Nominal Wall Thickness

The nominal wall thickness effects not only on the strength of plastic injectionparts, but also on the characteristics such as performance, surface aesthetics, appearance, moldability, and economics, so that the nominal wall is the ground floor for plastic injection part design. The best plastic injection part thickness is often a trade-off between strength versus parts weight, durability versus parts cost. For injection molding, it is important to keep the thickness of a part as constant, or nearly as constant. Constant wall thickness can assures that uniform mold shrinkage will prevent part warpage problem, and to obtain accuracy dimension of parts.

Fig 1 illustrates both poor and optimum object nominal wall thickness design. The first is to make a constant or nearly constant wall thickness. The second is that if there is a transition in wall thickness, the key consideration is to make wall transitions less drastic and abrupt. The corner should be design in a radius.

Fig 1 Wall thickness design

A rule of thumb is to avoid designing with nominal wall thickness above 4.0mm for most thermoplastics products. When the thickness is higher than 4.0mm that will cause excessively long cycle times for that the long cooling times is needed to remove heat from a thick wall, and will risks the increase of voids in a plastic injection part, affects the part performance negatively. If wall thickness higher than 4.0mm is required, it is best way to use other molding process technologies, such as structural foam or gas-assist injection molding.

Fig 2 illustrates the corner design of plastic injection parts. This modification is benefit to improve cooling of the part, and to reducing cycle times without sacrificing part structural integrity. Thick, heavy solid section such as are found in knobs and handles can be designed into two individual moldings and keeping the nominal wall as uniform as possible avoids voids in plastic injection parts. The special products such as handle usually are designed with texture in exterior for aesthetics.

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Fig 2 Corner of plastic parts