Electroplating involves lots of intricate processes, and yet the quality of the finished component is determined long before any hands-on plating begins. It’s determined at the design stage.
Decisions about wall thickness, corner geometry, surface texture and part structure will either set a component up for a flawless chrome finish, or create problems that even process experts struggle to overcome.
But there has been one theme present in all our successful plating projects – the design was considered early. This guide shares what every designer, moulder and product engineer needs to understand before powering the machine.
If it has to bend, don’t plate it
Once a component is plated, its flexibility is significantly reduced. ABS plastic, the material commonly used for electroplated components, loses strength through the plating process, and areas subject to repeated bending or flexing are likely to crack or fail.
In these cases, the practical approach is to plate only the visible areas, and leave the rest unplated to allow for stress relief. This is particularly true of assembly lugs and snap-fit features. Where a component has a snap fit, the lugs should always be left unplated, otherwise it could cause them to break off during assembly.
The same applies where heat-staking or ultrasonic welding are to be used. The relevant surface areas must be kept free of plating to allow the process to work correctly.
Long, thin sections require extra care
The design of long, thin sections, such as handles, requires particular attention. To compensate for the reduced strength that plating introduces to ABS components, these sections benefit from over-engineering, with thicker wall sections and strengthening webs. What works structurally as an unplated part may not be adequate once plating is applied.
Geometry matters
Component geometry must work with the plating process, not against it, in order to achieve a high-quality finish. The main principles are gently curved convex surfaces, radiused angles and the minimum of protuberances.
Sharp corners are a pain point in plating. They often lead to excessive metal build-up at extremities and chrome ‘burn’ or greying. An experienced plating engineer can sometimes address this using ‘robber’ bars within the plating bath, which helps absorb excess chrome near sharp corners or parts of components positioned close to the electrodes. Still, this is a workaround as opposed to a complete solution. It’s best practice to eliminate sharp corners at the design stage.
Deep recesses present the opposite challenge of inadequate plating coverage. Surface imperfections also become far more visible on large plain surfaces once chrome is applied. Components with deep recesses, sharp edges or textured surfaces often need the help of release agents during moulding. This is so a quick, clean tool release can be achieved.
However, release agents must not be used under any circumstances on components intended for plating. Designing with gentle curves and no sharp corners not only improves plating quality, but removes the need for release agents entirely.
Selective plating
Selective plating represents a significant step forward in the world of chrome plated plastic. Designers can deliver complex parts moulded as a single piece with a selectively chrome-finished surface, widening design possibilities that once required multiple components.
But selective plating introduces its own design requirements. If the edge of the plated area falls on a flat surface, higher current density at that point during the plating operation will cause the metal to encroach on the unplated area, producing a rough, ragged edge.
The boundary of the plated area must coincide with a natural edge, a groove or an undercut designed for that purpose. This way, current density is reduced and the plated edge comes out clean and sharp. Where they can, designers should make an effort to avoid stopping off the plating on an open flat surface.
The three most used groove approaches are:
- Simple or step groove: An electroformed mask is applied to define the boundary between painted and plated areas. The groove avoids a ragged edge and makes the fit of the paint mask less critical. The groove width is normally equal to its depth and should be a minimum of 1.5mm. A step groove can be used as an alternative, with the paint mask fitting at the bottom of the second step.
- Painted groove: A decorative paint finish coating the bottom of the groove conceals the boundary area, providing a clean aesthetic result.
- V groove: The preferred groove where there is a change of plane in the surface and where a step or simple groove is not appropriate. The lines are softer and less abrupt. The groove has a 45-degree angle with a minimum depth of 0.75mm, with the plating boundary occurring at the bottom. Where a painted finish is required, the boundary is slightly overlapped and remains invisible.
Important moulding considerations
It makes a world of difference when the designer, tool maker and plater work together from the start of product development. There are number of moulding guidelines that affect the quality of the finished plated product, so it pays for a collaborative effort.
Wall thickness
Although we strive for uniform wall thickness, it isn’t always achievable. Nominal wall thickness is anywhere in the region of 2.3mm to 3.0mm and should not fall below 1.9mm or exceed 4.8mm. Where thickness variations are unavoidable, transitions should be gradual and uniform. Uneven cooling from abrupt thickness changes can cause shrinkage and warping, both of which will be visible – and exaggerated – once the chrome finish is applied.
Hollow ribs, bosses and elevation changes support more complex designs, while maintaining nominal thickness throughout. Unsupported edges can be strengthened by turning the edge or adjusting the linear plane of the wall.
Ribs
Ribs are what reduce the bulk or weight of components without losing rigidity and strength. But ratio is critical. They must be designed at 50 to 60 percent of the wall thickness at the point of intersection.
A rib that is too thick relative to the wall will cause visible sink marks on the opposite face, which become strikingly obvious after plating. Ribs should also be designed with proper radii at the intersection to manage stress, and with a minimum draft of one degree to allow clean release from the mould.
Feed/gate areas and flow lines
When moulding ABS components for plating, feed and gate areas need to be considerably larger than standard practice in order to minimise moulded-in stress. Careful attention must also be paid to where flow lines will appear. With close co-operation between tool designer, moulder and plater, flow lines can be positioned where they will be less visible – at corners rather than along flat straight sides, reducing their visibility once the chrome is applied.
Radius and material flow
Radius is important at both internal and external intersections and should allow good flow of moulding material. Usually, where variations are necessary in a component, the material should be fed from the heavier to the thinner section. Large weight-to-surface ratios should be avoided.
Retrofitting plating to existing designs
There is a growing trend for non-plated components to be given a ‘makeover’. In other words, using chrome plating to refresh the look and quality of an ageing design. It’s an understandable impulse, but one that can create problems if the original design didn’t account for plating. Tolerance in particular can become an issue once the plating layer has been applied to a component that was conditioned for it.
If you’re considering introducing plating to a component later in its lifecycle, the same is expected as if it were a new design. Get us involved from the start of the process, as working together from the outset makes the production much smoother for everyone.
The benefits of early collaboration
None of these guidelines are especially complex in isolation. The challenge lies in applying them consistently, in combination, at the right stage. Once tooling is cut, changes become expensive. Once production has started, certain problems can only be managed, rather than removed.
That’s why Borough works closely with designers and product engineers from the first concept. We offer both injection moulding and electroplating from a single facility in Leigh-on-Sea, which means the people responsible for plating your components can advise on design before tools are made. It is how we keep quality high and reject rates low.
If you’re developing a component that will be chrome plated, or considering plating an existing part, speak to a member of the team on 01702 425425. The earlier we’re involved, the better the product will be.
