DSM Engineering Plastics

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Wall thickness

Specifying a parts nominal wall thickness is the first step in determining its manufacturability, performance and cost.

Wall thickness

Dimensioning

The recommended general wall thickness of injection molded parts depends on the size of the geometry, the material of choice and the desired performance of the geometry. Here, performance is used to sum up desired characteristics in the following fields:

  • Mechanics – Strength and stiffness.
  • Impact – Ability to absorb mechanical energy.
  • Insulation – Thermal and/or electrical.
  • Flammability – Ease with which a material ignites.

Typically, the wall thickness will be in the range 0.5 mm to 4 mm. In specific cases, wall thicknesses that are either smaller or bigger also occur. A basic design guideline is to keep wall thicknesses as thin and as uniform as possible. Where varying wall thicknesses are unavoidable for reasons of design, there should be a gradual transition as indicated in Figure 1.

In general, it’s relatively easy to remove metal from an existing metal mold cavity. Adding metal, on the other hand, can be difficult (expensive) or even impossible (requiring remaking the mold - expensive). Looking at this from the part’s wall thickness perspective, you can make it larger, but you can't make it smaller. So, in case of doubt, start thinner rather than thicker; this principle is called designing "steel safe" or "metal safe."

 

Influence of wall thickness

It’s important to carefully choose the nominal wall thickness. This is because, apart from structural performance, the wall thickness has an influence on the following:

  • Mold filling – If the wall thickness doesn’t fit the flow behavior of the thermoplastic material, it may be hard to completely fill the mold.
  • Part weight – Obviously, the greater the wall thickness, the heavier the part.
  • Cooling time – The greater the wall thickness, the longer it will take for the part to cool down after injection molding.
  • Part cost – both of the above, plus larger part volume and increased injection molding cycle time results in higher part cost.
  • Dimensional accuracy – Different areas of the part having different cooling rates, which is typically the case when the wall thickness is high or not uniform, leads to molded-in residual stresses that cause the part to warp after being ejected from the mold.
  • Part performance – Thick sections can cause voids to arise within the wall thickness.
  • Part aesthetics – If the (local) wall thickness is too high, non-uniform cooling rates may lead to sink marks (see Figure 3).

Material specific wall thickness

The recommended wall thickness depends also on the flow behavior of materials. The following material-related factors influence the flow behavior:

  • Viscosity at molding temperature.
  • Crystallization level and rate.
  • Presence of fiber filling and other additives.

To get a first impression of the flow behavior of a specific material, one can refer to spiral flow curves. These give a relative measure of the maximum achievable flow length for a given wall thickness and injection pressure. Spiral flow curves of widely used DSM materials are available in Plastics Finder

Figure 1 - Gradual transition of wall thicknesses.

 

 

Figure 2 - Voids due to large wall thicknesses.

Figure 3 - Sink marks due to large wall thicknesses.

Wall thickness and material viscosity

The flow behavior of  a molten plastic material is expressed in terms of its viscosity: a lower value for the viscosity means that the material flows better in the molten state. This is beneficial in case of molding parts with very thin wall sections. 

DSM’s Akulon® (PA6 & PA66) and Stanyl® (PA46)  product lines offer several improved flow grades. Higher flow translates into the following:

  • Easier mold cavity filling in case of thin sections.
  • Shorter cycle times.
  • Molding at lower temperature and/or using a lower-tonnage press.
  • Improved surface quality.

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