Creating Perfect Products, Sharing Experience of Zero Mistakes in RTM (liquid) Molding Process!

 

The 29th training – RTM (liquid) molding process training has ended. During the few days of service, I listened to the theoretical courses of teacher Zhang Yanfei almost throughout the whole process, and also followed the industry colleagues in the training class to experience the practical process of liquid molding under the guidance of Mr. Liu. The last time I did liquid induction molding was a year ago, also in Qingdao, and made a large lid. After learning, I reviewed the products made last year and found that there were indeed many details that needed to be improved when making the products before. Through this training, I also summarized the key points and precautions of liquid induction molding that I understand, hoping to learn from other industry colleagues, at least to avoid the pits we have stepped on before. The theoretical level is limited and the practical experience is still shallow. I hope that teachers and seniors in the industry will criticize and correct the shortcomings or mistakes.

 

1. Mold Selection

 

Mold materials should be selected comprehensively based on the production quantity, production efficiency, and production process characteristics of the product, such as aluminum molds, steel molds, fiberglass molds, wooden molds, etc. Both teacher Zhang Yanfei and Liang Qiushi, the co-organizer of Qingdao Zhongtan Mold, have elaborated on this in the theoretical lectures, so I will not repeat them in this article. Because the previous operations all used single-sided fiberglass molds, the following only uses single-sided fiberglass molds as an example for explanation.

 

Personally, I think that even if fiberglass is selected as the molding mold, the degree of mold preparation should be determined based on the product appearance requirements and whether it is post-processed. The same mold has different prices for different degrees of processing. For example, the upper cover made a year ago requires the product appearance to show the bottom pattern, and hopes that the subsequent processing is as simple as possible, without too complicated processes, and can be qualified for delivery by simply spraying paint in the spray paint factory or 4S shop on the market. Therefore, the inner wall of our customized mold is sprayed with gel coat, and the finish and flatness are very high, as shown in Figure 1 below.

 

(Figure 1. Due to commercial confidentiality considerations, only a brief display of the mold is given)

It can be seen that the mold surface effect is very good. Regardless of the final treatment of the product on the curved surface, corners, etc., the overall surface effect should be very good.

 

(Figure 2. Part of the product after demolding)

The mold we used this time was originally used for autoclave molding and has been used for a long time. Second, the product focuses on post-treatment of soil filling and painting after demoulding, so the surface quality of the mold is not very high, as shown in Figure 3 below.

 

(Figure 3. The mold used in this training)

Therefore, the appearance of the product after demoulding is relatively poor, as shown in Figure 4 below

 

(Figure 4. The appearance of the first sample made in this training)

Under normal circumstances, even if the mold is not sprayed with gel coat, washing the mold and spraying the release agent according to the specifications will not affect the demolding. It is just that the processing of the product in the later stage will be more troublesome. If the product does not show the bottom pattern, it will be sprayed with color paint in the end. It doesn’t matter how the product looks after demolding. Therefore, the extent to which the mold needs to be processed requires comprehensive consideration by mold development designers and mold users.

 

2. Material Preparation

 

After the mold is processed, the next step is material preparation. The materials used for fluid diversion molding generally include fiber cloth, epoxy resin, sealing strips, demolding cloth, diversion net, vacuum bag, spiral tube, tee, setting glue, etc. There may be other products, but the products mentioned above are indispensable. Let’s talk about some precautions for raw materials.

 

Woven cloth: mainly uses dry carbon fiber woven cloth and glass fiber woven cloth. The texture of the rolled cloth is very easy to loosen during the process of moving or cutting, which can easily cause the texture of the woven cloth to be distorted after forming. This type of cloth belongs to amorphous cloth. When cutting this type of cloth, one can stick tape or masking tape on the cutting route, as shown in Figure 5 below, and the other can spray shaping glue to play a certain shaping role.

 

At present, there are also manufacturers in the market that have launched a shaping cloth product to address this pain point. The carbon fiber woven cloth and glass fiber woven cloth used in this training are the shaping woven cloth provided by Anhui Yingtan New Materials Technology Co., Ltd. This kind of cloth is a silk thread that plays a shaping role in the weaving process of the cloth. Using such a woven cloth, even if it is pulled and deformed in one direction, it can be pulled in another direction and the cloth can be restored to its original state. However, it is difficult for amorphous cloth to maintain its original shape after transfer and pulling.

 

As a long-term partner of the Carbon Fiber Research Institute and a supplier of high-performance fiber fabrics and prepregs for training, Anhui Yingtan New Materials’ product quality has been highly recognized by students for a long time. You are welcome to visit our company for guidance.

 

(Figure 5. Cut along the textured paper, using a little more material)

 

If you don’t have textured paper or tape, and want to cut neatly, you can pull out one or two bundles of thread from the edge of the fabric. Press the fabric with your hand when pulling out the thread to prevent the fabric from wrinkling when the yarn is pulled out, as shown in Figure 6 below.

 

(Figure 6. Carbon fiber or glass fiber can be pulled out from the side of a few strands, easy to cut)

 

Release Cloth:

 

During this training, Yang Man, vice president of Qingdao Ruigao New Materials Co., Ltd., the co-organizer of the training, gave a detailed introduction to the classification and characteristics of release cloth. Ordinary polyester and nylon release cloths have been used all the time. Then the dark-colored Teflon release cloth was used. It was found that you get what you pay for. It is really easy to use and very easy to demold. It is recommended that manufacturers who have not used it but have the conditions can try it when doing liquid molding

 

3. Mold Care

 

New molds or molds that have not been used for a long time should be cleaned and wiped twice with mold washing water. The directions of the two wipes are vertical to ensure that there are no omissions. After wiping with mold washing water, prepare to apply mold release agent. For single-sided liquid diversion molding, it is recommended to apply sealing strips outside the cutting line of the mold edge before applying the release agent (the mold design and manufacturing manufacturers will extend a few centimeters on the basis of the molded product to leave enough space to paste the sealing strips and lay the materials). Do not remove the release paper of the sealing strips first, and then remove it when placing the vacuum bag.

 

The purpose of applying the sealing strip first is to apply the release agent to the position where the strip is applied, which will affect the adhesion effect and cause air leakage. Release agent: The new mold should be applied 2-3 times at an interval of about 10 minutes, and the recycled mold can be applied twice. Like the mold washing water, apply it in a vertical direction to avoid omissions, especially corners, curved surfaces, and joints between two surfaces that are not easy to demold. Application range: The entire mold, apply to the inside of the pasted sealing strip.

 

4. Material Laying

 

According to the product performance requirements and material requirements, the required materials are cut according to the product performance requirements and material requirements. For products with exposed bottom lines, sprayed transparent paint, and consistent lines, the cutting must be more strictly designed and calculated. The overlap position of the two pieces of material is selected at the corner and the junction of the two surfaces, and try to achieve a transition without leaving any traces. This really requires a lot of practical operations to master it. As an industry novice with limited practical experience, I can only summarize it to this extent.

 

 

 

Previously, (Figure 7, top, Figure 8, bottom. In Figure 7, the mold width exceeds the material width, so the two sides must be spliced ​​with other materials. This seam should be selected at the position where the two sides are combined in the box in Figure 8) The material is not easy to apply. First spray the setting glue and then apply the cloth. In order to avoid material waste, the values ​​of the length and width of the required material are calculated, not based on the length and width of the mold, but based on the length and width of the mold, to the range within the sealing strip. For more precise paving and seams, manual cutting methods will definitely not meet the requirements of high-quality products. Whether it is the upper cover made a year ago or the product made in this training, I am more determined to use a cutting machine for this type of product. I have been using the cutting machine of Jinan Ao Lei CNC Co., Ltd. Today’s cutting equipment is not only as simple as cutting prepreg, but also carbon fiber, glass fiber, carbon mat, carbon cloth, and even sandwich materials such as PVC and PMI.

 

To make perfect composite products, you really need a device like Ao Lei cutting machine. In addition, in addition to the need for seams to ensure the texture due to corners, curved surfaces, etc., some materials may also need to be spliced ​​due to widening issues. When splicing, pay attention to staggering the seams between layers as much as possible, especially for other layers except the first layer on the surface. Since they are inside, there are no requirements for seams, so the seams between layers must be staggered, as shown in Figure 9.

 

 

(Figure 9. The middle reinforcing fiber layer should be staggered when overlapping the plane while ensuring that the corners and seams are tightly fitted.) Five. Laying the demoulding cloth. Pay attention to the size of the demoulding cloth when laying the demoulding cloth. It should completely cover the preformed raw materials that have been laid below to avoid direct contact between the upper guide net and vacuum bag and the materials. In addition, the length of the demoulding cloth is about 5 cm longer at the position where the spiral tube is placed for glue injection and vacuum extraction. The purpose is that after the spiral tube is placed, the demoulding cloth can be folded inward to wrap the spiral tube to avoid the vacuum bag sucking the spiral tube during vacuum glue injection, which affects the operation effect. The overall picture is shown in Figure 10.

 

 

(Figure 10. Schematic diagram of the size of the demoulding cloth) When laying the demoulding cloth, the shaping glue should also be sprayed, and all positions should be applied one by one to avoid gaps and bridges.

 

6. Laying the Guide Net

 

The guide net also has different directions, and the flow rate in different directions is different. The longitudinal direction is the main guide direction, and it should be laid perpendicular to the guide tube. Guide net size: When the glue is injected from one end and the vacuum is drawn at the other end, the guide net should be laid to the position of the glue injection spiral tube, and the remaining sides are based on the thickness of the layer. The thicker the layer, the farther the edge of the guide net should be from the edge of the material. It is recommended that there is a 5 cm blank between the edge of the guide net and the edge of the material. The purpose of this is to ensure that the resin flow front of the surface layer of the guide net is not too different from the bottom layer flow front that seeps downward, forming a dry yarn. As shown in Figure 11.

 

 

7. Laying the Guide Pipe

 

The guide pipe for resin inlet should be placed on the guide net and fixed with tape, adhesive strips, masking paper, etc. In addition, the convection formed by two resin flows is also easy to cause dry yarn at the bottom of the convection position, so try to avoid resin convection when laying the spiral pipe. According to the specific situation of the mold, the injection method should also be adjusted accordingly. A more regular one can inject glue on one side and exhaust air on the other side. For a larger mold, it may be necessary to inject glue in the middle and exhaust air on both sides. A comprehensive judgment needs to be made based on the resin working window time, quality requirements, etc.

 

For products with weight requirements, the number of injection ports should not be too many. For the same process, more openings will increase the amount of injection. Therefore, if more injection ports cannot be opened, more pipes can be laid. If there are fewer ports, the injection of glue will be slow in some places. If the local injection is slow, blowing the area around the area with a hair dryer can accelerate the injection. This is also a process of weighing the pros and cons. As shown in Figure 12, there is one side injection and one side exhaust on the left, and the middle injection and exhaust on both sides on the right.

 

Figure 12. The left picture shows glue injection on one side and air extraction on one side, and the right picture shows glue injection in the middle and air extraction on both sides

 

8. Vacuum Bag

The area should be large, large, and large, about 1.5 times the area of ​​a single layer of paving material, to ensure that the vacuum bag can be completely applied to the surface of the material when vacuuming, without any bridging. You can fold a few more “ears” at the corners of the mold to increase the area and redundancy of the vacuum bag. As shown in Figure 13.

 

 

Figure 13. Students Practiced by Folding Ears at Several Corners

 

9. Mixing Resin, Vacuuming, and Injecting Glue

 

After the vacuum bag is laid, start vacuuming. After reaching the vacuum degree, close the valve to check for leaks.

 

Resin preparation: Confirm that there is no air leakage, and prepare the resin according to the resin amount calculated in advance. When the amount of resin is relatively large, it is recommended to use a special equipment for mixing resins. If there is no equipment and the amount is relatively large, it can be prepared in batches to avoid too much at one time. The uniformity of mixing cannot be guaranteed, and there may be accidents of resin explosion.

 

The choice of resin is also very important. I didn’t pay much attention to resin products before. I found out after reading the materials that the 5028AB two-component resin of Huibo New Materials was used for the upper cover when making samples for customers a year ago. This training is still prepared for Huibo New Materials’ 5028AB two-component resin. Different application scenarios and different users all chose Huibo New Materials. It must be said that Huibo New Materials is worthy of being a company that focuses on the research and development and production of high-end epoxy resins.

 

After the resin is prepared, bend the injection tube, bend out a long section and put it into the resin first. When the injection is ready, loosen the bend directly. This can effectively prevent a large amount of air from entering the preform along the guide tube during the short period of time when the guide tube is transferred to the resin barrel. Do not bend the guide tube or the exhaust tube repeatedly at one position. Tubes made of hard materials are very likely to be damaged and leak at the bend.

 

10. Curing

Whether it is room temperature curing or temperature curing, the vacuum state must be maintained throughout the curing process. Only by maintaining the negative pressure state throughout the process can the interlayer bonding force achieve the ideal effect. When the resin completely fills the mold cavity, continuous vacuuming will not cause the resin to be drawn away again, especially in the case of large curved surfaces and the resin is conducted from bottom to top. When the resin is filled, it is in a non-flowing state, and continuous vacuuming will not draw the resin away again.

 

For the complex inner surface of the mold, there is a concern that the material at the corners, curved surfaces, etc. is not tightly fitted with the mold and cannot be squeezed into place. In order to improve production efficiency and reduce the process and cost of surface treatment, you can consider adding a silicone rubber inner mold to press the product into place. Of course, if it is a flat surface, you need to consider the problem of resin being sucked away. In this case, you can try to set up two layers of vacuum bag film. The inner layer of the vacuum bag extracts the gas in the mold cavity and introduces the resin into the mold cavity. When the resin is almost completely infiltrated, stop the vacuuming of the inner layer of the vacuum bag, and continue to vacuumize the outer layer of the vacuum bag. While giving the material a certain pressure to solidify, the resin in the mold cavity will not be continuously sucked away, causing the resin to be sucked away.