PVA Frequently Asked Question
IS IT NECESSARY TO USE POLYVINYL ALCOHOL (PVA) IN ADDITION TO WAX?
The answer to this question depends on a number of factors but the bottom line is this: if you do not want a part to stick to and possibly ruin your mold, then use PVA over a wax mold release. If the mold is new or reconditioned, then you should definitely use a PVA such as Partall® Coverall Film or Partall® Film #10 to prevent styrene migration. If the mold is seasoned, then use of PVA is not absolutely necessary except as additional insurance against sticking as long as wax is applied properly and with sufficient frequency. If the mold is very large or intricate you may have difficulty achieving adequate coverage using wax only. PVA can be sprayed into hard-to-reach crevices and forms a visible barrier you can see on the mold surface.
For additional information on the benefits of using PVA (particularly on new molds), please read “Why Molds Stick” by Bob Lacovara (Fabrication News, Feb 1991).
A HARD WHITE BUILD UP HAS ACCUMULATED ON MY MOLD – WHAT IS IT?
The appearance of a hard white build up on the mold surface is commonly mistaken as evidence of styrene in the parting wax or wax build-up. The white substance is more likely styrene that has migrated from within the mold to the surface and broken through the wax barrier. Styrene molecules in the mold are attracted to styrene molecules in resins used to form the part and will bond if allowed to come into contact, causing the part to stick.
Waxes are applied to mold surfaces prior to the molding process in order to prevent bonding. However, heat generated during the molding process gradually softens wax and can inhibit its effectiveness as a barrier. In order to prevent styrene migration between mold and part a polyvinyl alcohol film (PVA) such as Partall® Coverall Film or Partall® Film #10 should be used in conjunction with wax. If applied properly, PVA creates a solvent resistant, yet water soluble barrier through which styrene molecules cannot penetrate. Wax and/or PVA must be applied properly and adequately in order to form an effective barrier.
The use of PVA is particularly necessary on new, repaired or reconditioned molds. Once a mold is seasoned wax alone is normally sufficient as a barrier when applied as needed but PVA can certainly be used as extra protection against costly and time-consuming molding hang ups, particularly on very large, intricate, or expensive molds.
If styrene migration does occur you will need to recondition the mold surface. This type of build up usually requires stripping down the mold with a proprietary mold cleaner or in extreme cases a power sander until styrene is no longer present on the surface. In some cases buffing or hand rubbing the mold with fine abrasives or finishing compounds may be sufficient. The key is to remove all traces of styrene build up from the mold surface. Keep in mind that a reconditioned mold should be treated like a new mold in terms of the waxing/PVA process.
HOW THICK MUST PVA COAT BE TO FORM A SUFFICIENT BARRIER?
Apply to a dry film thickness of at least 2 – 4 mils (50 – 100 µm) on new or reconditioned molds and at least 1 – 2 mils (25 – 50 µm) dry film thickness on seasoned molds. Two mils is approximately equal to the thickness of an industrial-type trash bag. The dry film should be continuous and non-porous.
HOW DO I KNOW IF PVA HAS BEEN APPLIED PROPERLY?
When applied correctly PVA should form a continuous film free of pinholes or air bubbles that is smooth and glossy when dry. Dry film thickness must be at least 2 – 4 mils (50 – 100 µm) on new / reconditioned molds and at least 1 – 2 mils (25 – 50 µm) on seasoned molds (2 mils is approximately equal to the thickness of an industrial-type trash bag).Each coat of PVA must be allowed to dry completely prior to applying additional coats or proceeding with molding. Keep in mind that entire coat – not just the surface – must dry completely in order to form an effective barrier. You may find it helpful to spray a test panel at the same time PVA is applied to mold in order to more easily check that coat has dried thoroughly.
HOW LONG SHOULD I WAIT BEFORE BUFFING WAX?
Begin buffing Formula Five® Mold Release Wax 10 – 12 minutes after application. If wax wipes off easily or “balls up” when buffed then it has not been allowed to form an adequate bond with mold surfaceBegin buffing Partall® Paste #2 and Partall® Hi-Temp Wax Hi-Temp Wax when moderately dry (immediately or within one minute after application or as soon as it hazes over). Buffing will be more difficult if Partall® Paste #2 and Partall® Hi-Temp Wax remain on mold surface for extended lengths of time before buffing.
CAN PVA BE USED ON A WOOD OR PLASTER MOLD WITHOUT SEALING MOLD FIRST?
No, both wood and plaster molds must be sealed prior to application of PVA. Primer-sealers and lacquers intended for composites use generally produce the best surfaces. Mold release wax should also be applied to wood molds prior to use of water-soluble PVA coatings such as Partall® Coverall Film or Partall® Film #10 to prevent moisture that resides naturally in wood compromising the PVA dry film integrity that could result in areas of the mold surface that are not adequately protected.
I’M SPRAYING PVA ON TOP OF GEL COAT AS PART OF A REPAIR PROCESS BUT THE PVA SEEMS TO BE “ATTACKING” THE GEL COAT – WHAT IS HAPPENING?
Two things are probably happening. The gel coat may not have been allowed to become tacky enough (is not hard enough) and the PVA is penetrating its surface. Also, you may be too close to gel coat surface while spraying or air pressure in the spray gun is too high forcing the PVA into the gel coat. Spray from a distance that allows PVA to atomize (mist) completely before coming in contact with the gel coat.
CAN PVA BE APPLIED USING A BRUSH INSTEAD OF A SPRAY GUN?
PVA can be applied using a brush, cloth, sponge or roller or the part may be dipped in the PVA and drained. A professional quality spray gun is recommended in order to produce a smoother surface on the mold. With practice and good spray equipment, a Class “A” automotive quality finish may be obtained. The use of an HVLP spray gun with about 20 PSI (~1.5 bar) pressure at the gun using a spray tip orifice size of 1.8 – 2.2 mm is recommended.
WHY DID PARTALL® FILM #10 “COBWEB” WHEN I SPRAYED IT?WHY ARE THERE AIR BUBBLES IN THE PARTALL® FILM #10 COAT I JUST SPRAYED?
Cob-webbing or spider webs coming out of a gun spraying PVA are due to lack of humidity and high ambient temperatures. The PVA is drying in the air before it gets to the substrate. This is more common in desert-like climates and during the hottest summer months.
Lack of humidity can also result in air bubbles or “fish-eyes” in Partall® Film #10. This happens when the PVA coating dries on the mold surface before air bubbles have had a chance to break.
Low humidity conditions can usually be resolved by adding water to Partall® Film solutions. Up to 10% water may be added by weight or volume. Use the cleanest water possible. Bottled drinking water or distilled water is better than city tap water. When Partall® Film solutions are diluted, the percentage of solids in the mixture decreases, sometimes requiring an extra coat of PVA to achieve adequate dry film coverage and protection.
Other hints to reduce cob-webbing: invest in climate control for the workspace; work in the cooler part of the day such as early morning hours; increase humidity level in the shop by sprinkling water on the floor; chill the Partall Film #10 before application; use the alcohol-free Partall® Coverall Film that has a slower drying time.
CAN I APPLY PVA AND THEN BAKE MOLD IN AN OVEN?
Partall® Film #10 and Partall® Coverall Film consist primarily of partially hydrolyzed polyvinyl alcohol (PVA) and thus can be baked following application to mold surface. The following are guidelines for approximate length of time at specific temperatures –
Temperature in oven Length of Time in oven
200°F (~95°C) 12 hours
225°F (~110°C) 6 hours
250°F (~120°C) 4 hours
275°F (~135°C) 2 hours
300°F (~150°C) 45 minutes
It is not recommended that Partall® Film #10 or Partall® Coverall Film be subjected to temperatures higher than 300°F (~150°C) as carbonization is likely to occur. Carbonization is a process by which molecules in the PVA film bond with molecules on the mold surface. As this carbonization takes place the PVA film becomes more difficult to remove. In extreme cases, sanding may be required to remove PVA from the mold or part surface. Even if carbonization does not occur PVA is likely to require hot water for removal when baked in an oven.
If you choose to bake a mold with PVA on it, keep a close eye on the process and use your own judgment as to when to remove mold from oven.
HOW DO I REMOVE WAX FROM MY MOLD?
Use manufacturer’s instructions for removal of wax from mold surface. If no instructions are provided, use a proprietary mold cleaner such as FORMULA FIVE® Mold Cleaner or FORMULA FIVE® Wax Stripper. Alternatively, a solution made by mixing one pound of Trisodium Phosphate (TSP) with one gallon of hot water makes an effective de-greaser and wax remover. TSP can be found at hardware stores and stores that sell paint-related items. Scrub mold thoroughly with mixture to remove waxes from the surface.
If suggestions outlined above do not remove wax residue or if styrene migration has occurred it may be necessary to use buffing or cutting compounds or sanding techniques to recondition the mold surface. Once reconditioning is complete mold should be treated as if new. Use a high quality composites-specific sealer such as FORMULA FIVE® Semi-Permanent Mold Sealer-S (Super), and then appropriate mold releases for your resin and process.
HOW CAN I ACHIEVE A “SATIN FINISH” USING PVA?
Partall® Coverall Film and Partall® Film #10 normally produce a smooth shiny finish on the mold surface and in turn on the molded part. In order to achieve a matte or non-glossy (i.e. – satin finish) you may add fumed silica to PVA solution. Silica is a very fine crystalline compound that does not dissolve in PVA, thus producing the desired texture.
Fumed silica should be added at a ratio of 1 ounce (~25 grams) per one gallon (3.79 liters) PVA. Mix immediately before use, then allow to sit for approximately one minute until any air bubbles disperse. Note that the addition of fumed silica will increase the viscosity of the PVA solution and adjustments to spray equipment may be required. Some common trade names for fumed silica are Cabosil and Aerosil.
CAN I PREP MY MOLD WITH PVA AND LEAVE OVERNIGHT PRIOR TO MOLDING?
Both Partall® Coverall Film and Partall® Film #10 are water-soluble solutions that may weaken if in contact with excessive moisture in the atmosphere. If you do prep your mold the night before molding be certain to carefully check the integrity of PVA film and make certain that its effectiveness as a release agent / protective barrier has not been compromised. The risk of surface contamination from airborne dust and oils increases with time. In extremely dry and/or cold climates the cured PVA film may shrink away from mold corners if left in place for extended periods.
CAN PARTALL® PASTE #2 BE USED IN HIGH HEAT APPLICATIONS?
No, Partall® Paste #2 is not designed for use in applications exceeding 120°F (~50°C) as wax may begin to melt and may not provide adequate protection. Use Partall® Hi-Temp Wax for higher process temperatures up to 350°F (177°C) or FORMULA FIVE® Semi-Permanent Mold Releases®. Partall® Hi-Temp Wax contains polytetrafluoroethylene (Teflon®) that bonds to the mold surface and continues protecting after waxes have melted away, resulting in high temperature resistance. Because Partall® Hi-Temp Wax does not contain silicone it can usually be used in applications where silicone-based waxes hinder post-finishing operations, such as painting and bonding.
IS IT OK TO DILUTE PARTALL® COVERALL FILM OR PARTALL® FILM #10 PRIOR TO USE?
Partall® Coverall Film and Partall® Film #10 are shipped ready to use and should not be diluted. The addition of water (or any other liquid additive) will decrease the amount of solids per unit of measure, thus requiring that additional coats be applied in order to achieve the same degree of release protection as would be obtained with undiluted product. Never add chemicals as they may not be compatible and may change the product’s flash point, boiling point, etc.
The information and recommendations contained in this bulletin are, to the best of our knowledge, accurate and reliable. No guarantee of their accuracy is made, however, and the products discussed are sold without warranty, express or implied and upon conditions that the purchasers shall make their own tests to determine the suitability of such products for their particular purposes and uses.