When considering chrome plating, the rule of thumb is; if it has to bend, don’t plate it. And if it is likely to be subjected to impact, only plate the areas that show, leaving the rest un-plated to allow for stress relief. This has particular reference to assembly lugs, so where a component has a snap fit, the lugs should be unplated. Plating them would very likely cause them to break off. (Similarly where heat-staking or ultrasonic welding are to be used, the surface needs to be free of plating.)
The design and moulding of long thin sections, like handles, also require careful consideration. They will need over-engineering with thicker sections and strengthening webs to compensate for the reduced strength of plated ABS.
To ensure an electroplated plastic component has a high quality finish, the designer must consider the need for gently curved convex surfaces, radiused angles and the minimum of protruberances.
Sharp corners can be a problem, as can the orientation of parts in the plating bath, both of which often lead to excessive plating at the extremities and chrome ‘burn’ or greying. The experienced plating engineer can sometimes overcome the problem by using ‘robber’ bars within the plating bath, to absorb excess chrome near the sharp corners or parts of components near to the electrodes.
Conversely, parts designed with deep recesses may be inadequately plated and surface imperfections will stand out on large plain surfaces. Moulding components with deep recesses, sharp edges or textured surfaces, will often require the use of release agents to ensure a sufficiently quick release from the tools. Whilst not normally an issue, when the parts are to be plated, release agents must not be used under any circumstances. Designing for plating, with gentle curves and no sharp corners not only makes the plating easier and better quality, but will also preclude the use of release agents.
Selective plating represents a great advance for the use of chrome on plastic components, allowing the designer new freedom to deliver complex parts moulded as one piece and selectively finished. But if the edge of the plated area lies on a flat surface, it is likely to take excess metal and become ragged and rough.
This problem is caused by a higher current density area at this point than other areas of the part during the plating operation. The metal as it is plated, tends to encroach on the resistance area, producing a rough edge.
If however the boundary of a plated area coincides with the natural edge of a part, or occurs at a groove or undercut provided for this purpose, then the current density at this point is lowered and a plated edge is produced sharp and free of irregularities. Consequently, when designing parts for selective plating, the designer should avoid stopping off the plating on a flat surface.
The initial process in selective plating is the application of a good line of definition between paint and plate and an electroformed mask is usually used for this operation. A simple or step groove avoids a ragged edge at the boundary and makes the fit of the paint mask less critical. The width of the groove is usually equal to the depth and normally should be a minimum of 1.5mm. The step groove can be used instead of the simple groove, with the paint mask fitting at the bottom of the second step so the resist boundary again 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. The V groove is useful where there is a change of plane in the surface and where the step or simple groove design is unacceptable. In this instance the lines are softer and less abrupt but still provide a slightly recessed area for the boundary.
The groove here has a 45 degree angle with a minimum depth of 0.75mm. The paint mask is designed so that the plating boundary occurs at the bottom of the groove, so if a painted finish is required, the boundary line of the plating is overlapped slightly and not visible.
More Moulding Considerations
In practice, the best approach to injection moulding is for the designer, tool maker and plater to work together from conception of the product to delivery of the final component. But a few important recommendations for moulders and tool designers will help ensure the quality of the finished products is high and the reject rate is low.
A completely uniform 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 exceed a maximum thickness of 4.8mm or a minimum of 1.9mm.
The incorporation of hollow ribs, bosses and elevation changes in the design will allow much intricacy while still maintaining nominal thickness throughout. Avoiding thickness changes will eliminate uneven cooling which could cause shrinkage and warping. When thickness variations are unavoidable transitions should be gradual and uniform.
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 is possible to allow for a good flow of the moulding material.
In general where variations are necessary in a component, it should be fed from the heavier to the thinner section. Large weight to surface ratios should definitely be avoided. Ribs are essential to reduce bulk or weight of components while retaining rigidity and strength, but should be designed so that they are 50 to 60 per cent 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 with a minimum draft of one degree to facilitate easy removal from the mould.
On the subject of stress, feed/gate areas need to be considerably larger when moulding ABS components for plating to ensure moulded-in stress is minimised. Careful attention also needs to be paid to where the flow lines will appear. Close co-operation between tool designer, moulder and plater should ensure the lines can be situated where they will be less visible, at the corners perhaps rather than along the straight flat sides, as again there is the risk the plating will highlight any deficiencies.
Currently there is a trend for non-plated parts to get the ‘makeover treatment’, using the chrome plating to breathe new life and quality into an aging design. Unfortunately, unless this possibility is considered at the time of the original design and tooling, there may be a problem with tolerance, once the plating has been applied.
If you are considering moulding for plating, even later in the component life-cycle, you get us involved from the start of the process and life for everyone will be easier and the quality will be improved.