My Opinion On HP-RTM Process

 

Composite materials are developing rapidly. As composite materials enter more and more fields, experts and scholars in various fields have their own views on the application, process and performance of composite materials based on their own characteristics. Therefore, experts and scholars in different fields often have different views and opinions on the same material ratio or even the same production process.

This phenomenon of a hundred schools of thought will play a positive role in promoting the development of my country’s composite materials industry. We all hope that China and the composite materials industry can find the right path as soon as possible and then flourish.

The following is Liu Wei’s opinion on the HP-RTM process from Changzhou BEST Composite Materials Co., Ltd. Experts and scholars are welcome to discuss the views, arguments and conclusions of the article.

You can leave a message at the bottom right of the article to participate in the discussion and express your own views. Welcome to participate in the discussion and submit your articles.

 

In recent years, the HP-RTM (high pressure RTM) process has been very popular. On WeChat Moments and industry websites, relevant information pops up every now and then. I have expressed the view that this process is a “pitfall” many times on WeChat Moments or in private chats with friends, which always attracts criticism from many people in the industry. As a technician in the composite material industry, I have said so many times that I feel it is necessary to sort out my personal opinions to avoid continuing to “talk nonsense”. I feel that there are few people who are engaged in the composite material profession and can really get a glimpse of the tricks, so I sorted out some of the superficial opinions published in WeChat Moments, hoping to give some advice to companies that intend to use this process, and also to accumulate merit for the development of the industry.

The first sentence of the “criticism of industry insiders” mentioned above is mostly: “BMW is using this process, why do you say it is not good”. Coincidentally, I have a friend who works for BMW. According to him, BMW only used this process in the i3 project ten years ago, and did not use it in the subsequent i4 to i7 projects. As for what process was used later, the friend said that the company kept it strictly confidential and he didn’t know. In fact, it is not difficult to understand. If BMW had not given up, how could it allow equipment developers to promote it extensively and transfer it to third parties?

 

HP-RTM For Carbon Fiber Automotive Parts

 

 

Source: BMW

 

When I first saw the introduction of this process, my first impression was that the company that developed this process should not be in the composite materials industry, but rather a heavy machinery industry company that “uses resources to change principles” (quoted from Mr. Liu Cixin’s science fiction novel “The Three-Body Problem”). Later, after investigation, I found that this was indeed the case. Dieffenbacher and KraussMaffei are the leaders in the field of European processing machinery.

 

Source: KraussMaffei

 

Let’s take a look at the introduction process of the HP-RTM process. When BMW planned to use carbon fiber composite materials to manufacture the body of the i3 car, what were the traditional processes used to produce carbon fiber composite materials? Generally speaking, there are the following: hand lay-up, vacuum induction and RTM, prepreg autoclave molding, and prepreg compression molding.

 

Long-term metal body manufacturing has formed a qualitative thinking in the automotive industry: stamping, welding, assembly, and painting, a total of four production workshops, and four major production processes. Due to this habitual thinking, for automobile factories, the production line cannot be stopped. Once the line is stopped, it will be a major accident. Then, automobile factories will naturally list production efficiency as an important condition that cannot be compromised.

 

 

Among the existing composite materials processes, the most efficient one is naturally the prepreg molding process, but it is still not fast enough. Why is it slow? The resin cures too slowly. Why does the resin cure slowly? Because the prepreg process determines that it needs to have sufficient storage time at room temperature or low temperature. If the curing time at high temperature is too short, the storage time at room temperature is not enough.

 

Although after ten years of technological development, there are some feasible solutions to solve both high-temperature rapid curing and low-temperature long-term storage, but ten years ago, it was still very technically difficult. How to solve the problem of rapid curing? Two-component on-site mixing, no storage time is required, and it can be fast. Ok! Then change to a two-component system (actually a room temperature or low temperature curing system).

 

What if the resin injection speed is slow? Increase the injection pressure. What if the resin injection pressure is too high and the flow rate is too fast, which will wash away the fiber? Fiber preforming. What if the glue content of the glue injection method is too high? Press it again after the glue injection is completed. Well, a new process is out. What should I call it? This process is quite similar to RTM, but with a higher pressure, so let’s call it HP-RTM.

 

Having written this far, I really have to sigh that it is so great to have money. You can do whatever you want. If the problem cannot be solved, just use money to throw it, and rely on brute force to solve the technical problem; as for the cost-effectiveness, you can ignore it for now.

 

If I were an engineer at China’s “Laoniu Company” and had no money to throw, and my boss gave me 2,000 yuan in research funds and asked me to copy the high-end and money-burning process introduced above, what would I do?

 

Buy a few pieces of carbon fiber cloth, spend 100 yuan, go to the street to find a shoemaker who repairs shoes, take an awl, and help me sew a few pieces of carbon fiber cloth into “fiber preforms”. Spend another five yuan to buy a brush, buy some room-temperature curing epoxy resin that can be cured in 0.5-5 hours at room temperature, and brush it on the carbon fiber. Find a metal mold, find a hydraulic press, put the carbon fiber brushed with resin on the mold, heat it to 150 degrees, press for 2 minutes, cure, demould, and get it done. In this way, you can save 1,000 yuan and have a good meal with your brothers.

 

From a basic theoretical point of view, is there any difference between our rustic copycat process and their high-end process? Just as all forms of energy available to humans before they could use nuclear energy are no different from the bonfires lit by primitive people (it seems to be the view of science fiction writer Liu Cixin, all of which use the energy generated by the change of electron cloud energy levels).

 

In essence, they all use sewing to make three-dimensional fabrics first; then use resin that has a very short storage time at room temperature to impregnate the fibers; and finally use high-temperature molding. A friend asked, seeing this, why does it feel like the “wet molding” process used to produce dining trays 30 years ago?

 

The late Professor Zhu Yiling, an old predecessor and expert in the composite material industry, first proposed that composite materials are both materials and structures, which is different from traditional materials represented by steel. My mentor Professor Wang Jun further clarified that composite materials should be a system engineering of materials, structures, and processes.

 

Only when the matrix material and the reinforcing material are organically combined under a specific process can a product with atypical structure and integrated materials be prepared. It is impossible for composite materials to exist independently of products. The creation of this kind of composite product should pursue a system engineering with high material and economic efficiency and balanced indicators in all aspects. What can the HP-RTM (High Pressure-Resin Transfer Molding) process really provide us?

 

The process naming of composite materials is very important. The naming process is actually an in-depth explanation of the process. For example, the composite material pultrusion process is powered by a traction mechanism, pulling the resin-impregnated fiber into the mold, squeezing out excess resin, generating in-mold molding pressure, and finally solidifying and molding. The core lies in “pulling”. The source power of the thermoplastic material “extrusion” process lies in the extruder, which relies on the rotation of the screw to drive the material through the mold to melt and cool and form. The core lies in “extrusion”. Therefore, we cannot name the composite material “pultrusion” process as “extrusion” or “extrusion and pulling”.

 

Have we ever thought about it, after saying “HP-RTM” so many times, is it really appropriate to call this process “HP-RTM“? What is the driving force of the HP-RTM molding process? Is it the injection pressure of RTM? If the answer is yes, then we should not need a 3,600-ton large press. Just clamp the mold in other ways and rely on high-pressure injection. But what problems will this bring? The fiber content is too low. Therefore, the final touch is compression molding. This process should be called “wet compression molding” or “injection compression molding”.

 

For the naming process of HP-RTM, I might as well make a bold guess. There are only two situations: one is that the namer lacks understanding of the composite material profession and mistakenly believes that this process should be classified as RTM; the other is that the namer deliberately uses the naming gimmick to attract attention for commercial considerations.

 

The HP-RTM process has always emphasized its fast molding, but every technician in the composite material profession should understand that too fast molding speed will inevitably bring problems – poor fiber impregnation and poor interface performance. Composite materials are composed of matrix and reinforcing fibers. Whether the interface between the two is well bonded greatly affects the final performance of the composite materials.

 

When glass fiber or carbon fiber is produced, in order to protect the fiber surface and enhance the interface bonding ability, a layer of impregnating agent (sizing agent) with polymer as the main component will be coated on the fiber surface. When the composite material is formed, the resin needs to fully impregnate the fiber and dissolve the impregnating agent (sizing agent) on the fiber surface so as to form a chemical bond with the fiber to obtain excellent interface performance.

 

The technicians who developed the prepreg process divided the impregnation and curing into two stages precisely because they knew this truth. In order to make the resin fully impregnate the fiber, they did not hesitate to increase the difficulty for themselves and put forward the “latent” requirement for the resin. Technicians all over the world are working hard to improve the “latent” of resin. Don’t they know that epoxy resin, which can react at room temperature, can be quickly cured by heating?

 

This article originated from a dispute in WeChat Moments. The writing was rushed, so please feel free to correct me if there are any shortcomings. Finally, let me summarize why I think HP-RTM is a “pitfall”:

 

You can you up，no can no BB：

 

1. The success or failure is due to Xiao He. The molding speed is too fast, the fiber impregnation time is too short, the fiber impregnation is poor, and the interface performance of the composite material is poor.
2. No technical breakthroughs have been made. The “layman-style” violent solution is purely money-throwing, “changing principles with resources”, “forcing the bow”, and has no aesthetics (science and art should be interlinked).
3. The cost-effectiveness is too high, and the one-time investment in equipment and molds is too large. The fields that can accept such high costs (aerospace) do not need such a fast speed, and the fields that need to be fast (automobile) cannot accept such a high production cost.
4. BMW’s practice actually proves the impracticability of this process, rather than proving the feasibility of this process. This can also be regarded as a side evidence.

 

You can you up, no can no BB, BB for so long, what solution do you have? In case someone asks me this, I will say this myself. As for the solution, I have also thought about it, but I have to make a living, so I will only briefly propose a few main points here without publishing the details. If you want to know the details, you can contact me and we can talk over tea. :

 

1. Make full use of existing materials, processes and equipment to reduce initial investment.
2. Break the boundaries of materials. Don’t say that composite materials are good just because you are engaged in composite materials. Give full play to the advantages of various materials. Not only do composite materials themselves need to be composited, but materials also need to be composited with each other.
3. For composite materials themselves, borrowing the views of my mentor Professor Wang Jun, it is that we should fully carry out the recombination of materials and materials, structures and structures, and processes and processes.