The Origin of RTM Technology
RTM technology has been used in the composite industry for more than 50 years. Although the time of the initial application of RTM is not very certain, there have been reports that British Aircraft Corporation began to use RTM technology in the late 1950s. According to Alan Harper, he has been exposed to RTM technology since 1973 when he worked at Aylesbury in the UK. At that time, they bought a 1:1 pump machine from K&C Mold Company and made the first RTM mold. Looking back on the early mold technology, Alan Harper described it as “unbearable to look back on.” He said that the technology at that time could only inject resin through a hole in the center of the mold. People didn’t really know that the mold was full of resin until they saw the resin flowing through the four corners of the mold.
So you can imagine that after repeated production, the exhaust port of the mold will be piled with thicker and thicker, long-dried, stalagmite-like resin, which will inevitably cause huge waste over the years. Although people did adopt better design solutions later, such as sealing the mold cavity and laying a guide tube around it to directly guide the resin to the resin collector connected to the top of the mold. But this still cannot avoid serious waste. Although the mold is closed, the exhaust port on the mold cavity will still release some styrene waste gas, which not only needs to be cleaned regularly, but also needs to be replaced after each cycle.
It was not until the mid-to-late 1980s that the resin injection process began to show some signs of automation. The main progress of the process at that time was that the operator did not have to climb over the mold with the injection valve, insert the injection head into the injection port of the mold, and hold the injection head tightly during the entire injection process. After the injection is completed, the injection valve is pulled out, and then the injection port of the mold is quickly plugged with a plug to prevent the resin from flowing back. When talking about the early RTM process, people may laugh, but in that era, such a process did exist for many years.
Figure 1: In the early RTM process, the operator had to hold the injection valve and insert it into the injection port of the mold.
Later, the invention of the automatic injection valve (AutoSprue) further increased the degree of automation of RTM because it replaced the manual insertion of the mold. However, the handling of the resin in the mold cavity still uses the old method.
Figure 2: Early design of complex mold injection valve
Figure 3: Today’s advanced turbocharger mold injection valve
With the further development of the RTM process, a more advanced lightweight resin transfer molding process, LRTM, emerged. The process was first used in Germany in 1970. By 1996, Jean Jacques Sick, director of Alwo, France, introduced the LRTM process to Charles Tur, a closed mold process expert at MVP, USA, and Jean Paul Galichet, a composite material lecturer at the French National Association of Adult Professionals (AFPA).
At that time, Charles Tur was also a technical support consultant for Socomer, a Belgian raw material supplier. He has been committed to promoting the progress of this new process and providing relevant training to their customers. AFPA lecturer Jean Paul taught this new process to students as a technical training lesson plan.
At the same time, Chomarat Composites also launched a hybrid felt, Rovicore, to the market. As it well met the needs of low-pressure injection molding resin infiltration process, it was well received once it was launched. By 1998, Shomara was traveling around the world to demonstrate to customers how to use the new LRTM technology and molds to make Rovicore hybrid felt.
That same year, Charles Tull introduced LRTM to Plastech (then Alan Harper’s company), which immediately saw the real advantages and potential of this new process and developed a set of RTM equipment and mold accessories (such as seals, etc.) to provide its customers with a complete LRTM tooling package. Plastech also included LRTM technology as part of its global RTM training courses.
When the LRTM process was introduced in the 1990s, it immediately brought a cleaner resin injection system to the market. In addition to the obvious cost and light weight advantages, the process also reduces the chance of contact with the liquid resin mixture. The emergence of LRTM represents the pinnacle of the development of the entire closed mold process. Today’s users have become more and more skilled in operation and can build molds accurately on demand. LRTM is very similar to RTM, also using a closed mold with two pieces, but the production work environment is cleaner than the traditional RTM process, and the waste rate is lower. In contrast to RTM, the LRTM process allows the resin to flow around the periphery of the mold first, and then the excess resin flows out from the outlet located in the center. All of these processes are carried out in a vacuum-sealed environment, so the emission of volatile organic compounds is almost zero.
Transition from RTM to LRTM
It can be said that LRTM has now become the mainstream closed mold process around the world. Although new RTM projects are still being implemented, it cannot change its trend of fading out. The reason is simple. LRTM can produce professional and dimensionally precise closed mold parts with only one-third of the time cost and one-third of the production cost of RTM. Of course, we cannot ignore that if on the same production and operation basis, LRTM requires less infrastructure installation than RTM, and has a uniform and reasonable cost investment.
In the past, people often thought that LRTM was limited to the preparation of relatively simple and small-scale parts, but facts have proved that LRTM can also manufacture some parts with complex shapes and even metal or foam cores. Even composite parts with 55-62% fiber content can be made using the LRTM molding process, and the injection and curing temperatures of the single-component resin system only need to reach 80°C and 180°C respectively.
In addition, the scale of the parts is basically unlimited, as evidenced by the fact that LRTM is already widely used in the shipbuilding industry.
A few years ago, a multiple insert tooling technology (MIT) was introduced, which inserts the A mold of RTM into a multi-layer sub-mold. While one layer is injected with resin, another layer can also be used as a gel, and the remaining layer is used to lay glass fiber. This may greatly increase the production volume of a set of mold equipment, thereby shortening the production cycle, which is the only factor that determines the injection and curing speed of the part.
During the pressing process, the position of the preload layer can be quickly adjusted to ensure that a new injection cycle can be put into place immediately. However, as the LRTM process became popular, people saw that the LRTM process could achieve low-cost mass production like MIT, and most importantly, people no longer needed expensive molding equipment.
A Brazilian MVC molding machine equipped with multiple LRTM molds actually produces 650 gel-coated products with a surface area of 1.5 square meters per day, which shows the efficiency of multi-mode LRTM. Nowadays, LRTM equipment manufacturers encourage their users to use at least two A molds and one B mold to double the production efficiency, while the cost only needs to increase by one third.
Figure 4: Telephone booth made with high-flow LRTM
Investing in pneumatically-controlled injection systems has become a trend in recent years. For example, the pure pneumatic PneuJect III injection molding machine model provides programmable catalyst increase control, automatic flushing and catalyst flow alarm, precision injection pressure/flow control, catalyst/solvent level control and alarm functions. All these functions do not require any electrical assistance and are very suitable for the FRP workshop environment. Pure pneumatic equipment requires the cooperation of a pneumatic pressure/vacuum sensor (PPVS), which senses the injection pressure in the LRTM mold, thereby achieving the purpose of safely controlling the output level of the equipment.
Figure 5: Pneumatic pressure sensor installed on the mold runner, using direct airflow to effectively control the injection rate of the equipment
Figure 6: Pneumatic mold filling sensor
The introduction of pneumatic equipment has also driven the development of pneumatic pressure sensors towards a more intelligent direction. Users can use it to accurately sense whether the resin fills the mold cavity according to the settings. The principle is to control the process of resin injection by sending signals to the equipment through the sensor, thus greatly saving the operator’s labor and reducing the inaccuracy of human estimation. In addition, the sensor can also automatically control multiple vacuum exhaust ports of large molds by sending a closing signal without operator intervention. One thing to note is that these pure pneumatic devices must be designed from scratch because there are no equivalent devices on the current market.
Hot press molding process (SMC/BMC/DMC) has always been a competitor of RTM. However, from the perspective of equipment investment, the necessary steel mold and high-pressure equipment investment have long put the molding process into the high-cost camp, while the RTM and LRTM processes that adopt low-pressure injection also have their own advantages and will continue to coexist in this market.
At present, some manufacturers are still moving towards the direction of resin film penetration process after getting rid of the mold opening process. However, compared with LRTM, the infiltration process is inevitably restricted by its own limitations, such as single mode, high cost, and difficulty in handling waste.
When companies first move to closed mold technology, the most common question is how to make a B mold that is accurate enough to form a complete closed mold tooling. Many people watch the live demonstration and think that as long as they use the correct sealant, slot counter, and filler, they can prepare an accurate mold, but in fact, most of them fail because they do not understand how to achieve accuracy within the dimensional tolerance.
This is a good training course for many people, and they can use this opportunity to explain all the technical issues such as machine output control, selection and placement of maximum seals, injection molding and vacuum exhaust, mold heating, mold material selection, and practical experience.
Vacuum leaks are also a common problem in LRTM. The cause can be insufficient vacuum or improper installation of injection connections. Obviously, mold manufacturers need to pay special attention to these points, but in fact, any common insert can help. What mold manufacturers need now is a standard unified insert connector. Therefore, it is necessary to unify the universal standard of inserts to apply to various accessories, including injection valves, injection tube connectors, flange vacuum connectors, resin collectors, secondary vacuum exhaust connectors, pneumatic or electronic sensors, and air injection connectors.
The shift from hand lay-up and injection molding to closed mold molding is a cultural shock for most companies. With increasingly stringent controls on processing temperature, gel time and production cycle, traditional open mold molding will inevitably be abandoned.
Where will RTM/LRTM go?
Existence is reasonable. Closed mold molding processes are popular because they can meet most needs. If not, there will be new regulations to limit volatile organic compound emissions. They also provide a much improved and socially accepted FRP working environment. This ability to reduce production scrap rates and help parts achieve professional double-sided quality should not be underestimated.
The recently popular central runner keyhole injection method, that is, injecting resin through a 1mm diameter hole on the B mold, can significantly reduce resin consumption and waste by avoiding the waste of filling around the mold cavity. Generally speaking, central runner keyhole injection is suitable for parts with a large height-to-width ratio, such as a bus panel with a length of 3.2 meters and a width of 0.3 meters. For example, the resin filling volume for the 7-meter perimeter mold cavity is at least 75% higher than the injection volume for the central runner hole. In addition, since the sealant can seal the perimeter of the molded product, it ensures near-zero waste and a net-size part throughout the production process.
Central runner hole injection is also beginning to be used in the manufacture of LRTM wind turbine blades, which are now mostly made by wet hand lay-up and resin film penetration. In addition, the production of leading and trailing edges of blades using LRTM’s sophisticated in-house design capabilities can achieve the desired streamlined shape and more precise engineering design.
RTM/LRTM will continue to shine for a long time to come with its advanced mold and control system technology, and its role in cost efficiency, waste and consumption efficiency, and in reducing volatile organic compounds. However, users of RTM/LRTM need to be aware that only efforts to improve applications and train corresponding employees can truly make this advanced process useful.
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