Learn About The Process, Key Equipment And Common Problems Of Lightweight Resin Transfer Molding In One Article

 

Foreword: In the previous article “A Brief Introduction to the Characteristics, Process and Applicable Scope of Several Composite Material Closed Mold Processes” (read the original text), the characteristics of resin transfer molding as a common closed mold process for composite materials were briefly introduced. In this article, the basics of Light Resin Transfer Molding (LRTM) are further introduced in detail.

 

The resin transfer molding (RTM) process first appeared in the 1970s. Because this process can effectively produce strong and high-quality parts, it has become increasingly popular. As a common closed mold process for composite materials, RTM has almost no material waste compared to most open mold processes, and the production environment is very clean, and HAP emissions are much less.

 

 

RTM is also a more consistent and repeatable process that can produce parts with high strength-to-weight ratios and dimensional consistency, with aesthetically pleasing surfaces on both sides of the part. Light RTM (LRTM) has recently gained increasing use because it allows the use of simpler, less expensive molds that do not have to withstand high cavity pressures, but can still complete a complete part in as little as 20 minutes, often without the need for subsequent finishing.

 

 

LRTM can produce large, complex parts with excellent quality and high precision

 

LRTM has been successfully used in multiple end markets served by the composites industry, including aerospace, trucks and buses, marine, wind energy and other markets. An important factor in the ability to consistently produce high-quality parts using the LRTM process is the precise control through the resin injection system, which is essential for good process control.

 

Magnum Venus Products (MVP), a global supplier of fluid delivery solutions for more than 80 years, has revolutionized the process by defining production details to save time, reduce costs and improve part quality.

 

1. LRTM process

An important feature of LRTM is that the investment required for equipment and tooling is relatively small, requiring only an injection or dosing system (such as MVP’s Patriot Innovator PLC injection system) to meter and mix the resin and catalyst and inject the mixture into the mold.

 

 

LRTM Process Flow

 

The mold consists of a base mold (“A” side) and a semi-rigid secondary mold (“B” side). The base mold is made of a relatively rigid composite structure and a very simple steel reinforced frame. The secondary mold is a thin 3 to 4 mm laminate with high rigidity around the flange that mates with the base mold. The secondary mold carries two seals that surround the peripheral flange. The outer seal is a neoprene or silicone “wing” seal that forms the main vacuum seal.

 

The inner silicone seal is a dynamic seal that prevents the injected resin from escaping from the mold into the flange area.

 

 

Schematic diagram of flange structure of LRTM process

 

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 hold the mold halves together. Once vacuum pressure is confirmed, the resin and catalyst mixture is injected at low pressure (<1 bar or 14.5 psi) into the peripheral feed channels that run around the entire edge of the component inside the inner seal.

 

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

 

 

A single base mold can be used on multiple base molds, allowing very efficient production if one base mold can be prepared while the other can process the part.

 

Using an MVP automatic valve or a simple 10 mm (0.39 in) injection tube connected to the appropriate injection port, resin is injected through the secondary mold into the resin channel that runs around the part along the dynamic seal. At the end of the injection, the injection tube is clamped and the injection machine is disconnected. Maintain vacuum on the collection tank 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 the resin from flowing into the water collection basin, and a longer gel time will allow too much resin to enter the collection tank.

02. LRTM process control equipment

 

LRTM can be completed using relatively simple pump, vacuum and tubing assemblies, while using more technologically advanced equipment can produce higher quality, repeatable processes and shorter cycles. MVP has been designing products specifically for this process since its introduction in the 1970s. Here are some important factors that contribute to a safe and efficient production 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 packs both into one compact package. The unit is designed to inject polyester, vinyl ester and methacrylate resins and catalysts under controlled pressure.

 

Material mixing is done on demand at the injection head, and the catalyst ratio is adjustable from 0.75% to 2.5%. The fully pneumatic automatic control system adjusts the number of pump strokes per injection to provide consistent material delivery and recirculation, and maintains material consistency, making the process easier to perform and more efficient.

 

Depending on the degree of automation of the process required, MVP also offers the option of PLC control. This allows precise control of injection volume, reducing material waste and saving costs. The PLC option is also equipped with RFID tracking to track consumables and injection formulas, allowing for safer injection and greater accuracy.

 

 

MVP Patriot Innovator Injection System with PLC

Pressure Sensing – Maintaining optimal injection pressure is critical to an efficient and safe process. Too little pressure will prolong filling time, too much pressure will warp the mold and cause part thickness; it may also separate the mold halves during injection or even damage the counter mold. MVP maintains precise pressure control with the system’s built-in mold pressure protection function and PPVS pressure sensor. If the pressure inside the mold is too high, the pump will automatically shut down until the pressure is released. After the pressure is normalized, it will restart the injection.

 

 

Using the Turbo Auto Sprue injection valve (center back of mold) and PPVS pressure sensor (left back) accessories can make the process more accurate and consistent.

 

Precise Material Metering and Flushing – To maintain consistent product quality and reduce waste, the amount of material injected at each shot must be precisely controlled, and the injection lines must also be kept clean and free of obstructions.

If the material is left in the line for too long, it will begin to solidify and cause inconsistent material flow or even block the line. Using MPV’s Turbo Auto Sprue (TAS) injection valve at each injection port can solve both problems: At the start of injection, an air signal is sent to the valve to open it.

Once the MVP injection unit reaches its preprogrammed stroke count, the machine will automatically shut down and send a signal to close the valve, which can accurately control the injection amount of each part; after the injection is completed, if the operator fails to flush the machine, a gel alarm will sound, which will prompt the operator to start the injection line flushing cycle, forcing approximately 300-cc of solvent through the valve into the line and into the flushing bucket.

After this process, a timed and adjustable air purge will be performed to ensure that all solvent is discharged from the injection circuit and is ready for the next shot. The safety interlock ensures that the flushing cycle cannot be activated when the machine is in injection mode.

 

 

03. Solutions to Common LRTM Problems

 

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

 

Technical Flange Design: This is the most important element to ensure that the mold and base mold are clean and can be used again after demolding. Proper flange design and processing include:

o Use vertical flanges to easily close large molds without the use of clamps.

o Use 45-degree angles at the part boundaries to accurately fit into the mold.

o Cover the flange area when spraying gel coat instead of using tape, which is time-consuming and expensive.

 

Loading reinforcements on seals: Since fiber reinforcements are typically loaded into the base mold cavity, but the seal is attached to the base mold, it is easy to inadvertently place fiber reinforcements in the flange space (between the inner and outer seals), which can disrupt the integrity of the cavity vacuum. Careful and accurate preparation and use of fiber reinforcement cutting templates will help avoid this problem.

 

Inconsistent reinforcement loading: Inconsistent fiber reinforcement loading or placement can lead to part quality variation. Make sure fiber reinforcements are placed in the right position and pattern, and cut accurately using patterns.

 

Thermal variability: Inconsistencies in temperature in the production equipment or resin can also lead to process variability, which can affect material flow rate and other factors. Using MVP’s auxiliary heaters, thermocouples, and resin recirculation systems can help improve thermal stability.

 

Poor additive mixing: Additives help improve the properties of LRTM parts, but can fall out of the resin if not mixed properly. MVP offers a range of mixers and recirculation loops to maintain material consistency all the way to the injection head.

 

Selecting the right injection point: Selecting 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 when starting injection and make the part easier to trim after demolding.

 

Mold construction: When constructing the mold, MVP recommends injecting a reusable calibrated part thickness instead of using flake wax in large molds. This option is faster, six times less expensive, and the calibrated part is reusable if multiple counter molds are made.

 

Having the right seal profile is also critical, especially for the resin channels. MVP recommends going two-dimensional to reduce resin waste. Finally, reduce and standardize raw materials on the mold and counter mold by using the same gel coat, resin, and glass.