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What is vacuum introduction?

Dec 26, 2018

What is vacuum introduction? This article starts with the principle of vacuum introduction, and provides a complete description of the whole process flow. It also compares with other processes from the aspects of mechanical properties and advantages, so that everyone has a clear understanding of the vacuum introduction process.


First, the vacuum infusion process (VIP)
The vacuum introduction process, referred to as VIP, is to spread the reinforcing material (glass fiber, carbon fiber, sandwich material, etc., different from the vacuum bag process) on the mold, then vacuum the bag, and extract the air in the system to form a cavity in the mold cavity. Negative pressure, using the pressure generated by the vacuum to press the unsaturated resin into the fiber layer through the pre-laid pipe, let the resin infiltration reinforcement material finally fill the entire mold, after the product is solidified, the vacuum bag material is removed, and the required material is obtained from the mold. Products.
The VIP uses a single-sided mold (just like a normal hand lay-up and spray mold) to create a closed system.
The vacuum introduction process has been published for a long time, and the process was patented in 1950. However, it has not been developed until recently. Since this process is introduced from abroad, there are various names in the naming, vacuum introduction, vacuum infusion, vacuum injection, and the like.

Second, the theory
The vacuum introduction process can be widely applied and has its theoretical basis. This is Darcy's law:
Darcy’s Law)t =ℓ2h/(2 kDP )

t is the introduction time, determined by four parameters; h-resin viscosity
It can be seen from the formula that the lower the viscosity of the resin used, the shorter the introduction time required, and therefore the viscosity of the resin used for vacuum introduction 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 discharge port, and the distance required for the long distance is also long.
DP-pressure difference, the greater the difference between the pressure in the system and the pressure outside the system, the greater the driving force for the resin, and the faster the resin flow rate, of course, the shorter the introduction time.
k = permeability, refers to the parameters of glass fiber, sandwich material and other infiltration of resin. A large k value indicates good wetting, like a multi-directional felt than a gingham cloth, and the chopped strand felt is easily infiltrated by a resin. Therefore, in order to make the resin easy to fill the system under the condition that the reinforcing material is compacted, some guiding grooves are generally artificially set, for example, punching holes in the sandwich foam.


Third, the advantages
In a typical hand lay-up, the reinforcing material is placed in a mold and manually infiltrated with a brush, roller or other means. Another improved method is to use a vacuum bag to suck excess resin from the laminate during hand lay-up. This increased glass fiber content results in a higher strength and lighter product. VIP has many advantages over traditional processes, such as hand lay-up, vacuum bag and vacuum introduction. Vacuum introduction has obvious advantages in mechanical properties.
This shows the advantages of vacuum introduction:
a. Higher quality products: The resin infiltrates the glass fiber in a vacuum environment, and there are few bubbles in the product compared to the conventional manufacturing process. There is no excess resin left in the system, and the glass fiber content is very high, reaching 70% or even higher. The resulting article is lighter in weight and higher in strength. Batch and batch are also very stable.
b. Less resin loss: With the VIP process, the amount of resin can be accurately budgeted, and it is difficult to control the hand lay-up or spray process due to the variability of the operator. VIP can minimize the loss of resin and, more importantly, it can save costs.
c. Resin distribution is uniform: For a product, the pressure generated by different parts of the vacuum is consistent, so the resin infiltration rate and content of the glass fiber tend to be consistent. This is critical for weight-reliable FRP parts.
d. Process is less volatile: there are no brushes or rollers in the production process, no spillage or dripping of the resin, and no large amount of odor. Therefore, it can provide a clean, safe and friendly working environment to protect the health of the operator.
e. Use single-sided mold: Only one side of the mold can be used to obtain smooth and flat products 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 applied to special fields such as aerospace, and later slowly applied to high-demand civilian products.

VIP process applications
1) Boat industry - hull, deck, rudder, radar shield
2) Wind power energy - blade, machine cover
3) Sports and leisure - helmets, windsurfing
4) Automotive industry - all kinds of roofs, windshields, compartments
5) Building area - building top parts, building formwork
6) Agriculture and horticulture--granary cover, agricultural machinery protection cover


Fourth, the process
4.1 VIP materials
4.1.1 Resin
When preparing for the vacuum introduction test, the first choice of resin is epoxy or unsaturated polyester resin. The resin introduced into the vacuum process cannot be replaced by ordinary resin. It has special requirements on viscosity, gel time, exothermic peak and wettability. For details, please consult the resin supplier.
4.1.2 Curing system
If it is an epoxy resin, use its corresponding special curing agent; the curing agent commonly used for unsaturated resins is methyl ethyl ketone peroxide. The quality of different manufacturers is different, and it is evaluated when selected. It cannot affect the quality of the whole product because of the small amount of materials.
4.1.3 Reinforcement materials
Glass fibers and carbon fibers are commonly used for reinforcing materials. For example, composite seam felt, single cloth, etc., according to the mechanical design. It is best to do the experiment when selecting. The permeability is different. Because the sizing agent used in the manufacturing process of the fiber and the different adhesives will not infiltrate the resin, the mechanical properties of the final product will be greatly different.
4.1.4 Sandwich material
Commonly used are Balsa wood, PVC foam, PUR/PIR foam, strong felt and so on. According to the needs of the product, choose the appropriate sandwich material.
4.1.5 Equipment, auxiliary materials
Vacuum pump, interface, pressure gauge, draft tube, release cloth, guide cloth, vacuum bag, etc. The materials used in the vacuum introduction process are not used casually, and each material is experimentally confirmed to determine whether it is suitable.
4.2 VIP process
In order to understand how the materials are used and how to arrange them. For the sake of convenience, we take the example diagram as an example.
The first step: preparing the mold - as with other lamination processes, high quality molds are also required for VIP. The surface should have a high hardness and a high gloss, and the edge of the mold should be at least 15 cm to facilitate the laying of the sealing strip and the pipeline. Clean the mold and then release the mold wax or mold release water.
The second step: construction of the gel coat can be based on the requirements of the product, you can use the product gel coat and sanding gel coat, the choice of the type of o-benzene, m-benzene and vinyl. Apply gel coat by hand brush and spray.
The third step: reinforcement material laying - use of reinforcing materials - glass fiber, carbon fiber, sandwich material ... which depends on the strength requirements of the product. Choosing a reinforcement material is an important step in the lamination process, but there are a few more considerations for the VIP. Although all fabrics can be used, different materials and weaves can affect the resin flow rate.
Step 4: Laying the vacuum bag material - first lay off 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 line, otherwise the resin will not be able to infiltrate. Be very careful when laying it so that some sharp objects do not pierce the vacuum bag.
Step 5: Vacuuming - After laying out these materials, clamp each resin tube, vacuum the whole system, evacuate the air in the system, and check the air tightness. This step is critical if there is a leak. When the resin is introduced, the air will enter the system, and the bubbles will infiltrate into other places at the leak point, and even the entire product may be scrapped.
The sixth step: with resin - after vacuuming to meet certain requirements, prepare the resin. According to the gel time, the corresponding curing agent is added. Remember to add the curing agent, otherwise it is difficult to make up. However, in general, the vacuum introduction resin contains a curing indicator, and it is possible to judge whether or not a curing agent is added from the color.
Step 7: Introduce the resin--Into the resin pipeline into the resin barrel, open the clip according to the feeding sequence, pay attention to the amount of resin, and replenish if necessary.
Step 8: Demoulding - After the resin gel is cured to a certain extent, the vacuum bag material is removed. The article is removed from the mold and post-treated.


V. Conclusion
Of course, any process can't be perfect. At present, a large part of the disposable consumables required for VIP needs to be imported to increase the material cost, but this part can reduce the balance of resin dosage. In addition, the skill requirements of the operator are higher. Each process is carefully followed by steps to enter the next step, otherwise it will cause irreversible losses.
Therefore, this process is currently used in high value added FPR parts and products, such as sporting goods accessories, yachts, wind power blades and the like. However, people have a large demand for higher performance materials, and the vacuum introduction process is being recognized and adopted by more and more people.

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QuanZhou Neweco High Polymer Material Co.,Ltd.

Neweco is one of the leading unsaturated polyester resin manufacturers and  suppliers,widely used in automobiles, ships, rail transit, wind power  generation, pipe cans, heat and corrosion protection, building  materials, stone repair, sanitary ware, artificial stone and other  industries. Suitable for hand lay-up, spray, pultrusion, winding,  molding, vacuum introduction / RTM, casting and other molding processes. Neweco Resin is committed to excellence and focuses on the application  of composite materials to provide customers with comprehensive  solutions.

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