Plastic Product Design Principles (Part 2)

This is the final piece of a two part series that focuses on plastic product design fundamentals and the impact they have on the manufacturing and production processes. Part 2 will look at the need for draft angles, corner radii and the gating position.

Design Draft Angle
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 means 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 certain 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, 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 know, this can have an effect of 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.

Mould Tool Design
Gating : It is often preferred to gate onto the thickest section of the component to reduce the possibility of sinking due to insufficient material packing. Fixing the gate location ultimately determines the filling behaviour, weld lines, shrinkage, warpage and surface quality of the moulded part. Weld lines are lines where two plastic flow fronts meet and form a relatively weak bond. These are the area's most likely to fail when the part is under stress. Complex mouldings will always contain weld lines and if the number cannot be reduced, they should be moved to non-critical areas of the component. This is typically achieved by moving the gate location or changing the part wall thickness.

Conclusion
Over the last two issues we have briefly looked at six plastic product design principles. While each of the points discussed are generic and cannot be applied to every scenario, they are certainly a solid base from where to start your next design. In the engineering world, for a project to be successful, it is a continual compromise between product design and production feasibility.

To review part one, please select HERE.

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