decoration | Plating on plastic
of 4.8mm or a minimum of 1.9mm. Avoiding thickness changes will eliminate uneven cooling which could cause shrinkage and warping of the part. When thickness variations are unavoidable, transitions should be gradual and uniform and the aim should be to feed from the heavier to the thinner section. Large weight to surface ratios should definitely be avoided. The incorporation of hollow ribs, bosses and
Above left: The first step towards a perfect plated part is a top quality moulded surface
designing parts for selective plating the designer should avoid ending the plated area on a flat surface. The first process in a selective plating process is the
application of a good line of definition between paint and plate. An electroformed mask is usually used for this operation, with a simple or step groove avoiding a ragged edge at the boundary and making the accuracy of fit of the paint mask less critical. The width of the groove is typically equal to the depth and normally should be a minimum of 1.5mm. A step groove can be used instead of a simple groove, with the paint mask fitting at the bottom of the second step so the resist boundary occurs at the bottom of the first step. Painting with a finish coat of decorative paint covering the bottom of the groove hides the boundary area. A V groove is useful where there is a change of plane in the surface and where a step or simple groove design is unacceptable. In the case of the V groove, the lines are softer and less abrupt but still provide a slightly recessed area for the boundary. The groove shown in the schematic has a 45˚ angle with a minimum depth of 0.75mm. The associated mask is designed so the plating boundary occurs at the bottom of the groove where, if a painted finish is required, the plating boundary line is overlapped and not visible.
More Moulding Considerations In practice, the best approach to development of a plated plastic injection moulding is for the part designer, tool maker and plating specialist to work together from conception to delivery of the final component. However, a few important general mould- ing recommendations can be provided for part and tool designers to help ensure the quality of the finished product is high and the reject rate is low. A completely uniform part wall thickness is the ideal
but, unfortunately, not always possible. Nominal wall thicknesses should be in the region of 2.3 to 3.00mm and should not generally exceed a maximum thickness
56 INJECTION WORLD | November/December 2014
elevation changes in the design will allow intricate design features to be incorporated while maintaining nominal thickness. Ribs should be designed so that they are 50 to 60% of the wall thickness they are supporting at the point of intersection. This wall thickness ratio is important to avoid visible sink marks on the opposite side of the wall from the rib, which will become strikingly obvious after plating. Ribs should also be designed with proper radii at the intersection to avoid stress and with a minimum draft of one degree to facilitate easy removal from the mould. 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. Feed and gate areas need to be considerably larger when moulding ABS components for plating to ensure moulded-in stress is minimised in the final part. Careful attention also needs to be paid to where flow lines will appear on the moulding as plating risks highlighting even the smallest surface feature. Close co-operation between tool designer, moulder and plating specialist should enable any flow lines to be situated where they will be less visible, such as at corners rather than along straight flat sides. Plating is frequently used to give product designs a
‘makeover’. Application of chrome plating can breathe new life into an ageing design and can elevate perceived quality. However, great care should be taken in such cases as, unless plating was considered as a possibility at the time of the original design and tooling, problems may well be encountered with regard to tolerances once the plating has been applied. If you are considering moulding a part for plating,
even later on in the component life-cycle, ensure that the designer, toolmaker, moulder and plating experts are involved at the start of the process.
For more information: David Coombes is managing director of UK-based Borough, which operates an automated plastics injection moulding and electroplating facility at Leigh-on-Sea. ❙
www.borough.co.uk
www.injectionworld.com
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