Designing molds with “holes

Covering metallic shafts, rolls and rollers with rubber is required for many applications such designs are used in the textile, printing, and many other industries. For producing coatings, molds with a vertical axis are often filled through inlet holes located along the diameters of the mold... 

Bosses are used to reinforce holes in the part and to provide mounting locations. Solid bosses are called studs. Bosses must be molded with rounded corners and adequate fillet radii. They usually are located in a corner or juncture of two or more surfaces to minimize the effects of sinking. The problem of shrink marks is lessened with GRTP because of their reduced shrinkage. It is generally recommended that the height of the boss be limited to twice its diameter. Vertical surfaces should have at least a 1 deg draft per side. Recommended designs are shown in Fig. 6-5. 

Be designed with 0.001 in. total clearance with hole-in-mold button. The holes for the mold button and pierce punches must be line-bored to obtain the close tolerances needed. 

Proper design and location of the gate ensures filling of the mold with the reaction mixture without entrapped air bubbles. Air bubbles must be avoided because they increase in size during foaming and rise to the surface caused by buoyancy. They can create pin holes or lentil-shaped air enclosures at or near the surface of the part. This can create rejects or requires costly post-molding finishing work at least. 

FIGURE 2.17 Design process for a slider mold, with the example of a small box with lateral bore holes (in accordance to [1])... 

Well-designed molds are vented, as entrapped air in the mold prevents good contact between the parison and the mold cavity surface. When air entrapment occurs, the surface of the blown part is rough and pitted in appearance. A rough surface on a shampoo bottle, for example, is undesirable because it can interfere with the quality of decoloration and can detract from the overall appearance. Molds are easily vented by means of their parting line, with core vents and with small holes. A typical mold parting line venting system is shown in Fig. 13.23. 

Covers for the battery designs in Figures 1 and 2 are typically molded from materials identical to that of the respective case, and vent plugs are frequentiy made of molded polypropylene. Other combinations are possible, eg, containers molded of polyethylene or polypropylene may be mated with covers of high impact mbber for use in industrial batteries. After the cover is fitted over the terminal post, it is sealed onto the case. The cover is heat bonded to the case, if it is plastic it is sealed with an epoxy resin or other adhesive, if it is vulcanized mbber. Vent caps are usually inserted into the cover s acid fiU holes to faciHtate water addition and safety vent gasses, except for nonaccessible maintenance-free or recombinant batteries. In nonaccessible batteries, the vent is fabricated as part of the cover. 

Moldable holes and inserts can complicate molding and may require extra postmolding operations. Thus, the most economical designs are those that minimize the number of holes. With many plastics, both external and internal threads can be molded, but sharp V threads should be avoided because they can cause the plastic to bridge, resulting in incomplete thread fill. Rounded or modified buttress threads will allow improved thread fill. 

Preform screens are usually made from 16-gauge perforated material with Vs in. holes on / 6 in. centers. This produces about 40% open area. For some operations, a more open area is required. Perforation patterns are also used to develop specifically designed reinforcement directional properties. The screen is usually designed so that the outside contour is identical with the contour of the mating half of the mold. A screen, which is not of the correct size, will cause a great deal of difficulty in... 

---