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Tuesday, November 5, 2013

3-Coat Reinforced and Proper Coating

A picking roller with 3-coat reinforced coating that was over baked.

DuPont’s premier coating system combines excellent durability and lubricity in a thin film (1.5mils). This 3-coat, reinforced, water based system, 857G-040, 857G-140 and 857G-240, differs from many of our other water based Teflon*coating options because it contains a primer, a mid-coat and a topcoat. However, it requires a high temperature, short duration bake cycle and due to the sensitivity of this cure cycle, the coating is susceptible to curing errors leading to re-work and lost money.

In order to control the sensitive cure of this DuPont Teflon® industrial coating system, it is highly recommended that the use of direct wired thermocouples attached to the substrate  to prevent the three most common curing errors for this system:  under-cure, over-cure, and improper part temperature ramp up.   The use of IR thermometers is discouraged because they create inaccuracies and oven heat loss.
From an application perspective, this coating is relatively easy to apply.  The primer is applied over an ambient part that has either been grit blasted or is smooth (aluminum) and force-dried at 250F to drive off the water. The mid-coat and topcoat are applied wet over wet, so there isn’t a cure or drying required between mid-coat and topcoat.
The last step in this process is a five minute cure at 800F, which is the highest cure of any DuPont Industrial Teflon*products.   This coating system does allow for a higher cure, 815F for three minutes, but peak temperature should never reach more than 825F and the amount of time at that high temperature should not extend past three minutes.

With an undesired film finish, it is imperative to understand the end result and how to rectify.   An under cured film will lack gloss and produce a soft film, which can be fixed by recurring at the proper oven temp.  An over cured film will display a haze and a yellowish color, but in extreme over bake situations the film will discolor to a blue/gray state.   Unfortunately, the only way to correct an over bake situation is to grit blast the part and start the coating application process again. 

The least common problem is an improper part temperature ramp up which can result in the coating experiencing too high of a temperature in an expedited timeframe.  This can cause a situation called “skinning” where there is a vast difference in the substrate temperature and the coating’s temperature or the substrate temperature elevates too quickly.   It is not recommended that a freshly coated, ambient part be placed in an oven that is at 815F.

How often do curing errors occur in your shop?  Is re-work a common issue for your coaters?

Register for an account on Intech Services to create your own private customer center portal and order a 3-coat reinforced coating for your needs.

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