August 2007 Design Tip

Resist the Twist (and that Sinking Feeling)

With a few notable exceptions (H2O for one) most materials shrink as they cool and solidify. This is true, to a greater or lesser extent, of virtually all plastic resins. Uniform, predictable shrinkage would be easy to account for in designing a part; you could simply design the part slightly larger than its desired size and the part would shrink to a perfect fit. In reality, however, shrinkage is rarely that simple.

Some resins shrink most in the direction of resin flow within the mould, while others shrink least in that direction. And, depending on the shape of the part, shrinkage of later-cooling areas of the part can pull against areas that have already solidified, causing sink or warping. To the extent that these problems can be anticipated, they can be minimized.

There are four primary factors that contribute to sink and warping:

  • Shrink characteristics of the resin — Resins differ in both tendency to shrink and shrinkage relative to direction of resin flow. For example, an alloy of polycarbonate and ABS is very resistant to shrinkage, while glass filled nylon not only shrinks, but shrinks less in the direction of resin flow within the mould than perpendicular to resin flow.
  • Shape of the part — Thick areas are particularly prone to sink when the surfaces closest to the mould solidify and are then pulled inward as the underlying resin cools and shrinks. Potential problem areas can be obvious, e.g., a thick wall. Or they can be subtle, e.g., a boss nestled in an inside corner.
  • Sudden transitions from thick areas to thin — These can result in stress and warping at the point of transition.
  • Poor placement of gates — Pattern of resin flow can lead to warping of the finished part.

Fortunately there are ways to address all of these issues and eliminate or reduce distortion of the finished part.

1. Know the shrink characteristics of your chosen resin
These can be found at www.ides.com. As a very general rule, shrink under 0.010 inches/inch (or mm/mm) is more forgiving, higher shrink demands a well designed part. If the material is too shrink-prone for the application, consider another material.

2. Put thick parts on a diet
Unnecessarily thick parts can sometimes be "put on a diet" to prevent sink. If the function of the part requires the larger shape, coring out the thick section can produce a hollow shape with thin walls, which will serve the same function (see figure 1).

Fig. 1

Fig. 1

3. Redesign and relocate
If placement of a feature, such as a boss, results in excess localized thickness, consider redesigning or relocating the feature (see figure 2).

Fig. 2

Fig. 2

4. Ramp transitions and unsupported geometry
Transitions from thick to thin can be ramped to reduce stress and eliminate warp. Whenever possible, gussets or some other 3D structure bracing such corners can prevent warping (see figure 3).

Fig. 3

Fig. 3

5. Use Radiused Corners
Un-radiused inside corners can overheat and stress the resin flow, causing distortion of the angles between walls. A radius in the corner is always good practice (see figure 4).

Fig. 3

Fig. 4

6. Place gates strategically
Gate placement can help control warp when using resins characterized by differential shrink in the direction of resin flow. The disk shown here resulted from a gate in the center of the disk. Placing gates at the edge of the disk reduced the warp (see figure 5).

Fig. 5

Fig. 5