Re-Understanding the Basics of Lightweight Resin Transfer Molding (L-RTM)

 

Like Other Closed-Molding Composite Processes, Lightweight Resin Transfer Molding (LRTM) offers reduced waste and emissions, efficient use of materials, and a consistent and repeatable process. But LRTM requires less expensive tooling and still provides excellent cosmetics on both sides of the part in an efficient manufacturing process.

 

Although Resin Transfer Molding (RTM) has been around since the 1970s, it continues to grow in popularity due to its ability to efficiently mass produce strong, high-quality parts. Like several other closed-molding processes for composites, RTM produces virtually no material waste and a very clean production environment, with significantly fewer HAP emissions than most open-molding processes. It is also a more consistent and repeatable process, producing parts with high strength-to-weight performance and dimensional consistency, with aesthetically pleasing surfaces on both sides of the part.

 

Lightweight RTM (LRTM) has been increasingly used recently because it allows the use of simpler, less expensive molds that do not have to contain high cavity pressures, yet still achieves short cycle times of as little as 20 minutes to mold a complete part, including initial cure, and typically without the need for subsequent finishing.

 

LRTM has been successfully applied in several end markets served by the composites industry, including aerospace, truck and bus, marine, wind energy and many others. An important factor in consistently producing high-quality LRTM parts is the precise control of the resin injection system, which is essential for good process control.

 

Magnum Venus Products (MVP), a global supplier of fluid motion solutions for over 80 years, has revitalized the process by defining production details to save time, reduce costs and improve part quality. Since the inception of LRTM, MVP has led process development with novel technologies, including Flexforming, Infusion and the latest process, Fast Flow LRTM, which utilizes a collapsible channel to reduce injection time.

 

MVP offers full-service solutions for closed molding, from comprehensive training and support to extremely precise injection equipment. Here is more information from MVP on important aspects of the LRTM process and how to consistently shape high-quality parts in a production environment.

 

The LRTM Process

The LRTM process essentially requires only an injection or ratio mix system, a vacuum system, and the mold itself. This shows the Flexoforming process, a variation of LRTM, with a reusable film counter mold. Developed by MVP, Flexoforming allows parts to be infused using an injection machine connected to a pneumatic attachment that is loaded onto a vacuum bag or reusable film, or both.

 

One of the great features of LRTM is the relatively modest investment required for equipment and tooling. All you ultimately need is an injection or ratio mix system, such as MVP’s Patriot Innovator PLC Injection System, which measures and mixes the resin and catalyst and injects the mixture into the mold, a vacuum system that seals and creates negative pressure between the mold and counter mold, and the mold itself.

 

The mold consists of a base mold (the “a” side) and a semi-rigid counter mold (the “B” side). The base mold is a relatively rigid composite structure with a very simple steel reinforcement frame. The counter mold is a thin 3 to 4 mm laminate with additional rigidity built in around the flange that mates with the base mold. The counter mold is constructed of two seals running around the peripheral flange. The outer seal is a neoprene or silicone “wing” type seal that forms the primary vacuum seal. The inner silicone seal is a dynamic seal that prevents injected resin from escaping the mold into the flange area.

 

To make the part, dry fiber reinforcement is loaded into the mold after mold release and gel coat are sprayed into the base mold cavity, and the two mating mold halves are closed. A vacuum is then pulled between the inner and outer seals to secure the two mold halves together.

 

Once vacuum pressure is verified, the resin and catalyst mixture is injected at low pressure (<1 bar or 14.5 psi) into the peripheral feed channels that extend around the entire edge of the part, just inside the inner seal.

Flange Detail of an LRTM Mold.

 

The vent is located at the final point of fill, usually in the center of the part, through which the vacuum is drawn. On large molds or molds with complex geometries, multiple exit vacuum points may be used. The mold cavity is evacuated at a slightly lower pressure level than the vacuum seal. A catch pot is located at the final fill point to collect any excess resin that would otherwise be drawn into the vacuum system. This center vent is usually built into the counter mold.

 

Resin is injected through the counter mold into the resin channel that surrounds the part along the dynamic seal using an MVP automatic valve or a simple 10 mm (.39 in.) injection tube connected to the appropriate injection port. At the end of the injection, the injection tube is clamped and the injection machine disconnected.

 

The vacuum is maintained on the catch pot until the resin cures. It is important to use the appropriate gel time to cure the part. For example, if the injection time is 10 minutes, the gel time must be 12 to 15 minutes. A shorter gel time will prevent resin from flowing into the catch pot; a longer gel time will allow too much resin to enter the catch pot.

For part families, a single counter mold can be used for multiple base molds. If one base mold can be prepared while another is machining the part, this creates very efficient production.

 

Equipment for Better LRTM Process Control

 

While LRTM can be performed with relatively simple pump, vacuum and tubing components, a higher quality, more repeatable process with shorter cycle and setup times can be achieved with more technologically advanced equipment. MVP has been designing products specifically for this process since its inception in the 1970s.

 

Here are a few important factors that contribute to production and a safe process:Injection Unit – To consistently produce high quality LRTM parts, precise control of injection pressure and catalyst to resin mix ratio is required.

 

The Patriot Innovator Injection System accomplishes both of these functions in one compact package. The unit is designed to inject polyester, vinyl ester and methacrylate resins and catalyst under controlled pressure. Material mixing occurs on demand at the injection head, and the catalyst ratio can be adjusted from 0.75% to 2.5%.

 

A fully pneumatic, automatic control system adjusts the number of pump strokes per shot to provide consistent material delivery and recirculation to maintain material consistency, helping to make the process easier to execute and more efficient.

 

Depending on how automated you want your process to be, MVP also offers a PLC control option. This provides precise control over pellet size, reducing material waste and saving costs. The PLC option also comes with RFID tracking of supplies and injection recipes for safer injections and greater accuracy.

 

 

MVP Patriot Innovative Injection System with PLC.

Pressure Sensing – Maintaining optimal injection pressure is important for an efficient and safe process. Too little pressure will extend the filling time. Too much pressure can warp the mold, resulting in a thicker part than desired. It can also separate the two halves of the mold during the injection process or even damage the counter mold.

 

MVP maintains precise pressure control with mold pressure protection built into the Patriot injection system and PPVS pressure sensor. If the pressure in the mold is too high, the pump automatically shuts off until the pressure is released. Once the pressure is standardized, the injection is restarted.

 

Precise Material Metering and Flushing – To maintain consistent product quality and reduce waste, you must precisely control the amount of material for each injection. You must also ensure that the injection line is clean and free of obstructions. If material stays in the line for too long, it will begin to solidify, resulting in inconsistent material flow and even clogging the line. Using MPV’s Turbo Auto Sprue (TAS) injection valves at each injection port can solve both of these problems.

 

When injection begins, an air signal is sent to the valve, opening it. Once the MVP injection unit reaches its pre-programmed stroke count, the machine automatically shuts down and sends a signal to close the valve. This provides precise control over the amount of material injected into each part. At the end of an injection, if the operator fails to flush the machine, the gel alarm will activate. This will prompt the operator to initiate an injection line flush cycle, which forces approximately 300cc of solvent into the line, through the valve, and out to the flush bucket.

 

This sequence is followed by a timed and adjustable air purge to ensure that all solvent has left the injection circuit and is ready for the next injection. A safety interlock ensures that a flush cycle cannot be initiated while the machine is in the injection mode. This metering and cleaning system produces very little waste, clean injections, and reusable valves and hoses.

 

Using accessories such as the Turbo Auto-Gate Injection Valve (mold back center) and PPVS pressure sensor (rear left) allows for a more accurate and consistent process. The vacuum collection canister is shown in the front corner of the mold. Troubleshooting Common LRTM Problems

 

While using better injection equipment will ensure a more accurate and consistent resin transfer cycle, there are other factors to consider when achieving a high-quality and efficient LRTM process. Here are some important aspects of the LRTM process that are often overlooked:

 

Technical flange design – This is the most important factor in ensuring that the mold and counter mold are clean and can be used again after the part is demolded. Proper flange design and processing include:

 

Using vertical flanges allows large molds to be easily closed without the use of clamps. This is common in Europe but less common in the United States.

 

Using 45-degree angles at the part boundaries allows the glass to be accurately loaded into the mold.

Spraying gel coat should cover the flange area instead of using tape, which can be time-consuming.

 

Careful Cutting and Placement of Reinforcement Material will Ensure a More Consistent End Product.

 

Loading reinforcements onto seals

Since fiber reinforcements are typically loaded into the mold cavity of the base mold, but the seal is attached to the counter mold, it is easy to inadvertently place fiber reinforcements in the flange space (between the inner and outer seals). This disrupts the integrity of the cavity vacuum. Taking care to accurately prepare and use the fiber reinforcement cutting template can help prevent this problem.

 

Inconsistent fiber loading

Inconsistent loading or positioning of fiber reinforcements can lead to variable part quality. Be sure to have a strictly followed procedure for where and how to place fiber reinforcements and use patterns to accurately cut the glass.

 

Thermal variability

Inconsistent temperatures in the manufacturing facility or resin can also lead to process variability, affecting material flow rates and other factors. Using MVP’s auxiliary heaters, thermocouples, and resin recirculation systems can help improve thermal stability.

 

Poorly Mixed Additives

Additives such as fillers, pigments, and accelerators offer a variety of opportunities to enhance the properties of LRTM parts, but can fall out of the resin if not properly mixed. MVP offers a range of mixers and recirculation loops to maintain material consistency all the way to the injection head.

 

Choosing the right injection point

Choosing the right injection point based on part shape and size is critical.

 

Thickness between part end and resin channel – Reducing the thickness between part end and resin channel will regulate resin flow at the start of injection and make the part easier to trim after demolding.

 

Mold Construction

When constructing the mold, MVP recommends injecting a calibrated part thickness that can be reused rather than using flake wax in a large mold. This option is faster, six times less expensive, and the calibrated part can be reused if multiple counter molds are built. It is also important to have the correct seal profile, especially for the resin channel. MVP recommends going two sizes to reduce resin waste. Finally, reduce and standardize raw materials on molds and counter molds by using the same gel coat, resin, and glass.

 

LRTM is able to manufacture beautiful parts with very high strength and weight characteristics at a very reasonable cost, making it an excellent choice for the efficient production of a variety of consumer and industrial products. With the right equipment and process control, these products can meet the highest quality standards time and time again.