Design Guidelines

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This section is primarily intended for product designers, who may be proposing a component be designed, injection moulded, then electroplated. As previously stated, the best approach for designing injection moulded components for electroplating is if the designer, tool maker and electroplater all liaise at the concept stage of the product’s life. It is also useful, for designers to have a basic guide to help them understand the special design requirements of components moulded for plating. An understanding of these requirements will enable them to avoid errors that would subsequently have an adverse effect on the plating quality achievable.

Firstly, a completely uniform wall thickness is ideal (), but unfortunately not always possible. Nominal wall thicknesses should be in the region of 2.3 to 3.0mm, but should not exceed a maximum thickness of 4.8mm or a minimum thickness of 1.0mm.

The incorporation of hollow ribs, bosses and elevation changes in the product design will allow intricacy, while still maintaining the nominal thickness values throughout. Avoiding thickness changes will help to eliminate uneven cooling which could cause shrinkage and warping of the finished plating. Obviously thickness variation is sometimes unavoidable, but when it must occur, then the transitions should be gradual and uniform. In general, where variations in a component are necessary, it should be fed from the heavier to the thinner section ().

To minimise problems further, large weight to surface area ratios should definitely be avoided. In (), the knob would have an unacceptable shrinkage in the moulding if it were moulded from solid. It would also suffer from serious thermal cycle problems after plating.

Borough understands that ribs are essential to reduce the bulk or weight of components while maintaining rigidity and strength, but there are important tips to be remembered when they are being designed. The ribs should be designed so that at the point of intersection with the wall they are supporting, they are at a maximum of 50% of the wall thickness. This ratio of wall thickness to rib thickness is important to avoid visible sink marks on the opposite side of the wall from the rib intersection.
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This shrinkage problem becomes strikingly obvious after plating, when every small defect is magnified by the metal coating. Ribs should also be designed with proper radii at the intersection to help minimise or avoid stress altogether. There should be a minimum draft of 1 degree to facilitate easy removal from the mould.

The comments about wall thicknesses and ribs applies equally well to bosses or studs on a moulding, used for the attachment of related components. Boss heights should not be more than twice their diameter. With practice and the right information it is possible to design quite intricate systems of structurally interdependent bosses and ribs. These will provide dimensional rigidity and allow the attachment of related parts, while maintaining wall thickness ratios and reducing material needs and moulding cycle times.

Unsupported edges may be strengthened by turning the edge or 'setting' the linear plane of the wall (). Radius is of great importance at both internal and external intersections and should be as generous as possible to allow for a good flow of the moulding material. For example, external radii should not be less than 1mm and internal radii not less than 0.5mm. Moulded threads should be rounded rather than sharp and are not recommended under 6mm. Mechanical threading or tapping is not recommended for plastic parts that are to be electroplated. Recesses should be at least twice as wide as they are deep. Finally, feed/gate areas need to be considerably larger than for normal ABS, and their position is important to avoid weld lines becoming visible.

Other important parameters are material drying, tool temperatures and actual melt temperatures, together with injection speeds. For a more detailed discussion of the special requirements for the successful injection moulding of components for electroplating, please contact the Borough Sales Team.

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