31
Jul

Injection Molding Wall Thickness

A wide variety of parts are manufactured using injection molding, from car dashboards and body panels to small components like AAA battery boxes. There are important factors to consider when designing parts and molding processes – one of these factors is injection molding wall thickness. The wall thickness of parts that are manufactured by injection molding must be thick enough to ensure proper strength for the application of the part. The reasoning to make it as thin as possible is to save weight and material cost.

The thickness also plays a part when it comes to the manufacturing process itself, to avoid defects that may be caused due to the wall thickness being too thick or too thin. When the wall thickness is too thin for parts with large and complicated structures it is difficult to fill in cavities due to the flow resistance of melt resin is too large. For parts where the wall thickness is too big, defects like bubbles inside the part and shrinkage marks on the surface is caused to the molded parts. This also results in an extended cooling time and a waste of material and cost.

Minimum wall thickness

The one question of how thick the wall of a product must be leads to more questions that needs to be answered first. Here are a few suggested questions that needs to be answered when determining the wall thickness of a molded part:

  1. Is the wall used for structure?
  2. Will it have sufficient strength and stiffness?
  3. Will the part be fragile in its application if it is too thin?
  4. Will the material choice affect the thinness of the wall?

The requirements for the minimum wall thickness of the part can then be determined when these questions are answered. With this being a common problem that people will face when designing injection molded parts, standards for the wall thickness of parts has already been determined when using certain types of materials.

Table 1: Recommended wall thickness of different materials

Uniform walls

Uniform thickness of parts walls is beneficial as this allows the molten plastic to flow unrestricted and fill the cavity more easily.

When the walls of a part is not uniform the thick section cools after the thin section and causes it to shrink. This builds stresses at the boundary area between the two sections. The thin section that cools first has hardened and it doesn’t yield. The thick section can however yield, this leads to warping or twisting.  If the warping and twisting is severe enough it can cause cracks.

If the part has design limitations and it is impossible to have uniform wall thickness, then the thickness should change as gradually as possible. There exists methods to eliminate the problem of not uniform wall thickness:

  1. The wall sections can be kept uniform by removing plastic from the thick area. This method is known as coring.
  2. The method of using support structures like gussets can be designed into the part to reduce the possibility of warping.

How to decide the injection molding wall thickness

To determine the wall thickness of injection molded parts is an important step in the design process of these parts. The thickness of the part’s walls are dependent on the application requirements of the part. These requirements include some of the following: assembling, the basic cost, the sizes stability, strength etc. The basic principles for part’s wall thickness is the following:

  1. The wall thickness needs to be smooth, not too thin or too thick. The wall thickness cannot change suddenly. If a different thickness is required, the transitions between the levels should be smooth. When the transition is not smooth it can cause shrinkage and, as previously stated, it affects the fluidity and strength of the part negatively.
  2. The recommended thickness of different materials are stated in Table 1.
  3. Most commonly the thickness is in the range of 2~3 mm, 1~5 mm is considered a reasonable range.
  4. Added support structures shouldn’t be too thick. It is recommended that the thickness of support structures be around 50% of the body parts.
  5. Do not design simple structures that can deform easily, like a single flat structure.
The transition diagram