Stiffening ribs

A special attribute of these processes is the abiHty to pre-position reinforcement, inserts, and core materials for stiffening ribs. Gel coatings can be apphed to the mold surface to eliminate post-mold finishing. Because both surfaces of the part are formed in a mold to close tolerances, accurate assemblies are possible, which is a requirement for many automotive or tmck body appHcations. 

Fig. 3 -26 Examples of how different plastics and processes affect the design details of a stiffening rib.

Use of rib stiffening was required. It was found necessary to stiffen the cylindrical shell against buckling under external pressure from ground water and dimpling from local earth pressure due to surface wheel loads. Sandwich wall construction was investigated as an alternative to use of stiffening ribs and found to be feasible but less cost effective. 

The design in Fig. 8.33c is an improvement on the concepts illustrated in Fig. 8.336. Here the bottom cross leg has been removed. That permits the upper portions to become nominal wall thickness (A). The inside radii (C3) have been increased to equal the nominal wall thickness, which is the ideal configuration. The outside radii are the inside radii plus a wall thickness (E = C3 + A). A stiffening rib has been added at the center of the part. Its height should not exceed the nominal wall thickness (H < A). Its thickness (J) should be designed using the same parameters as elsewhere where there are material build-ups (D = 1.25A). If the rib is greater than described by this equation, a sink will appear where there is a dashed line. This can be masked by... 

The Mark 9 clip has a centrally positioned stiffening rib, which aligns the stress correctly in service, and, in production, provides a good flow channel for the molten polymer during injection moulding. The stress now develops principally in the web section (B in Figure 5.2), and this can be calculated for various service conditions. 

Absence of sink marks enables thick sections to be produced with large bosses, webs, stiffening ribs, etc. on the reverse side of a moulding. Noryl SF computerized cash registers make a good example of this kind of exploitation in a demanding application, i.e. sink marks would be totally unacceptable on the surface of the product. 

The minimum section thickness of SF parts is approximately 5-6 mm since below this level the foam has no space in which to expand and solid plastic sections are produced. Above 6 mm thickness the cooling time increases dramatically. This, it was decided by the PDD of BIP to adopt a general wall thickness of 6 mm for the trolley platform. From aesthetic and mechanical requirements the depth of the platform was put at 60 mm and since the smallest platform area to be moulded was 1000 x 600 mm it was evident that some stiffening ribs would have to be incorporated. 

PRIMARY COOLANT FLOW CHANNELS FABRICATED WITH STIFFENING RIBS, DETAILS PROVIDED LATER. 

Epoxy laminating resins Preferred material for mid-size and large models Attention stiffening ribs or profile rack frames have to be laminated for stability reasons ... 

Anton Tedesko, designer of such significant concrete shell structures as the sports arena in Hershey, Pennsylvania, and the airport terminal in St. Louis, relates the story of inspecting fine hairline cracks that developed in a hyperbolic paraboloidal shell in Denver that he designed for I. M. Pei. The concrete shell does not possess the stiffening ribs that have been criticized as architec-... 

Damping can be achieved by fitting proprietary damping pads, stiffening ribs or by using double skin construction techniques. 

Large flat surfaces should be avoided due to distortion and difficulty to form. Use stiffening ribs. Internal walls need to be well spaced. 

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