Metal Replacement with Plastic

This review shows how to design a plastic to replace a metal in such a way as to optimize weight and cost reductions. The plastic used is a stampable reinforced thermoplastic sheet (STX) material (STX is a registered tradename). It is composed of a glass fiber-nylon RTP. Replacing metal with plastic while maintaining material efficiency and achieving cost reduction requires a degree of ingenuity and practicality in design. Direct replacement of 21 gauge (0.032 in.) sheet steel having a modulus of 30 x 106 psi would require that an STX material with a modulus of 1 x 106 psi (l/30th that of steel) have a thickness of 0.099 in. (3 times that of steel). These figures are derived as follows:

where E = flexural modulus and / = moment of inertia £, = 30,000,000 E2 = 1,000,000

(30,000,000)(2.7 x 10"6)t> = 1,000,000(/?3/12)6 81.92 = {hPllljx 106 h3 = 9.83 x 10"4 h = 0.099"

While this increase in thickness may have some disadvantages, (space limitations and cost among them), plastics can still be weight and energy effective. The beam problem in Fig. 4.104 illustrates the weight effectiveness achievable by using plastic instead of metal. To achieve comparable deflection, a plastic beam would require three times the height of a steel beam, but importandy, the weight of the plastic would be only 62% of the metal.

To use material effectively and, at the same time, overcome the space utilization of packaging problem that probably would be encountered with a large section beam, the designers can resort to the extremely simple addition of ribbing to a plastic part. What would be a very complex operation in sheet metal is an attractive option in plastic because the fabrication method permits incorporation of ribs in the molding process. Ribbing increases part section modulus with minimum weight increase. In most cases, ribbing can be very simply and easily incorporated into an STX or other RP constructions part with minimal weight addition and without a penalty in molding cost.

This principle of ribbing can be applied to other mechanical strengthening and stiffening requirements of an STX component. For example, to replace a normal sheet metal flange, a plastic flange would be designed, incorporating several modifications that are easy to make in plastic. The flange is thickened and an outer, down or upturned lip is added, increasing the moment of inertia and thus limiting torque type racking; in addition, gussets are added to distribute the bolt loading. Corrugations in the wall are valuable to provide stiffness to reduce vibration noises or tin canning, and in flange design, to increase the effectiveness of bolt loading by moving the wall to the bolt centerline, thereby utilizing the wall section's stiffness to distribute the bolt loading between bolt holes.

In contrast to sheet metal, where the thickness is uniform, STX can be molded with sections of varying thickness. While it is always recommended in design of plastic parts that section thicknesses be uniform, it is always the first rule to be violated. It is this, very design flexibility that becomes advantageous, and designers are usually unable to resist the ability to strategically modify section thickness to meet the load or stress requirements. If stress, temperature, creep, impact or any other environmental conditions require a heavier or specific section thickness, it can be done within prevalent design parameters.

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