Presentation on theme: "Rene Herrmann 2011. RTM process (resin transfer moulding) is the most used process for composite mass manufacturing. For biocomposites the basic idea."— Presentation transcript:
Rene Herrmann 2011
RTM process (resin transfer moulding) is the most used process for composite mass manufacturing. For biocomposites the basic idea is that the material is ’bio’ because of its resources are regenerating or/and it is bio degradable. Composites are not biodegradable, only the energy content can be regained by burning. In our case the fibers are biological and the resin is partially biological as opposed to organic. (observe that organic does not imply the material to be biological)
In composite mass manufacturing the parameters are similar to other plastic processes. The issues is the cycle time of the tool. (Most) Composites use termoset resins, their curing is exotermic and the heat released accelerates the curing. Thick laminates can become very hot. The resins have however a maximum curing temperature (e.g. 160C) beyond which the polymer undergoes molecular ’cracking’ (opposite to polymerisation). Operating at the optimum requires the ability to heat (start up) but also to cool (stop). A perfect processes is defined by rapid wetting of the fibers and a speedy curing closed to the limit of resin curing temperature and a followed release of the product such that the next product can be made.
The process of fiber wetting is coverned by the surface energy of both interacting constituents. The surface energy of a liquid (resin) can be measured using contact angle measurement setup. A droplet that is not wetting will remain a ball, a liquid that wetts well will however spread out on a surface. The wettable surfaces are the fibers. Biological fibers have variation in their properties and little can be done to them. However one aspepect has to be considered. Electrostatic charge: The fibers can become an electrically charged object like a condensator as they fibers themselfs are electrically nonconducting. If the fibers carry charge and their potential is the same as that of the resin both will repell each other. In order to remove or redistribute charge from/in the laminate an electrical current must flow. The only practical way is that the resin conducts current and redistributes this way any charges within the material.
The resins fiber wetting requires it to conduct charge. This is the case as long as the resin is uncured. However the number of charge carrier is the same as the number of free ions and that in turn increases as higher the temperature of the resin (liquid). Wettability increases therfore with resin temperature.
Resins fiber wetting also depends on the viscosity. The resin must rapidly flow to all places in the mold and wett the fibers, the flow velocity increases with decreasing viscocity. The viscocity of the resin drops dramatically if the temperature increases but this starts also a much faster curing of the resin, a decrease in geltime of the resin.
The process is a combined vacuum and pressure assistent lamination within a closed mold. The mold receives a release coating and the dry fibers. The mold is closed and the resin is ’injected’. The injection can happen by either, pressure and/or vacuum. If vacuum is used the mold must be vacuum tight as the laminate will draw otherwise not only resin but also bubbles. The result will be a product with a lot of voids. If the resin is injected by pressure the mold itself has to withstand that pressure and is most often more expensive than a vacuum tool.
If the resin is to be fed by pressure a resin pumping mechanism is needed. There are two kinds and they differ significantly. Alternative 1, a commercial resin pump. It has 2 separate containers, base and hardener and mixing chamber. Often there is also a tank with cleaner. The system is computer controlled and the flow rate and pressure is ajustable. Besides its prices (>10keuro) the system is CONTINOUS, because resin is mixed with its hardener continously, only the tanks have to be refilled. Consider a laminate having more than 1t of resin in itself and all should be mixed by hand in small buckets.
Resin pumping, alternative 2: If the piece is small and continous mixing is not needed a much cheaper solution is available to do the same. Consider a platic bottle in which both resin and hardener are mixed (batch), the bottle is closed and 2 separate hoses enter the bottle, one reaching the bottom of the resin mix and one stays above the substance. If the later receives air pressure the liquid mix will be pushed out of the other hose. (Bernoulli). This setup is feasable for upto 20kg of resin batches. There is 2 things to be considered. First the flow speed of the resin is ajustable by means of air pressure, however the tank has to withstand that pressure. The tool (as often home made) has to be tested with water first. Also the material has to withstand resin chemistry. Secondly, the tank has to be transperant because you will otherwise not now when the resin is all pumped out. This setup is in use (famous finnsh pampas) and it is reported that an improvement to the setup is that a third inlet can be introduced to even refill the tank during a momentary stop in pumping/depressurisation
If the tool is not metal than heating of the tool is impractical, however resin can be heated before pumping, normally after mixing of base and hardener. Fibers are cut using a model, in mass manufacturing the textiles are ordered cut to size. Laser cutting is there the refered choice of handling. The reason is not only the precision but also that the fibers melt on the edges/cuts and ’weld’ themself together, the textile does disintegrate while handled.
Study the following links: rbus-a340-carbon-composite-spoiler-made- with-rtm rbus-a340-carbon-composite-spoiler-made- with-rtm
You will receive a transperent 5 liter plastic container. Plan building of a resin pump that can be cleaned and reused. The MAXIMUM pressure is 1atm above enviromental pressure. Pressure is regulated using Arcada’s air pressure system.