RTM Molding Process and Classification Introduction

 

1. RTM Molding Process and Classification

RTM refers to a process technology in which low-viscosity resin flows in a closed mold, infiltrates reinforcing materials and solidifies into shape. It belongs to the category of liquid forming or structural liquid forming technology of composite materials. The specific method is to pre-place reinforcing materials that have been reasonably designed, cut or pre-formed by mechanization in the designed mold. The mold must have peripheral sealing and tightening, and ensure smooth flow of resin; after the mold is closed, a certain amount of resin is injected, and the desired product can be demolded after the resin is solidified.

 

SMC, BMC molding, injection molding, RTM, VEC technology are all closed mold molding processes. Due to the formulation of environmental laws and the improvement of product requirements, open mold molding of composite materials has been increasingly restricted, which has promoted the application of closed mold molding technology, especially in recent years. The innovation and development of RTM technology has been promoted.

 

2. Types of RTM

The RTM process began in the 1950s. At present, the RTM molding process has been widely used in construction, transportation, telecommunications, health, aerospace and other fields. The following introduces several RTM technologies.

 

1. RTM, resin transfer molding. This technology is derived from polyurethane technology. During molding, the mold is closed and resin is injected into the preform. The glass fiber content is low, about 20-45%.
2. VARIT, vacuum-assisted resin transfer injection molding. This technology uses vacuum to suck resin into the preform, and can also press the resin in. The vacuum degree is about 10-28 inches of mercury.
3. VARTM, vacuum-assisted resin transfer injection molding. The product generally has fewer pores and the glass fiber content can be increased.
4. VRTM, vacuum resin transfer molding.
5. VIP, vacuum infusion method.
6. VIMP, variable infusion molding method. The resin moves with the help of vacuum or deadweight, and the impregnation is compacted.
7. TERTM, thermal expansion RTM. Insert the core material into the preform, let the resin impregnate and heat the mold and the molded product. The core material expands due to heat, compacting the layer. This compaction effect is combined with surface pressure molding.
8. RARTM, rubber-assisted RTM. In the TERTM method, rubber is used instead of core material. The rubber mold presses the molded product, which greatly reduces the porosity and the glass fiber content can be as high as 60-70%.
9. RIRM, resin injection cycle molding. Vacuum and pressure are combined to alternately inject resin into multiple molds to circulate the resin until the preform is fully impregnated.
10. CIRTM, Co-Injection RTM. Co-injection RTM can inject several different resins, and can also use several preforms. Vacuum bags and molds with flexible surfaces can be used.
11. RLI, resin liquid impregnation. Resin is injected into the lower mold, and after the preform is inserted, the upper mold is covered, heated and molded with the molding pressure of the autoclave. Heating reduces the viscosity of the resin, improves fluidity, and is easy to impregnate.
12. SCRIMPTM Seaman Composites Company resin impregnation molding method. Patent application. The resin is pressurized and impregnated using a vacuum bag, with a fast impregnation speed and a wide area. The resin can also be fully impregnated in the thickness direction of the preform, and a vacuum bag and a soft-surface mold must be used.
13. RTM, resin transfer molding. This technology is derived from polyurethane technology. During molding, the mold is closed and resin is injected into the preform. The glass fiber content is low, about 20-45%.
14. VECTM virtual design The core technology of composite materials VEC is the patented “floating mold” concept. The composite material mold is installed between two liquid-filled steel pressure vessels, and the mold forms a seal along the entire length of the container. The container is filled with a compressible heat-conducting liquid, which is usually water.

 

3. RTM Molding Process and Process Characteristics

Features:

⊙ According to the requirements of the production scale, molds of different materials and grades can be used to minimize costs and obtain better economic benefits.

⊙ It is a closed mold resin injection method, which can minimize the harm of harmful components such as resin to the human body and the environment, and meet environmental protection requirements.

⊙ Low-viscosity fast-curing resin is used, and the RTM mold can be heated during the production process, thereby further improving production efficiency and product quality.

⊙ It is conducive to the preparation of large and medium-sized, complex shapes, and smooth overall structural parts on both sides, with good dimensional accuracy, high surface quality, and good mechanical properties.

⊙ The reinforced material preform can be selectively reinforced, locally reinforced, mixed reinforced, and embedded and sandwich structures can be used according to performance requirements, which can give full play to the performance designability of FRP.

 

4. Common Defects, Causes and Solutions of RTM Products

 

1. The surface of the product is partially rough and dull

The main reason for this phenomenon of RTM products is the production of mild mucous membranes. Touch the mold with your hand, and when you touch these parts, you will feel extremely rough. Usually, this problem will occur after the product has been produced for a period of time, and the mold needs to be cleaned in time. First, use water sand to polish the rough parts of the mold, then use cotton wool dipped in acetone to wipe the entire mold, and finally apply a release agent to the mold.

 

2. Wrinkling

This is one of the common drawbacks of gel coat products. The main reason for the wrinkling of the gel coat is that the gel coat resin is not completely cured before the resin is injected, and the monomer (styrene) in the injected resin partially melts the gel coat resin, causing expansion and wrinkles. Therefore, before injecting the resin, check whether the gel coat is cured.

 

3. Leakage

The main reason for leakage is that the mold is not tight after closing or the sealing gasket is not tight. Before closing the mold, check whether the sealing gasket is intact and whether there are cracks. If found, it should be replaced in time. Check the sealing condition when closing the mold. When closing the mold, the mold should press the sealing gasket tightly. Once leakage is found during pouring, the bolts around the leakage part should be tightened immediately.

 

4. Bubbling

The main reasons for this phenomenon: 1) The heat release of the resin in the mold cavity is too high, and the curing time is too short, so the gas in the mold cavity is not completely discharged. 2) When the resin enters the mold cavity, too much air is brought in, and the bubbles cannot be completely discharged within the injection time. 3) The viscosity of the resin is too high, and the bubbles cannot all overflow from the product during injection. 4) The pressure of the resin injected into the mold cavity is too high, causing the bubbles to be contained in the resin and difficult to be discharged. The corresponding solutions are: 1) Appropriately reduce the amount of resin curing agent used for infusion; 2) Design an exhaust port on the mold; 3) Test the viscosity of the resin at 25°C. Usually, the viscosity of the resin for RTM is 0.5~1.5PaS. If the resin viscosity does not exceed the standard, you should consider whether the ambient temperature is too low. If the temperature is too low, you can heat it to a constant temperature before the resin is poured, but the temperature selection should be appropriate. Too low resin viscosity will affect the mechanical properties of the product; 4) Reduce the resin injection pressure and increase the resin injection volume, thereby reducing the flow rate of the resin in the mold cavity and increasing the seepage volume.

 

5. Dry Spots Inside the Product

 

The main reason for dry spots inside the RTM product is insufficient glass fiber infiltration. If the dry spots appear in a certain part of a certain product in the same period, it should also be considered whether it is caused by contamination of the glass fiber cloth quilt. Usually, the dry spots inside the product are also related to the resin viscosity, so the resin viscosity should be analyzed and adjusted first. Check whether the mold flow channel is too long or too narrow, and modify the mold in time. Check the feed pipe, improve the feed pipe, improve the feed pipe, and increase the feed point.

 

6. Cracks

There is still a certain gap between the domestic RTM process and foreign countries. The fiber content of domestic RTM products is low (generally about 50%), and due to the immature technology of the process, the fiber content of some products is unevenly distributed, resulting in cracks in the parts with too low fiber content. From a theoretical analysis, there are two reasons for the occurrence of cracks: 1. The cut product is not completely cured in the mold cavity, and even after post-curing treatment, the inside of the product is still slowly curing, and the curing shrinkage rate of the resin is large. In this way, the strength of the load-bearing carrier is insufficient in the parts with low fiber content in the product, and cracks are formed on the surface of the product due to the effect of curing internal stress; 2. The product itself has been completely cured, but due to the large temperature difference during transportation, thermal expansion and contraction, the internal stress is large, and cracks are generated in the weak parts with the lowest fiber content of the product.

Therefore, the process parameters should be adjusted according to the actual process conditions to improve the uniformity of fiber content and fiber distribution. At the same time, attention should be paid to the curing shrinkage rate of the resin used in the RTM process. The large curing shrinkage rate of the resin not only affects the surface effect of the product, but also increases the difficulty of demolding due to the internal stress generated by curing shrinkage. At this time, low shrinkage additives should be used, or a mixed resin system should be used to reduce the curing shrinkage rate of the resin.

 

7. Core Material Movement

The main reason for this phenomenon in RTM products is the production of mild mucous membranes. When you touch the mold with your hands, these parts feel extremely rough. Usually, this problem will occur after the product has been produced for a period of time, and the mold needs to be cleaned in time. First, use water sand to grind the rough parts of the mold, then use cotton wool dipped in acetone to scrub the entire mold, and finally apply a release agent to the mold.

HP-RTM process production application

RTM molding process

 

 

HP-RTM Molding Process

▶The injection pressure is between 10 and 60 bar, and the process cycle is about 6 minutes.

▶Dieffenbacher and KraussMaffei, experts in the field of processing machinery in Germany, have jointly developed an automated production line for high-pressure resin transfer molding (HP-RTM). Including preforming, pressing and finishing processes. Compared with the traditional RTM process, the HP-RTM process reduces the number of resin injections, improves the impregnation quality of preforms, and shortens the molding cycle.

 

Main Advantages of HP-RTM

▶The resin quickly fills the mold cavity.

▶Improves the quality of resin-impregnated reinforcements.

▶Accelerating the resin reactivity system can obtain a short curing cycle.

▶It is of great significance to the removal of air and the reduction of porosity in the product.

▶The product has excellent surface properties and quality.

▶The thickness and three-dimensional shape size deviation of the product are low.

▶It has high process stability and repeatability.

▶Use internal release agent and self-cleaning system.

 

HP-RTM Can Meet

▶Good materials and high part performance.

▶The surface quality requirements of parts are very high.

▶The conditions and ability to use fast-curing resins.

▶Large-scale industrial production

▶Short processing cycle.

 

 

HP-RTM Future Trend

After 2015, with the reduction of carbon fiber cost and the emergence of fast-curing epoxy resin, coupled with the development of HP-RTM process for rapid manufacturing of thermosetting composite materials and the development of thermoforming automation equipment for thermoplastic composite laminates, by 2020, the comprehensive cost performance of carbon fiber composite materials will be better than that of metals, and will replace metal materials for chassis manufacturing of mass-produced vehicles, covering most high-end models and a small number of mid-range vehicles

 

 

HP-RTM Application Examples

Dow Automotive Systems will present its latest VORAFORCE™ 5300 epoxy resin solution (for high pressure resin transfer molding composites) at the JEC Europe Composites Show in Paris in 2015. With this Dow solution, the molding cycle of high pressure resin transfer molding (HPRTM) or wet compression process can be controlled to less than 60 seconds, thus realizing the mass production of carbon fiber composites.

 

Non-toxic, ultra-short injection and curing cycles, and its ultra-low viscosity can effectively improve the ability to integrate components in the production process of automotive structural composite parts, bringing an economical and lightweight solution that is truly suitable for mass-produced vehicles.

 

Henkel Japan Exhibits High-pressure RTM Roof Materials

The Japanese “Human and Car Technology Exhibition 2014” exhibited a molded product (car roof) made of carbon fiber reinforced resin-based composite materials using “Loctite MAX3”.

 

 

The new matrix resin “Loctite MAX3” has the same heat resistance of 125°C as epoxy resin, and has a shorter molding time than epoxy matrix resin. The elongation and toughness exceed those of epoxy resin, and the tensile modulus and tensile strength are the same as those of epoxy resin. The viscosity of Loctite MAX3 resin is low at high temperatures, and it is easier to penetrate into the fiber material during the resin injection process. The time from injecting resin into the molding mold to demolding (demolding time) is shortened. When the roof material is molded by the high-pressure RTM process, the demolding time is 5.5 minutes, which is about 1/5 of that of epoxy resin. Carbon fiber composite material developed by BMW

 

BMW developed a carbon fiber composite body for the i3 electric car. BMW and SGL have spent more than 10 years developing carbon fiber and carbon fiber cloth for automobiles. The composite material is manufactured using the HP-RTM process and is now in mass production with high quality standards, good cost performance and short production cycle.

 

BASF’s New Epoxy Resin System

 

 

Baxxodur System 2202 is specially developed for the production of structural parts using the high-pressure RTM process: at 120°C, the product has an injection time of only 45 seconds and excellent fiber impregnation, and can be cured within 2.5 minutes. This thermosetting resin uses a new curing mechanism that allows for rapid cross-linking reactions. For the vacuum infusion process of wind turbine blades, this epoxy resin series can meet the requirements of blades of various sizes for onshore and offshore wind turbines. The use of the Baxxodur system in the production of large blades helps to improve the productivity of the manufacturing process.