RTM Molding Process

 

RTM Molding Process The basic principle of the RTM process is shown in the figure below. First, the reinforcement material preform, core material and embedded parts are pre-placed in the mold cavity, and then the resin is injected into the closed mold cavity under pressure or vacuum force to wet the fiber. After curing, the mold is demolded and then secondary processing and other post-processing procedures are carried out.

 

Fiber preforming has many forms, such as manual laying, manual fiber laying plus mold hot pressing preforming, robot jet chopped hot pressing preforming, three-dimensional weaving, etc. The effect to be achieved is that the fiber can fill the mold cavity relatively evenly, so as to facilitate the subsequent resin filling process.

 

In the process of mold closing and locking the mold, according to different production forms, some mold locking mechanisms are installed on the mold, some use external mold closing and locking equipment, and vacuum assistance can be used to provide locking force while locking the mold. Mold vacuuming can also reduce the impact of the internal pressure generated by resin filling on mold deformation.

 

In the resin injection stage, the viscosity of the resin is required to be kept as unchanged as possible to ensure uniform flow and full impregnation of the resin in the mold cavity. After the filling process is completed, the resin in each part of the mold is required to be cured synchronously to reduce the influence of thermal stress generated by curing on product deformation.

 

This process feature puts forward relatively high requirements on the viscosity of the resin, the curing reaction process and the corresponding curing system.

 

Process Characteristics

 

RTM has been widely used in ships, military facilities, national defense projects, transportation, aerospace and civil industries due to its excellent process performance. Its main features are as follows:

 

(1) The mold manufacturing and material selection are flexible. According to different production scales, the equipment changes are also flexible. The product output is between 1,000 and 20,000. The RTM molding process can obtain the best production economic benefits.

(2) It can manufacture complex parts with good surface quality and high dimensional accuracy, and its advantages are more obvious in the manufacture of large parts.

(3) It is easy to achieve local reinforcement and sandwich structure; it is easy to flexibly adjust the type of reinforcement material and structural design to meet the requirements of different performances from civil to aerospace industries.

(4) The fiber content can reach up to 60%.

(5) The RTM molding process is a closed mold operation process with a clean working environment and low styrene emissions during the molding process, which is beneficial to environmental protection.

(6) The RTM molding process has strict requirements on the raw material system. It requires the reinforcement material to have good resistance to resin flow erosion and good wettability. It requires the resin to have low viscosity, high reactivity, medium temperature curing, low curing exothermic peak, low viscosity during impregnation, and quick gelation after injection.

(7) Low-pressure injection, generally the injection pressure is <30psi, and FRP molds (including epoxy molds, FRP surface nickel casting molds, etc.), aluminum molds, etc. can be used. The mold design has high freedom and low mold cost.

(8) The product has low porosity.

Compared with the prepreg molding process, the RTM process does not require the preparation, transportation, and storage of frozen prepregs, does not require complicated manual laying and vacuum bag pressing processes, and does not require heat treatment time. It is simple to operate. The development and expansion of the technology are active mainly because the resin and fiber are relatively separated in the early stage of the process, the combination of fiber materials is very free, different types of fibers and different structural forms of weaving methods can be used, and various types of resins can also be selected according to product needs. However, in the RTM process, since the resin and fiber are shaped through the impregnation process only in the molding stage, the flow of the fiber in the mold cavity, the fiber impregnation process and the resin curing process all have a great influence on the performance of the final product, which leads to increased complexity and uncontrollability of the process.

 

The following table lists the comparison of the applicability of Hand Lay-up, RTM, SMC/BMC molding methods

Comparison	Hand Lay-up	RTM	SMC/BMC
Production scale (pieces/year)	Less than 1000	5,000－10,000	10,000 or more
Molding Temperature	Room Temperature	40－60℃（Room Temperature is also Acceptable）	130－150℃
Molding Cycle	1－4h	5－30min	1-15min
Production Efficiency/8h	2－3	16－90	50－400
Mold Type	FRP	FRP or metal	Metal
Mold Fee (with Mold Opening as 1)	1	2－4	5－10
Product Surface Effect	One Side Light	Both sides are smooth	Both Sides Polished
Part Repeatability	Human Factors have a Great Influence	Better	Very good
Part Dimensional Accuracy	Human Factors have a Great Influence	Better	Very good
Resin and Fiber Ratio	Human Factors have a Great Influence	Better	Very good
Filler Content	High	Lower	High
Release Agent	External Demoulding	Both inside and Outside are Acceptable	Internal Demoulding
Pressure	Contact Pressure	0.1－0.25 MPa	4－10MPa

 

The Influence of RTM process Parameters on the Process

 

The process parameters that affect the RTM process include resin viscosity, injection pressure, molding temperature, vacuum degree, etc. At the same time, these parameters are interrelated and affect each other during the molding process.

 

(1) Resin Viscosity

The resin suitable for the RTM process should have a low viscosity, usually less than 600mPa•s. When it is less than 300mPa•s, the processability will be better. When the viscosity of the resin used is high, the resin and molding temperature are usually increased to reduce the resin viscosity to facilitate better filling process.

 

(2) Injection Pressure

The selection of injection pressure depends on the fiber structure and fiber content as well as the required molding cycle. Many research data show that lower injection pressure is conducive to full impregnation of the fiber and to the improvement of mechanical properties. The injection pressure can be reduced by changing the product structure design, fiber layer design, reducing resin viscosity, optimizing the location of injection port and exhaust port, using vacuum assistance, etc.

 

(3) Molding Temperature

The selection of molding temperature is affected by the heating method that the mold itself can provide, the resin curing characteristics and the curing system used. Higher molding temperature can reduce the viscosity of the resin, promote the flow and impregnation of the resin inside the fiber bundle, and enhance the interfacial bonding ability of the resin and the fiber. Data show that higher temperature can improve the tensile strength of the product.

 

(4) Vacuum Degree

The use of vacuum assistance during the molding process can effectively reduce the rigidity requirements of the mold, while promoting the removal of air during the injection process and reducing the porosity content of the product. According to experimental data, the average porosity content of the flat plate molded under vacuum conditions is only 0.15%, while the porosity content of the flat plate without vacuum reaches 1%.

 

RTM Derivative Processes

 

With the continuous expansion of application fields, RTM process has developed a series of derivative processes, representative processes include: Light-RTM, SCRIMP (Seemann Composites Resin Infusion Molding Process), RFI (Resin film infusion), etc.

 

1. Light-RTM Molding Technology

 

Light-RTM is usually called lightweight RTM. This process is developed on the basis of vacuum-assisted RTM process. It is suitable for manufacturing large-area thin-walled products. The typical feature of this process is that the lower mold is a rigid mold, while the upper mold adopts a lightweight, semi-rigid mold, usually with a thickness of 6 to 8 mm. The process uses a double sealing structure, the outer ring vacuum is used to lock the mold, and the inner ring vacuum is used to introduce resin. The injection port is usually a linear injection method with a runner, which is conducive to rapid mold filling. Since the upper mold uses a semi-rigid mold, the mold cost is greatly reduced. At the same time, when manufacturing large-area thin-walled products, the mold locking force is provided by atmospheric pressure, which ensures the uniformity of mold pressurization and the wall thickness uniformity of the molded product is very good.

 

2. SCRIMP Molding Technology

 

SCRIMP Molding technology is a vacuum resin injection technology patented by Seaman Composites in the United States.

There are many similar processes, but the names are different. The SCRIMP process is the most representative.

The process principle is: remove the gas in the fiber reinforcement under vacuum, and realize the impregnation of the fiber through the flow and penetration of resin.

The molding mold first lays one or several layers of fiber fabric on the mold, then puts various auxiliary materials, and then seals it with a vacuum bag, opens the resin valve to suck and inject glue, fills the mold and finally solidifies it.

Compared with the traditional RTM process, it only needs half a mold and an elastic vacuum bag, which can save half the mold cost and the molding equipment is simple.

Due to the effect of the vacuum bag, a vacuum is formed around the fiber, which can increase the wetting speed and penetration of the resin.

In contrast to the RTM process, it only needs to be impregnated and cured under atmospheric pressure; the difference between the vacuum pressure and the atmospheric pressure provides a driving force for the resin injection, thereby shortening the molding time.

Impregnation is mainly achieved through flow in the thickness direction, so thick and complex laminated structures can be impregnated, and even structures containing cores, inserts, reinforcements and fasteners can be injected and molded in one go.

The SCRIMP process is suitable for medium and large composite components, with safe construction and low cost.

 

3. RFI Molding Technology

 

RFI was first patented by L. Letterman (Boeing, USA), and was originally developed for molding aircraft structural parts. In recent years, this technology has improved the shortcomings of RTM, such as low fiber content, expensive mold costs, and easy defects. RFI also uses a single mold and a vacuum bag to drive the impregnation process. The process is to lay the prefabricated resin film on the mold, then lay the fiber preform and close the mold with a vacuum bag; place the mold in an oven or hot press to heat and evacuate, and when it reaches a certain temperature, the resin film melts into a liquid with very low viscosity. Under the action of vacuum or external pressure, the resin gradually infiltrates the preform along the thickness direction to complete the transfer of the resin; continue to heat the resin to solidify, and finally obtain a composite product.

 

3. RFI Molding Technology

 

RFI was first patented by L. Letterman (Boeing, USA), and was originally developed for molding aircraft structural parts. In recent years, this technology has improved the shortcomings of RTM, such as low fiber content, expensive mold costs, and easy defects. RFI also uses a single mold and a vacuum bag to drive the impregnation process. The process is to lay the prefabricated resin film on the mold, then lay the fiber preform and close the mold with a vacuum bag; place the mold in an oven or hot press to heat and evacuate, and when it reaches a certain temperature, the resin film melts into a liquid with very low viscosity. Under the action of vacuum or external pressure, the resin gradually infiltrates the preform along the thickness direction to complete the transfer of the resin; continue to heat the resin to solidify, and finally obtain a composite product.

 

Types of Reinforcement Materials

 

The fiber types used for RTM include E glass fiber, R glass fiber and S glass fiber, as well as various high-strength and high-modulus carbon fibers and aramid fibers. The glass fiber fabric structures used include surface mats, woven fabrics, chopped mats, continuous mats, stitched mats, multi-axial fabrics, RTM-specific composite mats, and three-dimensional woven fabrics. High-performance fibers such as carbon fibers usually use cloths with different weaving methods. In many high-performance component manufacturing occasions, three-dimensional stereoscopic fabrics are increasingly used.

 

1. Woven Roving Mat

Ginger fabrics are the most common woven fabrics, and other types of woven roving fabrics such as twill and satin can be used in the RTM process. Various types of woven fabrics are prone to wrinkles and twists when laying, and are not easy to lay in place. Therefore, woven roving fabrics are usually used for some products with relatively simple surface changes. In order to ensure the stability of the fiber in the mold, a specific adhesive can be used to fix the fabric, or manual sewing can be used to sew the fabrics together with polyester thread.

 

2. Chopped Strand Mat

The advantages of Chopped Strand Mat for RTM process are low cost and good deformation resistance, and the disadvantage is poor scouring resistance. However, if woven fabric is laid on the chopped strand mat near the injection port of the mold, the scouring of the resin on the fiber can be reduced. From the actual use, the combination of chopped strand mat and woven fabric can improve the shear performance between product layers and realize the complementarity of fibers in different distribution directions.

 

3. Continuous Filament Mat

Glass Fiber Continuous Strand Mat is an important glass fiber non-woven reinforced substrate. It is a certain number of continuous glass fiber strands randomly dispersed into a circle and evenly distributed on the mesh belt.

It is combined into a mat by the chain effect of the strands overlapping each other and a small amount of adhesive. The unit mass of continuous mat is 225~900g/m2 and the thickness is 2~5 mm.

Because continuous filament mat has the advantages of isotropy, good migration resistance, resin scouring resistance, good adhesion and high product strength, it has become a very important reinforcement material in RTM process.

The foreign continuous mat production process mainly adopts the “one-step” mating technology, that is, multiple forming process devices are arranged under the glass furnace wire drawing plate, and the fibers are pulled out by several wire throwing machines and directly spread on the moving mesh belt to form a mat blank, which is then sizing, drying, and rolled into a mat.

The characteristics of this process are good fiber splitting, large output, and high degree of automation. At present, the international “one-step” forming production process is represented by Owens Corning Company in the United States and Saint-Gobain Company in France.

 

4. Stitched Glass Fiber Mat

 

Stitched Mat is a fiber mat structure formed by stitching different types of fibers together using a stitching machine. Stitched mat can be used to achieve a variety of fiber fabric reinforcement structures through different stitching methods. It is the most widely used and low-cost reinforcement material in the RTM process. Various types of stitched mats include:

 

(1) Unidirectional Mat Untwisted roving is laid parallel to only one direction of the fabric length direction 0 (warp) or 90 (weft) and stitched into a fabric.

(2) Biaxial Mat Untwisted roving is laid parallel to any two of the four directions of 0, 90, and ±45 degrees to the fabric length direction. Each direction forms an independent yarn layer and stitches into a biaxial fabric.

(3) Multiaxial Mat Untwisted roving is laid parallel to any three or four of the four directions of 0, 90, and ±45 degrees to the fabric length, and then stitched into a multiaxial fabric.

(4) Stitched Chopped Strand Mat The untwisted roving is chopped and spread evenly using a chopped strand machine combined with a stitching machine, and then stitched into a mat.

(5) Stitched Combination Mat Any one of Unidirectional fabric, Biaxial fabric, Multiaxial fabric and Stitched Chopped Strand Mat can be stitched together on a stitching machine to form 2 to 5 layers of stitched composite mat.

 

5. Three-dimensional Braided(Sandwich) Fabric

Three-dimensional braiding is a three-dimensional seamless complete structure obtained by interweaving long and short fibers. Its process characteristics are that it can produce regular and irregular solid bodies and make structural parts multifunctional – that is, weaving multi-layer integral components. Three-dimensional fabrics are mainly used in the manufacture of aerospace structural parts with very high mechanical properties. The principle of the weaving process is: many fiber rolls arranged in the same direction are moved precisely along a predetermined trajectory on a plane through a yarn carrier, so that the fibers cross or interweave to form a network structure, and finally the interweaving surface is tightened to form a three-dimensional fabric with various morphological reinforcement structures.

 

Advantages of Three-dimensional Weaving:

 

(1) The special-shaped parts are woven and formed as a whole in one step, realizing people’s idea of ​​”directly designing materials”;

(2) The structure is not layered, the interlayer strength is high, and the comprehensive mechanical properties are good.

 

Disadvantages of Three-dimensional Weaving:

(1) High production cost and high consumption of manpower and material resources;

(2) Slow weaving speed;

(3) The size of the parts is greatly limited.

 

Resins for RTM Process Resins for RTM Need to Meet the Following Basic Requirements:

 

Viscosity: The viscosity of the resin ranges from 0.1 to 1 Pa•s, generally 0.12 to 0.5 Pa•s. Viscosity that is too high or too low may lead to poor impregnation, or form a large number of pores and unimpregnated areas, affecting the performance and quality of the product. Resins with too high viscosity require higher injection pressures, which can easily cause the fibers to be washed out.

 

Compatibility: The resin should have good wettability, matching and interface properties for the reinforcing material.

 

Reactivity:

The reactivity of the resin used in the RTM process should be manifested in two stages. During the mold filling process, the reaction speed is slow and does not affect the mold filling. After the mold filling is completed, the resin begins to gel under the curing temperature conditions and quickly reaches a certain degree of curing, so as to reduce the mold occupancy time and improve production efficiency.

 

Shrinkage:

The resin shrinkage should be low. Excessive resin shrinkage will increase the porosity and the chance of product cracks.

 

Modulus:

The resin modulus is moderate on the premise of meeting the mechanical properties. High modulus resins produce high thermal stress, which is easy to cause product deformation and cracks.

 

Toughness and Elongation at Break:

These two indicators of the resin are mainly proportional to the impact resistance and crack resistance of the product. Higher values ​​can improve the resin’s ability to resist heat cracks.