Composites manufacturers always have the need to produce mass quantities quickly. The cost and amount of mould release used in daily production in this industry is relatively insignificant compared with gelcoat, resin and reinforcement. Yet, mould releases are almost always ignored by even the most sophisticated manufacturers. As many composites manufacturers eventually learn, no matter how good the mould design may be or how advanced the moulding process or resin system, it is all in vain if the mould release agent does not perform its function properly.
Effective mould release :
An effective release should have the following.
• Chemical inertness to both mould surfaces. This is essential to counter the adhesion that commonly occurs between the two surfaces. It is influenced by factors such as penetration, chemical reaction and compatibility, surface tension, and the surface configuration and polarity between two materials.
• A low surface tension so that it wets out the mould forming a continuous film.
• Insolubility.
• Heat resistance to avoid being melted by the curing or processing temperature. A strong attraction or bond to the mould surface, while having a similar
• polarity with the wet resin so that it separates easily. Adhesion is often caused by the opposing polarities of the two surfaces.
• Resistance to other factors that can cause adhesion such as high static charges, vacuum formation between two smooth and glossy surfaces or if one of the surfaces remains tacky.
Comparison of labour and raw material costs for a theoretical component
What to look for
Selecting a mould release agent with the above criteria is important, but there are other elements that should be considered. It is the moulder’s natural instinct to place all the blame on the mould release if a part sticks. However, various other conditions should be investigated before automatically condemning the release agent.
Climate: In colder climates, if the mould surface is colder than the shop temperature, there may be an earlier cure on the outside surface of the resin or gel-coat. This would leave the interface surface under-cured or tacky. In effect, this phenomenon will not only cause a significant resistance to easy release, but it may also prevent free styrene monomer from vaporizing. Further, if there is residual, uncured styrene, it could act as a solvent to dissolve the mould release film.
Delayed cure at the interface. The same styrene monomer, which is often used as a mould stripper, slowly dissolves the mould release film. Although a good mould release agent will have some resistance to the styrene, the fact remains that the longer the residence time before gelation the better the opportunity for the styrene to adversely affect the mould release film. Many shops experience sticking, most notably during the change of seasons. At this time, it may be a result of an inadequate catalyst that results in a slower, longer cure. Or, the exact opposite may be true, i.e. too quick a reaction time, which raises the exothermic temperature. A fast, hot cure can melt the wax release film if it has a low temperature tolerance, consequently causing the part to stick to the mould.
Gel time. Gelcoat and laminating resins contain a large amount of styrene and a styrene monomer can be used to strip a mould. So, whenever a gel-coat is sprayed on a mould, a potential stripping agent is applied. Therefore, it is important that the gel time is not too long. Whereas a gel time of approximately II to 20 minutes is good, a 30 to 50 minute gel time may dissolve the mould release surface because of the presence of the styrene monomer. To our knowledge, every release on the market, with the exception of polyvinyl alcohol (PVA), is soluble in styrene.
MOLD BASICS :
Why things stick
• Mould design / construction
• Porosity
• Chemical reaction at the mould face
• Under-cured resin
How mould releases Work
• Vacuum / Static attract ion chemically inert
• Good heat resistance
• Surface tension – low enough to wet-out on mould, no
interference resin flow
Selecting a mould release
• Type of resin and catalyst Process
• Type of moulds (metal or FRP)
• Temperature conditions
• Type of Parts / Special requirements
• Current mould release
• How will the release be applied?
• Who will prepare and maintain moulds?
Case Study : New release agent means less mould maintenance for RTM parts in auto manufacturing
A maker of body components for a major automotive manufacturer has successfully tested and converted to Axel’s XTEND 19 ZAM semi-permanent release for resin transfer moulding (RTM) applications. One application utilizes a tough Modar® resin to produce FRP body and chassis components. The aluminum and epoxy tooling is heated and maintained at approximately 55°C for the female cavity and 45°C for the male plug. The previous mould releasing agent used resulted in extensive mould maintenance such as dry ice blasting and compounding and buffing to prevent resin residue build-up. This process to prepare the moulds for next day production was time consuming and labour intensive. Also, after cleaning, a minimum of three coats of sealer had to be applied, followed by a minimum of three coats of the release at the moulding temperature.
According to Axel, using XTEND 19 ZAM will mean a better release performance in both open and closed mould processes. Four applications of XTEND 19 ZAM are wiped on the clean mould, with application of XTR sealer. In general a 30 minutes dry/cure time is recommended between each coat and after the final coat. The release is applied to the moulds at the moulding temperature. The parts release cleanly and easily from the moulds with a minimum of residue and scumming. Applying a light coat of the XTEND 19W between moulding ensures an even better release and reduces build-up. ln addition, because XTEND 19W does not require a sealer there are significant time savings in mould preparation. The switch to XTEND 19 ZAM has significantly decreased the labour requirements for mould maintenance, says Axel.
Comparison of raw materials and the number of parts moulded from a particular release agent
