IX Международная студенческая научная конференция Студенческий научный форум - 2017

Physical dimensions

Physical dimensions of many processed parts must be held to fairly close tolerances to ensure proper assembly of parts into a complete structure, as, for example, molded fender panels bolted to steel chassis cars, plastic screw caps for glass jars, etc. In general, the final dimensions of the processed part will differ from the dimensions of the mold cavity or the pultrusion die. Such differences are somewhat predictable, but are usually unique to the specific material and to the specific process. The dimensions of a mold cavity for a phenolic part requiring close tolerances will often be different from dimensions of a cavity for an identical polyester part. Both the part designer and the mold or die designer must have a full understanding of the factors affecting final dimensions of the finished product, and often need to make compromises in tolerances of both part and cavity dimensions (or even in plastic material selection) in order to achieve satisfactory results with the finished product.

The following subsections will address the behavior characteristics of plastics that affect dimensional tolerances.

Mold shrinkage

Shrinkage of a plastic as it polymerizes is a fact of life, and is often specified as “parts per thousand.” Such “mold shrinkage” is reasonably easy to compensate for by making the cavity proportionately larger in all dimensions as compared to the desired part dimensions.

But shrinkage in many materials is different when measured transverse to the material flow as when measured longitudinal to the flow. In reinforced or heavily filled materials, this difference is significant. Gate location and size, and multiple gates in some instances, must be considered for cavity and part design to minimize effects of such mold shrinkage.

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.

Nonuniform hardening of material

Nonuniform hardening of the material during residence in the mold generally produces internal stresses in a molded part which, after removal from the mold, may distort the part widely from the intended dimensions. Flat panels become concave, straight parts may curve, round holes may elongate, etc. Part and cavity design can generally accept some necessary compromises to accommodate such deformations, yet still yield a part meeting its functional requirements.

Deep molded parts

Deep molded parts may require design considerations to ensure minimum stresses during the ejection phase of the molding process. Imagine a straight-walled plastic drinking “glass” of internal and external diameters unchanging from top to bottom. As the part hardens in the cavity, it will tend to shrink around the force, or male part, of the cavity. When the mold opens, the part will stay with the force. To remove it from the force will require considerable pressure, either from ejector pins or air pressure coming out the end of the force against the bottom of the glass or from an “ejection ring” moving longitudinally from the inner end of the force. The pressure exerted by either of these ejection methods will be considerable until the open end of the “glass” finally slips off the end of the force.

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.

Parting lines

Parting lines on molded parts require special consideration in part and mold design, especially where two molded parts must come together as, for example, on each half of a molded box with a hinged opening.

In compression molding or injection-compression of thermoset parts, the mold is fully closed only after the material has been placed into the cavity. More often than not, some material is forced out of the cavity onto the land area before the mold is fully closed, metal-to-metal. In effect, then, the mold closing is halted short of full close, perhaps by as much as 0.1 mm or more. Such overflow hardens, leaving “flash” on the molded part. Under these circumstances, the molded part dimension perpendicular to the mold parting surface will be 0.1 mm, or more, greater than intended. If the cavity and land area is so designed that such flash is perpendicular to the parting surfaces of the mold, the correct part dimension perpendicular to the mold parting surfaces may be achieved by removing such flash during a secondary operation of tumbling, blasting, or machining after the part has reached room temperature. More often, the tolerance of such compression molded parts is kept very wide in deference to the inherent characteristics of the process.

When an assembly of two molded parts is ultimately required, even if the materials and the molding processes are the same, it will be virtually impossible to achieve a perfect match where the parts come together. Slight variations in shrinkage or warpage will yield an easily noticeable or “feelable” mismatch. Intentionally designing mating surfaces with an overlap or a ridge enables ingenious camouflaging of the nonuniformity of mating areas in the final assembly of the two contacting parts.

Ejection of molded part

In designing molded parts, and the molds used to produce them, it is necessary to consciously determine how the part will be removed from the mold cavity or force and to maintain positive control of the part during mold opening, such that it is ejected as intended. This positive control is especially vital in automatic molding.

Assuming that the decision has been made that the molded part must be ejected from the moving half of the mold (as opposed to the fixed half), then it is necessary to make provisions such that the part will not remain in the fixed half of the mold during mold opening, but will invariably remain with the moving half.

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 mold half during the opening stroke. After mold opening, the ejector pins or mechanism in the moving half of the mold will then have to push hard enough to allow the molded part to distend sufficiently to be pushed off the undercuts and away from the moving half of the mold.

References

1. Charles A. Harper Modern plastics handbook

2. GOST 11645-73

3. Billmeyer, F. W., Jr., Textbook of Polymer Science, 2d ed., John Wiley and Sons, Inc., New York, 1962, p. 439.

4. Strong, A. B., Plastics: Materials and Processing, Prentice-Hall, Englewood Cliffs, N.J., 1996, p. 193.

5. MacKnight, W. J. and R. D. Lundberg, “Research and ionomeric systems,” in Thermoplastic Elastomers, 2d ed., G. Holden et. al., eds., Hanser Publishers, New York, 1996, p. 279.

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