Vacuum Infusion Molding Process and Application

 

 

Among the current materials, composite materials are widely used because of their light weight and high strength. The manufacturing process for composite materials is also constantly improving and innovating. From the initial hand lay-up to mechanized spraying, pultrusion, molding and other processes, the vacuum infusion process is now emerging.

 

The processes related to vacuum infusion include resin transfer molding (RTM), vacuum assisted RTM (VARTM), vacuum bagging, SCRIMP, SRIM (Structural Reaction Molding), RTI (resinfilm infusion), but there are some differences, which have been introduced in many articles, so I won’t go into details here.

 

Vacuum Infusion Process (VIP)

 

Vacuum infusion process (VIP) is a process that involves laying “dry” reinforcement materials (glass fiber, carbon fiber, sandwich materials, etc., which are different from vacuum bag process) on the mold, laying vacuum bags, and extracting the vacuum in the system to form a negative pressure in the mold cavity.

 

The pressure generated by the vacuum is used to press the unsaturated resin into the fiber layer through the pre-laid pipeline, allowing the resin to infiltrate the reinforcement material and finally fill the entire mold.

 

After the product is cured, the vacuum bag material is removed to obtain the desired product from the mold. VIP uses a single-sided mold (like the usual hand lay-up and injection mold) to establish a closed system. The vacuum infusion process has been public for a long time, and the process was patented in 1950.

 

However, it has only been developed in recent years. Since this process was introduced from abroad, it has many names in terms of name, such as vacuum infusion, vacuum infusion, and vacuum injection.

 

Theory

 

The vacuum infusion process can be widely used, and it has its theoretical basis, which is Darcy’s Law (Darcy’s Law) t=｛2h/(2KDP)

 

t is the infusion time, which is determined by four parameters.

 

h-resin viscosity. From the formula, it can be seen that the lower the viscosity of the resin used, the shorter the infusion time required. Therefore, the viscosity of the resin used for vacuum infusion is generally not too high. This allows the resin to quickly fill the entire mold.

 

｛-injection length, which refers to the distance between the resin feed port and the outlet. The longer the distance, the longer the time required.

 

DP-pressure difference. The greater the pressure difference between the system and the outside of the system, the greater the driving force on the resin, the faster the resin flow rate, and of course the shorter the required infusion time.

 

K＝permeability, which refers to the parameters of glass fiber, sandwich materials, etc. for resin infiltration. A large K value indicates good infiltration, such as continuous felt and multi-directional felt are easier to be infiltrated by resin than gingham and chopped felt. Therefore, in order to make the resin fill the system conveniently when the reinforcement material is compacted, some guide grooves are generally set up artificially, such as drilling holes on the upper and lower sides of the sandwich foam.

 

Advantages

 

In the usual hand lay-up process (han lay-up), the reinforcement material is laid in the mold and the reinforcement material is manually impregnated with a brush, roller or other means.

 

Another improved method is to use a vacuum bag to suck out the excess resin in the layer during hand lay-up. In this way, the glass fiber content is increased, and higher strength and lighter products are obtained.

 

VIP has many advantages over traditional processes. As shown in the figure, hand lay-up, vacuum bag and vacuum introduction are taken as examples. Vacuum introduction has obvious advantages in mechanical properties.

 

From This, We Can See The Advantages Of Vacuum Introduction

 

Higher Quality Products:

In a vacuum environment, the resin impregnates the glass fiber, and there are very few bubbles in the product compared with the traditional manufacturing process. There is no excess resin left in the system, and the glass fiber content is very high, which can reach 70% or even lower. The resulting product is lighter and stronger. It is also very stable from batch to batch.

 

Less Resin Loss:

With the VIP process, the amount of resin can be accurately estimated. For hand lay-up or spraying processes, it is difficult to control due to the variability of operators. VIP can minimize the loss of resin, and more importantly, it can save costs.

 

Uniform Distribution Of Resin:

For a product, the pressure generated by the vacuum in different parts is consistent, so the resin impregnation speed and content of glass fiber tend to be consistent. This is very critical for FRP parts with stable weight requirements.

 

Less Process Volatility:

There are no brushes or rollers in the production process, which will not cause the resin to splash or drip, and there will be no large amount of odor. Therefore, it can provide a clean, safe and friendly working environment to protect the physical and mental health of the operator.

Use A Single-Sided Mold:

Only one side of the mold can get a smooth product on both sides, which can better control the thickness of the product. Save mold manufacturing costs and time.

 

Because the products made with the VIP process have such advantages, they were first used in special fields such as aerospace, and later gradually applied to high-demand civilian products.

 

Application Areas Of VIP Technology

1) Ship Industry – hull, deck, rudder, radar shield

2) Wind Power Energy – blades, engine room cover

3) Sports and Leisure – helmets, sailboards

4) Automobile Industry – various roofs, windshields, carriages

5) Construction Field – building top parts, building templates

6) Agriculture and Horticulture – granary dome, agricultural machinery protection cover

 

Resin

When preparing to start the vacuum introduction test, first choose the right resin, epoxy resin or unsaturated polyester resin. The resin of the vacuum introduction process cannot be replaced by ordinary resin. It has special requirements for viscosity, gel time, exothermic peak, wettability, etc. For details, please consult the resin supplier.

 

Curing System

If it is epoxy resin, use its corresponding special curing agent; the commonly used curing agent for unsaturated resin is methyl ethyl ketone peroxide. The quality of different manufacturers is different. When selecting, evaluate it. The quality of the entire product cannot be affected by the material with small amount.

 

Reinforcement Materials

Reinforcement materials are usually glass fiber and carbon fiber. For example, continuous felt, composite seam felt, single cloth, etc., which should be designed according to mechanics. It is best to do an experiment when selecting to see how permeability is, because the different impregnants and adhesives used in the manufacturing process of the fiber will have different impregnation with the resin, resulting in great differences in the mechanical properties of the final product.

Sandwich

Generally, wood boards, Balsa wood, PVC foam, PUR foam, strong core felt, etc. are commonly used. Choose the appropriate sandwich material according to the needs of the product.

 

Equipment and Auxiliary Materials

Vacuum pump, interface, pressure gauge, guide tube, demoulding cloth, guide cloth, vacuum bag, etc.

The materials used in the vacuum induction process are not used casually. Each material must be confirmed by experiments to determine whether it is suitable. I will not elaborate on it here, and will discuss it as a special topic in future articles.

 

4.2VIP Process Flow

Then you need to understand how the materials are used and arranged. For the convenience of explanation, we take the schematic diagram as an example.

Step 1: Prepare the Mold

Like other lamination processes, high-quality molds are also necessary for VIP. The surface should have high hardness and high gloss, and the mold edge should be kept at least 15 cm to facilitate the laying of sealing strips and pipes. Clean the mold and then apply mold release wax or mold release water.

Step 2: Apply Gel Coat Surface

Depending on the requirements of the product, product gel coat and polished gel coat can be used. The types of selection are orthophthalic, isophthalic and vinyl. Apply gel coat by hand brush and spray.

Step 3: Lay Out Reinforcement Materials

Choose reinforcement materials – glass fiber, carbon fiber, sandwich material… This depends on the strength requirements of the product. Choosing reinforcement materials is an important step in the lamination process, but for VIP, you need to consider a few more points. Although all fabrics can be used, different materials and weaves will affect the resin flow rate.

Step 4: Lay Out Vacuum Bag Materials

First, lay out the release cloth, then the guide cloth, and finally the vacuum bag. Before closing the vacuum bag, carefully consider the direction of the resin and the vacuum pipeline, otherwise the resin will not be able to penetrate some places. Be very careful when laying to prevent some sharp objects from puncturing the vacuum bag.

Step 5: Vacuuming

After laying these materials, clamp the resin inlet pipes, vacuum the entire system, try to evacuate the air in the system, and check the air tightness. This step is very critical. If there is a leak, when the resin is introduced, air will enter the system, and bubbles will penetrate to other places at the leak, and even the entire product may be scrapped.

Step 6: Resin Preparation

After the vacuuming reaches a certain requirement, prepare the resin. Add the corresponding curing agent according to the gel time. Remember not to forget to add the curing agent, otherwise it will be difficult to make up. However, generally, the vacuum-introduced resin contains a curing indicator, and you can judge whether the curing agent has been added by color.

Step 7: Introduce the Resin

Insert the resin inlet pipeline into the prepared resin barrel, open the clamps in sequence according to the feeding order, pay attention to the amount of the resin barrel, and replenish it in time.

 

Step 8: Demolding

After the resin gel is cured to a certain extent, remove the vacuum bag material. Take the product out of the mold for post-processing.

 

Conclusion

Of course, no process is perfect. At present, a large part of the disposable consumables required for VIP needs to be imported, which increases the material cost, but this part can be balanced by reducing the amount of resin used. In addition, higher skills are required for operators. Each process must be carefully followed step by step before entering the next step, otherwise it will cause irreversible losses.

 

Therefore, this process is currently used in FPR parts and products with high added value, such as sports equipment accessories, yachts, wind power blades, etc. However, people have a large demand for higher performance materials, and the vacuum infusion process is being recognized and adopted by more and more people.