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Tuesday, August 31, 2010

Technical Tip from Intech

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 Edge Pull / Shrinkage Overview

A common issue that many coaters encounter is edge pull or shrinkage of the coating material from a parts corners or sharp edges.  With the assistance of the DuPont R&D team and several other sources, we reviewed this phenomenon and compiled the following information to help coaters understand and control the variables that contribute to edge pull.

Overall, edge pull is a function of many factors:
     • Type of metal
     • Thickness of metal
     • Radius of the corner or edge
     • Thickness of fluoropolymer
     • Type of fluoropolymer

In many situations, the underlying cause of edge pull is the cooling difference between the metal and coating material.    During the curing and cooling process, fluoropolymers can potentially shrink ten times more than the metal surface.  This difference in shrinkage can cause the coating to pull away from corners and outer edges where there is more stress on the coating material from the shrinkage.  

Shrinkage, or edge pull, is much more pronounced in thick film coatings (25-40 mils) due to the increased coating stress.  For coatings 10-25 mils thick, edge pull can be seen, but is usually not a major factor.  For coatings under 10 mils, edge pull is minimal.  The photo below illustrates severe edge pull on a thick film coating of DuPont Ruby Red PFA.

The type or molecular weight of fluoropolymer does not seem to significantly affect edge pull, although ETFE seems to show less edge pull than PFA.  Because FEP and PTFE are typically very thin film coatings, they typically do not exhibit signs of shrinkage.   

Also, surface preparation does not seem to have any affect on minimizing edge pull.

Edge pull problems can be minimized by adopting procedures which address one or both of the following areas:
     1) Cooling rate of the part– A slow cooling rate between the final bake temperature and the melt-flow temperature of the fluoropolymer.  For example, if a final bake is 750F and the melt-flow temperature of the fluoropolymer is 585F, then the cooling rate should be slow from 750F down to 585F.  Once below 585F, edge pull is less likely to occur.

     2) Radius of the edge or corner - Thin film coatings can take a smaller radius (sharper edge) than thick film coatings.  High build coatings, 25-40 mils, should have corners that are properly prepared in order to increase the likelihood of a successful coating.  Concave corners should have smooth radii that blend into the sides of the corners.  It is recommended that 3/8” - 1/2” radius welds be used for the best performance. Convex corners should be rounded over to 1/4” - 3/8”radius or the thickness of the metal, whichever is less. 
The drawing below is a simplified version of good preparation and poor preparation.



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