Undercuts, part design

Other than poor thermal conductivity, fluoropolymers have much higher coefficients of linear thermal expansion than metals. This means that any type of heat buildup will cause significant expansion of the part, resulting in overcuts or undercuts, thus deviating from the desired part design. 

The draft specification controls the way the mold is built as the direction of draft normally indicates the direction of draw (removal of the part from the tool) since reverse draft would be an undercut condition. For injection molding, the mold designer will attempt to locate the core on the back, or movable, half of the mold. That is the side where the molding machine s ejector bars are located, and locating the core on that half makes the ejector mechanism available to push the part off the core. Part designs with cores from both sides of the part may require slight undercuts to keep them on the side of the mold which has the ejector mechanism. The draft specification will also control the location of the parting line, which should be indicated. 

In mold design, the core is (1) a channel in a mold for circulation of heat transfer media or (2) a part of a complex mold that molds undercut parts. In the latter case the cores are usually withdrawn to one side before the main sections of the mold open (also called a core pin). Cores are usually withdrawn before the main sections of the mold are opened. A core drill is a device for making cooling channels in the mold. 

Undercuts, whether external or internal, should be avoided if possible. In cases where it is essential to incorporate them in part design, a great many can be realized by appropriate mold design in which either sliding components on tapered surfaces or split cavity cam actions will produce the needed undercut. This obviously means increased tool cost, in the neighborhood of some 15 to 30 percent. 

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... 

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. 

Successful removal of the part from the mold is a major factor in mold design. Many packaging components, such as closures, contain undercuts, portions of the molded object that protrude into the side wall of the mold (Fig. 11.5). When these are present, the object being molded cannot simply slide out of the mold, since it is restrained by the protruding area. Therefore, some mechanism for releasing the object must be provided. Sometimes the molded object is flexible enough, and the undercut small enough, that it can be blown or popped loose from the mold. If this... 

A part which contains a recess or undercut requires slides or moveable cores in the die, otherwise the casting cannot be ejected. These slides and moveable cores greatly increase die costs and slow down the rate of production. As a general rule, the design of a part should avoid undercut sections. 

Undercut - k9t (1859) n. A lateral indentation in a molded part (or protuberance in a mold) that tends to impede withdrawal of a molded part from the mold. Articles of flexible materials such as plasticized vinyls can often be removed without difficulty from molds with severe undercuts, but undercuts must either be avoided in rigid materials or, where they must be part of the design, the mold must have movable parts that withdraw side draws) before the part is to be ejected. Slight undercuts are sometimes deliberately designed into one half of a mold to cause a part to remain in that half when the mold opens. 

One common way to accomplish such positive part control is to provide undercuts in the cavity or force of the moving half. These undercuts will enable plastic to flow into them and harden there before the mold is opened. Upon opening, the hardened plastic in the correctly designed and sized undercuts will hold the molded part in the moving... 

Injection molds are often classified by the basic design of the mold. A standard two-plate mold opens in one direction, and the part is demolded by gravity, ejector pins, or ejector sleeves. The sprue, runner, and parts are connected after ejection with the sprue and runners forming part scrap. Two-plate molds are used for all kinds of moldings that do not contain undercuts and provide the best overall part properties. 

Care must be used when designing a mold so as not to create undercuts that would make removal of the part impossible without damage to the part or mold or both. Molds can be made with movable features to assist with removal. 

Local stress concentrations are often caused by notches (see chapter 4). Notches may be part of the design (e. g., at bearings or at undercuts), may be caused during manufacture (e.g., tool marks caused by metal cutting), or may be due to imperfections in the material (e.g., casting pores, brittle precipitates). 

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