Introduction to Resin Infusion Process and Process Material Requirements – 2

 

 

Resin

… 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. Generally, stitched fabrics and random fiber mats have the highest number of pores, while woven fabrics will restrict resin flow. If woven fabrics must be used, high-heald weaves (up to 8 healds for twill fabrics) work best because they reduce the number of fiber bends.

 

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

 

 

Foam Cores

 

As with the resin 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 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 should also have sufficient temperature resistance to withstand temperatures higher than the normal exotherm temperature.

 

The core 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 cores used in 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 surfaces 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 plate 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

 

This surface treatment was first invented in the late 1970s and involves cutting the core material in a square grid at 90o on both sides (offset by half the grid width on one side). The cuts are slightly deeper than half the core thickness (60% of the total thickness) and small holes are formed at each intersection with the cut on the other side. This helps the resin flow from one side of the core to the other side, and the resin evenly impregnates the entire laminate. The small cut width minimizes resin pick-up and less print-through than other cutting methods. Cores with a hyperbolic surface treatment are ideal for large radius and/or small curvature shapes. NAVICEL rigid PVC foam cores with densities up to 200kg/m3 can be cut surface treated.

 

Surface Grooving

 

A surface treatment 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, which flows to the other side of the core. The wider the saw kerf, the more resin flows into the panel faster. Surface Grooving works best on thicker cores and flat layers with little core deformation. This surface treatment can be processed for all densities of NAVICEL cores.

 

The scratching and cutting method of the scratched board is to use surface scribing (shallow tool cutting method, with an angle of about 75 degrees to the original cutting method). This scratching and cutting process can be used for all the surface treatments mentioned above, allowing the resin to pass through the foam blocks to avoid insufficient surface resin. All core materials of JFC with a density of less than 200kg/m3 can be processed with this scratching process.

 

 

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, the resin can be soaked throughout the part, and no separate resin distribution medium is needed. The core material itself is the flow medium. This can save costs, remove some waste, and also reduce the amount of resin used 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 manufacturing process of composite materials, various processes can be flexibly adopted on the basis of mastering the main heating, pressurizing and resin intervention methods. If the liquid resin intervention 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 flow medium can be used as an external belt, or the flow-guiding 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 flow.