Molded undercut

Polymerization with more than raie species of monomer, which can react with (Mie another, forming a copolymer. Either a male element in die that produces a hole or recess in a part, a part of a complex mold that molds undercut parts (cores are usually withdrawn to one side before the main sections of the mold open), a channel in a mold for circulation of a heat-transfer medium or the central part of a laminate. 

Fig. 9-12. Jiggler pins for molding undercuts. Courtesy Robinson Plastics Corp.)...

Because certain plastics, such as PET, must be cooled below their glass transition temperature (see Chapter 2) before they can be removed from the mold, undercuts of any significant depth will not strip from a mold. Moldings of PET can usually be... 

Undercuts removal from the mold will solve the sticking problem. In case of mold undercuts insufficient draft has to be... 

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). 

The geometry for snap joints should be chosen in such a manner that excessive increases in stress do not occur (Table 4-3). The arrangement of the undercut should be chosen in such a manner that deformations of the molded product from shrinkage, distortion, unilateral heating, and loading do not... 

Surfaces of plastics may be provided with designs that can give a good grip or that can simulate wood, leather, etc. These types of surfaces should be specified in a manner that will not create undercuts to the withdrawal action from a mold. The undercut effect can be responsible for stresses and marring. A similar condition applies to lettering, and the location of such lettering should conform to smooth withdrawal requirements. 

Many injection molded products will influence the final product s performance, dimensions, and other characteristics. The mold includes the cavity shape, gating, parting line, vents, undercuts, ribs, hinges, and so on (Table 3-17). The mold designer must take all these factors into account to eliminate problems. At times, to provide the best design... 

Control temperature of mold surfaces, to chill plastic to rigid state From side holes, slots, undercuts, threaded sections... 

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)... 

Bottle caps made from different plastics are extensively used. Some closures are of the simple cork snap type design, but most are of the screw type. Strong, accurate threads can be molded, which represent undercuts. Simple designs should be used when permitted, such as wide-pitch threads. The thread should be designed to start about 1 /32 in. (0.08 cm) from the end of the face perpendicular to the axis of the thread. It is usually practical to mold up to 32 threads per in. more than this number can give certain molders trouble. 

Undercuts and reentrant shapes are possible in many designs. They require movable or collapsible mold members, but with small undercuts they can often be sprung from a female mold while the formed product is still warm. This type of action works best when the plastic has some flexibility, as do the TPEs, or the material is very thin. Guidelines for the maximum amounts of undercutting that can be stripped from a mold are as follows 0.04 in. (0.1 cm) for acrylics, PCs and other rigid plastics 0.060 in. (0.15 cm) for PEs, ABSs, and PAs 0.100 in. (0.25 cm) for flexible plastics such as the PVCs. 

Fig. 8-13 Methods of molding holes or openings in sidewalls without undercutting mold movement.

The simplest molds consist of two parts one fixed and one moveable. We use such molds to fabricate items, such as disposable stadium cups, compact disks, and coat hangers, which have simple designs with no undercuts. As the complexity of the product increases, so must the complexity of the mold. The molds used to produce large items, such as dishwasher tubs and automobile bumpers, may consist of multiple moveable components and weigh several tons. Even apparently simple items, such as screw tops for bottles, require a complex ejection system that unscrews the product from part of the mold. In order to facilitate cooling, virtually all molds are designed with liquid cooling channels. 

Mold A manufactured cavity which preserves a negative impression of a specimen. The cavity can be filled with uncured polyurethane blends. Can be manufactured from a variety of materials depending on the production requirements, such as steel, aluminum, polyurethane, epoxy, FRP, silicone rubber, or latex. It can be manufactured in "one piece" or in multiple interlocking pieces. Multi-piece molds are used when the cast has a complex shape or undercuts which would make demolding from a one-piece mold difficult or impossible. 

Undercut optimum cure. A part of a mold, cast, or specimen which deviates from a sloping or vertical surface and turns back onto itself. 

Polyurethanes play an important role as molds for pavers used in gardening as well as molds for other concrete items with minimal undercuts. 

Where applicable an undercut insert in the mold can remain as an insert in the finished part. Such inserts can be held in place by the undercut. If possible, they should be made from the same material used in forming the part so there will be no difference in thermal expansion and contraction. 

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