Building a Better Powder Coating System: Pretreatment Equipment

Posted on Wednesday, February 6, 2019

You can have the greatest, most beautiful coating in the world, but without decent pretreatment, it’ll be worthless and flaking off in sheets within days. 

At PEM, our favorite event of the year has always been the FABTECH show. We love spending time with our finishing industry family and enjoy the chance to share stories and experiences. While working our booth at the show, we’re always amazed at the amount of confusion people have when it comes to pretreatment equipment. Potential customers constantly approach our booth with bewildered looks on their faces wondering where to start. 

Even in today’s world, it’s not uncommon for someone to ask us for help when they’ve been scrubbing parts with solvent or spraying with a garden sprayer in order to get by. Their idea of a pretreatment system is limited to a huge, expensive multi-stage washer and it has always scared them off. With this article we hope to remove this fear by getting to the equipment basics and walking you through the process. We’ll only scratch the surface with the space allowed in the article, but our goal is to give you the basic information you need in order to have an informed conversation. You can follow up with any chemical or equipment supplier to discuss your specific needs in detail. 

The basics of any pretreatment system are simple. Get the parts clean and apply a pretreatment that will enhance adhesion and corrosion protection. 

When planning a pretreatment system, it’s important to consider production requirements, part size, and coating performance. Ask yourself: How Fast? How Big? How Good? The answers to these questions will guide you to whether you should pretreat with a batch system or a conveyorized system. Either method provides a quality pretreatment; your production needs and part size will determine which is right for you.

A conveyorized system is typically used to pretreat medium to large volumes of parts. This consists of a monorail conveyor carrying parts through a tunnel of spray nozzles that douse the parts with various chemicals and rinses. Parts pass through a series of spray zones that are separated by drip zones. 

A batch system is used for smaller quantities of parts or parts that are too large to fit economically into a conveyorized system. The batch system consists of a pressure washer that sprays chemicals through a spray wand, while the operator maneuvers the wand to manually apply the chemical onto a part that sits stationary. This is done inside a spray bay or an enclosure that contains the moisture. 

Determining pretreatment quality is the next step. The difficulty of removing your soils and the level of corrosion protection required will determine your equipment’s performance requirements. Your chemical supplier can guide you through this process in detail and provide you with anything from a simple one-step cleaner/coater to multi-stage processes that will provide years of corrosion protection. 

In general, a one-step cleaner/pretreatment such as a cleaner phosphate followed by a rinse does a nice job for a light-duty application. If you need to remove heavy oils or require intense outdoor corrosion protection, you may need to split the cleaning and pretreating into two steps. The reason for this is that a cleaner/pretreatment is generally acidic to provide the pretreatment, but acids are not ideal for cleaning. An alkaline normally cleans better but is not great for pretreatment. By splitting the cleaner and pretreat, you get the best of both worlds by cleaning with an alkaline and pretreating with an acid. 

Three-stage or five-stage washers have historically been the most common sizes seen in the industry. A common threestage setup would be: 1) heated cleaner/phosphate, 2) rinse, 3) rinse. Typical five-stage would be: 1) heated cleaner, 2) rinse, 3) heated phosphate, 4) rinse, 5) rinse. There could also be many variations of this depending if nanochemistry such as zirconium is used or if sealers are added to enhance corrosion protection. 

It’s important to remember that all of these stages can be done in either a conveyor or a batch application. The conveyorized washer will have a tank for each stage with nozzles above spraying chemicals onto parts as they pass through. In the batch application, the chemical is simply injected into the spray wand stream after the pump, so a spray wand can quickly change from one chemical or rinse to another. The part doesn’t move. The operator just switches between chemicals to provide the various stages. 

If a batch process is desired, the part size will determine the area of the spray bay. This spray bay can be as complicated as a booth structure that contains run-off and pumps it to a holding area or it can be as simple as a shower curtain near a drain. Your chemical and equipment suppliers can help you with this since your specific need will depend on your application. When considering a spray wand, you’ll want to consider pressure, heat, consistency of chemical injection, and reliability. There are also several options available such as multiple hoses for quicker chemical changes, hose reels, drum top pumps for sealers, etc. Chemical and equipment people can educate you on these options as well. 

If the conveyor process is best for you, the equipment gets a little more complicated. The part size will dictate the height and width of openings and the length of drip zones. Openings will have to be large enough to allow parts to pass through freely when hanging, while the drip zones will have to be long enough, so parts can clear one spray zone before entering the next to avoid cross-contamination. The length of the spray zones will be determined by the line speed and the time required in each zone. For instance, if your parts need to be exposed to a cleaner for one minute and your line is running six feet per minute, the spray zone has to be six feet long. 

In the past, it was common to build washers out of mild steel in order to keep initial costs down. Even with the higher costs, stainless steel is becoming more common today because of its longevity and ability to withstand today’s harsh chemicals that are involved in nano pretreatments. 

Poly washers are another recent alternative. Washers made of this bright white material are chemical resistant, easy to clean, and less costly than stainless steel. Shrouds are removable for easy maintenance and stages are modular, so they can be quickly installed or easily modified for future expansion. 

We hope this quick overview gets your pretreatment plans underway for building a better powder coating system. Understand that we just scratched the surface. We didn’t touch on other considerations such as heating equipment, filtration equipment, RO water treatment equipment, chemical monitoring equipment, spray nozzle types, and rinse counterflow options just to name a few. Chemical and equipment suppliers are all great sources of information on these topics and can help determine the pretreatment approach that best fits your needs. 

byTed Schreyer, president of Associated Finishing Inc. and vice president, PEM.