The nominal wall thickness effects not only on the strength of workpiece, but also on the
haracteristics 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 betwen strength versus workpiece weight, durability versus work piece 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
Fig 2.2 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
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 moldin
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,#2.5% variation of wall thickness is allowable. If shrinkage is greater than 0.010 mm/mm, a thickness variation of#1% is acceptable
The other problem encountered in nominal wall design is a workpiece designed with thin areas
surrounded by thick perimeter sections(Fig. 2. 3), 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
Fig.2. 4 illustrates the comer design of plastic workpiece. This modification is benefit to improve cooling of the workpiece, and to reducing cycle times without sacrificing workpiece structural integrity. Thick, heavy solid section such as are found in knobs and handles can be designed into two individual moldings(Fig. 2.5)and keeping the nominal wall as uniform as possible avoids voids in workpiece. The special products such as handle usually are designed with texture in exterior for aesthetics. Tab. 2. 2 recommends thickness of products.
