What is the RTM Composite Closed Molding Process?

 

The so-called closed mold molding process is a process method for molding composite components when the female and male molds are closed. SMC, BMC molding, injection molding, RTM, VEC and other technologies are all closed mold molding processes. Due to the formulation of environmental laws and the improvement of product requirements, the open mold molding of composite materials has been increasingly restricted, which has prompted the application of closed mold molding technology, especially the innovation and development of RTM technology in recent years.

 

RTM refers to a process technology in which low-viscosity resin flows in a closed mold, infiltrates reinforcing materials and solidifies into shape. It belongs to the category of liquid forming or structural liquid forming technology of composite materials. The specific method is to pre-place reinforcing materials that have been reasonably designed, cut or mechanized pre-formed in the designed mold. The mold must have peripheral sealing and tightening, and ensure smooth flow of resin; after the mold is closed, a certain amount of resin is injected, and the desired product can be demolded after the resin is solidified.

 

This process has many advantages. It can use a variety of fiber-reinforced materials and resin systems, and has an excellent product surface. It is suitable for manufacturing high-quality products with complex shapes, and has high fiber content, less volatile components during molding, less environmental pollution, strong adaptability to production automation, low investment, and high production efficiency. Therefore, RTM technology has been widely used in the automotive industry, aerospace, defense industry, mechanical equipment, and electronic products. The primary factor that determines RTM products is the mold. Since RTM molds generally use the male and female mold matching method, finding ways to improve the surface quality and dimensional accuracy of the male and female molds has become a key factor in determining product quality.

 

1) RTM, resin transfer molding. This technology is derived from polyurethane technology. During molding, the mold is closed and resin is injected into the preform. The glass fiber content is low, about 20-45%.

 

2) VARIT, vacuum-assisted resin transfer injection molding. This technology uses vacuum to suck the resin into the preform, and can also press the resin in. The vacuum degree is about 10-28 inches of mercury.

 

3) VARTM, vacuum-assisted resin transfer injection molding. The product generally has fewer pores and the glass fiber content can be increased.

 

4) VRTM, vacuum resin transfer molding.

 

5) VIP, vacuum infusion method.

 

6) VIMP, variable infusion molding method. The resin moves with the help of vacuum or deadweight, and the impregnation is compacted.

 

7) TERTM, thermal expansion RTM. Insert the core material into the preform, let the resin impregnate, and heat the mold and the molded product. The core material expands due to heat, compacting the layers. This compaction effect is combined with surface pressure molding.

 

8) RARTM, rubber-assisted RTM. In the TERTM method, rubber is used instead of core material. The rubber mold presses the molded product, greatly reducing the porosity, and the glass fiber content can be as high as 60-70%.

 

9) RIRM, resin injection cycle molding. Vacuum and pressure are combined to alternately inject resin into multiple molds to circulate the resin until the preform is fully impregnated.

 

10) CIRTM, Co-Injection RTM. Co-injection RTM can inject several different resins, and can also use several preforms. Vacuum bags and molds with flexible surfaces can be used.

 

11) RLI, resin liquid impregnation. Resin is injected into the lower mold, and after the preform is inserted, the upper mold is covered, heated and molded with the molding pressure of the autoclave. Heating reduces the viscosity of the resin, improves fluidity, and is easy to impregnate.

 

12) UVRTM, ultraviolet (curing) RTM. Similar to the SCRIMP method, it cures quickly and must use a UV light source, a vacuum bag that can transmit ultraviolet light, and a soft mold.

 

13) VECTM Virtual Design of Composite Materials The core technology of VEC is the patented “floating mold” concept. The composite mold is installed between two liquid-filled steel pressure vessels, and the mold forms a seal along the entire length of the container. The container is filled with a compressible heat-conducting liquid, which is usually water.

 

 

⊙ According to the requirements of the production scale, molds of different materials and grades can be used to minimize costs and obtain better economic benefits.

⊙ It is a closed mold resin injection method, which can minimize the toxicity of harmful components such as resin to the human body and the environment, and meet environmental protection requirements.

⊙ Low-viscosity fast-curing resin is used, and the RTM mold can be heated during the production process, thereby further improving production efficiency and product quality.

⊙ It is conducive to the preparation of large and medium-sized, complex shapes, and smooth overall structural parts on both sides, with good dimensional accuracy, high surface quality, and good mechanical properties.

⊙ The reinforced material preform can be selectively reinforced, locally reinforced, mixed reinforced, and embedded and sandwich structures can be used according to performance requirements, which can give full play to the performance designability of FRP.

 

 

RTM Molding Process Mold Technology

 

1) RTM overall process route: RTM has three important components: 1 raw material system; 2 injection equipment; 3 mold system.

 

2) RTM molding mold technology: All RTM products require a mold suitable for the process, and RTM is no exception. RTM molds can be made of aluminum steel and FRP. Since aluminum steel molds are not easy to deform but are expensive, they are not introduced here. The following mainly introduces FRP molds.

 

1. RTM Mold Type and Material

 

FRP molds are used for RTM. According to general regulations, a 7-10mm thick mold layer is made, and then a heating core is installed under the surface of the base mold to form a sandwich structure. The total thickness of the mold layer is 20mm. Since this thickness is not enough for the strength required by the RTM molding process, it needs to be further enhanced. Box-shaped steel profiles are much cheaper than composite materials and are generally reinforced with box-shaped steel profiles.

 

FRP mold practice has proved that the service life of molds made with inferior resins is extremely short, and it also has a direct impact on product quality, so the mold surface is required to be made of heat-resistant and chemical-resistant materials. The cost of mold making is mostly labor, and the cost of material selection has almost nothing to do with the total mold cost. Currently, vinyl ester mold resin system and gel coat are generally selected, and it has been proven to have better service life and temperature resistance than traditional epoxy materials. According to foreign data, molds made with vinyl ester mold resin have been molded more than 18,000 times and are still in use.

 

2. Mold Heating

 

Most resins used in the FRP industry have a curing curve that is directly related to temperature, so it is of great practical significance to find a way to control the temperature of the production mold, which helps to optimize production efficiency. In fact, for every 10°C increase in mold temperature, the gel time will be halved. Therefore, at ambient temperature (20°C), a molded part may take 60 minutes to cure and demold, while at 50°C, the same resin system, molding the same part, can be demolded within 7.5 minutes.

 

Electric heating cloth heating: The electric heating method of applying heating cloth to the memory of the mold in RTM molds has been used for many years. The heating is relatively uniform, and the mold can easily be heated to more than 100°C, but the standard application maximum mold temperature is 75°C.

 

Liquid heating: Liquid heating is to install a hot and cold circulation system into a suitable mold structure, which can replace electric heating. This also provides a cooling system for the mold. The temperature can be controlled by the circulating medium in the pre-buried pipe.

 

3. Mold Sealing

 

All closed molds for RTM and vacuum molding (VM) require a seal to control resin overflow at the edge of the mold cavity. In the case of VM molding, an additional external mold flange vacuum seal is required.

 

There are many different forms of seals, but the seal material is required to have high temperature resistance and consistent recovery rate. At present, organic silicone materials are the most successful for basic mold resin sealing. If used correctly, it is enough to provide a service life of more than 1,000 times.

 

Passive seal: Almost dedicated solid silicone rubber seal section, designed to compress 1.0-1.5mm at its “Z” end closed position when the mold is closed. To achieve effective sealing without increasing the mold closing force, the hardness and compression size of the seal are critical. However, because the required pressure is too large, it is easy to cause mold deformation. The mold contact surface is generally 3-5mm, which is enough to prevent resin seepage and achieve effective sealing.

 

Dynamic seal: The dynamic form is better than the passive seal form. It can permanently control the cross-sectional changes. The cross-sectional size change of the illustrated form can be up to 4mm. This allows the vertical sealing track to be embedded in the mold flange. Otherwise, the passive seal will only “plastically deform” and wear extremely. Once the inner section of the dynamic seal ring is pressurized, it can provide a good adjustment of the sealing effect. When the mold is closed or opened, the seal ring deforms elastically with the vacuum gap, which prevents wear caused by “plastic deformation” in the vertical direction.

 

4. Injection Port

 

Technicians engaged in RTM molding process pay special attention to the inlet position of injection molding resin. Practice has proved that it is most reliable to set the RTM injection port in the center position (depending on the shape of the mold cavity).

 

5. Mold Accuracy

RTM molds are often under stress, so for a successful closed mold, mold accuracy is one of the key factors. The mold cavity accuracy control of ±0.2mm is the target accuracy of the closed mold. Failure to achieve this accuracy will inevitably lead to glue shortage and unforeseen resin filling, and the molded part size is out of tolerance. The most common is overthickness, and the material shrinkage parameter must be taken into account.

 

1. RTM Process Operation

The injection operation of the RTM process is generally required to be completed within 1/4-1/2 of the gel time, the transfer time is 2-15 minutes, and the transfer pressure is 0.3-07Mpa.

The resin transfer pressure is the main parameter that should be controlled in the RTM process. This pressure is used to overcome the resistance encountered when injecting into the mold cavity and soaking the reinforcement material. The time for the resin to complete the transfer is related to the system pressure and temperature. A short time can improve production efficiency. However, if the resin flow is too large, the glue will not have time to penetrate the reinforcement material, and it may cause accidents due to the increase in system pressure.

 

Therefore, it is generally required that the rising speed of the resin liquid level entering the mold during the transfer process is not more than 25mm/min. Monitor the resin transfer process by observing the discharge port. It is generally believed that when all the observation ports on the mold have glue overflowing and no bubbles are discharged, and the actual amount of resin added is basically the same as the expected amount of resin added, the transfer process is completed. Therefore, the setting of the discharge port should be carefully considered.

 

2. Resin Selection

The selection of the resin system is the key to the RTM process. To get the resin into the mold cavity and quickly soak the fiber, the viscosity of the resin should be 0.025-0.03Pa•s. Polyester resin has a low viscosity and can be completed by cold injection at room temperature. However, due to different performance requirements of the product, different types of resins will be selected, and their viscosities are not the same, so the size of the pipeline and injection head must be designed to meet the fluidity requirements of the special components.

Resins suitable for RTM process include polyester resin, epoxy resin, phenolic resin, polyimide resin, etc.

 

3. Selection of Reinforcement Materials

In the RTM process, glass fiber, graphite fiber, carbon fiber, silicon carbide and aramid fiber can be selected as reinforcement materials. According to the design requirements, the varieties can be selected from chopped fibers, unidirectional fabrics, multiaxial fabrics, weaving, knitting, core materials or preforms.

 

Common Defects, Causes and Solutions of RTM Products

 

1. The Surface of the Product is Partially Rough and Dull

The main reason for this phenomenon of RTM products is the production of mild mucous membranes. Touch the mold with your hand, and when you touch these parts, the hand feels extremely rough. Usually, this problem will occur after the product has been produced for a period of time, and the mold needs to be cleaned in time. First, use water sand to polish the rough parts of the mold, then use cotton silk dipped in acetone to wipe the entire mold, and finally apply a release agent to the mold.

 

2. Wrinkling

This is one of the common drawbacks of gel coat products. The main reason for the wrinkling of the gel coat is that the gel coat resin is not completely cured before the resin is injected, and the monomer (styrene) in the injected resin partially melts the gel coat resin, causing expansion and wrinkles. Therefore, check whether the gel coat is cured before injecting the resin.

 

3. Leakage of Glue

The main reason for leakage of glue is that the mold is not tight after closing or the sealing gasket is not tight. Before closing the mold, check whether the sealing gasket is intact and whether there are cracks. If found, it should be replaced in time. Check the sealing condition when closing the mold.

 

4. Bubbling

The main reasons for this phenomenon:

1) The heat release of the resin in the mold cavity is too high, and the curing time is too short, so the gas in the mold cavity is not completely discharged.

2) When the resin enters the mold cavity, too much air is brought in, and the bubbles cannot be completely discharged within the injection time.

3) The viscosity of the resin is too high and the bubbles cannot be completely overflowed from the product during injection.

4) The pressure of the resin injected into the mold cavity is too high, causing the bubbles to be contained in the resin and difficult to be discharged.

 

5. Dry Spots Inside the Product

The main reason for dry spots inside the RTM product is insufficient glass fiber infiltration. If the dry spots appear in a certain part of a certain product in the same period, it should also be considered whether it is caused by contamination of the glass fiber cloth quilt. Usually, the appearance of dry spots inside the product is also related to the viscosity of the resin, so the viscosity of the resin should be analyzed and adjusted first. Check whether the mold flow channel is too long or too narrow, and modify the mold in time. Check the feed pipe, improve the feed pipe, improve the feed pipe, and increase the feed point

 

6. Core Material Movement

The movement of the core material during injection is caused by the instability of the flow. It can be solved by opening a hole in the core material or ensuring that the closing pressure applied to the core material is much greater than the pressure of the resin at that location. The movement of the reinforcing material, such as the flushing of the fiber, is also due to the insufficient mold closing pressure relative to the injection pressure.