Gas Catalytic IR: The Technology Enabler

Disruptive technology in manufacturing comes along rarely, but when it does, it can have a huge impact. The use of infrared (IR) in curing powder was pioneered around 25 years ago and prompted a major rethink within the entire finishing industry. Many described that technology as disruptive, and it is fair to say that there has been pushback from various corners of the finishing industry, some would say to the fear of fear itself.

Now, as manufacturing technology continues to improve and after years of practical experience, clever thinking is still being applied, and new applications are still being found for old disruptive technologies. In the case explained here, three companies working together have developed an improved coating process that provides a highly corrosion-resistant finish to ferrous substrates.

It Starts with a Zinc-Rich Primer

The story begins with Miller Electric Manufacturing Co. of Appleton, Wisconsin, the world’s largest manufacturer of arc-welding products. If you ever come across a blue-arc welder, it is probably from Miller. As the equipment is utilized in many industries and applications, its mild steel casings and panels need to be as corrosion-resistant as possible.

Enter TCI Powder Coatings of Ellaville, Georgia, a major manufacturer of powder coatings in North America and long-standing supplier to Miller that offers a widely used zinc-rich primer. A zinc-rich powder primer can provide superior protection to zinc electroplating when correctly cured and topcoated.

Focused on continuous improvement, Miller needed to improve the efficiency of its finishing plant. It was using a two-cycle process that necessitated running the parts twice through to apply and cure both primer and topcoat.

Miller began considering whether gas catalytic IR could shorten cure times, deliver energy savings, and improve quality, processing time and work space. It brought IR specialists from Heraeus to the table for their advice.

Soon, the powder supplier and IR supplier started working together, exchanging ideas and carrying out some testing. One solution involved the removal of a separate final cure process from the priming stage, and the results were dramatic. Removing just this single step enabled the reduction from a two-pass to a single-pass process, and this reduced the overall cycle time by 50 percent.

Reducing Process Steps

How can the removal of full primer curing be a smart move? Take a look at what is going on through the whole process: First, the parts are coated with the zinc-rich powder. Then, in a usual powder curing process, the parts go through five stages in the oven. However, when zinc powder-coated parts enter the IR pre-gel oven, they only go through four of the five stages:

1. Melt point: Powder particles begin changing from a solid to semi-liquid state.
2. Flow stage: Powder is fully liquefied, reaching its lowest viscosity and allowing the resultant film to smooth out.
3. Cross-linking stage: Sufficient coating temperature then triggers a large-scale reaction within the film, initiating the total chemical/physical change of product.
4. Gel stage: When sufficient cross-linking has occurred, the film then solidifies from a liquid to a solid (gel) .

Normally, in stage five, the “gelled” primer would then be left in the oven to fully cure before being topcoated and sent through the IR oven again for final cure. Some smart thinking, research and testing by the team in this story showed that a new, improved process could continue as follows:

• Once gelled, parts are ready to accept the topcoat.
• This is the clever part: The topcoat powder is applied while the zinc primer is still at the gel stage.
• The part, with the primer at the gel stage and the newly applied topcoat, now enters the convection cure oven. During this final cure process, both the primer and topcoat continue through the final stage of cure development in which cross-linking continues until the full cure properties are reached.

Benefits of Gas Catalytic IR

This process would not be possible without gas catalytic IR. From a manufacturing perspective, the removal of the final cure stage of the zinc primer saves time, energy and space. But gas catalytic IR helps in four more ways:

1. Improving the bond between primer and topcoat. Trials have shown that when a zinc-rich primer is applied to a ferrous substrate, it does not have to be fully cured before the topcoat is applied. In fact, a better bond is achieved when the iznc-rich primer has just gelled and then the topcoat is applied, and both coats are then fully cured. Gas catalytic pre-gel ovens are not new; they have been in use for quite a few years now and were designed to be placed just in front of convection ovens to gel the powder before the coated part enters the conventional convection oven. This can improve the efficiency of the coating line, reduce cross-contamination of powder and save space.
2. Making it easier to facilitate a redesigned production line. All gas catalytic IR ovens inherently save space. Thanks to the 3:1 rule (1 minute in an IR oven is the same as 3 minutes in a convection oven), IR ovens are shorter and can be modular in construction. This means that the introduction of a gas catalytic IR oven into an existing powder coating line causes less disruption and can be achieved with minimum downtime.
3. Cost saving. By eliminating the need for a full cure of the zinc-rich primer coat, operating costs are reduced. Add this to 50 percent overall cost savings that gas catalytic IR can achieve when compared to conventional convection ovens, and the savings can really start to add up.
4. Quality improvements. These are well-known: Less air movement means less cross-contamination, less out-gassing means an improved surface finish, and even heat distribution means a consistent cure.

A Proven Finishing Technology

The use of IR in industrial thermal applications has proven to be ground-breaking over the years since it was first introduced. Even now, new, innovative ways of using IR are still being discovered. In the past, powder manufacturers have understandably been reluctant to embrace IR, mainly due to fear of warranty issues. Every powder comes with a well-tested cure schedule, based on curing performance in a conventional convection oven. This forms the boilerplate instructions upon which any warranty is based. Any deviation from this potentially offers a get-out clause for the powder manufacturer.

However, the advantages of IR are such that the technology is now widely accepted across many manufacturing industries. The case outlined here shows how forward-looking powder manufacturers are embracing the technology and finding innovative ways to improve the performance of their products to the benefit of manufacturers and end users.

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Chris Chapman is a technical consultant in gas catalytic IR for Heraeus. For information on Heraeus, visit catalyticovens.com.