Nominal Wall Thickness

The nominal wall thickness effects not only on the strength of workpiece, but also on the characteristics such as performance, surface aesthetics, appearance, moldability, and economics, so

that the nominal wall is the ground floor for workpiece design.The best workpiece thickness is often a trade-off between strength versus workpiece weight, durability versus workpiece cost. For

injection molding, it is important to keep the thickness of a workpiece as constant, or nearly as constant. Constant wall thickness can assures that uniform mold shrinkage will prevent workpiece warpage problem, and to obtain accuracy dimension of workpiece.

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 comer should be design in a radius.

A rule of thumb is to avoid designing with nominal wall thicknesses above 4.0 mm for most thermoplastics products. When the thicknesses is higher than 4.0 mm 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 workpiece, affects the workpiece performance negatively. If wall

thickness higher than 4.0 mm is required, it is best way to use other molding process technologies,such as structural foam or gas-assist injection molding.

Some variation in nominal wall thickness is allowed in designing workpiece, but the variation is connected with the shrinkage of plastic materials. As a general rule, for thermoplastic materials, if

the shrinkage is less than 0.010 mm/mm,+25% variation of wall thickness is allowable. If shrinkage is greater than 0.010 mm/mm, a thickness variation of +15% is acceptable.

The other problem encountered in nominal wall design is a workpiece designed with thin areas surrounded by thick perimeter sections， that creates race tracking. In this case, the molten material flows rapidly around the thick section of the workpiece, which means the thick wall, the material flows faster than the material in the thinner section that will create a pocket in the form of a gas trap. The defects such as short shots or incompletely filled parts will occur. The thick sections can also be cored out to achieve uniform walls.