Take you to know the vacuum introduction process of composite materials

- Jun 01, 2019-

Take you to know the vacuum introduction process of composite materials


Since the introduction of vacuum introduction technology in China, after more than ten years of development, has been a very mature technology in China. In recent years, due to the strict inspection of environmental protection, the closed die process is getting more and more attention.


Starting from the principle of vacuum introduction, this paper gives a complete description of the whole process flow, and compares it with other processes in terms of mechanical properties and advantages, so that everyone has a relatively clear understanding of vacuum introduction process.




1. Vacuum infusion process (VIP)

Import vacuum technology, hereinafter referred to as VIP, spread reinforced material in the mold (glass fiber, carbon fiber, sandwich materials, etc., is different from the vacuum bag process), and then spread vacuum bag, and extraction system in the air, to form a negative pressure in the mold cavity, by using the vacuum pressure of the unsaturated resin before paving pipe pressure into the fiber layer, to fill the whole mold, with resin infiltration reinforced material last products after curing, vacuum bag material is uncovered, get the required products from the mold.


VIP USES a single side mold (like the usual hand paste and spray molds) to build a closed system.


The vacuum introduction process has been around for a long time. It was patented in 1950. Only in recent years, however, has development begun. Due to the introduction of this process from abroad, so in the name of a variety of names, vacuum introduction, vacuum infusion, vacuum injection, and so on.



2, the theory

The vacuum induction process can be widely used, and it has its theoretical basis, which is darcy's law:
Darcy 's Law) t = ℓ 2 h / 2 (kDP)

 t is to import 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 time required for the introduction. Therefore, the viscosity of the resin used for vacuum introduction should not be too high. This allows the resin to fill the mold quickly.


- injection length, refers to the resin inlet and the distance between the arrived at the discharging port, of course also the time needed for long distance.

DP- pressure difference, the greater the pressure difference between the system and the system, the greater the driving force on the resin, the faster the resin flow rate, of course, the shorter the lead-in time.


K = permeability, refers to the parameters of glass fiber, sandwich material, etc. on the infiltration of resin. Large k value indicates good infiltration, like multi-directional felt than square cloth, chopped felt is easy to be resin infiltration. Therefore, in order to make the resin can easily fill the system when the reinforcing material is compacted, some diversion channels, such as drilling up and down the sandwich foam, are usually set up artificially.



3, advantages

In the normal hand lay-up process, reinforcement is spread in a mold, manually infiltrating the reinforcement with brushes, rollers, or other means. Another improved method is to use vacuum bags to absorb excess resin from the layup. This increases the fiberglass content, resulting in stronger and lighter products. Compared with traditional technology, VIP has many advantages. As shown in the figure, hand paste, vacuum bag and vacuum introduction are taken as examples. In terms of mechanical properties, vacuum introduction has obvious advantages.




It can be seen that advantages of vacuum introduction:
. higher quality product: resin impregnated glass fiber in vacuum. There are very few bubbles in the product compared with traditional manufacturing process. There is no excess resin in the system and the glass fiber content is very high, up to 70% or higher. The resulting products are lighter and stronger. Batch to batch is also very stable.


B. less loss of resin: with VIP process, the amount of resin can be accurately budgeted, and for hand paste or injection process, it is difficult to control due to the variability of operators. VIP can minimize the loss of resin, and more importantly, save costs.


C. uniform distribution of resin: for a product, the pressure generated by vacuum in different parts is the same, so the infiltration rate and content of resin to glass fiber tend to be the same. This is critical for weight - stable FRP parts.


D. Less volatilization in process d: there are no brushes or rollers in the process of production, which will not cause the spilling or dripping of resin, nor will there be a large number of odors. So it can provide a clean, safe and friendly working environment to protect the physical and mental health of operators.


E. use single side mold: only use one side mold can get smooth and flat products on both sides, which can better control the thickness of products. Save mould manufacturing cost and time.

It is because of the advantages of VIP technology products, first used in aerospace and other special areas, and then slowly applied to high-demand civil products.


4. Process flow


Materials used by VIP
When preparing for the vacuum introduction test, the appropriate resin, epoxy or unsaturated polyester resin, should be selected first. The resin of vacuum introduction process cannot be replaced by ordinary resin, which has special requirements on viscosity, gelation time, exothermic peak and wettability. Please consult the resin supplier for details.


Curing system
If it is epoxy resin, use the corresponding special curing agent; The curing agent commonly used in unsaturated resins is methyl ethyl ketone peroxide. Different manufacturers of its quality is not the same, the selection of its evaluation, not because the use of less material and affect the quality of the entire product.


Reinforced material
Reinforcement materials commonly used are glass fiber and carbon fiber. Such as composite edge felt, single cloth, etc., the specific design according to the mechanics. When selecting, it is better to do an experiment, and how is the permeability? Because different sizing agents and adhesives used in the manufacturing process of the fiber will have different infiltration on the resin, resulting in great differences in the mechanical properties of the final product.


Sandwich materials
Commonly used are Balsa wood, PVC foam, PUR/PIR foam, strong felt and so on. Choose suitable sandwich materials according to the needs of products.


Equipment and auxiliary materials
Vacuum pump, interface, pressure gauge, guide tube, stripping cloth, guide cloth, vacuum bag and so on. The materials used in the vacuum lead-in process are not readily available, and each material has to be tested to determine whether it is suitable.


Step 1: prepare the mold

Vacuum infusion process requires the mold to have high quality, high surface gloss and hardness, mold edge at least 15 cm, easy to lay sealing strip and pipeline. Clean the mold
Clean, then wax or apply the mold water.


Step 2: construction of gelcoat surface

According to the requirements of the products, choose suitable gelcoat, hand brush or spray. (some products are directly laid with dry cloth without making gelatine coat surface)


Step 3: reinforced material laying

The choice of reinforcing materials depends on the strength requirements of products (glass fiber, carbon fiber, sandwich material.. ), different materials and weave methods will affect the resin flow rate.


Step 4: vacuum assisted material laying

Successively lay demoulding cloth, flow guide net, vacuum bag... Before closing the vacuum bag, take into account the direction of resin and vacuum tube, or there will be bad infiltration phenomenon. Care should be taken when laying so as not to Pierce the vacuum bag with sharp objects.


Step 5: vacuum

After material laying, clamp each resin tube, start to vacuum, try to take out the air in the system, and check air tightness, this step is very critical, if there is a leakage point, when the resin is introduced, the air will enter the system, bubbles will leak into other places, and even the whole product may be scrapped.


Step 6: mix the resin

After the vacuum has reached certain requirements, resin preparation begins.
According to the different gelation time of resin, the corresponding curing agent was prepared.


Step 7: import resin

Insert the pipe into the resin bucket, and open the clip in turn according to the order of feeding. Pay attention to the amount of the resin bucket, and timely supplement.


Step 8: demoulding

When the resin gel is cured to a certain degree, the vacuum material is removed. Remove the product from the mold and conduct post-treatment.


No process can be perfect. At present, a large part of the disposable consumables needed by VIP needs to be imported to increase the material cost, but this part can be balanced by reducing the amount of resin. In addition, it requires higher skills of operators. Each process must be carefully followed to proceed to the next step, otherwise irreversible losses will be caused.
So this process is now used in high value-added FPR components and products, such as sports accessories, yachts, wind turbine blades and so on. However, there is a great demand for higher performance materials, so more and more people are recognizing and adopting vacuum introduction technology.