Resin Infusion Process

 

 

The RFI process uses a press to apply pressure, a resin film to insert the resin, and heat conduction heating of the mold.

 

For a certain process, multiple heating, pressurization or resin introduction methods can also be used. For example, NLR used a combination of prepreg introduction and resin liquid infusion RTM process, mold heating, and mold pressurization in its research on aircraft composite mouth covers.

 

Vacuum infusion (also known as VARTM, SCRIMP, VIP or various other abbreviations) is a lamination process used to produce fiber reinforced plastic (FRP) parts. Dry materials (felt, fabric, stitched felt and foam core) are stacked on a male or female mold surface and sealed around the perimeter of the part using a thin plastic vacuum bag or semi-rigid counter mold.

 

A vacuum pump is used to evacuate the dry materials and apply atmospheric pressure to compact the dry materials and create a “vacuum gap”. Resin is then injected into the gap through strategically placed resin injection pipes. The pressure difference between the vacuum gap and the external atmospheric pressure presses the resin into the porous material until it is completely infiltrated. The vacuum is maintained until the part is cured to ensure density.

 

 

Introduction to Resin Infusion Process

 

The RFI process uses a press to apply pressure, a resin film to insert the resin, and heat conduction heating of the mold.

 

For a certain process, multiple heating, pressurization or resin introduction methods can also be used. For example, NLR used a combination of prepreg introduction and resin liquid infusion RTM process, mold heating, and mold pressurization in its research on aircraft composite mouth covers.

 

Vacuum infusion (also known as VARTM, SCRIMP, VIP or various other abbreviations) is a lamination process used to produce fiber reinforced plastic (FRP) parts. Dry materials (felt, fabric, stitched felt and foam core) are stacked on a male or female mold surface and sealed around the perimeter of the part using a thin plastic vacuum bag or semi-rigid counter mold.

 

A vacuum pump is used to evacuate the dry materials and apply atmospheric pressure to compact the dry materials and create a “vacuum gap”. Resin is then injected into the gap through strategically placed resin injection pipes. The pressure difference between the vacuum gap and the external atmospheric pressure presses the resin into the porous material until it is completely infiltrated. The vacuum is maintained until the part is cured to ensure density.

 

 

From this equation, we can see that viscosity is inversely proportional to the flow rate of the resin, or in other words, the higher the viscosity of the resin, the longer it takes to wet the part. Conversely, the more porous the material (higher K value), the greater the pressure difference, and the faster the resin flows. Therefore, in theory, to optimize the infusion process, the resin needs to be very thin (low viscosity), the material needs to be very porous, and the pressure difference needs to be as large as possible.

 

Classification of Vacuum Infusion Processes

 

There are different methods to achieve vacuum infusion. The main difference between them is the way the resin is injected into the dry fabric layup and whether pressure is added to the resin. The different methods and process parameters are given in the table below.

 

 

The higher the pressure of the resin, the more rigid the mold needs to be. RTM molds are made of steel or aluminum. They are usually equipped with a heating system to ensure a fast and stable production process. Vacuum infusion molds are usually made of wood or fiberglass and can of course be heated.

 

Soft Film Vacuum Infusion

 

This method is particularly suitable for customers who are switching from hand lay-up to vacuum infusion, as well as for single parts or proofing. The existing mold can still be used, and only some necessary modifications are made at the flange position so that the vacuum bag can be laid on the mold. In addition, a vacuum pump, low-density polyethylene tubing (or other semi-rigid plastic), a resin collection tank and standard materials for making vacuum bags (sealing tape, vacuum bags and peel ply, etc.) are required, and then the vacuum infusion process can begin.

 

SCRIMP Method

 

The SCRIMP™ process uses an additional resin flow medium outside the sandwich structure. The SCRIMP resin flow medium is a porous layer placed on the surface of the part. During the infusion process, the resin flow medium quickly distributes the resin to the surface of the part and then penetrates into the sandwich structure part from the surface. The resin distribution network is used to distribute the resin more evenly throughout the component and provide channels for the resin to flow. The flow medium and resin distribution network are usually removed after use.

 

Because the resin flow medium cannot be part of the sandwich structure, a peel ply or breathable membrane is placed between the surface of the sandwich component and the flow medium. Both materials are designed to be used once and need to be removed after the resin is cured.

 

The following diagram shows the schematic diagram of the laying of vacuum-assisted infusion using the SCRIMP™ process. The resin flow medium (green) is separated from the surface of the sandwich structure by the peel ply (red). The vacuum pump causes the membrane to generate surface pressure and suction for the resin. The example here uses an open-cell foam core. Allowing the resin to flow from one side of the core to the other side forms a closed resin system.

 

 

However, the SCRIMP™ process also has some disadvantages. The flow medium and other process layers generate a lot of waste. In addition, more resin is required for the part.

 

New reusable silicone vacuum bags have been developed to reduce waste. However, the mold needs to be redesigned and a specially produced liner is added to the runner mold. The fact that this liner is reusable brings advantages for the manufacture of small or medium batches.

 

In addition to using a flow medium outside the sandwich structure, it is also possible to use resin flow felts, which directly act as a flow medium. Continuous fiber mats, or mixed mats of glass fiber and polyester fibers. Using these types of mats will reduce waste, but increase the resin content of the composite material, which means less fiber content.

 

The figure below shows a lay-up diagram using resin flow felt (dark green). Similar to the SCRIMP™ process, the foam core must be perforated to allow uniform impregnation of both sides of the sandwich structure.

 

The disadvantage of resin flow felts is that the sandwich structure forms a resin-rich panel, and the fiber content is reduced to 20% or even lower.

 

FRP Closed Mold Vacuum Infusion (Lightweight RTM)

 

The closed mold vacuum-assisted infusion process allows the production of composite parts with two smooth surfaces in one step. In this process, a few millimeters thick FRP counter-mold is used instead of a flexible vacuum bag. The mold must be fixed at the edge of the base mold, either with a clamp or with a separate vacuum system. The principle of laminate pressurization is similar to the flexible vacuum bag resin infusion process.

 

It is easy to check the resin flow through the FRP mold panel. The applied vacuum draws the resin in and distributes it inside the mold. If the vacuum is too high, the gaps in the laminate will close and the resin flow will be significantly slowed down. This is why the vacuum level in this process cannot exceed 0.5 bar, which depends on the specification of the fiber laminate. The resin infusion time can also be reduced by injecting a small amount of pressure on the resin. During the pressurized infusion process, the counter-mold must not be damaged or displaced.

 

 

RTM-Process

 

The resin transfer molding process is a process where resin is injected into a mold using high pressure. The mold can be for individual laminates or sandwich structures. The high temperature and high pressure environment associated with this process requires the foam to have high mechanical and temperature resistance properties.

 

Requirements for the main material of the resin infusion molding process

 

Resin

 

The resin matrix exists in liquid form and is the flowing material in the infusion process. In contrast to the hand lay-up process, the viscosity of the resin must be low enough to easily penetrate into the gaps of the dry laminate. In order to prevent incomplete wetting of the laminate, the gel time of the resin must be longer than that of the hand lay-up process.

 

Typical vacuum infusion resins have a viscosity of no more than 300-350 centipoise (cps) and are non-thixotropic. Some vinyl ester resins can have a viscosity as low as 100 centipoise (cps).

 

Gel time is also a major problem for vacuum infusion. When the resin gels, its viscosity rises sharply and the flow rate decreases. When first infusing a part, it is best to use a medium to long gel time (45-60 minutes). Later, once the proper injection location and timing are determined, the gel time can be reduced. Because the entire layup is cured simultaneously, it is important to achieve a low peak exotherm and a gentle exotherm prior to gelling (in order to reduce the risk of solvent attack on the core). Lower exotherms can be achieved by adjusting the formulation of the resin itself, or by using a cumene hydroperoxide (CuHP) initiator.

 

Most resin manufacturers have such resins available. They can also provide formulations that increase the gel time without changing the final properties of the resin.

 

Fiber Mats

 

The choice of dry reinforcement is also critical for vacuum infusion. Any type of fiber commonly used in composites can be used in infusion: for example, E-glass, S-glass, Kevlar, carbon fiber and polyethylene fibers.

 

The important aspect of dry reinforcement is to use the right type of fabric. In general, stitched fabrics and random fiber mats have the highest number of pores, while woven fabrics restrict resin flow. If woven fabrics must be used, high-heald weaves (up to 8 healds for twill fabrics) work best, as they reduce the number of fiber bends.

 

CSM (chopped strand mat), woven fabrics or stitched mats can all be used for infusion, but resin flow through these mats or fabrics is slow. This is why special infusion fabrics have been developed. These fabrics can be integrated into the layup to speed up resin flow. The different infusion fabrics are listed below in order.

 

… Most fabrics will compact up to 30% of their cross-section thickness under vacuum, so a hand lay-up that is designed for a particular application may no longer have the required stiffness if switched to vacuum infusion.

 

Foam Cores

 

As with resins and fibers, the choice of core material is very important. Cores suitable for vacuum infusion need to be closed-cell, such as the most commonly used polymer foams, such as NAVICEL, most structural plastic foams, and balsa wood. Most honeycomb cores are not suitable for vacuum infusion because they have large and open pores.

 

The core material must also be solvent resistant to some extent, because it will be exposed to styrene (if the resin is polyester or vinyl ester) for a long time during infusion. Since the entire sandwich structure is cured at once, the core material should also have sufficient temperature resistance to withstand temperatures higher than the normal exotherm temperature.

 

The core material should also have sufficient compressive strength to not be crushed by the pressure generated by the vacuum. However, this is not a major consideration because almost all core materials used for this composite material have a compressive strength greater than 0.1MPa (maximum atmospheric pressure or pressure difference).

 

In summary, the following three points:

 

100% closed cell structure

 

Styrene resistance (if unsaturated polyester and vinyl resins are used)

 

Ability to withstand vacuum pressure (compressive strength > 0.1MPa)

 

When using core resin infusion process, it is worth noting that the foam core in the sandwich structure can act as a resin distribution medium, the grooves on the foam surface allow the resin to flow quickly, and the open cells of the foam can help the resin flow at the same speed on both sides of the laminate.

 

In order to obtain sufficient flow and infusion effect, infusion using core materials also requires special cutting, grooving and/or perforating the core material (to increase the permeability of the core material). These cutting/grooving/etc. surface treatments allow infusion injection without the need for additional infusion media. JFC offers a variety of surface treatments for vacuum infusion. The following surface treatments are developed specifically for vacuum infusion and are used in this process. Other existing treatments, such as contour board materials, can also be used for infusion, but it is usually recommended to use them in conjunction with one or more of the following surface treatments.

 

Hyperbolic Sheet

Surface Grooving

 

Developed in the 1980s, the core is grooved in one direction with a saw blade, or in a 90-degree rectangular groove, or on both sides. Shallow grooves (usually 1.3mm wide x 3mm deep) can be accompanied by punching holes to evenly distribute the resin and allow it to flow to the other side of the core. The wider the saw kerf, the more resin will flow into the panel faster. Surface Grooving works best on thicker cores and flat layers with little core deformation. This surface treatment can be applied to all densities of NAVICEL core.

 

Scratched Board

 

The Scratching Cutting method is to use surface scribing (shallow tool cutting method, about 75 degrees from the original cutting method). This Scratching Cutting process can be used for all surface treatments mentioned above, allowing the resin to pass through the foam block to avoid insufficient surface resin. All JFC cores with a density of less than 200kg/m3 can be scored.

 

 

Perforated Sheet

 

Hole punched or perforated in a 50 x 50mm (standard) square through the material thickness and a hole in the middle of the square. Perforations, used in conjunction with surface grooving and/or contoured sheet core finish, allow resin to flow into the mold side of the laminate (not the mold side where the resin injection line is installed) during a “top-down” infusion process.

 

 

Slotted Foam

 

Because the core material is cut/slotted/punched to allow the resin to soak into the entire part, there is no need for a separate resin distribution medium, the core material itself is the flow medium. This saves costs, removes some waste, and can also reduce resin usage and shorten preparation time.

 

 

The left panel in the above figure is a typical SCRIMP process, and the right panel is a core infusion process with surface grooves. These panels started infusion at the same time, with the left panel less than 1/2 infusion and the right side almost completely infiltrated.

 

Foam grooves allow resin to flow easily and quickly through the grooves and holes and be evenly distributed in the sandwich structure. Grooved foam does not require additional resin infusion felt or flow medium. For almost all applications, groove infusion is recommended.

 

Usually, one or both sides of the foam are cut in a rectangular pattern with a knife or saw. The ratio of the width to the depth of the groove is very small. The depth of the groove is greater than the width, which will reduce the risk of printing through the gel coat. The spacing of the grooves can be designed based on the layup, resin viscosity, and the overall dimensions of the part to avoid dry spots or areas of incomplete resin infiltration. In addition, the foam is opened with holes of about 2mm diameter and the spacing is about 50mm. The holes can ensure that the resin can flow through and evenly infiltrate both surfaces of the foam core.

 

The principle of infusion of foam core as resin flow medium is shown in the figure below.

 

Summary

 

In the composite material manufacturing process, various processes can be flexibly adopted on the basis of mastering the main heating, pressurizing and resin introduction methods. If the liquid resin introduction process is adopted, different molds can be used, such as two-sided molds; one side hard mold and one side soft film; one side hard mold and one side semi-rigid mold.

 

In terms of the specific resin introduction medium material, further distinction can be made, and the resin guide medium can be used as an external belt, or the guide adhesive inside the laminate, or the core material of the sandwich structure. If the core material of the sandwich structure is used, punching, grooving or scratching can be used to help the resin guide.