Why Is Uniform Wall Thickness Important?
Uniform wall thickness is a critical factor in injection molding techniques. Without it, your product may have a variety of critical and non-critical design failures.
What Is Wall Thickness?
Wall thickness is how thick each plane of your molded item is. Different materials have different average thicknesses, but a typical range is 2 mm to 4 mm (0.080” to 0.160”). Each wall should be at least 40%-60% of the thickness of all adjacent walls. It’s best for each wall to remain the same thickness throughout; any change in thickness should be gradual.
Why Uniform Wall Thickness Is Important
A variety of problems can arise from having walls that aren’t a uniform thickness, including:
- Warping
- Sink
- Twisting
- Short shot
- Cracks
- Cosmetic imperfections
- About Warpage
Warpage occurs when the part does not shrink in a uniform manner. Differential shrinkage often results from drastic changes in part thickness, as well as pressure differences within the part during injection and packing. ‘Hold’ is used to keep the polymer in the mold cavity until the gate freezes. This is often referred to as the third stage of injection. The hold pressure is typically less than both the injection and packing pressures. The hold time must be long enough to allow the gate to freeze off. Longer hold times force additional material into the runner, thus wasting both material and electricity.
An injection mold must perform four major functions; transfer the polymer from the nozzle to the part cavity, form the part, cool the part, and eject it from the mold. When designing multiple cavity molds, each cavity must be treated equally. This is achieved by using the same flow length, cooling line layout, gate position, and wall thickness. The mold design must allow the cavities to be filled easily. For this to happen, a naturally balanced runner system must be used.
The cooling line layout should provide even cooling to each mold cavity. Baffles and bubblers can be used to transfer the heat from the hard to cool areas. Additional cooling lines are required in thick sections to even the part’s overall cooling rate. To ensure that the part reaches the desired tolerances, be sure the mold provides adequate draft without any sharp thickness transitions. The correct material shrinkage values should also be verified by using either a prototype mold or mold filling software.
About Shrinkage
As the polymer is heated during plastication, the polymer expands, and the density decreases. After the polymer is injected and cools, the density of the polymer increases, which causes Shrinkage.
Shrinkage occurs in virtually all standard injection molding processes. Many designers expect Shrinkage to occur evenly in all directions. Unfortunately, Shrinkage is greatly effected by variations in the Material, Orientation, Cooling, and Part Thickness. Therefore, the effects of Shrinkage can be difficult to predict.
Differential Shrinkage causes stresses in the part, and the part warps to help alleviate the stress. Therefore, Warpage is a result of non-uniform stresses in the part.
Another factor to consider is that thicker cross sections have larger Shrinkage factors. The below graph depicts the Shrinkage range for a Semi-crystalline polymer with varying part thickness.
As the polymer is injected into the mold cavity, the material on the surface cools quickly and shrinks little. The material in the center takes longer to cool. The increase in thickness increases the cooling time and Shrinkage.
Tips for Uniform Wall Thickness
Below you can find our top tips about uniform wall thickness to keep in mind as you design your product:
- With most materials, a 10% increase in wall thickness results in a 33% increase in stiffness
- Avoid long unsupported part spans and sharp internal corners
- Use ribs to provide stiffness without thickness
- Remove weight by coring out thick areas
- Keep draft consistent
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