Most paint shops suffer from a small number of rejects due to “dirt” problems. Plastic finishing shops are particularly prone to defects due to the tribo-charging effect on plastic components. Tribo-charging is the build-up of electro-static charge due to friction. Most people have tried the school boy test of rubbing a balloon on a jumper to generate sufficient electrostatic charge to hold the balloon to that jumper.
Unfortunately air movement over a plastic automotive bumper can generate a similar effect. Thus any airborne particles in the paint shop have a high degree of probability of being attracted to the parts to be painted. The electrostatic attraction can hold on to the particle even through an “air curtain”.
Automotive manufacturing paint shops go to extreme lengths to avoid dirt contamination on vehicle bodies and typically get the reject rate down below 2%. Plastic finishing plants, presumably because of these electrostatic issues can have much higher reject rates, more than 10% is not that unusual. The cost of rejects and rework cannot be over emphasised.
An analysis of the rejects gives further clues to the problem. Typically more than 70% plus of the inclusions that causes rejects are fibres, which are light and prone to the effect of electrostatic attraction.
Reducing the failures
In order to produce as few rejects as possible:
- Reduce and avoid electrostatic problems.
- Stop dirt getting into the spray booths.
- Avoid fibres in the painting area.
Reducing electrostatic problems
Painting metal parts normally does not represent a problem as they usually earth any charge away through the hooks and jigs. Plastic parts in contrast are of themselves insulators and they also make the charge from tribo-electric effects. We can help to discharge this static charge by wiping with high polarity liquids. Alcohols work very well. In order to have a balance between remaining wet long enough to give effective cleaning, rapid drying to minimise the time before painting and low application cost, then iso-propanol is often the best option. Not only is this important before painting, but it can also be very helpful to wipe the parts before they enter the paint cabin. Otherwise the electrostatic forces will already carry charged fibres and dust particles into the spray area to be re-deposited back on the work.
Removing anti-static i-Wipes from the container.
The choice of fabric is also very important. We chose various types of fabric depending on the solution chemistry to minimise lint. The fabric shown below is a competitors commercially available wipe which when torn can be seen to generate lint fibres with disastrous consequences to the painting process.
A poor quality wipe showing how lint fibres can be produced.
C T Supplies manufacture W-700 Wipes that are impregnated with a mixture of water, alcohol and a light surfactant for cleaning before entry into the spray cabins. Once inside the spray-cabins the parts may benefit from a second wipe with our W-711 i-Wipes that are impregnated with 100% solvent to again clean and remove static, noting that this time drying speed is of the essence - so no water is included. We use as near to a lint free fabric as possible for the W-700 and i-Wipes.
The spray shop itself needs to be organised to minimise dirt. The conveyor entering the spray zone should enter via an air knife if possible to blow off as many contaminants as possible.
Most substantial spray booths now have heated warm air make up systems providing filtered warm air. The water wash system should be well maintained to avoid the build-up of dry overspray (as powder). If the water system becomes fouled than dry areas may appear on the “water curtain” that then becomes fouled with dry over-sprayed paint dust. This can then be picked up by the fast moving air and can conceivably get back into the “clean zone” causing further failures. Paint coagulation and denaturing chemicals can be used in the water wash sump to ensure the continued functioning of the booth. The chemistry can be adjusted to help the treated overspray sink or float depending on the design of the booth. Our C-130, caustic based product, provides a very simple to control product to maintain the booth in good order. The idea is that atomised paint particles that are captured in the water system are attacked by the chemistry to make a “soapy layer”. This stops the paint particles all agglomerating together to form sticky mass that will block the pumps and stick to the booth curtain.
A water wash system demonstrating dry, treated overspray.
C-177 WR Booth paste can be used to protect the subfloor areas and capture over spray dust. Peelable (C-200) or Tacky Booth Coating (C-180) can be regularly applied to keep the spray-booth walls free of overspray dust. Tacky spray booth coating effectively makes the walls and ceiling act like a giant fly-paper!
C-190, Tacky oven coating can be applied to panels that pass through the oven to catch fibres and airborne dirt particles. It is worth noting that Tacky coatings will slowly dry out (especially in hot ovens) so it is best to apply them to something that is easily removed from the oven. In this way the coating can be easily and frequently removed and replaced. Do not coat the oven itself as the tacky coating will eventually dry out and start to flake causing more problems than it solves!
Personal protective Equipment
Within the spray shop, the operators can further help avoid contaminants entering the booth by wearing conductive safety boots that are designed to avoid the build-up of static charge (This is doubly important when using highly flammable liquids because electrostatic sparks can also cause fires). Wherever possible racks, conveyors and metal parts should all be earthed.
Before entering the spray shops lint free overalls must be used. Hoods can hold back hair (and dandruff!), gloves are also advisable from the point of view of cleanliness and personal protection. I note that cotton gloves can produce lint and surgical gloves probably risk static charging. On balance, I think surgical gloves are still the best option.
At the entrance to the spray areas our Clearpad Tacky Floor Mats provide a low cost way of capturing dirt from an operator’s shoes. We offer a 30 sheet mat so that the top layer can be peeled off, say after every shift and disposed of, ensuring you start each shift, dirt free.
Finally, before the paint coating is applied We supply T-659 lightly impregnated Tack Cloths (i.e. Cloths that have been impregnated with a light layer of none drying adhesive) to capture any dust left on the surface. The T -659 cloths are a considerable improvement on conventional cotton tack cloths as they are made from a tough woven synthetic fabric that is very difficult to snag and shred on any roughness of the component.
Use of a tack cloth to catch any surface contaminents.
Cheap Tack Cloths made from loose woven poor quality cotton can leave lints behind and cause almost as many problems as they solve!
As specialists in providing technology to maintain the paint spraying area, C.T. Supplies are well placed to support customers that paint plastics.
Original published August 2014, reproduced courtesy of Finishing Magazine.