by Liz White, editor
“Release agents – no-one likes them, everybody uses them.” This was the opening gambit in a recent presentation from Dr Peter Prochnow of Acmos Chemie AG, based in Bremen, Germany.
But Prochnow has a solution to this conundrum, in the form of a newly developed plasma-coating technique for moulds that (almost) eliminates the need for release agents.
It also allows users to make polyurethane parts with a Class A surface finish, as needed for automotive components.
This development is not suitable for all types of polyurethane materials, Prochnow said, noting that it is also most suited to long runs of components.
Pointing out that release agents are extensively used because they are needed to demould components without sticking, Prochnow said: “We consider we have found a solution, with IFAM,” (the Fraunhofer Institute for Manufacturing Technology and Applied Materials Research, IFAM, in Bremen), in a project subsidised by various groups.
The solution lies in using plasma – the phase between a gas and another radical phase, with the material prepared in a closed vacuum plasma reactor (see photo). Acmos can coat moulds in this reactor with the new easy-release coating, in a low-pressure, high-frequency process, at about 50°C.
The method takes the fragments of the reactive gas and applies them to the mould surface. Good bonding to the mould surface is essential, and the mould surface has to be “very, very clean,” with all grease and release agent residue removed before applying the plasma, he said.
Acmos says the patented technology, called ‘Coverel’ is an, “environmentally friendly” coating that forms a permanently release-active plasma-polymer interface. This coating gives Class A surfaces, and the concept has now been developed with luxury car maker BMW AG into a production-ready method.
A two-year qualifying project
BMW spent two years on a joint project with Acmos to qualify the new plasma tool coating for in-mould-coated PU spray instrument panels for the BMW 5 series GT, explained Dr Sven Stein of BMW. Stein and Prochnow made a joint presentation at the 10-11 Nov meeting of the FSK, the German association of foam plastic producers, in Würzburg, Germany.
BMW makes the instrument panels at its Landshut plant. Previously it used an in-mould coated (IMC) sprayed skin, with release agent and a 2-component IMC lacquer with 60-70 percent water content, Stein said. When these coats were dry, a 1 mm thick layer of PU was applied and moulded and the part then demoulded.
“The need to clean the moulds concerned us,” from the point of view of cost, Stein said.
Moulds needed cleaning after 40-50 cycles, or 3 hours in use. Such cleaning interrupts production, Stein said, adding that the project with Acmos was aimed at cutting down on this, while still giving parts with high-gloss, Class A surfaces.
Prochnow said the aqueous release agent used in this approach is only there to ensure the desired surface values such as colour, shine and feel do not change, while ensuring the necessary lubrication in geometrically difficult areas to ease ejection and to limit distortion.
While some release agent is still needed, cleaning is much reduced: 80 percent less release agent is needed, and the resulting components are higher quality, Stein said.
Now, after using the technique successfully in the 5-series, BMW will also use plasmacoated tools in the new 3-series, Stein added.
Some parts need no release agent
Mould service life — the interval between cleaning — can be increased by over 400 percent using plasma coating. Trials have shown that geometrically simple parts can be released 700 times without using release agents on the tool, Prochnow said.
Determination of the average service life of the plasma coating is still pending, Prochnow noted. With other plastics, mould release numbers of up to 5 digits are feasible, he said.
Such coating costs from €3000- €4000 and Prochnow said that, given the costs of release agents, even if the coating only lasts six months before it needs replacing, it is still advantageous when cleaning costs and productivity gains are also accounted for.
Acmos has tested the coating with other PU materials and some it works well with, while others – shoe formulations for example, it does not, commented Prochnow.
For the coating to be worthwhile financially it probably needs to be used on long part runs of 20 000 – 50 000 parts, Prochnow added.
Users must evaluate the component/release agent/mould surface combination with real materials, Prochnow emphasised.
Qualities of the plasma coating
The Coverel coating is very thin, at only 0.5 μm thick, and the plasma covers every surface that is exposed to it, so that corrosion protection is guaranteed, said Prochnow. The coating is also resistant to thermal shock up to 300°C, to organic solvents, acids and alkalis, he noted.
The thinness of the coating allows the finest surface details to be replicated.
No coating is transmitted from the mould to the components. The plasma coating leaves no residue of release agent on the mould or part surface, so cleaning is reduced, as are emissions.
Another advantage is that mould life-time is extended, so that productivity rises and costs are cut, said Prochnow.
Coverel coating can be used for: Thermoplastics such as TPU or PVC; Carbon fibre composite components; Epoxy plastics; and PU systems, in particular IMC coatings (it is not suitable for abrasive PU systems).
RELEASE AGENTS HAVE ISSUES
Mould release agents are standard technology in plastics moulding, but they do have disadvantages. Remnants of the agent can build up in the mould and form a film. This can reduce surface qualities such as ‘feel,’ gloss and colour.
Release agents have other disadvantages: Costs of release agents; Time delay for applying and drying agents; Contamination of surrounding area; Time delay for regular cleaning; Release agent remnants reduce adhesion; and Costs of cleaning moulds and/or moulded parts.
But, Prochnow said, without release agent it is not possible to demould parts non-destructively.
Also, without release agent, the desired Class A surface finish cannot be achieved.