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Design fundamentals for plastics injection moulders: Part II

01 March 2010

In the first of this two-part series by Marc Freebrey we studied the benefits of consistent wall thickness, rib design and gating. In this issue we will look at three more product design fundamentals for plastics injection moulders – draft angle, part radii and boss design

The need to add draft angle to a model is well understood, but often ignored during the design stage. While this may seem like a trivial task, if the taper is not added at the right point within the history tree (if applicable) or complex fillets are subsequently added, this task becomes a great deal more complex.

Draft angle is an important feature that allows a moulded part to be extracted from a mould cavity without issue. The high pressures of injection moulding and material contraction mean that it is often difficult to remove the part. While it is possible to mould parts with zero draft (or even negative draft) using side cores, lifters or two-stage ejection, these features dramatically influence the complexity and cost of the tool.

Although no exact formula exists for defining the correct draft angle for a particular part, there are many factors that have an impact on the optimum value. Generally, thin-walled parts that undergo high-pressure injection moulding need more draft as the material is forced in, which results in a tighter grip on the cavity. Equally, parts that are subjected to lower-pressure moulding can have less draft.

For smooth surfaces, generally a minimum of 0.5 degree draft per side is recommended although experience has shown that a draft angle of 1 degree per side provides easy ejection for most surfaces. Textured surfaces are slightly different as the non uniform texture will drag and scuff, ruining the required effect if the draft angle is not sufficient. As a general guideline, a minimum of 1.5 degrees per 0.025mm depth of texture needs to be allowed for, in addition to the normal draft amount.

The depth of draw (deep ribs) is a very important consideration because as the distance of draft becomes greater, ejection becomes easier but the thickness of the geometry also becomes thicker, and, as we have already learnt from a previous article, dramatic changes in model thickness may cause internal voids, surface sink marks and unpredictable warpage. As an example, a draft angle of 1 degree over a drop of 100mm would increase part thickness by 1.75mm per side.

Although at the product design stage, the moulding polymer may not be known, this can have an effect on the required draft angle. For example, materials with fillers (glass filled) tend to have a reduced shrinkage value and will therefore not move away from the cavity wall. In this case, greater draft angles are required.

Holes are easy to produce in moulded parts and are typically created using core pins. However, blind holes with zero draft often create a vacuum effect at the top of the core pin during ejection (more prone to parts with a polished finish). In this case, a small draft angle will break the seal and improve ejection. Ultimately, the easier it is to remove the part from the mould, the fewer the number of ejector pins required.

Part radii
A significant number of plastic parts fail due to sharp corners or insufficient radius. Sharp corners create localised stress concentrations, which will promote crack initiation and cause premature part failure. The addition of fillet radii to all sharp corners will not only reduce stresses, but also improve plastic flow. As a general rule, at corners, the inside radius is 0.5 x material thickness and the outside radius should be 1 x material thickness plus the part thickness - a larger radius should be used if the part design allows it.

Bosses are a fundamental component of plastic part design as they offer strengthening properties and provide alignment during assembly. Similar to rib design, it is important to consider the wall thickness when designing bosses. The design guidelines listed below will help avoid surface imperfections such as internal voids, surface sink marks and unpredictable shrink rates.

*Boss thickness should be 60% of the nominal wall thickness. If the part thickness is greater than 4mm, the boss thickness can be reduced to 40% of the nominal wall thickness.

*Boss height should not exceed 2.5 x the diameter of the hole in the boss.

*Corner bosses integral to side walls will result in excess material accumulation.

In these articles we have briefly looked at six plastic product design principles. While each point discussed is generic and cannot be applied to every scenario, they are certainly a solid base from which to start your next design project.
In the engineering world, for a project to be successful, it is a continual compromise between product design and production feasibility.

Marc Freebrey is with Vero Software

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