Injection Moulding Part Design Infographic
The injection moulding process is quite simply, injecting molten plastic into a space between two tightly clamped pieces of metal with a hole inside (cavity). The part cools and solidifies, the mould opens, the part is ejected then the cycle starts again.
Good injection moulding product design reduces material & time and therefore cost.
Good design strategy:
1. Functional efficiency.
Injection mould tools are expensive. If you design to make 6 parts instead of 1, tooling costs will be higher. When the parts are made, they might need putting together by someone. Design your parts to minimise assembly, parts and material
2. Material Selection.
Based on the operating environment, any special requirements and cost restraints, several suitable material candidates should be chosen.
3. Material efficiency
Almost always in injection moulding part design, using the minimum amount of plastic that satisfies structural, functional and appearance consideration is the right design. This contrasts sharply with subtractive manufacturing (e.g. CNC machining) where a solid block is shaped into the required parts.
THE most all-encompassing and important topic in almost all plastic design. Just about every other topic revolves around or relates to this one. Ignore these factors at your peril!
Always estimate mechanical loads applied to walls at the actual service temperature during the design stage. At higher service temperatures, the selected polymer is often shown to be under specification. Possible solutions might be to change wall section or to switch to a more thermally resistant polymer.
Material Melt Flow
Melt flow ratio or spiral flow data will tell you how far the plastic will be able to flow under normal conditions within the wall sections of your part. If the melt flow range isn’t high enough, the tool will have to be “blown apart” or over-packed during moulding just to fill the cavity. Many processing defects arise due to over-filling the cavity.
With a wall section, the cooling time can be calculated from the specific heat capacity of the chosen polymer. Cooling time directly affects the running cycle time of the tool and therefore price. 1mm can make a difference of 5-10 seconds in some polymers like polypropylene, adding significant cost to the parts.
Down to Design
Incorporate these ideas into your design:
Sharp corners in plastic parts, particularly inside corners cause poor flow patterns, reduced mechanical properties, increased tool wear and moulded in stresses as the material shrinks onto the corner. Inside radii of 1.2mm are recommended as a minimum. Outside the inside radius + wall thickness.
Almost all moulded parts have features that have to be cut into the mould at a perpendicular angle to the parting line of the mould tool. To get the part out of the tool, parts are designed to have a taper in the direction of the moving half of the tool. When the part cools, it shrinks. Without draft, the part may stick to the core. A highly polished surface finish is also easier to release than an unpolished mould.
The thick wall section on picture 1 causes sinking marks on the surface of the part. The void is a hole in the centre caused by the shrinking plastic. It is a weak point and to be avoided.
Areas of heat build-up
Zones with different wall thickness and sudden changes between them.
Used for this infographic (and excellent further reading) are:
Ticona’s Plastic Product Design Manual
Dupont’s Design for Polymer Engineers Manual
Thanks also to Peter Cracknell for his excellent course materials.