An extra layer: Carclo discusses different techniques for applying coatings to injection moulded plastic components
Together Martin Day, director of European sales and marketing, Ross Elliot, process optimisation manager at the Carclo Technical Plastics (CTP) facility in Brno, Czech Republic and Michal Benes, business development manager discuss the different techniques for applying coatings to injection moulded plastic components.
Coatings have traditionally been used to provide functional and aesthetic properties to a substrate for centuries. For example, paints and precious metals are used to decorate a range of surfaces and lacquers are used to provide a hard and protective surface to wooden articles.
This practice is routinely extended into the plastic manufacturing sector across a range of applications from the automotive industry to defence. Metallic coatings can be applied to provide reflective, conductive or hard wearing surfaces, polytetrafluoroethylene (PTFE) can be used to reduce the friction across a surface and elastomers can provide a “soft touch” feel.
Hard coating
Over many years, CTP has developed a particular expertise in applying hard coatings (lacquers) to plastic materials such as polycarbonate. In one such application, a lacquer is applied by a flow coating process to CCTV camera domes to increase surface hardness, thereby rendering them resistant to vandalism. In another, the lacquer is applied by dipping or spraying technology to helmet visors for aeronautical and firefighting applications. Such lacquers are primarily intended to enhance surface hardness and abrasion and mar resistance. They can also be formulated to provide anti-mist performance for the user and resistance to ultraviolet, elevated temperatures, solvents and other chemicals while not impacting optical clarity of the substrate.
Coating thicknesses are generally six to twelve microns but are dependent on customer specification. Hard coatings make plastic substrates hard and brittle but, by balancing the coating thickness, a level of flexibility in the component can be retained; this is especially important with facial protection products such as visors, manufactured to meet demanding international standards for personnel protection.
In both cases, there are a number of considerations to maximising product quality and ensuring health and safety compliance.
For example, the substrate needs to be kept clean. This can be assured by manufacture in controlled conditions, such as a classified cleanroom in order to minimise particulate contamination. CTP’s lines operate to ISO class five or six with control of both temperature and humidity. However, product handling between the plastic manufacturing line (e.g. injection moulding machine) and the coating line need careful planning to ensure the article enters the coating line as clean as possible. Once coated, the component may require drying at room or elevated temperature and, once again, good handling is essential.
In addition, component and process design need to consider coating at an early stage rather than as an afterthought. This is because minimising in-moulded stresses is critical to achieving a high quality coating. In some cases, a sprue/runner can be used to hold the component through the coating process but will need to be removed afterwards. If a full hot runner system is preferred for injection moulding the component, an alternative means of holding it through the coating process will be required. Deciding which process to use requires consideration of the customer’s specification, critical areas of the component, and analysis of moulded-in stress amongst other considerations. CTP can advise on design change requirements to ensure maximum coating performance. One important consideration is lacquer run-off points. Where the component has multiple run-off angles, CTP has developed automated, six-axis robotic coating capabilities for flow and dip coating complex components, giving the ability to supply at maximum performance and cosmetic levels.
When designing a coating line or facility, consideration must be given to handling the lacquer and any waste and extraction of fumes. Many coatings are solvent-based, and attention must be paid to local regulations.
Hydrophilic coating
CTP also applies functional coatings to medical diagnostic components. These can include surfactants for hydrophilic purposes. In commercial production, the same processes can be used however, in one lower volume and low complexity application, the component is manually dipped into the surfactant and allowed to dry at ambient temperature.
Specifiers of coatings should be aware that the coating materials are generally proprietary.
Conclusion
Coatings are often a critical aspect of product performance; however, careful consideration should be paid to all aspects of design for manufacture, material selection and process design and optimisation to ensure the maximum performance of the coated product.