolled edges for reinforcement. It can have structural ribs molded in or it can be a sandwich panel construction made up of two molded parts cemented together. If can also be a structural foam molding, which is a sandwich panel construction. In each of the configurations there are tradeoffs of stiffness and strength which 

---

In structural applications for plastics, which generally include those in which the part has to resist substantial static and dynamic loads, one of the problem design areas is the low modulus of elasticity of polymeric materials. Even when such rigid polymers as the ladder types of polyesters and polyamides are considered, the elastic moduli of unfilled polymers are under one million psi as compared to metals where the range is usually 10 to 40 million psi. Ceramic materials also have high moduli. Since shape integrity under load is a major consideration for structural parts, plastics parts must be designed for efficient use of material to afford maximum stiffness. 

---