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When designing for plastic injection moulding, there are several design considerations to study, this article shines a spotlight on coring and drafts. A draft is the degree of taper of a mould-cavity sidewall or the angle of clearance designed to facilitate the removal of parts from a mould. Generally, all plastic components should be designed with draft where possible. Coring is the removal of excess material from the cross section of a molded part to achieve a more uniform wall thickness. 

Drafts

When designing parts for plastic injection moulding, applying draft to the faces is key to improving mouldability – even ribs, bosses, depressions and holes must have draft.

Without draft, plastic parts are more likely to have surface defects and may bend, break, or warp due to moulding stresses caused by the plastic cooling.

It is also worth noting that without draft the parts may not eject from the mould tool correctly, thus damaging both parts and possibly the mould itself, which would be costly and time consuming to rectify.

Depending on the design of your part, we’ve identified some benefits to adding draft:

  • Draft reduces the risk of damage to the part during release from the mould
  • Draft reduces wear and tear and chances of damage to the mould
  • Draft ensures a uniform, smooth and unscratched surface finish
  • Draft reduces overall cooling time by lessening or eliminating the need for unconventional ejection setups
  • Most, if not all, of these benefits offer either direct or indirect overall production cost reductions

Our designers will apply the following rules when using draft:

  1. A draft angle of 1½ to 2 degrees is required for most parts

This rule applies to parts with a mould depth of up to 2 inches. At this size, draft as low as 1½ degrees will allow for an easy, non-damaging release from the mould.

  1. Draft should average about an additional degree for each inch of depth

More draft is required for deeper or larger parts, to account for the additional surface area and potential for friction upon mould release.

3) Draft should occur toward the “top” of the mold

The draft angle should follow the direction of the mould moving up or away as it separates.

4) Textured parts require more draft

When designing parts with intricate textures or other raised or recessed surfaces draft should be added to the initial optimal 1½ to 2 degrees at the rate of another 1½ degrees per thousandth-inch of surface depth.

Coring

As we’ve mentioned in our other articles, a uniform wall thickness is desired when it comes to plastic injection moulding. Sometimes to achieve uniform wall thickness, thick-walled areas of the component may need to be cored. Coring reduces wall thickness to avoid sink marks and saves on weight and cost. When designing a part, our team we will analyse the wall thickness and if too thick, we would suggest coring. 

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Looking for more information? Check out our blog, News & Views for useful articles, tips and tricks on plastic injection moulding.