RTM (Resin Transfer molding) is a process method of infiltrating and solidifying the resin into the closed mold, which is suitable for many varieties, medium batch and high quality advanced composite materials.

This advanced technology has many advantages, can use a variety of fiber-reinforced materials and resin systems, has excellent product surfaces.

It is suitable for manufacturing high-quality and complex shape products and has high fiber content, less volatile components in the forming process, less environmental pollution, strong production automation adaptability, less investment, and high production efficiency.

Therefore, the RTM process has been widely used in the automotive industry, aerospace, defense industries, mechanical equipment, and electronic products.

The first factor determining RTM products is the mold because the RTM mold is generally used in the yin-yang mode of the method, so finding ways to improve the yin and yang mold surface quality and dimensional accuracy has become a key factor in determining product quality.

Material selection is a key to the quality of the mold, and material selection is based on the characteristics of the RTM molding process.

(1) Plastic coat layer: RTM Heat release is high, 4mm thick products can generally reach 120°c above. This requires that the resin has a heat-resistant impact performance, gloss, and heat-resistant properties. This process chooses the vinyl mold plastic clothing, its hot deformation temperature of 160°c-172°c has a good mechanical property.

(2) Surface layer: Mainly consider heat and crack resistance, the use of 30g/m² surface felt and 300g/m² no alkali short cut felt(CSM) as reinforced materials, resin selected bisphenol A epoxy vinyl ester resin. The resin has good temperature resistance and a low shrinkage rate.

(3) Enhanced layer: emphasis on strength and shrinkage, the selection of reinforced materials for 0.4 of the alkali-free cloth and 300g/m²-free chopped strand mat(CSM) as reinforcement materials, using 0 shrinkage resin as the matrix material.

(4) Reinforcement layer: increase the overall stiffness of the mold, easy to open and close the mold operation, the use of steel frame reinforcement method. The production of the original mold has been a lot of the original mold of FRP mold used ⽯ gypsum,wood, cement, paraffin, etc. as the base material, the use of hand-made methods.

Using these materials and production process of the Master mold surface is difficult to achieve a level surface, dimensional accuracy can not be controlled, the production process is complex, long cycle, easy to produce defects, poor flatness, only for those with low precision, surface quality requirements of the FRP mold production.

If the above method is adopted, the production requirements of the RTM model can not be achieved.

In order to meet the requirements of the RTM model, a block machinable resin can be used as raw material through NC machining.

Block machinable resin is commonly known as wood. Using epoxy resin, ABS plastic, glass beads, aluminum dioxide, carboxymethyl fiber mixed after mixing evenly, butyl fat, etc. according to a certain proportion of ⽐ mixing, heating so that the mixture dissolved into paste slurry, after the vacuum defoaming treatment into the mold for molding, after the formation of the solid after heating to form a solidified material. The wood has the advantages of good thermoplastic, strong stability and good machining performance.

When making the model with Demu as material, the three-dimensional model of the product should be drawn by using three-dimensional drawing software such as pro/e, UG and CATIA according to the design requirement of the product.

In the three-dimensional design of the product, in order to ensure product accuracy, should be based on the selection of resin shrinkage to leave a certain margin.

Then through the numerical control processing will be the original mold processing, the original mold general processing for product samples, product size and thickness to meet the design requirements.

In this way, the process of making the mold cavity is omitted, so that the size and thickness of the product are greatly improved.

Mold Production:

Material selection and original mold making provide a prerequisite for the successful fabrication of an RTM mold. However, it is not possible to ignore the control of the production process.

The original mold through the numerical control processing, the dimensional accuracy has reached the request, but the processing knife mark still has a very big influence on the mold surface. It needs to be polished and polished to make the model A-level surface required.

First, to start the first piece of mold plastic coating layer of production, for the plastic coat layer in order to obtain a good surface effect, the coating needs to adopt spraying method of production.

Spraying plastic clothing, the general dosage of plastic clothing for 1000g/m², 3 times for spraying, the first layer of plastic coat dosage for 200g/m², color difference in the latter two times, after two times the dosage of plastic clothing for 400g/m².

The airflow of the glue-coat gun is uniform, the glue is in mist and sprayed evenly on the model surface. After the last spray gel, then the surface layer of the paving.

The surface layer is made of a 30g/m² surface felt and a 4-layer 300g/m² chopped strand mat(CSM).

The surface felt should be paved with no lap and folds, for the edges and corners to be the particularly small ⼼ heart, with a brush angle to carry out full pressure, driving away from the bubbles.

The surface layer after curing needs to pick the bubble work, the bubble will be punctured and then sanded to the burr and wipe with acetone, and then you can make the following paste system.

4-layer 300g/m² chopped felt(CSM) composed of 2 times paste system to complete, before each paste system to confirm the first time the full curing and pick bubbles to work completed.

The enhanced layer of paste system can be used cloth and felt alternate, 4-layer curing one way to paste system.

The paste system must be flat, the seams should be staggered, not at the edges of the overlap, to strictly control the dosage of each layer of resin glue, to be able to fully infiltrate the fiber, but not too much.

Paste system to design thickness can be reinforced steel frame paste system, steel frame and mold cannot have clearance, preferably on-site production, paste before the steel frame needs to be treated to eliminate welding stress.

After the first mold is cured for 24 hours, it can be demolding. When stripping, be careful not to leave the original mode out of the mold. If it is found that the die can be used to vacuum the method of the original mold and the first piece of the mold again.

After a simple trimming, the design of the location of the injection of plastic and out of the mouth can start the next piece of mold production.

After the completion of the second mold, the whole mold into the oven from room temperature gradually heating to 75°c, after 2 hours of heat preservation, stop heating, gradually cooled to room temperature. The welding guide can be used to process the demoulding.

Mold Post Treatment:

After stripping, remove the excess flying edge, the small particles in the mold with compressed air blown clean.

Sanding: 400# with a water soaked in water, wrapped in a smooth grinding plate, back and forth repeatedly polished. Until the first layer of plastic coat is all sanded off;

Clean the mold with water, use 600# water sandpaper, according to the above method of polishing, until the abrasive surface can not see the apparent thickness of the obvious scratches;

Wash the mold, with 800# water sand, according to the above method grinding, until the visual mold surface no obvious scratches, feel smooth and delicate so far;

Rinse with water and then use 1200# sandpaper to grind the above method repeatedly.

Polishing:
1000# the polishing paste evenly on the mold surface, 2-3 minutes after the polishing machine with a cloth wheel began to repeat back and forth again and again until the visual mold surface specular reflection and no residue of the abrasive surface (generally need to polish 2 times);

Clean water after washing, will 2000# polishing paste evenly on the mold surface, 2-3 minutes after the polishing machine with wool wheel polishing, until the visual mold surface height clear specular reflection.

After polishing, you can wash the polished cream with washing powder and rinse with water to dry.

Sealing Agent:

After the mold surface is completely dry, with a clean cloth dipped in the sealing agent, evenly smear on the mold surface, interval 20-30 minutes after painting again.

Release Agent:

The use of a High-performance liquid release agent, with clean cotton gauze soaked with a release agent, but do not drip. Wipe a layer of smooth coating, wet, coated, wait 20-30 seconds later, ⽤ another piece of cotton cloth to erase the excess release agent, but not too hard, with the palm from the pressure to the mold can be. This repeat operation 6 times each interval 20-30 minutes.

It can be seen from the whole production process of RTM mold that the design and fabrication of the original mold lay the foundation for the whole mold production.

Because of the new prototyping technology, the prototype has the advantages of fast, high precision, simple process, simple operation, and no mold cavity making, which shortens the construction period and satisfies the development trend of modern enterprises ‘ rapid response to market demand.

With the selection of materials and the control of mold making, the application of the RTM process will be more and more extensive.