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Wall Thickness
Did you know?
If there was only one rule for part design of an injection molded part it would be to maintain a uniform wall thickness. Nominal wall thickness is dependant on factors such as the material being molded, application (structural and electrical load requirements) and moldability (size of the part and the ability of the material to fill the furthest point from the gate).
Figure 1 shows some general guidelines for wall thickness in various classes of unfilled thermoplastics.
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Figure 1. Typical Nominal Thickness for Various Classes of Thermoplastics
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Wall thicknesses should be as thin as possible to shorten the molding cycle, obtain low part weight and optimize material usage.
When filled thermoplastics are used, generally an increase in wall thickness is required. To strengthen a part, instead of using a thicker wall, use additional structures such as ribs or gussets (see figure 2) or a stronger material. As filled plastics are stronger than unfilled, wall thickness can be reduced. All factors such as these are considered in the design stage for the part.
Uniform thickness in walls allows for an even flow of material into all sections. This improved flow provides for a consistent cooling and shrinking of the part. This results in reduced internal stresses as evidenced in a reduction in warpage. Thin wall sections cool faster and therefore shrink less.
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Figure 2. Solutions to Varying Wall Thickness and Thick Walls |
Did you know?
Where thicker sections are imminent, you can use a coring out approach on the wall to reduce localized masses of material (see figure 3). This term essentially means adding steel to the mold for the purpose of eliminating plastic in that area.
Reduced heavy sections will reduce part weight, improve filling, provide for a more uniform shrink and avoid sink marks.
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Figure 3. Coring Out a Thick Wall Section to Provide a Uniform Wall Thickness |
Did you know?
When wall thickness transitions cannot be avoided, the transition should be made gradually, on the order of a 3 to 1 blend (see figure 4).
The gradual blending of wall surfaces, allows for a more even filling and avoids stress concentrations and abrupt cooling differences.
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Figure 4. Transition Guideline if Coring is not an Option |
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