Light Resin Transfer Moulding (LRTM) - A424

 

Light Resin Transfer Moulding (LRTM)
Document Type	Article
Document Identifier	424
Tags	Deposition
Prerequisites	Materials deposition and consolidation management (MDCM) Viscosity (resin) Darcy's law

Introduction

Light resin transfer moulding (LRTM) is a closed mould, liquid composite moulding, infusion process that shares commonalities with resin transfer moulding (RTM) and vacuum assisted resin transfer moulding (VARTM) with some key differences. Dry fibre mats are placed in a rigid mould and covered with a semi-rigid counter mould. Resin is injected into the mould to infuse the dry fabric and then allowed to cure.

Significance

LRTM is a suitable process to increase productivity, reduce emissions, and improve quality compared to traditional wet layup. Understanding the scientific theory behind this process and the practical implications is critical to successfully produce parts.

Scope

This article will provide a detailed description of the LRTM process and explain the relevant scientific theory to successfully produce parts. The article will then describe the materials, shape, tooling, and equipment required for this manufacturing method. Finally, the advantages and disadvantages of adopting LRTM manufacturing will be presented.

Process Description

LRTM tooling consists of a rigid base mould (A-side) and a semi-rigid counter mould (B-side) as shown in Figure 1. After the moulds have been cleaned, released and gel coat applied, dry fibre mats are placed into the lower mould.

After the material is loaded into the base mould, the counter mould must be aligned evenly on all sides and lowered onto the part. During this process it is important to ensure the material in the base mould does not move. As it is lowered, the counter mould should be pushed down until the outer vacuum seal contacts the lower mould.

A vacuum line is then attached to the counter mould between the outer vacuum seal and the inner resin seal. Full vacuum is applied to the vacuum channel to clamp the counter mould to the lower mould.

The resin inlet port is located on the counter mould and inside of the inner resin seal. Usually there is space left open for a resin channel to surround the part cavity. This helps to ensure even distribution of the resin throughout the part.

A vacuum line is then attached to a port located within the part cavity. This vacuum line is connected to a resin catchpot to ensure that no resin reaches and damages the vacuum pump. All connections must be sealed using tape to prevent leaks. Typically, half vacuum level is applied to the part cavity. Vacuum integrity is then tested on the catch pot, part cavity, and the outer vacuum seal.

After vacuum integrity is confirmed the injection gun is attached and the injection process is started. The resin may be injected using the pressure differential caused by the vacuum, or it may be injected using positive pressure to increase the pressure differential. Applicatino specific LRTM pumps are available. Once the appropriate amount of resin has been injected to ensure the part is fully infused and meets the required fibre content requirement, the injection hose is pinched off and the gun disconnected. The vacuum lines must stay connected during the curing process. Typically, the demoulding process can begin once the counter mould has returned to room temperature.

Figure 1 - LRTM Tooling

Theory

Darcy’s law is used to model fluid flow through porous media, and like RTM and VARTM processing, the law can be used to model flow with LRTM processing.

\(Q = -\frac{KA}{\mu}\frac{\Delta P}{x}\)

Where,

\(Q = \) Volumetric flow rate

\(K = \) Preform permeability

\(A = \) Preform cross-sectional area

\(\mu = \) Resin viscosity

\(\Delta P = \) Pressure differential across preform

\(x = \) In-plane flow distance of pressure differential

Darcy’s law defines the critical parameters that need to be controlled to successfully produce a part using LRTM. The resin viscosity term shows that a lower viscosity will increase the flow rate. This helps to reduce fill time and prevent dry spots within the part.

A higher pressure differential also increases the flow rate and reduces fill time of the part. This explains why some LRTM parts are injected using positive pressure. However, care must be taken when setting the pressure differential because if the value is too high the flow rate may be too fast to allow the escape of volatile gases. This can result in porosity within the part.

Darcy’s law also shows that the flow rate is reduced as the in-plane flow distance is increased. This is why the distance from the resin injection port to the vacuum port in the part cavity should be minimized when designing the tool. It is also the reason why a resin channel that surrounds the part cavity is included. The resin channel allows for the resin to easily flow around the perimeter of the part and therefore shortens the flow distance through the fibre mat.

A higher preform permeability also increases the flow rate and is the reason why the fibre architecture of the mats needs to be carefully chosen. Ensuring high permeability helps to reduce the probability of dry spots in the part. It also demonstrates why the part cavity thickness and number of plies must be selected carefully. The number of reinforcement plies may be increased to improve the strength and stiffness of a part, but adding more plies to a pre-defined cavity thickness reduces the permeability of the preform. If the permeability of the mat stack is reduced too much then the result is dry spots in the part.

The use of Darcy’s Law and numerical methods can be used to model flow during LRTM processing. A challenge for modelling flow through LRTM tooling is the fact that the counter mould is semi-rigid. This means that the counter mould can flex depending on the number of plies stacked in the part cavity and the injection pressure. As the counter mould flexes this changes the permeability of the reinforcements and the cross-sectional area, therefore changing the volume flow rate. The amount of flex in the counter mould is dependent on the thickness and construction of the tool . These factors have to be taken into consideration to accurately model the injection process of the part.

Materials

Reinforcements

A variety of fibre mats are commonly used for LRTM manufacturing. Continuous filament mat and chopped strand mat are the two most common due to their low cost and high permeability. Unidirectional and bidirectional stitched mats are often used when increased stiffness and strength are required.

In many applications assistance is needed to help the resin flow through the part and avoid dry spots. In these cases, mats that consist of a synthetic non-woven core sandwiched between two layers of chopped strand mat are a good option. The non-woven core allows for easy resin flow while the chopped strand mat provides stiffness. These types of mats have the additional bonus that the non-woven core adds thickness to the part without adding the full weight of a glass ply.

Another option to improve resin flow is the inclusion of polyester-based mats that include resin channels. Similar to the non-woven cores discussed above, these mats improve resin flow while adding thickness to the part. They can also be used to aid in print blocking.

Resin

Polyester and vinyl ester resins are most commonly used during LRTM processing, although epoxy resin is also an option. Low viscosity resins are best suited to help ensure complete wet out of the part. Typical viscosity levels are in the range of 50 to 200 mPa-a (centipoise). Resin gel times need to be long enough to allow for degassing and for the mould to fill completely.

Core

Core materials suitable for LRTM processing include plywood, balsa, and a variety of foams including PVC, PET, and polyurethane. Foam-filled honeycomb is another possibility, but open cell honeycomb is not suitable as the cells would fill up with resin resulting in a dramatic weight increase. Depending on the part and tooling design, the core may hamper the flow of resin through the part. To improve resin flow shallow grooves may be added to the core. Another option is to perforate the core which allows the resin to transfer from one side of the core to the other.

Shape

LRTM processing is suitable for most geometries, but undercuts should be avoided. This is because the semi-rigid counter mould can’t be removed when undercuts are present. In some other processing methods split tools can be used when undercuts are present, but this is not recommended for LRTM as it comprises the vacuum integrity of the tool. Additionally, very tight geometry should be avoided because it is challenging to construct a B-side mould to accommodate these features. Tight geometry is also more likely to lead to fibre bridging and resin rich areas. The draw depth should also be considered. Although deep draws are possible, they increase the difficulty level to deposit the reinforcing layers, and to close and open the B-side tool.

Tooling

LRTM tooling consists of a rigid base mould (A-side) and a semi-rigid counter mould (B-side). The A-side tool is typically made from thick fibreglass and reinforced with framing as needed. The B-side is also made from fibreglass but is thinner than the A-side. It contains a number of key features. The first is the outer mould seal that contains the vacuum within the tool. The second is the vacuum/resin seal. The area between the outer and inner seal is the vacuum channel. When vacuum is applied to the channel atmospheric pressure clamps the B-side mould to the A-side mould. Most B-side moulds contain one or two vacuum ports in the vacuum channel depending on the size of the part.

The inner seal surrounds the resin channel and the part cavity. It prevents resin from travelling from the resin channel and part cavity into the vacuum channel. The placement of the vacuum and resin ports within the part cavity are critical to manufacture a quality part. Once resin enters the part cavity it will flow along the path of least resistance. The ports should be placed so that all material will be infused before the resin reaches the exit vacuum port. As an example, the placement of the ports in Figure 2 will likely result in dry spots as the resin will quickly travel to the vacuum port and cut off areas for gases to escape. In comparison, the placement of ports in Figure 3 allows for the resin to easily travel around the resin channel and then fill the part cavity evenly before reaching the vacuum port.

Figure 2 - Port Placements Resulting in Dry Spots

Figure 3 - Port Placements Resulting in Complete Fill

Equipment

The two primary pieces of equipment for this type of processing are a resin injection system and a vacuum source. There are two options for supplying resin under pressure. The first is a pressure pot where the resin is placed inside the pot within a disposable container. The injection tube is fed through the lid of the pot and into the resin container. Compressed air is then used to create pressure within the pot and force the resin to flow. The second option is a standard resin injection system. Most of these systems use pumps driven by an air motor. The equipment automatically mixes the resin and the catalyst at a specified level and then injects the resin at the required pressure level. Controls on the equipment can also be set to count the appropriate number of strokes so that a consistent amount of resin is injected for each part.

There are also two options to supply vacuum. The first is a venturi that uses compressed air to produce a vacuum. This is the cheapest option, but the flow rates are small. The second option is an industrial vacuum pump. This is a more expensive alternative but provides much greater flow rates. It is helpful if the vacuum source has the ability to adjust the vacuum level as for many LRTM applications full vacuum is applied to the vacuum channel to clamp the tool, while half vacuum is applied to the part cavity to draw the resin. It is also critical that the compressor and tank are sized appropriately depending on the size of the parts being manufactured.

Advantages

• Can produce near-net shapes (limited trimming required)
• Produces consistent parts (thickness and fibre content)
• Produces smoother B-side surface than open mould methods
• Higher production rates than wet layup
• Reduced labour compared to wet layup
• Closed moulding leads to reduced emissions of volatile organic compounds (VOCs)
• Less expensive capital equipment than RTM
• Lower tooling cost than RTM

Disadvantages

• Lower fibre content than RTM and VARTM processing
• B-side surface finish not as good as processes that use metallic tooling
• More expensive tooling than open mould since it requires a top mould
• More equipment required than wet layup (vacuum pump and resin injection systems)
• Lower cycle times than RTM or compression moulding

Related pages

• See Webinar on Composite materials engineering - Manufacturing processes - Liquid composite moulding
• See Page on Practice for troubleshooting a Light RTM step
• See Page on Process Selection for a Composite Transit Vehicle Door
• See Page on Resin flow

References

About	Help

Light Resin Transfer Moulding (LRTM) uses a semi rigid tool half (A side) that is similar to those used in VIP and a semi flexible tool half (B side) (typically made of thin fibreglass).

For polymer matrix composites (PMCs), resin refers to the matrix; the continuous material phase that binds the reinforcement together, maintains shape, and transfers load. Resins are divided into two main groups: thermosets and thermoplastics.

Engineered materials (designed to have specific properties) made from two or more constituent materials with different physical or chemical properties. The constituents remain separate and distinct on a macroscopic level within the finished structure.

Resin transfer moulding (RTM) involves loading a preform into a two (or more) piece, matched tool, closing it, and injecting resin under pressure (~15-100 psi, or ~1-7 bar).

Well suited to small to medium sized parts, limited to large sizes due to injection pressure loads and tool cost.

Vacuum assisted resin transfer moulding (VARTM) - also known as vacuum assisted resin infusion (VARI), vacuum infusion process (VIP) or often just resin infusion. VARTM is a liquid composite moulding (LCM) closed mould process with a single side tool and vacuum bag where the resin is drawn through the preform using vacuum.

‘Preform’ is the term for the fibre reinforcement. This is the stage between the raw material form after it is processed into an architecture (fabric, mat, etc.) and becoming a composite.

In composites processing, viscosity is an indicator of how easily the resin matrix will mix with the reinforcement and how well it will stay in place during processing. The lower the viscosity, the more easily resin flows. Resin viscosity ranges considerably across chemistries and formulations.

By scientific definition, viscosity is a measure of a material’s resistance to deformation. For liquids, it is in response to imposed shear stresses.

Permeability refers to the resistance to fluid flow through a porous material.

• Resin flow through fibre
• Gas flow through prepreg

Fibre architecture refers to how the fibre is assembled.

Most common architectures:

• Unidirectional
• Chopped strand mat (CSM) and chopped fibre (chopper gun)
• Continuous filament mat (CFM)
• Woven fabric
• Non crimp fabric (NCF)
• Braided
• Prepreg

The use of multiphysics models to predict the outcome of real-world scenarios. May be analytical (closed form, "hand calculations") or computational (implementation on computers is required due to the large number of calculations involved. e.g. finite element method, finite difference method)

Most often in composite materials engineering, modelling refers to either:

• Process modelling (predicting manufacturing outcomes, or the inverse problem of predicting manufacturing parameters to achieve a desired outcome), or
• Performance modelling (predicting the stiffness/strength/suitability of a structure, or the inverse problem of the material properties and dimensional requirements to achieve a desired stiffness/strength/suitability of a structure)

(same as "Simulation")

A single unit of fibre reinforcement that cannot be separated further without breaking it. It often has a circular cross section that is formed by the dies used to create it or its precursor material. Many filaments are manufactured in parallel to produce a strand or tow. The number of filaments in a strand or tow is often references by how many thousands of filaments comprise the tow. e.g. an 18k tow is made up of ~18000 filaments.