DESIGN FOR MOULDABILITY (PART 2)
NOMINAL WALL THICKNESS
An important consideration in the design of any plastic injection moulded part is the wall thickness. A wall section that is too thin can lead to structural failure or poor insulation characteristics.
A wall section that is too thick can result in appearance defects and an overweight or over-engineered part. With the latter point it is also worth remembering that wall thickness governs the moulding cycle time – the thicker the section the longer the cycle time and therefore the more expensive the part becomes. Furthermore, plastics shrink during cooling which in thick sections can result either in the surface of the part forming a sink mark or an internal void.
In most applications a thin, uniform wall with ribs is preferable to a thick wall. There are general guidelines for how thick a part should be, typically 0.75mm to 3mm for filled materials and 0.5mm to 5mm for unfilled. However, this does depend on the design and function of the part concerned. It is possible to mould much thicker sections, if required. Rutland Plastics has successfully moulded void free parts up to 120mm thick.
Gas Assisted Injection Moulding can be used in some instances where ribbing is not possible or desired. Chemical foaming agents are also an option with thick section mouldings.
Where changes in thickness occur the aim should be to have the flow of the material from a thicker to a thinner section. Sharp corners should also be avoided as they cause stress which could lead to failure of the component. In cases where the thickness of a part must be increased, this should be less than 25%.
RADII
Worst Case - sharp corners could lead to serious weakness.
A small radius is a better option.
A large radius is the best option.
A transition.
Alternatively, if it is a change in levels that is required then coring out underneath will maintain a uniform section.
As explained above, radii are important to alleviate stress in the part. Generous radii should be used in areas of major change in section or direction. Furthermore, sharp corners should be avoided because:
Facilitates ejection of the part from the mould. Sharp internal corners tend to
stick in the mould as the part shrinks onto the core.
Provides smooth flow. Resin flows more easily around smooth radiused corners
than sharp ones.
Reduces shear. Shear around sharp corners can degrade the material and cause
aesthetic problems.
An internal radius should be at least half the wall thickness and preferably in the range of 0.6 to 0.75 times wall thickness. However, this can result in a thicker section at the corner which goes against what was stated in the previous section. The result can be longer cycle times and a greater risk of distortion and sink marks. In an ideal situation the internal radius should be matched externally so maintaining a constant section.
Where a rib meets a wall there should be a radius on both corners. Again, this will result in a thickening of the wall at this point. There are various options in such circumstances (see article Design for Appearance).
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