Vacuum – pressure injection process is a closed-mold molding process for composite material making. It is a process of laying glass fiber reinforcement material between the positive and negative molds, closing the mold and then vacuuming it, and injecting liquid resin into the mold cavity under pressure to saturate the glass fiber, and then curing the mold. Because of this process can directly close the molding, so it keeps the inside and outside surface of the product clean. Wall thickness tolerance is small, which can reduce the exposure of harmful substances to the operator and improve labor conditions. Since the 1960s, foreign countries have conducted in-depth study on the vacuum-pressure injection process, and made rapid progress. Especially in the airliner and fighter jet nose redone, etc., obtained very good performance. Also as early as in the 1970s, China’s some material research institutes researched the development of vacuum – pressure injection process combined with national conditions, and received good results in the development of civilian products.
1.The vacuum – pressure injection process basic characteristics
(1) The process of molding products with a small voyage, generally below 0.2%. And because there is no need for machining, it reduce the surface of the FRP water absorption path, so the water absorption rate of the product is low. According to the laminate blister test results, the water absorption rate of vacuum-pressure injection products is 0.04%, while the water absorption rate of manually paste products is 0.12 to 0.50%, molded products of 0.38 to 0.39%, phenolic pressing process products is about 0.45 to 1.70%.
(2) Due to the fixed closed mold cavity space, and thus can strictly control the resin content. Practice has proved that the precision of the laminate and its products resin content within ±1.5.
(3) The quality is good, with high finish, good dimensional stability and repeatability. And according to the actual measurement of laminate thickness, the average error is less than 0.1mm.
- The vacuum – pressure injection process molding process
The process includes inspection of the mold, application of molding agent, glass cloth sleeve sewing, mold closing, inspection of mold airtightness, vacuuming, glue distribution, pressure injection, curing, mold release, etc. Since the mold cavity will be vacuumed, the mold must be guaranteed to be free of sand holes when casting. Also the processed mold should be carefully inspected and the air leakage should be partially repaired. Meanwhile, the mold needs to be cleaned before molding to remove oil and dust before spraying the mold release agent, and after closing the mold, the air tightness should also be carefully checked. In order to prevent loosening, the glass fiber cloth or woven sleeve still need to be tied after setting in the negative mold. After closing the mold need to warm up to 70℃, under reduced pressure for drying, and then cool down to 50℃ to maintain a certain period of time.
The resin used in the process, in addition to requiring a suitable viscosity, also requires that the resin gel time more than the pressure injection time of at least 5 hours. When dispensing, the resin, curing agent and other additives are added to the resin mixing kettle in a certain proportion, and after stirring well, connecting the vacuum system and pumping out the air bubbles in the resin and heating to the injection temperature of about 50℃. Then, use the pressure or pump to press the resin mixture from the lower part of the mold into the cavity, when the upper part of the mold flows out of the glue, the resin has filled the cavity. So you can stop the pressure injection and seal the outlet. Curing and post-treatment according to the curing system, cool to room temperature and then demold.
3.Factors affecting the vacuum-pressure injection process
The viscosity of the resin used in the molding method directly affects the injection pressure and production cycle. When the resin viscosity is high, the injection pressure is required to increase accordingly. When the resin viscosity is too high, it is inevitably required to increase the pressure resistance of resin delivery pipes and molds, which increases the difficulty of designing and processing the system and molds. In addition, when the viscosity increases, it takes longer to fill the mold cavity, and the production efficiency will be reduced accordingly.
The vacuum-pressure injection molding process is also very strict in terms of mold requirements. The rigidity of the mold and the deviation of the inner cavity are too small, both have a large impact on the quality of the products. Small mold rigidity will cause changes in mold dimensions during the injection process, resulting in uneven resin and fiber content in the product, which may deform the product. In addition, the uniformity of the layered density of the reinforcing material will also have a great impact on the quality of the product.
4.Vacuum – pressure injection process development and application
As the process product performance is more excellent, compared with the hand paste and pressing process, its tensile strength, tensile modulus, bending strength, bending modulus, inter layer shear strength, extrusion strength and other performance indicators have improved, so there is a great prospect for development.
The process has unique features compared with the RTM process. For example, the U.S. Venus equipment using hydraulic injection process, which using the pump to transport the rubber, resin viscosity requirements in 0.5 Pa.s (500 centipoise) or less. But when more than the requirement, the molding is very difficult. However, the vacuum-injection process does not require strict resin viscosity, it can be below 30 Pa.s and the most appropriate range of 0.2 ~ 5 Pa.s, thus domestic unsaturated polyester resin, epoxy resin, vinyl ester resin can be applied.
For example, the reinforcing materials used in foreign RTM process are continuous fiber mat, short-cut fiber mat, untwisted roving cloth, preformed blank or surface felt, especially continuous fiber mat is the most widely used. Due to poor deformation and imitation ability, short-cut fiber mat and non-twisted roving cloth usually can only be used for the preparation of flat, and the shape of simple products. There are currently some problems in production, quality and variety of China’s continuous fiber mat and short-cut fiber mat. For this reason, in recent years, most of the introduction of Venus hydraulic injection molding machines failed to play the role. And this vacuum – pressure injection molding process, which use the reinforcing materials, is applicable to the domestic production of many plain cloth, twill fiberglass cloth, non-twisted roving cloth, and glass fiber woven sets, etc., fully suitable for China’s national conditions and fit for widely use.
For example, in terms of product performance, foreign RTM process products contain more than 60% of glue, and their mechanical properties are similar to hand paste, but the resin content of the vacuum-pressure injection process is 30% to 42%, so its mechanical properties are more excellent.
This vacuum-pressure injection process has successfully developed a variety of products, for example, radome, antenna, fan blade, pipe, agricultural sprayer, etc. In addition, there are great prospects for development and application in engine blades, helmets, building siding, oil storage tanks, automobile parts, seats, bath tubs, as well as surfboards, sampans, skis, etc.
More information about epoxy resin applications in Vacuum – Pressure Injection, please contact us via email: sales@yqxpolymer.com, or voice to us at: +86-28-8411-1861.
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