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To provide a review of the latest trends in the field of machinery for closed-mould technology, Alan Harper, Plastech’s chief, describes the technology now being presented to the industry. Without doubt, the main developments are focused on the Light RTM process, although the higher-pressure traditional RTM technology also derives advantage.
(Published on May 2005 – JEC Magazine #17)
BY ALAN HARPER, MANAGING DIRECTOR, PLASTECH TT LTD
The use of closed-mould peripheral injection with the resin converging towards a central mould vent or vent points is now a very common method. To connect the resin delivery from metermix machine to the mould periphery is now more commonly carried out using a self-cleaning injection valve. These eliminate the need for disposable connection pipes and provide a truly clean process.
There now exist about five different suppliers of such valves, all having very similar styles of operation. The new Turbo Autosprue injection valve departs from convention in that it is considerably smaller and less expensive than current models. Its unique instant push-and-lock method of mould attachment and internal doublevortex flow characteristics bring further benefit to the user. The valve has eliminated the initial resin-flow air entrapment characteristic of other earlier valve designs, because it has no internal void catchment zones. The forward flow of resin is thus delivered air free. After injection, flushing is equally effective with water-based solvents as it is with acetone (fig.1, 2, 3, 4).
The need to vent closed moulds has always been a labour-intensive requirement, as either vacuum catchpots or disposable tubing have to be maintained. The use of a low-cost automatic vent in the form of a self-cleaning valve is now available to eliminate this manual operation. Based upon the success of the Turbo Autosprue, the same valve is used to switch between vacuum venting and self-cleaning once sufficient air and resin venting has occurred. A non-intrusive resin sensor is used to automatically activate the system once resin vents the mould, and thus safely and automatically clean the system as with the injection valve previously described.
Flow and pressure
With the Light RTM process now enjoying tremendous application in all composite moulding sectors worldwide, a universal need has arisen to eliminate the guesswork from operator machine control of flow and pressure into these vacuum-clamped moulds. A new “fire-and-forget” technology simply allows the operator to confidently start mould fill, in the knowledge that the machine will not exceed a safe fill rate and over-pressurize the mould cavity. An inexpensive PVSensor system is now available for the production mould; easily installable, it uses micro-electronic sensors housed in a robust PTFE housing. The sensor’s signal provides instant commands to any injection machine to slow or stop the machine, should pressures exceed the current atmospheric mould clamping pressure. Built for continuous production use “and abuse”, the new sensors and systems are specifically designed to survive the rigors of the average composite-moulding shop (fig.5).
New microcontroller-based systems provide highly reliable intelligent control. The new electronic packages are smaller than a box of matches. They are rugged, and consume very little power. However, their application on resin meter-mix injection machine controls is staggering in terms of computing power, allowing greater security and reliability at low cost. One example is the new intelligent system to provide the operator with accurate alarm conditions of resin gel time. This not only monitors the safe working time of the mixed resin system within the mixing head of the machine, but also monitors resin flow rate and constantly computes safety margins before switching to different alarm conditions to provide greater machine-use security.
Catalyst flow alarms with control
It is without doubt every moulder’s concern that during production use, his meter-mix machine should not fail to pump and mix catalyst with the resin. The new CATAL2 catalyst monitor system now provides accurate flow monitoring and intelligent decision-making before automatically stopping the pump in the event of catalyst failure.
Based upon the fact that normal flow of catalyst from reciprocating meter-mix machines is intermittent and can be as low as 5 cc per minute, the new system is programmed to differentiate between normal intermittent flow and actual loss of flow, even at the lowest of flow levels. Intelligent monitoring and machine control of this important fluid without unnecessary stoppages due to false alarms is thus provided.
Improving meter-mix ratio accuracy
All multi-component wide-ratio meter-mix machines for the composite industry use positive-displacement metering pumps. Conventionally, most of these employ metal-to-metal internal valves to ensure forward flow through the pump. It is not uncommon for these valves to fail if kept partially open due to even the smallest foreign particle in the fluid. The result is that the metering accuracy diminishes rapidly. Furthermore, even with no particles but under other operating conditions such as very slow pumping rates, these valves do not always provide the necessary sealing quality required.
A new system of valve seating has now been introduced to greatly improve the accuracy of these metering pumps. The system employs soft elastomeric seating in addition to the metal-to-metal seating. Pumps provided with this form of sealing have been shown to provide perfect vacuum sealing for days, whereas the former metal seats would not seal vacuum for more than a few minutes at best. The main reason to break away from conventional, unreliable metal/metal pump seating was greater demand for metermixing of epoxy resin systems, which requires much higher ratio accuracy than any normal polyester system.