Brief Introduction Of RTM Process And RTM Mold Making Process

 

RTM (Resin Transfer Molding) is a process method that injects resin into a closed mold to infiltrate the reinforcing material and solidify it. It is suitable for the molding of multi-variety, medium-volume, high-quality advanced composite materials.

 

This advanced 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 complex-shaped products, with high fiber content, low volatile components during molding, low environmental pollution, strong adaptability to production automation, low investment, and high production efficiency.

 

Therefore, the RTM process 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.

 

Material Selection

The quality of the mold depends on material selection. The material selection is based on the characteristics of the RTM molding process.

 

(1) Gel coat layer: RTM molding has a high heat release, and the heat release of a 4mm thick product can generally reach above 120°C. This requires the gel coat resin to have heat shock resistance, gloss and heat resistance. This process uses vinyl mold gel coat, which has a heat deformation temperature of 160℃-172℃ and good mechanical properties.

 

(2) Surface layer: mainly consider heat resistance and crack resistance, use 30g/㎡ surface veil and 300g/㎡ alkali-free chopped strand mat as reinforcement materials, and use bisphenol A epoxy vinyl ester resin as resin. This resin has good continuous high temperature resistance and low shrinkage.

 

(3) Reinforcement layer: focus on strength and shrinkage, use 0.4 alkali-free cloth and 300g/㎡ alkali-free chopped strand mat as reinforcement materials, and use zero shrinkage resin as the base material.

 

(4) Reinforcement layer: increase the overall rigidity of the mold, facilitate the opening and closing of the mold, and use steel frame reinforcement.

 

Production Of Original Mold

Many original molds of FRP molds have been made of gypsum, wood, cement, paraffin, etc. as base materials by hand-made methods.

 

The surface of the master mold made with these materials and production processes is difficult to reach Class A surface, the dimensional accuracy cannot be controlled, the production process is complicated, the cycle is long, defects are easy to occur, and the flatness is poor.

 

It is only suitable for FRP molds with low precision requirements and low surface quality requirements. If the above method is used, the production requirements of RTM models cannot be met at all.

 

 

In order to meet the requirements of RTM models, block-shaped machinable resin can be used as raw material for CNC machining. Block-shaped machinable resin is commonly known as wood substitute. Epoxy resin, ABS plastic, glass beads, aluminum dioxide, and carboxymethyl fiber are mixed and stirred evenly.

 

After dibutyl ester and other ingredients are mixed in a certain proportion, the mixture is heated to dissolve into a paste slurry. After vacuum defoaming treatment, it enters the mold for molding.

 

The molded solid is heated to form a solid. Wood substitute has the advantages of good thermoplasticity, strong stability, and good mechanical processing performance.

 

 

When making a model with wood substitute as the material, first of all, according to the design requirements of the product, use 3D drawing software such as Pro/e, UG, and CATIA to draw a 3D model of the product. When designing a 3D digital model of the product, in order to ensure the accuracy of the product, a certain margin should be left according to the shrinkage rate of the selected resin.

 

Then the original mold is processed by CNC machining. The original mold is generally processed into a real sample of the product, and the product size and thickness must meet the design requirements.

 

In this way, the step of making the mold cavity is omitted during production, which greatly improves the size and thickness accuracy of the product.

 

 

Mold Production

Material selection and original mold production provide the premise for the successful production of an RTM mold. However, the control of the production process cannot be ignored.

 

The original mold is processed by CNC, and the dimensional accuracy meets the requirements, but the knife marks of the processing still have a great impact on the surface of the mold. It is necessary to fine-tune and polish the model to meet the A-level surface requirements.

 

 

First, start the production of the first mold gel coat layer. In order to obtain a good surface effect for the gel coat layer, the gel coat layer needs to be produced by spraying. When spraying gel coat, the general amount of gel coat is 1000g/㎡, which is sprayed in 3 times.

 

The amount of gel coat for the first layer is 200g/㎡, and the color is different from the next two times. The amount of gel coat for the next two times is 400g/㎡. The airflow of the gel coat spray gun should be uniform, and the glue liquid should be in a mist form and sprayed evenly on the surface of the model.

 

After the last spray of glue liquid gels, the surface layer is laid.

 

 

The surface layer is composed of a layer of 30g/㎡ surface mat and 4 layers of 300g/㎡ chopped mat. The surface mat should be laid flat without overlap and wrinkles.

 

Be especially careful at the edges and corners, and press the corner of the brush to drive away bubbles. After the surface layer is cured, it is necessary to pick out bubbles, puncture the bubbles, then sand and remove burrs with sandpaper and wipe with acetone, and then you can paste the bottom.

 

The 4 layers of 300g/㎡ chopped strand mat are pasted in 2 times. Before each pasting, it is necessary to confirm that the previous glue is completely cured and the bubble picking work is completed. The reinforcement layer can be pasted by alternating cloth and mat, and 4 layers can be cured at a time.

 

The paste must be laid flat, the joints should be staggered, and not overlapped at the corners. The amount of resin glue in each layer should be strictly controlled to fully wet the fiber without being too much. After the paste reaches the designed thickness, the reinforced steel frame can be pasted.

 

There should be no gap between the steel frame and the mold. It is best to make it on site. The steel frame needs to be treated to eliminate welding stress before pasting.

 

 

After the first mold is cured for 24 hours, it can be demoulded. Be careful not to let the original mold leave the mold when demoulding. If you find that it is separated from the mold, you can use the vacuum method to re-fit the original mold with the first mold.

 

After simple trimming, the glue injection port and glue outlet are placed at the designed position, and then the next mold can be made. After making the second mold, put the entire mold in an oven and gradually heat it from room temperature to 75℃, then keep it warm for 2 hours, stop heating, and gradually cool it to room temperature.

 

After welding the guide, you can do the demoulding.

   

Post-processing Of The Mold

 

After demolding, remove the excess flash and blow away the small particles in the mold with compressed air.

 

 

Grinding: Soak 400# water sandpaper in water, wrap it on a smooth sanding board, and grind it back and forth repeatedly. Until the first layer of gel coat is completely sanded off; wash the mold with clean water; use 600# water sandpaper to grind according to the above method until there are no obvious scratches of uneven thickness on the surface of the mold; wash the mold, use 800# water sandpaper, and grind according to the above method until there are no obvious scratches on the surface of the mold and it feels smooth and delicate; wash with clean water and finally use 1200# water sandpaper to grind repeatedly once according to the above method.

 

 

Polishing: Apply 1000# polishing paste evenly on the mold surface. After 2-3 minutes, use a polishing machine with a cloth wheel to polish repeatedly back and forth until the mold surface is mirror-like and there is no residual sanding surface (generally, polishing is required twice); after washing with clean water, apply 2000# polishing paste evenly on the mold surface. After 2-3 minutes, use a polishing machine with a wool wheel to polish section by section until the mold surface is highly clear. After polishing, you can wash off the polishing paste with detergent water and then wash with clean water and dry.

 

 

Sealing Agent: After the mold surface is completely dry, dip the sealing agent with a clean cloth and apply it evenly on the mold surface. Apply it again after an interval of 20-30 minutes.

 

Release Agent: Use a high-efficiency liquid release agent and soak a clean cotton gauze with the release agent, but do not drip it. Wipe a layer of smooth coating, wet it, apply it all over, wait for 20 to 30 seconds, and then wipe off the excess release agent with another piece of cotton cloth, but don’t use too much force, just press the mold with the weight of your palm. Repeat this operation 6 times, each time for 20 to 30 minutes.

 

 

From the entire production process of the RTM mold, it can be seen that the original mold design and production lay the foundation for the production of the entire mold. Due to the adoption of new prototyping technology, the production of prototypes has the advantages of fast, high precision, simple process, easy operation, and no mold cavity production, which not only shortens the construction period, but also meets the development trend of modern enterprises to quickly respond to market demand. Coupled with the control of material selection and mold production, the application of RTM molding process will become more and more extensive.