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Draft Angles and Analysing Draft

by Luke Seal


When designing a part it is critical to think about the manufacturing process. All manufacturing processes which include moulds will require some amount of draft to remove the part from the mould.


Draft is an angled taper to the part that has a relationship between the direction that the part is being removed from the mould. This is known as the die line and should be defined in the design of the part.



An example of finished part for male and female moulds are shown below:  


Manufacturing processes that require draft on the part include:

Injection Moulding

Casting

Pressing / Stamping

Deep Drawing

Blow Moulding

Vacuum Forming

Pre-impregnated (Prepreg) Composite Forming

Resin-Transfer Moulding


Draft angle is dependant on both the supplier capabilities and the manufacturing process. There is no table of standards that can provide this information, although general design rules of thumb can be used when designing parts. During the design process, the designer should have regular communication with the supplier regarding their Design for Manufacture (DFM) capabilities.


A common process before releasing a part for manufacturing is to analyse the draft on the part to show any areas which are non-conformance with the required draft angle. This can be done in most 3D Computer-Aided Design systems. An example of draft analysis can be seen in the video below using CATIA V5’s draft analysis tool.


The result of not including a draft angle for a manufactured part will be damage to the part or the moulding tool. Additionally, if draft is not taken into consideration when designing a part within the context of an assembly, this may cause assembly issues further down the line.


Luke T Seal Engineering