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Peel Ply Explained and When to Use Each Style - Part 1

Aug 25th 2021

Peel Plys, otherwise known as Release Fabrics, have many uses throughout the composite industry ranging for achieving consistent surface finishes. Peel plys are generally added after all fabric layers have been laid up. However, peel ply may also be laid up on the tool surface to achieve a needed surface consistency. Peel plys are helpful to achieve overall surface finish of composite laminates or placed in general areas of secondary bonding. Most peel plys work by absorbing a small amount of resin during the cure process. When the cure is completed, the peel ply is pulled from the laminate surface, providing the desired surface finish. Peel plys also minimize mistakes in secondary processes as grinding or grit blasting processes are generally needed on parts using peel ply.

The questions around peel plys are often which type or brand of peel ply is best when different bagging types and layups techniques may be performed. Selection of peel plys are determined by how easy a peel ply will release from a surface, which can be used for a layup technique, which may be best secondary bonding, the material’s temperature range, drapability, porosity ranges, and the overall ease in using a given peel ply.

Porous Vs Non-Porous

When a porous peel ply is placed on top of a part that is placed under vacuum, the part can “breathe” in a sense. A porous peel ply will allow the vacuum to pull excess resin, air, and other volatiles to escape from the part during cure. It allows the vacuum to pull possible entrapments or defects in the part. When controlling excess resin, it can be used in conjunction with the breather to catch the excess resin within the part during cure.

A Non-Porous peel ply system is not going to breathe during a cure cycle and resin will not be pulled from the part. Resin that comes to the surface will cure and leave a consistent glossy surface finish. Note that excess resin and air may become trapped inside the part during its cure cycle. If using non-porous peel ply under vacuum, care should be taken to ensure it is laid flat and bagging is done meticulously. Non-porous peel plys should not be used for vacuum infusion processes but are fine with traditional layups and vacuum cures.

Coated vs Non-Coated

Non-Coated peel plys are usually used for the sole purpose of “texturing” the laminate surface. Because a non-coated peel plys are not treated with any release agent, it is not going to peel from the cured laminate as easily as one that is coated. Care and patience must be taken on during the peel ply removal process of the peel ply to not damage the underlying ply surface, especially with uni-directional laminates. However, non-coated peel plys will not leave release agent residue on the cured laminate surface, which will mitigate possible contaminants in secondary bonding surfaces.

A Coated peel ply on the other hand will be coated in a release agent such as silicone or Teflon (PTFE). These release agents will often make the process of removing the peel ply off cured laminates much easier. Often without any additional tools such pics or razor blades. Care should be taken with secondary bonding processes to ensure all release agents left, usually invisible to the naked eye, are removed from the laminate before bonding.

Best Practices for Peel Ply Use

  • Tools such as a dental pick set, and a razor blade are often used when removing peel ply. When used effectively they are beneficial to removing sticky peel ply easily. Care must be taken to not to damage underlying plys during removal.
  • Extend peel plys an inch or more beyond the lay-up surfaces to peel them off effectively.
  • Remove all air bubbles between the peel ply and the fabric surface.
  • Use caution to ensure peel plys are not bridged in female radii or complex contours of the part. This can cause quality issues with bagging and not getting adequate pressure to the part surface.
  • Use multiple pieces as needed. One piece will come up easier once cured on simple laminates. However, there is nothing to say peel ply must be one continuous ply.
  • Tighter weaves often leave finer surface finishes but may not be as drapable over complex contours.
  • Though data sheets boast that little to no sanding or prep is needed for secondary bonding, it is proven best practice to still roughen surfaces and prep composite parts per “Bonding to Metal and Other Substrates”. Peel ply will get you most of the way there, but the extra care will ensure there are no traces of peel ply remnant left from “peel-off” and ensure a clean bonding surface.

In Part 2 of this short series we'll cover specific products and the characteristics of them.