What is the difference between L-RTM and vacuum infusion VIP process? – Be Sure to Read the Last Paragraph

 

For many friends who know the two processes, they should know the difference between them. But many friends who have been doing hand lay-up process still have some doubts about the difference between the two processes.

 

Some time ago, I wrote an article: [Long Words] Hand lay-up to closed mold process, should I choose vacuum infusion or RTM? At that time, in order to make everyone make a better choice, I compared the vacuum infusion and RTM processes, but some points were not detailed enough, and there were some differences between L-RTM and traditional RTM.

 

It happened that I had a little understanding of both L-RTM and vacuum infusion processes, so I decided to write about the similarities and differences between them from my perspective, hoping to help novices better understand, or bring some new ideas to veterans.

 

The vacuum infusion process uses a single-piece mold. After the mold surface is cleaned, demoulding is performed. First lay the reinforcement material on the mold, then lay various vacuum auxiliary materials, and finally seal it with vacuum bag film, and evacuate the air to form a negative pressure vacuum state in the mold cavity. The resin is introduced through the resin tube, the resin infiltrates the reinforcement material, and the product is demolded after curing.

 

The L-RTM process uses a double-piece mold, that is, an upper and lower mold. After the mold surface is cleaned, demolding is performed. The reinforcement material is laid in the lower mold cavity, and the vacuum cavity is sealed by the flange after the mold is closed. The air in the cavity is extracted by vacuum, and the resin is injected into the injection port using a glue injection machine. The resin infiltrates the reinforcement material, and the product is demolded after curing.

 

These are the molding processes of the two processes. You can see obvious differences, but there are also similarities. For a better comparison, I will compare them from several dimensions below.

 

1. Molds

Quantity

The most intuitive difference in the number of molds is that vacuum induction uses a single-piece mold, while L-RTM uses a double-piece mold. To put it simply, L-RTM has an extra “lid” – the upper mold.

 

But with multiple “lids”, things are not simple.

 

The upper mold is generally made by turning the lower mold, so it is necessary to pre-arrange the product cavity, runner, vacuum cavity, limit groove, etc. in the lower mold. At this time, wax sheets are needed to stack these thicknesses. On the one hand, wax sheets are easier to follow the shape, and on the other hand, they can also ensure the dimensional accuracy of the thickness. However, the cost of wax sheets is high, and sticking wax sheets is also a technical job, which is time-consuming and laborious.

In terms of quantity, there is only one more upper mold, but the technical requirements and costs of L-RTM mold turning are higher.

 

Flanging

The frp mold used in the hand lay-up process also has flanges, which are generally about 5-8cm.

In the vacuum induction process, sealing tape needs to be attached to the edge of the mold to bond the vacuum bag film to form a closed cavity, so its flange needs to be reserved wider, generally designed to be about 15cm.

 

The edge of the upper mold of L-RTM has flow channels, limit grooves, vacuum cavities, internal and external sealing strips, etc. Therefore, its flange needs to be wider, generally designed to be more than 25cm. If the flange position is not enough and the vacuum cavity is too small, it will lead to weak clamping and the image of “mold explosion” may appear during glue injection.

In actual operation, adjustments can be made according to the specific product. If the flange is too small, it will not be conducive to the operation when making the product, but if it is too large, it will cause waste. Make more molds, learn to summarize experience, and you can solve this problem.

 

Embedded Parts

Embedded parts may also be used in hand lay-up process or vacuum introduction process, but they are mainly used when making products.

 

In the L-RTM process, because the upper and lower molds need to be closed, and glue needs to be injected later, the positioning pins, glue injection ports and exhaust ports need to be embedded in advance when the mold is turned over.

 

The purpose of the positioning pin is mainly to ensure that the upper and lower molds can be accurately positioned when the upper and lower molds are closed, and no offset will occur. Therefore, how to design and arrange can be designed according to different products. In general, the positioning pin is at the edge of the mold.

 

The glue injection port is used for glue injection when making products, and the number of settings is also determined according to the product and the glue injection method. The glue injection port is required to be durable, easy to clean, and leak-proof.

 

The exhaust port is set in the product cavity and the vacuum lock mold cavity for exhaust. The requirements are the same as those for the injection port: durable, easy to clean, and leak-proof. A vacuum port is sufficient in the vacuum lock mold cavity, and the number and position of the product cavity are set according to the product structure and injection method.

 

2. Equipment

The vacuum introduction process only requires one vacuum pump, and the vacuum negative pressure can reach -0.1Mpa. A cheap single pump can be done for a few thousand yuan, and the expensive dual-pump system can also be tens of thousands of yuan. Choose the appropriate vacuum pump equipment based on your own needs and production rhythm.

 

L-RTM also requires vacuum pump equipment to provide vacuum for clamping and cavity exhaust. The two vacuums are slightly different. The vacuum pressure of clamping needs to reach -0.1Mpa. The main purpose is to ensure that the upper and lower molds are tight and to ensure good air tightness. The exhaust pressure of the cavity only needs -0.05Mpa, the purpose is to exhaust and guide the flow of resin.

 

In addition, the L-RTM process also requires a glue injection equipment, and the injection pressure is within 0.1Mpa, which belongs to low-pressure injection. The more popular equipment on the market has systems such as automatic glue mixing and pressure feedback, which can more accurately control the entire glue injection process.

 

Some companies do not choose glue injection equipment when doing L-RTM process, and directly use vacuum to guide the resin to infiltrate the reinforcing material. For some relatively simple products, this production method is feasible, but when making some complex or larger products, or products with large batches, it is still recommended to have glue injection equipment to speed up production efficiency and improve production stability.

The requirements for product resins used in vacuum resin infusion and L-RTM processes are basically the same. The viscosity is low, generally 100-300Pa·s, and the wettability is good. The gel time should be relatively long to facilitate the infiltration of the reinforcing material, but the curing speed should be fast to achieve high production efficiency. Because the product is formed in one step, the exothermic peak of the resin should be low to prevent excessive temperature from causing product deformation and thermal shock to the mold.

Commonly used reinforcing materials in the L-RTM process of fiber materials include: axial cloth, composite mat, continuous mat, closed mold mat, strong core mat, surface mat, etc

Strong Core Mat 

In the vacuum infusion process, closed mold mat is less likely to be compacted under vacuum, so it is less likely to be chosen. The other materials are basically universal.

 

Whether it is vacuum infusion or L-RTM, sandwich materials belong to closed mold processes. If sandwich materials are designed in the layer, they generally need to be processed to make the resin flow better.

For example, common PVC, PET, balsa wood, PMI, etc. can be processed by slotting, punching, etc. to facilitate better flow of resin and impregnation of fibers.

The vacuum infusion process of auxiliary materials requires many vacuum auxiliary materials (for common vacuum auxiliary materials, you can read previous articles: [Long Words] What are the commonly used vacuum process auxiliary materials?) These auxiliary materials are basically disposable.

 

Only a small amount of vacuum auxiliary materials are used in the L-RTM process, and the commonly used ones are mainly sealing tape, resin tube, tee, and resin collector. Compared with the vacuum infusion process, the types and quantities are much less.

 

Some other auxiliary materials, such as release agents, initiators, promoters… The requirements of the two processes are basically the same, and there is no obvious difference.

4. Cost

Through the above comparative analysis, it is easy to see the cost difference between the two.

 

The initial investment cost of the L-RTM process is relatively large, and the investment in molds and equipment is much larger than that of the vacuum infusion process.

 

The auxiliary material cost of the vacuum infusion process is relatively large, because the commonly used vacuum auxiliary materials are disposable.

 

As for the comprehensive cost, which one has more advantages, I have made a detailed comparison in the previous article on how to choose. ([Long Words] Hand lay-up to closed mold process, should I choose vacuum infusion or RTM?)

04. Cost

Through the above comparative analysis, it is easy to see the cost difference between the two.

The initial investment cost of the L-RTM process is relatively large, and the investment in molds and equipment is much larger than that of the vacuum infusion process.

The auxiliary material cost of the vacuum infusion process is relatively large, because the commonly used vacuum auxiliary materials are disposable.

As for the comprehensive cost, which one has more advantages, I have made a detailed comparison in the previous article on how to choose. ([Long Words] Hand lay-up to closed mold process, should I choose vacuum infusion or RTM?)

L-RTM has higher technical requirements, especially the design and manufacturing of molds. If this technology can be mastered, the technical level of the enterprise will be qualitatively improved. Externally, it also represents the technical strength of the enterprise.

The L-RTM process uses equipment injection, which has better stability and can realize the assembly line operation of the robot. Do you expect the robot to help you apply the sealing tape?

The characteristics of L-RTM products on both sides are bright. On the premise that the basic performance such as mechanics can be met, the appearance is definitely a plus. Who doesn’t love high appearance?

Of course, many people may say that what’s the use of good looks? Customers can’t accept the price, no matter how beautiful it is, it’s useless.

Enterprises did not choose the L-RTM process. Putting aside factors such as product structure, many of them are because the products they made are not profitable, that is, the market cannot accept products with higher costs.

But everything is developing, people are moving higher, and the aesthetics of this era are constantly improving. Just like our lives, when conditions are not good, we think about solving food and clothing. After life improves, we will think about how to wear better, and even pursue famous brands and luxury goods.

 

 

The future market is multipolar. Good products must have high thresholds and high profits, while ordinary products have ordinary profits.

 

Just as a car can run at a speed of 120 kilometers per hour, some people will choose a luxury car that costs millions of yuan, while others will choose a small car that costs tens of thousands of yuan. Will the cost of these two cars differ by hundreds of times, and which car would you prefer to produce?

Some industries have changed from hand-layup technology to closed mold technology. When most companies are still hesitating, a certain company dares to be the first in the world and chooses to break through the L-RTM technology, and finally succeeds. In fact, they don’t have many orders for L-RTM products now, but with the title of “Industry No. 1”, they have virtually established a benchmark image in the industry and become a leading company.

There are many similar cases. How to calculate this account, decision makers should look further ahead

I remember a joke. A poor student asked a top student how to get 98 points in the exam? The top student said after a long time, isn’t it okay to get one question wrong? The poor student: ..

If you have strength, but no opportunities, you can also create opportunities. Without strength, even if an opportunity knocks into your arms, it will still slip away.

Wouldn’t it be good to be a top student and attack with dimensionality reduction?