Internal undercut

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)... 

Internal undercut n. Any restriction that prevents a molded part from being directly removed from its core (force). 

Products with internal undercuts (e.g., threaded closures, housings with a retracted edge, parts with snap-in noses. 

FIGURE 1.18 Mold for transport box with external and internal undercuts which are demolded using splits [1]... 

Internal undercuts can be found when the inside of the molded part has projections which will hinder the part to get removed from the mold. Internal splits (Figure 1.19), internal sliders which are externally operated and inclined segments (which are used in collapsing cores. Figure 1.19 on the right) can be used. 

Molded parts with very complex internal contours, for example with multiple internal undercuts, can t be produced using the conventional injection moiding because the needed compiex cores can t be puiied out. If the cores are made from a iow-meiting metai aiioy - preferably by a die- casting process - and are inserted into the moid, they can be melted out of the finished moided part e.g. using electric induction heating or an oil bath. 

Threaded holes represent internal undercuts that are usually released by the means of an unscrewing mechanism. In general, a rotation movement as well as a corresponding axial movement has to be reached. The axial movement can be executed by the threaded core or by the molded part itself. 

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. 

A major advantage of the electroless nickel process is that deposition takes place at an almost uniform rate over surfaces of complex shape. Thus, electroless nickel can readily be applied to internal plating of tubes, valves, containers and other parts having deeply undercut surfaces where nickel coating by electrodeposition would be very difficult and costly. The resistance to corrosion of the coatings and their special mechanical properties also offer advantages in many instances where electrodeposited nickel could be applied without difficulty. 

Interconnecting pits were observed inside the pipe in the ruptured area. The pits showed striations and undercutting features that are associated with microbial corrosion. A pit profile showed the chloride in the pits increased steadily from top to bottom. Sulfate-reducing, acid producing, general aerobic, and anaerobic bacteria were present in the deposits taken from two pit areas in the piece of line where internal corrosion was noted after the accident 630 m downstream of the rupture site. 

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] ... 

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. 

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. 

The feed section of the rubber extruders has to be designed specifically to the feed stock characteristics of the material. The extruder may be fed with either strips, chunks, or pellets. If the extruder is fed from an internal mixer (e.g., Banbury, Shaw, etc.), a power-operated ram can be used to force the rubber compound into the extruder. The feed opening can be undercut to improve the intake capability of the extruder. This can be useful, because the rubber feed stock at times comes in relatively large particles of irregular shape. When the material is supplied in the form of... 

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