Plastic

When discussion problems in minimizing or increasing load-bearing requirements in wall thickness of plastics, ribbing is recommended if it is determined that space exist for adding ribs (Figs. 4.6 and 4.7). If there is sufficient space, the use of ribs is a practical, economic means of increasing the structural integrity of plastic products without creating thick walls. Table 4.2 shows how a rib structure rigidity (2 in. rib spacings) compares with non-ribbed panels where each are 1 x 2 ft. The rib height is 0.270 in. with a thickness of 0.065 in. If it is determined after a product is produced that ribbing is required adding ribs after the tool is built is usually simple and relatively inexpensive since it involves removing steel in the fabricating tool. With thinner walls a major cost saving could develop since processing them reduces processing time and provides more heat uniformity during processing.

Handbooks reviewing Stress and Deflections in Beams and Moments of Inertia provide information such as the moment of inertia and resistance to deflection that expresses the resistance to stress by the section modulus. By finding a cross section with the two equivalent

- Example of a design where thin wall with ribbing supports high edge loading

- Example of a design where thin wall with ribbing supports high edge loading

Secondary rib Section B-B

factors, one ensures equal or better performance of the fabricated product. The moment of inertia can be changed substantially by adding ribs and other shapes such as gussets as well as their combinations.

There are available basic engineering rib-design guidelines. The most general approach is to make the rib thickness at its base a minimum of one-half the adjacent wall's thickness. With ribs opposite appearance areas, the width should be kept as thin as possible. In areas where structure is more important than appearance, or with very low shrinkage materials, ribs are often 75 or even 100% of the outside wall's thickness. A goal in rib design is to prevent the formation of a heavy mass of material that can result in a sink, void, distortion, long cycle time, or any combination of these problems.

.. Example where ribbing is beneficial

Property

Steel

Solid plastic

Structural foam

Thickness (in.)

0.040

0.182

0.196

E (psi)

3 x 107

3.2 x 105

2.56 x 105

1 (in.4)

0.000064

0.006

0.0075

E x 1 (rigidity)

1,920

1,920

1,920

Weight (lbs.)

3.24

1.98

1.78

Comparing a rib design strengthwise and weightwise with other materials

Comparing a rib design strengthwise and weightwise with other materials

K--«20-)l K-.620—)) |<-.620-J|*T

Zinc

Area = 0.283 in. Area = 0.489 Wt/in = 0.446 oz. Wt/in = 2.01 oz

Plastic-PC E -- 1.2 x 10° I = 0.0424 EI = 5.08 x 10^ Area = 0.170 Wt/in = 0.149 oz

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