An Introduction of Attentions of Plastic Mold Parts


    Plastics Guide bush are much softer and less rigid than metals. The sectional shapes and sizes should be carefully chosen in this regard. If needed, a high section modulus can be achieved using principles similar to that used in I-beams and tubes. Curves can be used to stiffen large, flat surfaces. Venetian blinds, for example, have very thin slats that are gently curved despite their rigidity. We will use a garden tractor as an example of a simple design approach by comparing a sheet metal and a plastic hood. By reinforcing with fibers or particles, it is also possible to achieve lightweight and stiff designs.
    An individual process for shaping or molding a part is often determined by its overall shape and thickness. When a particular process is selected, the parts and dies should be designed so that they won’t present difficulties in generating the proper shapes, in controlling dimensions, or finishing the surfaces. Since the properties of thermoplastics are not as stiff as metals, dimensional tolerances (especially for thermoplastics) are not as tight as with metalworking. Injection molding, for instance, has much smaller dimension tolerances than thermoforming. It is crucial to control the flow of materials in the mold cavity when casting metals and alloys. When a polymer is processed by extrusion, thermoforming, or blow molding, the impact of molecular orientation should also be taken into consideration.
    The shape should not be changed in an abrupt way, or with large variations in area and section thickness. As an example, the top piece in FIG. 1 shows sink marks (pull-in). Thicker sections solidify last, thus the reason for 19.31c. Furthermore, plastic parts, as well as metal ones, tend to have porosity as a result of contraction in larger cross sections, which affects them negatively. On the other hand, if the part is too thin, it may be more difficult to remove it from the mold after shaping.
    Low elastic moduli of plastics further require proper shape selection for component stiffness, especially when a significant amount of material saving is essential. The considerations for designing polymer castings and forgings are similar to those that apply to the design of metal castings, as is the requirement for drafts (typically below one degree for polymers) for removing parts from molds. A general rule of thumb is that the recommended thickness for small parts is 1mm (0.04 in.) and the thickness for large parts is 3mm (0.12 in.).

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