Wednesday 5 July 2023

5 Important Design Rules for your Next Plastic Injection Moulded Part Project

Hello and welcome back to a new blog post. In today’s post we highlight some important design guidelines for successful injection moulded parts. 

Since part design is one of the five critical success factors for obtaining an injection moulded part, it is important to follow the design recommendation for plastics. All factors (Part design, Material selection, Mould design and construction, Moulding machine selection, and Moulding process) can be summarised in the Polymer Product Pentagram

In general there are 10 holy design rules and today we picked the 5 most important ones. 

Overview - 5 important design rules for injection moulded plastic parts

1) Uniform wall thickness: aim to have a wall thickness as thin as possible as well as uniform as possible. Avoid your part to go from thin to thick regions (worse) and thick to thin (also bad). Best case scenario is a thin wall with ribs. For reinforced plastics wall thickness should range between 0.75 mm to 3 mm maximum. For unfilled plastics, you can aim for 0.5 mm to maximum 5 mm. 

2) Rib design: as I have mentioned to add ribs to your walls, it is important that they have the right geometrical dimensions too. Let us take a part with no ribs as an example. Such a part consumes approximately 15% more material than a part with ribs and the cycle can be up to 70% longer compared to the thinner part with ribs. For unfilled materials, rib thickness should be less than 70% of the nominal wall thickness. If you have a rib thickness larger than 70% of the wall thickness, material gets drawn away from the center of the opposite wall during cooling and as a result internal voids or sink opposite of the rib occur. Furthermore, ribs are getting effective when they are 5-10 times higher than the nominal wall thickness.

3) Draft angles on ribs: as important as the rib thickness is, draft angels need to be placed in order to eject the part after moulding. Usually a minimum draft angle of 0.5° is necessary. 1° to 2° is commonly applied according to material supplier recommendations. If you use glass-fiber plastics, a higher draft angle might be better. The same is true for low shrinkage materials. On the other hand, highly flexible materials such as PVC need less draft angles. Also, if you have rough surfaces on your part, recommendation is 1° for each 20 mu surface roughness. 

4) Radius: avoid sharp corners and make a radius, together with fillets. In general the radius should be 0.9 to 1.2 times the nominal range of the part. 

5) Undercuts: there are four different design features where undercuts play a role. There can be a window in a side wall, an overhang above the bottom wall of the part, a horizontal boss, and a snap finger. However, if possible try to avoid them since a more mould mechanism must be considered, as well as machined to have a proper ejection of the finished part. 

I hope that these 5 design rules will help you in your next plastic part project and if you have any questions around your part design or want to create your own plastic compound, pl. reach out here to support you. Also, if you need plastic sample material for testing, you can reach out here. 

Thanks and #findoutaboutplastics


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[2] Design guide for plastics by Tangram

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