China mold maker and supplier

Part thickness
Part thickness drives quality and cost
Many factors need to be taken into account when designing a part. These include
functional and dimensional requirements, tolerance and assembly, artistic and esthetic
appearance, manufacturing costs, environmental impacts, and post-service handling.
Here we discuss the manufacturability of thermoplastic injection-molded parts
considering the influence of part thickness on cycle time, shrinkage and warpage, and
surface quality.
Cycle time increases with thickness
Injection-molded plastic parts have to be cooled sufficiently before being ejected from the mold to
avoid deformation due to ejection. Parts with thick wall sections take longer to cool and require
additional packing.
Theoretically, Cooling time is proportional to the square of the heaviest part wall thickness or the
power of 1.6 for circular features. Therefore, thick sections will prolong the press cycle, reducing
the number of parts per unit time and increasing the cost per part. 

Thick parts tend to warp
Shrinkage is inherent in the injection molding process. Excessive and non-uniform shrinkage,
however, both globally and through the cross section of the part, will cause the part to warp.
Warpage is a distortion where the surfaces of the molded part do not follow the intended shape of
the design. The diagrams below illustrate how part thickness affects shrinkage and warpage. 

                                      FIGURE 1. A thick part (left) can lead to excessive shrinkage and warpage (center). A recommended
alternative design with uniform thickness (right).   

Thin, uniform parts improve surface quality
A combination of thin and heavy part cross-sections can easily produce a “race tracking” effect,
which occurs because melt preferentially flows faster along thick sections. Race-tracking leads to
Air traps and weld lines (see Weld lines and meld lines) that produce defects on part surfaces. In
addition, Sink marks and voids will also arise in thick sections without sufficient packing.
Reducing part thickness
To shorten the cycle time, improve dimensional stability, and eliminate surface defects, a good rule
of thumb for part thickness design is to keep part thickness as thin and uniform as possible. The use
of ribs is an effective way of achieving rigidity and strength while avoiding heavy cross-sectional
thickness.
Part dimensions should take into account the material properties of the plastics used in relation to
the type of loading and operating conditions the part will be subjected to; the assembly requirements
should also be considered.The diagrams shown below provides several examples of designs we
recommend vs. designs you should avoid.    

FIGURE 2. Designs with thick sections (left) and alternative designs that use ribs to achieve uniform
thickness (right).