External undercuts

The injection mol ding process eliminates the restriction of straight-sided components required when parts are ejected from a die, and offers opportunities for external undercuts and threads. A wide variety of alloys can be processed, including alloy steels and stainless steels. Material properties of injection molded parts are available (32). 

External undercut Any recess or projection on the outer surface of a molded part that prevents its direct removal from its mold cavity. Parts with such undercuts may be molded by splitting the mold vertically and opening the split to withdraw the part or by providing side draws, i.e., mold parts that are withdrawn to the sides to relieve the undercuts. 

As a closed-mold process, both surfaces of compression-molded parts can be finished. Draft for long-fiber compression-molded parts should be 3° per side, but less can be used for depths under 6 in—down to 1° per side in extreme cases. Molded-in holes are feasible in the plane of the parting line as are bosses, corrugated sections, molded-in labels, and raised lettering. Molded-in holes within a two-diameter distance from the outside edge of the part will need to be drilled as a secondary operation. The solid material thickness between two holes should not be less than one diameter (the largest). Core pulls, slides, split molds (for external undercuts), and metal inserts are recommended only for the SMC and BMC varieties. 

Parts containing undercuts are not usually ejected directly after the mold is open. With flexible polymers, the part can be stripped off the core. For vinyls, polyruethanes, polyolefins, and elastomers, this requires an angle of 33° on molded threads, and the threads should be interrupted to give better flexing of the outer wall. Slide core molds are used for parts with internal and external undercuts such as ribs, gaps, openings. 

Figure 11-49. The mold for external undercuts to retain part in the required female cavity prior to ejection. When they are required to be retained on a male cavity undercuts are on the male plug. 

Figure 11-52. External undercuts with this split-mold design eliminate the undercut problem but develop a parting line.

Products with external undercuts (e.g., spools and bobbins, transportation boxes with handles, handled vessels, parts with external threads). 

External undercuts normally have to be demolded using lateral moving elements. Sliders, splits and core pullers can be used for this application. 

FIGURE 1.16 Mold for small parts with external undercuts... 

External and internal undercuts for snap-fit assembly parts should be also taken into consideration in design. External undercuts are demoulded using retracting segments of the mould while for internal undercuts the wall of the mould should be free to flex outwards during injection. 

Demoulding of parts with local undercuts (slide mould) During opening stroke thrust in direetion of demoulding, ejection with pins, sleeves or stripper plate after release of undercut Mechanical, cam pins, slide mechanism Flat parts with external undercuts (threads)... 

Demoulding of large, full-side undercut (split-cavity mould) During opening stroke thrust in direction of demoulding, ejection with pins Mechanical, hydrauhc, springs, links, pins, cams Parts with external undercuts (ribs) or opening in side wall... 

The weld was riddled with mildly undercut, gaping pits. Attack was confined to fused and heat-affected zones, with a pronounced lateral or circumferential propagation (as in Fig. 6.10). The resulting perforation at the external surface was quite small. Pits were filled with deposits, friable oxides, and other corrosion products. Black plugs embedded in material filling the gaping pit contained high concentrations of iron sulfide. Bulk deposits contained about 90% iron oxide. Carbonaceous material was not detected. 

Undercut Whether external or internal, undercuts should be avoided if possible to reduce mold cost (by about 25 to 30%) and simplify melt flow during molding. However many molds use external and/or internal undercuts. In cases where it is essential to incorporate them in a design, appropriate mold design is required. The mold will include action such as sliding components on tapered surfaces, split cavity cam actions to produce the needed undercut, etc. (Chapter 8, TOOLING)... 

There are two basic ways of joining the hard and soft components of a composite, chemical and mechanical [14], A purely mechanical joint is obtained by overmolding apertures or undercuts in the hard component. The use of internal or external adhesion promoters permits the production of composites with liquid-tight frictional joints [15], A bond formed by chemical cohesion has a number of advantages over a mechanical joint formed by interlocking [16] ... 

PS can be readily molded with metallic inserts, however the inserts should be pre-heated to 80 to 120° C (176 to 248° F) prior to being placed into the mold in order to avoid molded-in stresses occurring. All metallic inserts should be thoroughly degreased and have some form of undercut feature on their external surface (i.e. in the form of a knurl or groove). 

Step fitments. Step fitments are another way to reduce dependency on tight tolerances provided one of the parts is more flexible than the other. They can be used externally or internally depending on whether the more flexible part is the internal or external fitment. Both styles require more expensive tooling. The external variety shown in Fig. 8.25a calls for a split cavity and the internal version in Fig. 8.256 needs a collapsing core unless the undercuts are small enough for the part to be stripped off the core. 

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