Comparative Introduction and Differences of Several Closed Mold Forming Processes for Composite Materials

 

FRP closed mold technology refers to a process method that injects resin into a closed mold and completes the curing process without being exposed to the outside air. There are several types of closed mold technology: vacuum injection, resin transfer molding (RTM), structural reaction injection molding (SRIM), injection technology, reinforced reaction injection molding (RRIM), etc.

 

For the three closed mold processes of vacuum injection, RTM, and SRIM, the equipment investment is the highest for SRIM, followed by RTM, and the lowest for vacuum injection. These process methods also have certain regularities in terms of processing cycle, automation level, batch size, etc.

 

For example, the processing cycle is the shortest for SRIM (about 8 minutes), followed by RTM (about 10 to 80 minutes), and vacuum injection molding is the longest (more than 1 hour to more than 10 hours). The automation level is the highest for SRIM, and its annual production capacity is also the highest, and the labor cost contained in the production cost is the lowest.

 

All of the above processes use the method of resin injection. An important difference is whether the reinforcing material is put into the mold before closing the mold or before resin impregnation. This is undoubtedly a very important factor in controlling the content and directional arrangement of the reinforcement material. RTM and SRIM processes are more favorable for controlling the glass fiber content due to the use of mold technology. It must be pointed out that when using closed mold technology, the resin used is also crucial. Usually, vacuum injection special resin, low shrinkage RTM resin, and mixed resin are used.

 

 (1) Vacuum injection molding process

This process uses resin injection technology and is assisted by vacuum to facilitate the injection process. Usually, there is a deformable membrane as the other side of the mold. Half of the mold can generally be used as a hand lay-up mold or an injection molding mold with only slight modifications. This process does not require a large amount of mold or equipment investment, and the production batch is large and the product quality is high.

 

The reinforcement material used in the vacuum injection molding process can be Chopped Glass Fiber Mat, or aramid fiber or carbon fiber fabric. The advantages of this process are summarized as follows: no styrene volatiles; clean workplace; easy bonding of core materials and inserts; low equipment investment, and can use hand lay-up or injection molds. However, this vacuum injection molding process also has disadvantages. For example, the vacuum injection process must use mat as a reinforcement material, which is slightly more expensive than untwisted roving; for example, the positioning of mat and film is more difficult, and it takes more time than injection molding.

 

The products made by the vacuum injection molding process are mainly used in boats, transportation, construction and other industries. For example, it can manufacture various fiberglass truck body sandwich panels and bus integral sandwich panels, windmill sails (11m long, 68% glass fiber content), offshore oil platform sandwich panels, building roofs, etc. The vacuum injection molding process is suitable for manufacturing medium-sized batches, medium or high mechanical performance requirements, and large components over 2 meters.

 

 (2) Resin transfer molding (RTM)

 

This process can be regarded as a resin injection process under low pressure, with a pressure of about 0.3 to 0.6 MPa. In recent years, many RTM-specific resins have been developed abroad, and high-quality Class A surfaces can be obtained. These resin systems, with the addition of low shrinkage additives, have an optimal curing temperature of about 70°C, which is much lower than the curing temperature of 140 to 160°C of the usual SMC low shrinkage system.

 

Currently, most RTM processes use gel coat resins to obtain better surface quality. However, when using a low shrinkage RTM system, gel coat resins are not required, and thus no volatile styrene is produced.

 

The development market for RTM products is mainly the automotive and construction industries, as well as other fields that require higher surface quality. At present, foreign countries mainly develop and establish RTM complete machine systems, including: Class A surface quality RTM-specific resins; higher temperature RTM process parameters; injection equipment evaluation, etc. Indupol, the largest fiberglass manufacturer in Belgium, has successfully developed RTM resins and their corresponding process technologies, which not only ensure the reproducibility of product production, but also reduce working hours. DSM Resins has developed a hybrid resin of unsaturated polyester and polyurethane, which has the following properties: viscosity below 200-400 MPa.s (at room temperature); fast curing speed at low temperatures (gelling time of 1 minute at 70°C);

 

(3) Structural Reaction Injection Molding (SRIM)

 

The molding equipment for this process should include the following parts: reinforcement material preforming machine, production mold, fixture and injection machine, etc. In industrial production, SRIM often needs to be equipped with a reinforcement material preforming machine, and the preforming cycle is about a few minutes. There are usually two preforming methods:

 

One is the continuous chopped mat hot forming method, which uses infrared to heat the mat sheet and adds a binder to heat and press to cure the preforming; the other is the direct injection glass fiber method, which uses chopped fibers and a small amount of powdered binder to be sprayed together and adsorbed onto the sieve screen, and then melted, preformed, cured and cooled. Today’s remote control technology can accurately lay the fibers where they are needed. For some products, such as flat plates, preforming may not be required, and often online preforming can be performed. The partners complete the preforming process according to the shape of the product.

 

SRIM molds can be made of different materials. Usually, the annual batch of SRIM process can reach more than thousands of pieces, so nickel molds are often used, and aluminum molds or alloy molds can also be used. These non-steel molds can not only reduce the applied pressure and temperature, but also save a lot of mold costs. Generally speaking, SRIM has a tighter mold closing effect, a wider range of applied pressure, and a lower injection pressure, with an average value of about 1MPa. The injection molding machine usually has two pre-mixers, which generally mix the resin in a non-critical state, and the mixing ratio is wide. After the mixed resin passes through the pre-mixer, it is injected into the heated mold. The price of the injection molding machine is between the injection molding machine and the PU mixer.

 

The characteristics of the SRIM process are as follows: no styrene volatilization; good product quality, bright on both sides, no exposed fibers; the content and positioning of the reinforcing material in the product are relatively accurate, so its strength is higher; the equipment investment is relatively low. The main markets for its product development are transportation, construction electrical and electronics industries.