By Design: Part design 102

Selecting the optimum wall thickness for a new part is a critical decision that affects cost, function, and processing. Once a wall thickness is determined, the next important challenge for the designer is to maintain the uniformity of that chosen wall thickness. The ideal wall thickness is a compromise between function and manufacturing requirements. Both are important, but function takes priority over ease of manufacturing. If the product does not function as required, it will not sell, and there will be nothing to manufacture.

The complex parts now being designed for injection molding may require variations in the wall thickness. This may be desirable from a functional perspective, but it is not good from a molding point of view.

Injection molding is a melt flow process. As the plastic, or melt, flows through the cavity for a part with a consistent wall thickness, the flow of the melt will be uniform in all directions. This would be an ideal situation. If the cavity contains some sections that are thicker than others, the melt will flow through the thicker sections before flowing into the thinner sections. In some cases, the melt will surround a thin section and create gas traps.

In other instances, the melt will start flowing into a thin section and then stop. The melt continues to flow through the thicker sections, which have less resistance to flow. After the thick sections are filled, the injection pressure increases to the point where the melt in the thin sections is forced to start flowing again. The problem this nonuniform cavity filling creates is the melt that stopped flowing has an opportunity to cool. That cooler melt can cause unsightly hesitation marks and flow lines.

Thin sections in a cavity require increased injection or packing pressures. These higher pressures tend to overpack the thick sections with a minimum of packing in the thin sections. All other things being equal, the higher the pressure on the melt, the higher the level of molded-in stress in the molded part. Molded-in stress is one of the primary causes of post-mold warpage.

Factoring Mold Shrinkage
During the heating portion of the injection molding process, the plastic material expands. During the cooling part of the cycle, the plastic material contracts or shrinks. The amount the plastic shrinks is referred to as the material's mold shrinkage factor. For a given plastic material, the mold shrinkage factor will vary depending on part thickness and how that material is molded. The higher the temperature the material is heated to, the more it will shrink as it cools down.

The higher the packing pressure on the melt, the lower the mold shrinkage factor will be. The theory is high packing pressures force more molecules into a given space and that reduces the mold shrinkage factor.

With all other variables equal, thick-walled parts will be highly packed and the shrinkage factor will be low. A thinner part would not be packed as much, and its shrinkage factor will increase. If a part contains both thick and thin sections, the degree of packing and the mold shrinkage factor will vary from one location on the part to another. This is a recipe for molded-in stress and part warpage.

If the melt flows into a relatively hot cavity, it will be slow to cool, and that material will shrink the maximum amount. If the cavity were colder, the material would cool faster and the mold shrinkage factor would be lower.

The mold shrinkage factor of a given plastic material is affected by the combination of the melt temperature, the packing pressure, and the temperature of the mold selected by the molder. Whenever a molder changes one of these molding machine settings, there will be a corresponding change in the mold shrinkage factor of the part being molded.

Each of the different families of plastic materials has its own unique mold shrinkage factor. In general, the amorphous plastics, such as polystyrene, ABS, acrylic, polycarbonate, and polysulfone, have relatively low mold shrinkage factors. The semi-crystalline plastics, such as high-density polyethylene, nylon, polypropylene, acetal, and the polyesters, have higher mold shrinkage factors. This increase in shrinkage is due, in part, to the fact that the molecules coalesce as the crystals are formed.

These semi-crystalline materials have the additional problem of different shrinkage factors parallel to and perpendicular to the direction of melt flow. This problem is further compounded by variations in the degree of crystallinity caused by changes in molding conditions. If the material is cooled slowly, there will be an increase in crystallinity and mold shrinkage. Rapid cooling will result in less crystallinity and less shrinkage.

Generally speaking, it is easier to maintain precision dimensions when molding plastic materials with low mold shrinkage factors. However, high mold shrinkage factor materials like nylon are specified for gears and bearings, which are precision components. Tight tolerances can be held while molding high mold shrinkage factor materials, but it is easier to maintain dimensional reproducibility with low mold shrinkage factor amorphous materials.

The Nominal Wall
A design-related consideration that also affects the mold shrinkage factor of a plastic material is the nominal wall thickness of a molded part. The published mold shrinkage data is based on a molded part with a .125-inch wall thickness. The mold shrinkage factor of that material will be different for a thicker or thinner wall. For example, the mold shrinkage factor for a common material, nylon 6/6, is .015 inch/inch for a .125-inch wall thickness. That same material will shrink only .010 inch/inch if molded into a .031-inch-thick part. A .250-inch-thick nylon 6/6 part would shrink .022 inch/inch.

The explanation of why the same plastic material shrinks more in a thick wall than in a thin wall is that the thick-walled part takes longer to cool. The longer the melt remains in the liquid state, the greater the time the molecules are free to move. This allows the molecules to undo the molecular orientation created by the melt flow process. This, in turn, allows the material to shrink more.

It is not uncommon for some portions of a mold, such as a hard-to-cool core pin, to be hotter than the rest of the mold. When this happens, the plastic in the hot portions of the mold will shrink more than the material in the cooler parts of the mold. An ideal situation would be when all surfaces of a core and cavity are maintained at the same temperature.

If an engineer designs a thin-walled nylon part, the mold shrinkage will be low, and that part could be dimensionally stable. Designing the same part with a thicker wall would result in a higher mold shrinkage factor. It would be more difficult to maintain precision dimensions on the thicker part.

If an engineer designs a nylon part that is thick in some areas and thin in others, the thick and thin areas will shrink different amounts. These variations in shrinkage make it difficult to maintain uniform dimensions. For example, consider a 10-inch-long, 2-inch-wide nylon part designed with a .125-inch thickness for half of its width and a .250-inch thickness for the other half. The thin half of the part would shrink 10 inches by .015 inch/inch for a total of .150 inch. At the same time, the thicker half would shrink 10 inches by .022 inch/inch or a total of .220 inch. This would represent a difference in shrinkage of .070 inch. If this part were 20 inches long, the difference in shrinkage would be .140 inch.

The resulting differences in the length of the thick and thin sections can be allowed for by lengthening the cavity that forms the thicker half of the part. This would solve the dimensional problem, but there will still be a difference in mold shrinkage, and that results in another problem.

During the cooling portion of the molding cycle, the .125-inch-thick section cools quickly, and its mold shrinkage is minimized. The .250-inch-thick section takes three to four times longer to cool, and this allows the maximum amount of mold shrinkage. In this scenario, the thin section will have cooled, stopped its in-mold shrinkage, and turned from a liquid to a solid. While this is happening, the thicker section will still be cooling and shrinking. This creates a very high level of molded-in stress at the junction between the thick and thin sections. There is a high probability a part of this shape will warp. The molded-in stress will also result in a decline in the molded part's impact strength and heat deflection temperature.

If this same 2-inch-by-10-inch part were molded in a low mold shrinkage factor plastic, such as poly-styrene, the difference in shrinkage and the resulting molded-in stress would be reduced.

Golden Rules
While designing injection molded parts in high mold shrinkage factor materials, the designer should try to limit wall thickness variations to 10 percent. A 15 percent variation in thickness would be enough to cause processing and quality problems.

The allowable wall thickness variation for the low mold shrinkage factor materials can be up to 25 percent. Less of a change in thickness would be desirable, but in most cases, the lower mold shrinkage rate will tolerate this much of a variation.

Part design guidelines, such as the allowable wall thickness variations, have evolved over the years by trial and error. These are the golden rules of the road that leads to success. There are exceptions and situations where success depends on molding an acceptable quality part that does not have a uniform wall thickness.

Faced with the need to produce a part with a nonuniform thickness, a designer should try to core out the thick section. If this cannot be done, the change in thickness should be made in several steps. It is also helpful if each of those smaller steps is blended with chamfers and radiuses. A gradual changing from one wall thickness to another spreads the difference in mold shrinkage over a broader area on the part and reduces the stress that results from an abrupt change in thickness.

For parts with wide variations in wall thickness, the designer must provide some location for the gate on the thickest wall of the part. The injection molding process is at its best if the melt enters the cavity in the thickest area and flows toward the thinner areas. If this is not done, it will be virtually impossible for the molder to uniformly pack out the molded part.

Injection molded parts with wide variations in wall thickness will have the best chance of success if they use low-shrinkage materials.

The allowable wall thickness variations cited earlier represent what is practical for the injection molding process. There are commercially acceptable injection molded parts produced with wider variations in wall thickness. These designs are in the possible range, which is beyond the practical range. These parts are the troublesome parts with a narrow processing window and are always higher in cost and lower in quality than the same part designed within the practical range. A good designer is one who is clever enough to be able to achieve the desired result while staying well within the practical range.