In PVC coating, a suitable component is encased in a continuous layer of soft PVC, which forms the functional surface of the component. Coating is therefore a variation of the dipping process: whilst in the dipping process a heated core is coated with PVC and the resulting PVC skin is then peeled off as a separate dipped part, in coating the PVC layer remains on the workpiece being coated.
To ensure a good coating result, the areas of the components supplied that are to be coated should be free from dirt, oil and grease on delivery. If the components are not sufficiently clean, HAMCO can carry out the necessary cleaning and preparation. A clean surface is important, as contaminants can impair the adhesion of the PVC coating.
A special workpiece holder may be required for the complete coating of a component or for the targeted coating of individual areas of a component. It ensures that the component is held in the intended position during the immersion process if its orientation at the time of immersion affects the coating result. Depending on the component’s geometry, the component can be suspended from or bolted to the workpiece holder.
When the heated component is immersed, a layer of PVC forms on its surface. The thickness of this layer is influenced, amongst other things, by the temperature of the component, the dwell time in the immersion bath and the viscosity of the PVC plastisol used.
Depending on the Covytec® grade used, a plastisol with the relevant formulation is processed. The properties of the various Covytec® grades and the corresponding material data sheets can be found on our Covytec® overview page.
However, it is generally not possible to achieve a uniform coating thickness across the entire component when using dip coating. The area of the component that is first immersed in the plastisol and is the last to emerge from the dip bath when it is withdrawn usually has a greater coating thickness than the edge of the coating. During withdrawal, excess plastisol may run off, causing the coating thickness to vary across the coating.
The geometry of the component also influences the distribution of the PVC. Edges, transitions, radii, recesses and other geometric features can lead to material build-up or areas with a thinner coating. These variations are inherent to the process and must be taken into account when designing and evaluating the coating.
Once the excess soft PVC plastisol has been removed and allowed to drain off, the coated component is reheated together with the workpiece holder. During this heat treatment, the PVC gels completely and acquires the rubber-like elastic properties characteristic of the material in question.
Once cooled, the coated component can, if necessary, be further processed or printed on. Functional surfaces which, for process-related reasons, must initially be coated along with the rest of the component can subsequently be cut away, provided that their function requires an uncoated surface. Depending on the geometry and requirements, the uncapping is carried out using a punch, a cutting jig or a hand-held knife.
The resulting cut edge differs from the coating edge. Whilst the coating edge is formed by the flow of the plastisol and is correspondingly rounded, the cut edge is sharp. Furthermore, when cutting by hand with a knife, the cut edge may be uneven.
As the component is immersed, the viscous PVC plastisol, figuratively speaking, rolls along the metal surface until the maximum immersion depth is reached. As the component is withdrawn, the excess plastisol runs off the PVC layer that has already formed. At the point where immersion ends, the edge of the coating forms with a rounded transition to the uncoated metal surface.
Due to the shape of the coating edge, it is possible to insert a thin object beneath the PVC layer and lift the edge away from the component, particularly in the case of soft PVC with low degrees of hardness. If such detachment is not desired, the relevant area can be pre-treated with an adhesion promoter – known as a primer – before coating. As the primer may be clearly visible on the metal surface, the coated area is designed so that the edge of the PVC coating slightly overlaps the primer area. This ensures that, after coating, the primer remains as completely concealed as possible beneath the PVC layer.
The adhesion promoter ensures a very strong bond between the component surface and the PVC layer. It can be applied selectively to specific areas where, for example, torsional forces are to be expected and a high level of resistance to twisting is required.
Components made from materials that can absorb the required thermal energy, store it sufficiently during the coating process and release it only slowly are particularly well suited. In industrial practice, this applies above all to metallic components.
The materials coated include, amongst others, components made of steel, stainless steel, aluminium, copper and brass, as well as glass and other suitable materials.
In principle, heat-resistant plastics can also be coated. However, in the case of plastic parts, it is necessary to check whether they can withstand the required process temperatures.
Very thin structural components cannot store the required thermal energy in the same way as more substantial workpieces and cool down more quickly during the coating process. In particular, on the end faces of thin sheets less than 0.8 mm thick, it is not possible to build up a sufficient coating thickness, meaning that the coating may be incomplete in those areas. If in doubt, suitability should be checked by carrying out a test coating.
To ensure a good coating result, the areas to be coated must be clean and free from oil and grease residues. Furthermore, a technically smooth, but not polished, surface is generally suitable for coating. To ensure the best possible adhesion, particularly for metallic components, a suitable mechanical pre-treatment is recommended, for example by sandblasting or glass bead blasting.
In the case of special surfaces, pre-coatings or galvanised components, suitability for PVC coating must be verified through practical testing. The quality of the surface pretreatment has a significant influence on the final coating result.
The properties of the finished coating are largely determined by the Covytec® material used. Hardness, abrasion resistance, damping, slip resistance and other properties can be specifically tailored to the application.
For specific requirements, materials are available which offer, amongst other things, increased UV resistance, ESD properties, detectability or increased temperature resistance. Special types of material for medical, therapeutic and food-related applications are also available.
The choice of a suitable material should therefore not be based solely on colour or hardness, but on the actual requirements of the finished component.
Further information on design guidelines, bonding agents, the effects of colour and surface requirements can be found in the comprehensive knowledge section of our Product page: Coatings.
HAMCO carries out the PVC coating process using a salt-free heat transfer system. No lithium nitrate or potassium nitrate is used in the process.
This results in particularly clean component coatings. At the same time, it facilitates health and safety at work and the subsequent recycling of the high-quality PVC materials.
As component geometry, material, surface and the desired coating properties all influence the result, a test coating is recommended for new applications.
This enables the layer structure, adhesion, surface finish and functional properties to be assessed under real-world conditions, and the process parameters for subsequent mass production to be defined.
HAMCO supports customers every step of the way, from component assessment and the selection of the appropriate PVC material right through to series production.
You now know how the PVC coating process works. However, the crucial question for your component is: is PVC coating really the right solution for your application? HAMCO supports you in selecting and implementing suitable coating solutions – from assessing the component and the desired function, through to selecting the appropriate PVC material, right through to coating and series production. Find out more about the Range of services by HAMCO.
Large and small quantity requirements Direct processing
Large and small quantity requirements
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