Controlled spray and non-atomized resin application (NARA) are best practices that apply to open molding processes, including gel coat application. These practices were developed by ACMA to reduce emissions during the open molding process and should be viewed as “lean” or “green” initiatives in a manufacturing operation.
The benefits of controlled spray are described in ACMA’s Certified Composites Technician (CCT) Open Molding program as follows: “While emissions can be reduced with lower styrene content resins, shorter gel times and the use of suppressants, the single greatest effect in reducing emissions is controlled spraying.” Controlled spraying is a method of increasing material transfer efficiency and reducing styrene emissions through minimization of atomization and overspray loss. This is accomplished by three means:
- Operation of the spray gun at the lowest applicable fluid tip pressure,
- The use of proper spray gun handling techniques, and
- The use of close capture mold configurations that minimize overspray surface area.
While reducing styrene emissions is important in any composites operation, controlled spray has the additional benefit of reducing waste and other non-value activities associated with overspray. The work area will require less frequent cleanup, and personal protective equipment will stay cleaner and can be used longer before it needs to be replaced.
While controlled spray focuses on the open molding process, NARA is based on the equipment design and proper operation of the equipment. NARA is referenced in the Composites Maximum Achievable Control Technology (MACT) Standard with the following requirements:
- Equipment must have documentation provided by its manufacturer or user that the design of the application tool will qualify as non-atomizing resin application (NARA).
- The equipment must be operated according to the manufacturer’s direction, including instructions to prevent the operation of the device at excessive spray pressure.
When selecting NARA equipment, it is important to know the flow rate requirements – or amount of material that will be sprayed in a given period. The flow rate is primarily controlled by the size of the spray tip along with pump pressure, resin viscosity and resin temperature. The flow rate requirements are determined by the part size and geometry.
Smaller parts with detailed shapes may be easier to spray with lower flow rates using smaller orifice spray tips. Large parts with simple planer surfaces can be most effectively made with larger orifice spray tips on spray systems that deliver higher volumes of resin at lower spray pressures and utilize high-volume glass choppers.