Allen Har Cooper (Alan Harper), an expert in the field of resin transfer molding (RTM), is a member of the American MVP limited. This article is about his origin, development, and application of the RTM process, as well as the later light resin transfer molding process (LRTM).

Traceability RTM Process

The RTM process has been used in the composite industry for more than 50 years. Although the timing of the initial application of RTM is not quite certain, there have been reports of British aircraft companies starting to apply the RTM⼯ process in the late 50.

According to Ais Leng Harper, he has been in contact with the RTM process since 1973, when he worked in Aylesbury, UK. At that time, they bought a 1:1-pump machine from the K&C mold company, making the first RTM mold.

Looking back at the early tooling process, Allen Har Cooper used to describe it. Using the craft at the time, he says, the resin can only be injected through a hole in the center of the mold, until the four corners of the mold are ⼀ through the resin, and people really know that the mold is full of resin.

So you can imagine, after repeated production, the exhaust nozzle of the mold will be piled thicker, has long been dry, such as stalagmites of resin, the perennial tired month will inevitably cause huge waste.

Although later, people did take a better design, such as sealing the die cavity and laying a diversion tube around it, directly draining the resin into the resin collector outside the top of the mold.

But doing so still cannot avoid serious waste, although the mold is closed, the vent on the cavity will still release a little styrene waste gas, which not only needs regular cleaning but after each cycle needs to be replaced.

Until the late 80, the resin injection process began to show some signs of automation. The main development of the process was that the operator did not have to carry the injection valve to the top of the mold, insert the injection head into the injection mouth of the mold, and tightly grasped the injection head throughout the injection process.

After the injection is completed, the injection valve is pulled out, then the plug is quickly plugged into the injection port of the mold to prevent the resin from flowing backward. When it comes to the early RTM process, ⼈ people may laugh, but in those days, the process did exist for many years.

Then, the invention of the automatic injection valve (AUTOSPRUE) increased the degree of RTM automation, as it replaces the steps that are manually inserted into the mold. However, the resin in the cavity is still used to manage the old method.

With the further development of the RTM process, a more advanced lightweight resin transfer molding process-LRTM was developed, the first application of which was in Germany in 1970.

By 1996, Jean Jacques (Jean Jacques Sick), head of the French alwo company, introduced LRTM process to the American MVP die-closure expert Charles Tours(Charles Tur) and the French National Association of Adult Professionals (AFPA) composite material lecturer Jean-Paul ( Jean Paul Galichet).

At the time, Charles Tours had another status as a technical support consultant for Belgian raw material supplier Socomer, who has been working to advance the new process and to train their clients. AFPA lecturer Jean-Paul the new process as a technical training lesson to students.

At the same time, Chomarat composite material Company (Chomarat composites) also will be mixed felt-rovicore to market, as well as the low pressure injection resin penetration process demand, once listed on the popular acclaim. By 1998, Chomarat company began to run around the world to show customers how to use LRTM new technology and mold manufacturing Rovicore mixed felt.

In the same year, Charles Tours introduced L-RTM to Plastech (then Allen Har Cooper’s company) and immediately saw the real advantages and potential of the new process.

Plastech Company has developed a set of RTM equipment and mold fittings (such as seals, etc.) to provide its customers with a complete LRTM tooling package. Plastech also LRTM technology as part of its global RTM training course.

As a result, the LRTM process, once introduced in the 90s, immediately brought a cleaner resin injection system to the market. In addition to its obvious cost and lightweight advantages, the process also reduces the chance of contacting a liquid resin mixture.

The advent of the LRTM represents the culmination of the entire closed-mold process, and today’s users have become increasingly adept at operating and building molds on demand.

LRTM is similar to RTM, but also takes the upper and lower two pieces of closed mold, but more than the traditional RTM production work environment cleaner, and the waste rate is lower.

In contrast to RTM, the LRTM process allows the resin to flow through the periphery of the mold, and the last excess resin flows out of the central outlet. All these processes are carried out in a vacuum-sealed environment so that the emissions of volatile organic compounds are almost nil.

RTM transition to L-RTM

It can be said that L-RTM has now become the mainstream closed-die technology around the world, although the new RTM project is still in place, it can not change its fading trend.

The reason is simple, L-RTM can use only one-third of the time cost of RTM and One-third of the production costs, the preparation of professional strong, ⼨ inch precision closed-die parts.

Of course, we can not ignore that if the same production and operation on the basis of L-RTM⽐ then RTM need to install less infrastructure and have the cost of homogenization rationalization.

In the past, it was often thought that Lrtm was limited to the preparation of relatively simple and small parts, but the fact has proved that LRTM can also make parts that are complex in shape and even in metal or foam cores.

Even if the fiber volume up to 55-62% of the composite parts can be completed by the LRTM molding process, and single-component resin system injection and curing temperature only need to reach 80°c and 180°c respectively.

In addition, the size of the workpiece is also basically unrestricted, LRTM has been widely used in the shipbuilding industry is one of the best evidence.

French shipbuilders Jeanneau have been using the LRTM process for more than 6 years, and are now able to inject 142-foot-long yacht decks with complex shapes. At present, the company continues to invest, in order to further extend the process to the entire hull parts manufacturing.

A few years ago, a multi-layer insert Die Technology (multiple insert tooling technology-mit) was introduced, and a module of RTM was inserted into the multilayer accessory mode. When one layer is injected with resin, another layer can be used for gel, and the remaining layer is used to pave the glass.

This may greatly increase the production of a set of tooling equipment, thereby shortening the manufacturing cycle, knowing that the production cycle is the only factor determining the injection and curing speed of the workpiece.

In the process of compaction, the position of the preload layer can be adjusted quickly to ensure the new injection cycle can be injected immediately.

However, as the LRTM process became popular, people saw that the LRTM process could deliver low-cost production like MIT, most notably because people no longer needed expensive molded equipment.

A Brazilian MVC molding machine equipped with multiple LRTM molds produces 650 sheets of 1.5 square meters of plastic coating products per day, and the preparation efficiency of multimode Lrtm is evident.

Today, LRTM equipment manufacturers are encouraging their users to use at least two a-mode with a B-mode to double production efficiency, while costs only need to increase by one-third.

Investment pneumatic Control injection system (pneumatically-controlled injection system) has become a trend in recent years.

For example, the pure pneumatic project III injection Molding machine model can provide programmable catalysts for increased control, automatic flushing and catalyst flow alarms, precision injection pressure/flow control, catalyst/solvent level control, and alarm functions.

The implementation of all these functions does not require any electrical assistance, is suitable for the FRP workshop environment. But the pure pneumatic equipment needs the pneumatic pressure/vacuum sensor (PPVS) the coordination, namely through it perceives LRTM mold The injection pressure, therefore achieves the safety control equipment the output level the goal.

The introduction of pneumatic equipment also drives the pneumatic pressure sensor toward a more intelligent direction, the user can according to the setting through it accurately perceive whether the resin is filled with the mold cavity.

The principle is to send signals to the device through the sensor to control the process of resin injection, thus greatly saving the operator’s labor and reducing the inaccurate factors of manpower estimation.

In addition, the sensor can realize the automatic control of many vacuum vents of large die by sending off signal without operator intervention.

It is important to note that these pure pneumatic devices must be designed from scratch because there is no comparable equipment in the current market.

All along, hot-pressing molding technology (SMC/BMC/DMC) is the competition of RTM, however, from the equipment input, due to the necessary steel mold and high-pressure equipment investment has long been the molding process into a high-cost camp, and the adoption of low-pressure injection of RTM and LRTM process have their own advantages, will continue to coexist in this market.

There are still some manufacturers in the direction of the resin membrane infiltration process after they get rid of the open mold technology. However, compared with LRTM, the infiltration process is inevitably limited by its own limitations, such as single-mode, high cost, and difficulty to deal with waste.

The main challenge for RTM/ L-RTM is that many FRP makers have not yet waited for the relevant procedures to mature due to stringent emission levels, and have immediately emerged from the traditional open mold process (such as hand lay-up and spray).

And to do this, especially in the current economic environment, is the need for the government of the industry’s cash-strapped enterprises to provide training related to closed-mode technology.

If they are simply barred from stopping ⽌ volatile organic compounds without any financial assistance to help move towards more modern models of closure, they are likely to push the overall fiberglass industry to the brink.

At the beginning of the enterprise steering closed-die process, the most common problem is how to make a precise B-mold form a complete set of closed-die tooling.

Many people who saw the live demo thought that using the right sealant, the slot-mode counter, and the filler could produce accurate molds, but the fact was that they were largely unsuccessful because they did not understand how to achieve accuracy within dimensional tolerances.

And that’s a good training course for a lot of people, this opportunity can be used to explain all of the technical issues, such as equipment output control, the largest selection, and placement of seals, injection molding and vacuum exhaust, mold heating, mold material selection, and practical lessons learned.

Vacuum leakage is also a common problem in Lrtm. Causes can be caused by the inadequate vacuum or improper installation of injection connections.

Obviously, mold manufacturers need to pay special attention to these points, in fact, choosing any kind of ordinary inserts can help.

Now the mold manufacturer needs a standard, uniform insertion connector. Therefore, it is necessary to unify the universal standards of inserts to apply to a variety of accessories, including injection valves, injection tube connectors, flanged vacuum connectors, resin collectors, two-stage vacuum exhaust connectors, pneumatic or electronic sensors, and air-jet connectors.

Turning from hand lay-up and jet technology to closed die technology is a technical and cultural shock for most companies. With the processing temperature, gel time and production cycle more stringent control, the traditional open mold process will be abandoned.

Where will RTM/ L-RTM go? Existence is reasonable. Closed-die processes are popular because they can satisfy most needs, and if they are not, there will be a new law and order to limit the emissions of volatile organic compounds.

Also because they can provide a much improved and universally accepted FRP working environment. This not only reduces the production scrap rate but also can help the workpiece to achieve the professional double-sided quality of the ⼒ force that can not be underestimated.

Recently heated central sprue hole injection (runner keyhole injection) mode, that is, through the B-mode of a diameter of 1 mm hole injection resin, as a result of avoiding the mold cavity around the waste of filling, so can significantly reduce resin consumption and waste.

Generally speaking, the central gate hole injection is suitable for the high width and larger parts, such as 3.2 meters x 0.3 meters wide bus board.

Another example, the surrounding 7 meters of the mold cavity resin filling volume than central sprue hole injection volume of at least 75% higher.

In addition, due to the sealing agent can be molded products around the sealing effect, so as to ensure that the entire production process of nearly 0 waste rate, and the preparation of the net size of the workpiece.

The central sprue hole injection has also been applied to the manufacture of LRTM wind turbine blades, and today most of the blades are made by wet hand lay-up and resin membrane infiltration.

In addition, the use of LRTM precision internal design capabilities to produce the leading edge and trailing edge of the blade can achieve the ideal streamline shape and more accurate engineering design.

It is believed that RTM/ L-RTM, with its advanced tooling and control system technology, and its role in cost efficiency, waste and consumption efficiency, as well as in reducing volatile organic compounds, will continue to write magnificently for a long time to come.

However, RTM/ L-RTM users need to be aware that only by trying to improve the application and training the corresponding staff can we really make this advanced process work.