Design of the Molded Part

It is often possible and desirable to incorporate fastening mechanisms into the design of the molded part itself. The two most common methods of doing this are by interference fit (including press-fit or shrink- fit) and by snap-fit. Whether these methods can be used will depend heavily on the nature of the plastic material and the freedom one has in part design. 

At lower bound transverse limit at 0 = 90, this is uneconomic over design of the molded part. 

The degree of difficulty in demolding the parts is a function of the design of the mold and the correct functioning of the mold release. Any levers used must be free of burs and cracks. Compressed air is often used to assist in breaking the seal between the polyurethane and the mold. The air lines and gun must always be in good order. Accidental misuse of the compressed air is very dangerous. 

Antistatic additives are designed to be present on the surface of the molded part to achieve the full antistatic benefit. The types of additives used to enhance antistatic properties include quaternary ammonium salts, alkyl sulfonates or phosphate plus alkali metals, ethoxylated amines, or gylcerol esters. Antistatics are typically used at higher levels than other additives such as antioxidants. Therefore, antistatics are likely to increase light scattering, making it more difficult to achieve the higher chroma colors. 

There are a few possible ways to eliminate melt fracture, such as reducing the velocity of melt, increasing the critical velocity, and reducing heat losses (see Table 6.1). These remedies are limited in the way that they affect the design of the mold, fabricated part properties, and degradation of the polymer. 

A three-plate mold design (Figure 2.10) features a third, movable, plate which contains the cavities, thereby permitting center or offset gating into each cavity for multicavity operation. When the mold is opened, it provides two openings, one for ejection of the molded part and the other for removal of the runner and sprue. 

A slide bearing to be used in a water treatment plant is to be made from a molded UHMWPE compound containing powdered brass filler (25% by weight). The brass powder has been added to improve heat transfer created by the friction from a rotating shaft. Assuming the brass powder has a density of 8.59 g/cc, what would be the estimated density of the molded part What would be the maximum amount of brass powder that could be added if the design specifies the UHMWPE compound must be able to float on water ... 

Sink marks. Sink marks in a molded part often occur in relatively thick sections, usually reflecting progressive hardening of the molded part from the cavity wall to the inside area. The outside wall hardens while the mass of plastic in the thick section is still somewhat fluid. As this inside mass subsequently hardens (and shrinks, as most plastics will), the cured outer wall is distorted inwards, resulting in a sink mark. The best way to avoid such deformation is to avoid thick sections wherever possible. Often one or more judiciously designed thin ribs in select locations will give a part adequate strength and thickness without the need for thick sections. 

To minimize such ejection stresses, forces for deep molded parts are designed with an appropriate draft or taper, up to 5° in some cases, such that very slight movement of the molded part with respect to the force will suddenly free the part from its strong grip on the force, and the remainder of the ejection stroke exerts almost no stress on the part. Such draft is advisable on all plastic parts, even those with depths of only 6 mm, to minimize ejection pressures and to prevent possible localized damage where the knock-out pins push against the not-yet fully hardened plastic. 

The design of the mold should take into consideration placement of the gates to prevent wrinkled or washed overlays. The overlay may be held in place in the mold by cutting it so that it fits snugly or by electrostatic means. Blow-molded parts can also be printed or decorated by in-mold processes. The ink or paint image is placed on a carrier film or paper and placed in the mold. As the hot plastics expands, filling the... 

Particularly in automotive applications, the design engineer selects materials that will endure periodic fluctuations of both tensile and flexural type loads for the service lifetime of the molded part. Under-the-hood automotive conditions include extremes in ambient temperature combined with possible localized vibrations from the engine and rotation of molded parts (cooling fans) under continuous load. Although deformation will occur due to creep, there is concern regarding fatigue type failure after repeated fluctuations of the applied load. 

The design engineer of plastic component parts needs more than just shortterm stress-strain data for anticipating long-term deformation behavior. For example, the useful service lifetime of the molded part is curtailed by onset of... 

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. 

The other families of users who need to he accommodated when one is developing a polymer (or blend) are the design engineers and end users, the people interested in the performance of the molded part or devices made from it. Their concerns can include the following. 

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