RTM Process Mold Design and Production Process

 

 

 

RTM molding technology is one of the fastest growing molding technologies in the current world FRP industry. All RTM products require a mold suitable for their process. RTM molds can be made of aluminum, steel, and FRP. Aluminum and steel molds are not easy to deform but are expensive, while FRP molds have lower costs and shorter production cycles. They are currently the mainstream RTM molds. The following is an introduction to FRP molds.

 

 

 1.1 Introduction

The mold is the mirror of the product, and the primary factor that determines the RTM product is the mold. Since RTM molds generally use the method of matching the male and female molds, 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. In addition, whether the design of the mold’s fastening, sealing, injection port and exhaust port is reasonable is also an important factor in determining the quality of RTM products.

 

 1.2 Mold Design Steps

 

1. Requirements for mold manufacturing environment and selection of raw materials

 

1. Environment

Ambient temperature: The ideal range is 25±3℃.

Relative humidity: Not greater than 60%.

Production workshop: A higher degree of cleanliness and daily maintenance should be maintained than the product production area.

 

1. Materials

Since RTM molds are required to have many characteristics such as high strength, small deformation, long life, and accurate cavity size, there are special requirements for mold materials. The main materials are: gel coat, mold resin, surface mat, alkali-free chopped mat, balsa wood, strong core mat, insulation core block, wax sheet. Auxiliary materials: polyester sealer, easy-to-grind primer, high-gloss gel coat, glass beads, butanone, acetone, gingham cloth, curing agent, accelerator, various mesh sandpaper, positioning parts, fasteners, injection ports, exhaust ports, injection plugs. Tools: gel coat spray gun, stirrer, paint spray gun, brush, plastic basin, various rollers, curing dosage cup, putty knife, scissors, utility knife, steel ruler, etc.

 

2. Production of Original Mold

The materials of the original mold can generally be wood, substitute wood, gypsum, polyurethane foam, or it can be supported by the product. Wood requires relatively low moisture, and it is best to use dried pine wood. Gypsum involves the problem of water volatilization and needs to be sealed. Polyurethane foam also involves surface treatment. Substitute wood and wood are preferred for products with high precision requirements. The processing of the original mold can be done manually or by a machining center. If the machining center platform is not that large, it can be divided and processed, and finally assembled. For the original mold that needs to be reused, it is recommended to use substitute wood.

 

The processing of R angle should be considered during the production of the original mold. The relationship between R angle and product thickness is shown in the table below.

 

In addition, the demoulding slope should also be considered. Products with demoulding slope have low demoulding force, smooth demoulding and are not easy to damage the mold surface. According to the appearance of the product and the technical conditions, the demoulding slope should be increased as much as possible. If the technical conditions are not met, it is also necessary to ensure that a demoulding slope of 3‰ is reserved.

 

3. Mold Design

⑴ Determination of production plan

In this step, we mainly consider the parting surface of the mold, whether to part the mold or make a live block. This step will affect whether the product can be produced smoothly, and whether the operation of the opening and closing process is smooth.

 

⑵ Design of Mold Structure Layer

In general, the FRP structure layer of the FRP mold is 3 to 5 times the thickness of the product. The following are two reference solutions:

A: 2 layers of gel coat + 1 layer of 30g/m2 surface mat + 1 layer of 300g/m2 chopped mat + 2 layers of 300g/m2 chopped mat + 2 layers of 0.4 woven fabric + balsa wood + 1 layer of 300g/m2 chopped mat, 6 layers of 0.8 woven fabric (all layers after balsa wood are made of RTM special mold resin) + 2 layers of 300g/m2 chopped mat (reinforced steel frame)

B: 2 layers of gel coat + 1 layer of 30g/m2 surface mat + 1 layer of 300g/m2 chopped mat + 2 layers of 300g/m2 chopped mat + 2 layers of strong core mat + balsa wood + 1 layer of 300g/m2 chopped mat, 6 layers of 0.8 woven fabric (all layers after balsa wood are made of RTM special mold resin) + 2 layers of 300g/m2 chopped mat (reinforced steel frame)

 

(Note: Each plus-shaped layer is completed in one go.)

 

4. Design of the Skirt

 

After the production plan and the layer design are completed, the production of the first mold (i.e., the negative mold) begins. The first step is the production of the skirt. Generally, the design width of the RTM mold skirt is 8~12cm. For the dead corners that are inconvenient to operate, sufficient space should be reserved for smooth transition. In order to facilitate the smooth fitting of the sealing strip, the position of the sealing strip should be reserved on the skirt, otherwise it will cause unnecessary trouble during production.

 

5. Treatment of the Original Mold Surface
6. Use 120-mesh sandpaper to remove the burrs on the surface of the original mold.
7. Scrape and polish the wood grain or nail holes on the surface of the original mold with atomic putty.
8. Spray a layer of easy-to-polish primer on the whole, and polish and level it.
9. Wax the mold surface

 

6. Treatment of the position of the sealing strip and the mold cavity

Use wax sheets to mark the position of the sealing strip, and then use putty or plasticine to smooth the edges.

 

7. Preparation of Mold Gel Coat

The amount of gel coat is generally controlled at 850g/m2, the thickness at this time is between 0.6~0.8mm, and the gel time is controlled at about 2 hours. Geling too quickly will lead to high porosity in the gel coat layer. The gel coat is sprayed twice, and the interval between the two times does not exceed 30 minutes. The spray gun pressure is controlled at 70psi during spraying. Mold gel coat construction procedure: First check whether the spraying equipment is in good condition, check whether all sealing conditions are good, and whether the pressure regulator is flexible.

 

8. Mold Stacking

Before making, make sure that the fiber products are not damp and oily. Ensure that the fiber content is controlled at about 60%.

9. Balsa Wood

Balsa wood or foam can be added for reinforcement.

10. Mold Reinforcement

According to the mold structure and the condition of the reinforced fiber fabric, the mold is reinforced with square steel. The quality of mold reinforcement directly affects the life of the mold.

 

11. Design of exhaust port and injection port

Generally, the exhaust port and injection port are set in the upper mold. For molds with regular geometric shapes, the injection port is set at its geometric center. For irregular molds, it is generally located at the bottom of the product under the injection state of the mold. The exhaust port design is generally placed at the highest point of the product and the position that is not conducive to exhaust.

 

If necessary, the air release can be placed in a suitable position to help the demolding of the product. In short, it is necessary to analyze according to the specific shape of the product, fully consider the flow mode of the resin, and make a comprehensive balance to properly arrange the position of the injection port and the exhaust port to ensure the successful injection of the product.

 

12. Making of the Positive Mold

When the wax sheet is laid and the positive mold is made, the paste of the structural layer is the same as that of the negative mold, but there are the following precautions:

 

1. When spraying the gel coat, pay attention to the humidity in the operation room not exceeding 60%, otherwise it will seriously affect its curing and easily cause wrinkles in the gel coat. Therefore, the gel coat cannot be sprayed under high humidity.

 

1. During the production of the positive mold, the amount of resin and the speed of lamination must be strictly controlled, otherwise the size of the mold and the cavity cannot be guaranteed.

 

13. Welding of Fasteners and Positioning Parts

 

After the FRP structure layer of the male mold is completed, the steel structure of the mold must also be strengthened. The operation is the same as the female mold, but it should be noted that the injection port and exhaust port should be left open during welding. At the same time, the spacing of the square steel at the injection port should be smaller to increase the rigidity of this position.

 

Generally, only two positioning parts are required for the male and female molds, and they are welded in symmetrical positions. Fasteners are used at other positions. The spatial distance of all these metal welded parts is about 40cm. If the mold is thicker or the steel frame of the skirt is not flat enough, a steel plate with a thickness of about 10mm is needed for connection. When welding, it is necessary to ensure that the force is in the same direction when the mold is tightened. Of course, it is also necessary to flexibly handle it according to the specific situation of the mold to achieve the best effect.

 

14. Post-curing Treatment

At this point, our mold making process is complete.

 

Due to the incomplete curing of the resin during the mold making process, and the welding of fasteners and positioning parts, etc., there will be a large stress in the mold. If it is put into production immediately, it will seriously affect the quality and life of the mold. Therefore, the mold needs to be post-cured.

 

The method is as follows: Heat treatment is carried out at a temperature 40-50℃ higher than the mold use temperature, generally with 60-80℃ as the reference temperature, and curing for 8 hours to eliminate the internal stress of the mold and improve the curing degree of the resin, so that the mold can achieve the best performance. After the post-curing treatment, if there is enough time, the mold can be placed for a week before demolding and processing before being put into production.

 

15. Post-Processing of the Mold

After the post-curing of the mold is completed, the mold can be demolded for post-processing.

⑴ Remove the mold and remove the wax sheet (the wax sheet is attached to the positive mold).

⑵ After removing the wax sheet, clean the mold with a mold cleaning agent.

⑶ Start from 600 mesh sandpaper and increase to 2000 mesh or more, and polish the mold according to product requirements.

⑷ After the mold surface is polished, start polishing.

 

16. Mold Trial

⑴ Before mold trial, make sure that the mold release agent is fully coated and do a demolding experiment.

⑵ Cut the mat according to the shape of the product.

⑶ Close the mold, tighten it in place, and prepare for injection.

• The pressure during injection can be adjusted from small to large.

⑸ After the product is solidified, demold it and check the entire mold and the product. Adjust it in time when necessary until you are satisfied. At this point, our entire mold production is completed.

 

17. Mold Maintenance, Maintenance and Storage

Since RTM injection molds are molds with relatively high precision, regardless of whether they are produced for a long time, the molds need to be maintained and maintained as necessary.

⑴ General principle: maintenance is more important than repair, and repairing is difficult to get a good mold.

⑵ Maintenance method: Maintenance while using, mainly re-grinding, polishing, and maintaining smoothness; molds that have been used for a long time may have mold release wax or mold release agent deposits on their surfaces, and special cleaning agents are needed to remove these deposits to make the mold look brand new.

⑶ After the mold is used, if it needs to be kept, pay attention to the following points: a. The mold must be closed and fastened in place before storage, and the mold cavity must be kept clean when closing the mold. b. It is not allowed to be placed in the open air.

 

Summary:

As a new method for processing reinforced plastics, RTM still has many problems that need to be studied in depth. Through our mold making process, we also know that the production of RTM molds is much more complicated than that of open mold FRP molds. Strict control of each link is required to obtain satisfactory results. Therefore, it will take a relatively long time to perfect and improve this production process.