Why manufacturers should think carefully about sanding for surface preparation
The need to lightweight vehicles is now under urgent investigation by all the OEM’s in the automotive industry. The regulations on emissions that come into force in 2020 coupled with punitive fines for companies that don’t comply mean that this is an issue that can’t be ignored. The move from steel to aluminium is already happening and the light weighting benefits from doing so are well documented; however, it is generally acknowledged that this move isn’t enough. The use of lighter and stronger materials such as carbon fibre composites is therefore appealing, but the adoption of carbon fibre composites in automotive revolves around one key point – cost. The cost of raw materials feedstocks as well as the time spent moulding parts is currently being addressed, resulting in advances in lower cost carbon fibre and snap cure resin systems, which can generate composite parts in under 5 minutes. One aspect which has had less attention, but which is equally time consuming in the manufacturing process, is how you prepare these materials for bonding and painting once formed.
Surface preparation of composites is essential both to ensure that the material is clean prior to painting or bonding and to promote adhesion of the paint or adhesive to the material. The most common way of achieving this type of surface preparation is through the use of mechanical abrasion, such as sanding or vapour/shot blasting. These subtractive processes remove containments, such as mould release, from the surface as well as providing increased adhesion to the material, thanks to defects in the surface, created by the abrasion process, into which a paint or adhesive can flow and set. Once cured, these act like ball and socket joints, anchoring the paint or adhesive to the substrate surface.
While mechanical abrasion is used across a wide range of industry sectors, it is not always a practical solution in niche and volume manufacturing.
One of the biggest problems with sanding is the length of time it takes, especially as the process is manual rather than automated. Even on a small scale this can reduce production efficiency; in volume manufacturing slow processes that create bottlenecks are just not economically feasible. Indeed, the move to snap cure resin systems for composite manufacture means lengthy surface preparation techniques are un-feasible. In addition, materials must be thoroughly cleaned post sanding to remove any loose debris, thus further extending the time taken to prepare a part.
Obtaining the same level of sanding across a material is not a straight forward matter and between batches or operators further levels of discrepancy can occur. These factors are compounded by the fact it is difficult to assign a level of sanding that is required for a given operation. Too little abrasion and the adhesion will be poor, too much and you run the risk of substrate damage (and hence wastage).
The physical shape and design of part can complicate abrasion processes. Designs containing narrow or closed apertures will readily fill with grit or abrasive during blasting surface treatment; designs with a 3-D nature and dimensionality prove difficult to hand sand. This increases both the time needed to prep the part and the likelihood that the consistency of sanding will be compromised.
The dust created by sanding creates several issues for manufacturers: Firstly, employees must not breathe in large amounts of dust. If working at scale, specialist containment equipment may be required, therefore adding cost to the process. Secondly, dust can also lead to contamination issues; failure to clean the substrate after sanding will increase the likelihood of bond failure as it causes weak points at the substrate-coating interface, adding yet more time to a process. This is particularly true if there are multiple coating layers that need to be applied to achieve the desired finish as sanding could be required between these layers.
Advanced new materials demand new methods of surface preparation; to persist with old-school sanding just doesn’t make sense technically, economically or environmentally. And there’s the rub.