Dear Joe,

I have some brake calipers that I powder coated and I would like to add lettering then then add a clear powder coat and put in the oven and bake it. I would like to know of any material (vinyl, decal or anything) that can withstand 200 ⁰C to be powder coated over.

Thanks,
Ismael Bahrain

Dear Ismael,

This is a tricky proposition. First of all, PVC (vinyl) decals will not take 200 ⁰C. So they're out. There are a number of materials used to make decals and the table below provides type and heat resistance.

Decal Materials and Heat Resistance

Polymer Type

Synonyms

Heat Resistance (C)

Polycarbonate

PolyCarb

65

Polyester

 

120

Polyvinyl Chloride

PVC, vinyl

150

Polyethylene Terephthalate

PET

180

Polyimide

 

500

For your application I would consider PET or polyimide as the decal polymer. PET may be borderline for heat resistance, however you should be able to use a clear powder coating that can cure around 180 ⁰C and avoid exceeding its heat resistance threshold.

Otherwise you will have to use a polyimide-based decal, which easily outperforms all other decal materials for heat resistance. Polyimide tapes are regularly used as masking for powder coated articles so I can attest to their resistance to typical powder curing temperatures.

Preparing the surface for the decal and choosing a powder coating are also important considerations for the success of your project. The surface should be wiped clean with a good solvent. I recommend either denatured alcohol or acetone. Both are readily available at your house paint store or DIY.

I would recommend a low-temperature-cure polyester powder coating as your clearcoat. Epoxies discolor with heat, and polyurethanes have a tendency to outgas at thick films. Polyesters capable of low temperature (i.e. around 160 ⁰C to 170 ⁰C) are commonly available. These products have good heat resistance and can be applied at relatively thick films to give a high gloss with excellent distinctness of image. Please ensure that the coating is fully cured to make certain that it provides full durability performance.

Please let me know if you need anything more.

Best regards,

Joe

 

Hi Joe!

I am having an issue that I haven’t seen before. We manufacture fairly standard welded metal cabinets and carts. The issue I am having is with a particular cart that would resemble a single-drawer file cabinet with a pencil drawer that most people have in a cubicle. The bottom of the cart fits over sides, creating a slight “lip” or edge if you will. My vendor claims that there is “oil” trapped in the seam and when the part runs through the cure oven the oil seeps, causing issues with the finish.

We hang these upside down using the caster holds for hanging points. So, the seam is upside down.

We are using a 6-stage iron-phosphate, dip, prewash system. The dry-off oven runs at a fairly consistent 200 ⁰C. As does the cure oven. The vendor wants to blast the seam with something resembling a WWII flamethrower!  This solves the oil issue, but contorts the part in some cases.

Do you have any recommendations as to how we can resolve the issue without ‘breaking the rules of the Geneva Convention?’

Thank you and kind regards,

John Zhongshan, China

Hey John,

The unseemly woes you describe are rather common. Fabrication lubricants and machine oils can get trapped in seams and cause headaches further down in the finishing process. There are a few approaches you can consider to alleviate this issue, although flame-throwing, whilst it undoubtedly is an exciting operation, seems a bit extreme and hazardous solution.

The first consideration is to eliminate the problem at its source. This entails cleaning the metal thoroughly before the seams are made. Seems simple (simple seams?) but this would guarantee no oil seepage when the powder is curing in your vendor's oven.

If this is not possible you may want to consider stronger impingement of cleaning solution at the seams. It seems like the use of a power washer could dislodge any residual oil and reduce the problem. This would be a manual process that would slow productivity and incur additional cost. It may require modest capital expenditure as well.

It seems to me that the best option to reduce this unseemly issue is to attack it at the source. Keeping the metal clean before the seams are established should alleviate the problem without compromising the Geneva Convention or any subsequent international treaties.

Kind regards,

Joe

 

Dear Joe,

A colleague of mine is having a problem with a powder coater out here. She gave them a table that was rusty in some parts and asked them to powder coat it for her. Well, they did and after a few weeks of wear and tear (I believe they keep it outside) it is rusting again. So, they fixed it for her again, but it is rusted again. Long story short any suggestions on what she should do?  I guess the table means a lot to the family and she isn't afraid of the costs involved, she just doesn't want it to keep rusting.

Thanks,

Donna, Pennsylvania

Hi Donna,

The problem with the premature rusting is a combination of two or three issues. 1.) It would appear that the original rust may not have been completely removed prior to the application of the powder coating. 2.) The metal may not have been chemically pretreated prior to the application of the powder coating and 3.) The film thickness of the powder may be low or in-continuous in areas.

To correctly powder coat the table the coater needs to:

1. Clean the metal 100%. I recommend shot blasting (aluminum oxide or silicon carbide).
2. Chemically pretreat the metal before application of the powder. A reputable jobcoater will have an iron phosphate system that will do a good job if they keep the process in control.
3. Thoroughly coat the table with a high-quality powder coating. This sometimes will involve preheating the table and spraying the difficult-to-coat areas while it is still warm. Alternately they can apply two coats of powder (primer followed by a topcoat). This isn't necessary if the coater is careful in applying the powder in the first place.

Please tell your colleague that it's really important to ask her coater about how they prepare the metal and control the film thickness. I hope that this helps.

Kind regards,

Joe