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Pc Mold Design

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Pc Mold Design

1. 1. Chapter 5 Product and Mold Design Product Design Mold Design 55
3. 3. Figure 27 – Stress Concentration Figure 25 – Suggested Design as a Function of Wall Thickness and for Wall Thickness Transition Zone Corner Radius P = Applied Load R = Fillet Radius T = Thickness P 3.0 Stress-Concentration Factor R 2.5 T Inappropriate Appropriate 2.0 1.5 1.0 0 .2 .4 .6 .8 1.0 1.2 1.4 Figure 26 – Suggested Design for Corner Radius R/T T Minimum T Figure 28 – Exaggerated Draft Angle Radius 4 Release Inside Radius + T Draft Recommended Not Recommended 57
4. 4. 5 Product and Mold Design Ribs and Gussets Figure 29 – Example of Rib Design When designing ribs and gussets, it is important to follow the proportional thick- ness guidelines shown in Figures 29 and 30. If the rib or gusset is too thick in relation- ship to the part wall, sinks, voids, warpage, weld lines (all resulting in high amounts of molded-in stress), longer cycle times can be expected. f d The location of ribs and gussets also can affect mold design for the part. Keep gate location in mind when designing ribs or c e gussets. For more information on gate loca- a tion, see page 66. Ribs well-positioned in the line of flow, as well as gussets, can improve b part filling by acting as internal runners. a = wall thickness d = 2.5a minimum b = 0.5 to 0.75a e = 0.25a (radius corner) Poorly placed or ill-designed ribs and gussets c = 3a maximum f = 1⁄ 2° per side, minimum can cause poor filling of the mold and can (if more stiffness (draft angle) is required, add result in burn marks on the finished part. additional ribs) These problems generally occur in isolated ribs or gussets where entrapment of air becomes a venting problem. Note: It is further recommended that the Figure 30 – Example of Gusset Design rib thickness at the intersection of the nom- inal wall not exceed one-half of the nominal b wall in HIGHLY COSMETIC areas. For ex- d ample, in Figure 29, the dimension of the rib at the intersection of the nominal wall should c e f not exceed one-half of the nominal wall. Experience shows that violation of this rule significantly increases the risk of rib a read-through (localized gloss gradient difference). a = wall thickness b =a c=a d = 2a e = 0.5 – 0.75a f = 2.5a minimum 58
5. 5. Figure 31 – Recommended Design of a Bosses Boss Near a Wall (with Ribs and Gussets) Bosses are used in parts that will be as- sembled with inserts, self-tapping screws, drive pins, expansion inserts, cut threads, and plug or force-fits. Avoid stand-alone bosses whenever possible. Instead, connect the boss to a wall or rib, with a connecting rib as shown in Figure 31. If the boss is so far away from a wall that a connecting rib is Recommended impractical, design the boss with gussets as shown in Figure 32. Figures 33 and 34 give the recommended dimensional proportions for designing bosses at or away from a wall. Note that these bosses are cored all the way to the bottom of the boss. Not Recommended Figure 32 – Recommended Design of a Boss Away From a Wall (with Gussets) Recommended Not Recommended 59
6. 6. 5 Product and Mold Design Figure 33 – Recommended Dimensions for a Boss Near a Wall (with Rib and Gussets) a = wall thickness b = diameter of core h (at top of boss) j c = 2.5b d = 3a e = 0.9d min f = 0.3e max f = e f c g = 1⁄2° per side (draft angle) b h = 0.6a (at base) i = 0.25a (radius corner) j = 0.6a (at base) g d e i a Figure 34 – Recommended Dimensions for a Boss Away From a Wall (with Gusset) i a = wall thickness b = diameter of core (at top of boss) c = 2.5b d = 3a e = 1⁄2° per side (draft angle) f = 0.25a (radius corner) max g = 0.95d c min h = 0.3g e max h = g h b i = 0.6a (at top of gusset) g d f a 60