Machinery's Handbook, 31st Edition
DESIGNING PLASTIC PARTS 591 Use of ribs allows the structural characteristics of a part to be tailored to suit its function, but ribs can increase the size of the product and cause warping, as well as appearance problems, so they are best avoided when not structurally necessary. If the first parts pro - duced require strengthening, ribs can be added or thickened without high cost after the tool is finished because the work consists only of removing steel from the mold. In general, ribs should have a base thickness of about 70 percent the thickness of the adjacent wall and be kept as thin as possible where they are positioned near faces that need to have a good appearance.
Fig. 17. (a) Original Beam; (b) Original Central Section; (c) Cross Section of Center with Rib Added to Reinforce the Structure Note: Dimensions in parentheses are for example on page 590.
Where structural strength is more important than appearance, or when materials are used that have low shrinkage, ribs can be made 75 or 100 percent of the wall thickness. However, where the rib base joins the main molding, there is locally increased thickness, forming a heavy mass of material. Shrinkage of this mass can produce a cavity or void, a hollow area or sink, or can distort the molding. If the mass is very large, cooling time may be prolonged, leading to low output from the machine. Large masses of material in other parts of a molding are also best avoided. These problems can usually be addressed by good mold design. Ribs need not be of constant height or width and are often varied in proportions to suit the stress distribution in the part. All ribs should have a minimum of 1 ⁄ 2 degree of draft per side for ease of removal from the mold and a minimum radius of 0.005 inch (0.127 mm) at the base to avoid stress-raising corners. Higher draft angles are needed when molding textured surfaces or fiber-filled resins that are difficult to eject. Draft and thickness requirements will usually limit the height of the rib, which can be from 1.5 to 5 times the base thickness, and several evenly spaced ribs are generally preferred to a single large one. Smooth transitions (chamfers, fillets, and gussets) should be made to other structural features such as bosses, walls, and pads.
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