Let You Understand Composite Material LRTM Molding Process with Various Process Introductions

 

The main composite material forming processes include:

 

Hand lay-up process, RTM process, vacuum bag process, autoclave process, molding process, tube rolling process, pultrusion process, roll forming process, winding process, pressure bag process, etc.

 

Must-read technology for composite materials: RTM process

 

Process Introduction

Resin transfer molding (RTM) technology is a low-cost composite material manufacturing method, which was originally used mainly for aircraft secondary load-bearing structural parts, such as doors and inspection covers.

In 1996, the U.S. Defense Advanced Research Projects Agency conducted research on low-cost RTM manufacturing technology for high-strength primary load-bearing components.

RTM technology has the advantages of high efficiency, low cost, good product quality, high dimensional accuracy, and low environmental impact. It can be applied to the molding of large, complex, and high-strength composite parts.

It has become one of the most active research directions in the field of aerospace material processing in recent years.

 

The main derivative technologies at present are vacuum infusion molding process (VIMP), flexible auxiliary RTM and co-injection RTM.

These technologies retain the advantages of traditional RTM process such as infusion molding of large components with sandwich, reinforcement, and embedded parts, and have the advantages of wide range of production components, stable product quality, easy combination with other weaving processes, and low cost manufacturing.

 

 

RTM Process Products

Principle Introduction

The main principle of the RTM process is to lay a preform of a reinforcement material designed according to performance and structural requirements in a mold cavity (the mold cavity needs to be pre-made into a specific size), and use an injection device to inject a special resin system into the closed mold cavity within a certain pressure range, and form it through the infiltration and curing of the resin and the reinforcement.

The mold has a peripheral seal and fastening as well as an injection and exhaust system to ensure smooth resin flow and exhaust all gases in the mold cavity and thoroughly infiltrate the fibers; it also has a heating system that can heat and cure to form composite components. It is a forming method that does not use prepreg or autoclaves.

 

Process Diagram

Process Diagram 1

Process Diagram 2

 

Requirements for Resin

(1) The resin for RTM Process is liquid or solid at room temperature and can exist stably at room temperature. The chemical composition and properties of the resin will not change during storage.

(2) The resin system has an appropriate viscosity at the process temperature (about 0.2-0.8Pa•s in the working range). Too high viscosity will lead to difficulties in resin flow and fiber/resin impregnation; too low viscosity will lead to unstable resin flow and diffusion, forming pores.

(3) Sufficiently long gel time to meet the requirements of resin flow filling and fiber impregnation. At the same time, the low viscosity retention time of the resin should be greater than 40 minutes to form the low viscosity platform characteristics required by the RTM process resin.

(4) No volatile products are generated during the injection and curing process of the resin, and the resin should have good wettability and adhesion to the reinforcement material.

 

Typical Advantages

 

Compared with the traditional molding process, the biggest advantage of the RTM process is that it replaces the two-step or multi-step impregnation process of the traditional molding process with one-step impregnation, reducing the prepreg preparation, lamination, vacuum bag and curing in the autoclave, thereby greatly reducing the molding time and molding cost.

 

Derived Process Introduction

 

Vacuum infusion molding process (VIMP) is a new type of low-cost liquid molding technology for large-size composite parts developed from RTM process. Its process principle is to use a flexible vacuum bag film to cover and seal the fiber reinforcement material on a single-sided rigid mold, use vacuum negative pressure to remove the gas in the mold cavity, and drive the resin flow through vacuum negative pressure to achieve resin impregnation of fibers and fabrics. The principle is shown in the figure below.

 

VIMP Packaging Principle Diagram

 

Flexible-assisted RTM process is a molding technology that uses the manufacturing of hollow structures and the compaction of preforms by flexible molds to process advanced composite materials. According to the way the flexible mold expands, it is divided into airbag-assisted RTM process and thermal expansion soft mold-assisted RTM process.

 

Compared with the traditional RTM process, this method solves the problem that the inner cavity structure is too complex to be demolded, and at the same time, the fiber volume content of the workpiece is increased and the product performance is improved. This process has unique advantages in molding complex composite components. The airbag-assisted RTM process is an advanced process that is currently being studied more abroad. It is formed by placing the preform in a sealed airbag, compacting the preform by pressurizing the airbag, and attaching it to the inner surface of the mold cavity.

 

 

At present, the manufacturing of small and medium-sized composite RTM parts has been widely used, and large RTM parts have also been successfully used on JSF’s vertical tail. The advantages of this method are environmental protection, good laminate performance and good double-sided quality. Its application in aviation can not only reduce the labor itself, but also reduce the assembly workload because it can form large integral parts. However, parts formed by pressure injection of resin into the mold cavity have defects such as large pore content, low fiber content, uneven distribution of resin in the fibers, and insufficient resin impregnation of the fibers. Therefore, this technology still has potential for improvement.

 

Application of RTM Process in Carbon Fiber Composite Materials

 

Due to its small fiber diameter and high surface inertness, carbon fiber generally adopts the prepreg/autoclave molding process. In order to reduce costs, there are currently carbon fiber products and epoxy resin models mainly used in RTM processes abroad. Domestic research in this area is relatively less.

Compared with the mature prepreg/autoclave molding process, the RTM process has a significant effect in reducing material costs and manufacturing costs, and has more advantages when manufacturing small and complex-shaped structures. It is reported that RTM technology has been gradually applied to domestic T700 grade carbon fiber composite materials. The use of RTM process molding can simultaneously meet the requirements of high performance, low cost and localization of aviation materials. We will continue to pay attention to more content later.

 

Introduction to Lightweight RTM Process

 

The traditional RTM process, because it is a closed-mold process, has the advantages of reducing volatile organic compounds (VOC) emissions, expanding the range of available raw materials, reducing labor, being environmentally friendly, and producing products with smooth surfaces on both sides.

However, in the RTM process, the resin is injected at a higher pressure and flow rate, so we need to make the structural strength and rigidity of the mold large enough not to be damaged or deformed under the injection pressure.

Sandwich composite materials with steel pipe supports are usually used, or aluminum molds or steel molds processed by CNC machine tools, which increase manufacturing costs.

Only products with a large enough output can offset the mold costs. In addition, in order to close the mold, it is necessary to have sufficient clamping capacity around the perimeter or use a pressure system to close the mold.

The above factors all limit the application of RTM technology on large products, otherwise the mold will become very heavy. And the investment will be huge.

 

Lightweight resin transfer molding (RTM-Light) is also known as LRTM, ECO, Vacuum Molding or VARTM. It is a low-cost manufacturing process that has developed rapidly in recent years. At present, its application in the fields of ships, automobiles, industry and medical composite materials has surpassed the RTM process.

 

The LRTM process retains the mold-matching process of the RTM process, thus retaining almost all the advantages of the RTM process. However, its upper mold is a semi-rigid fiberglass mold with a thickness of generally 6-8mm.

It usually does not need to be reinforced with steel pipes. The mold has a rigid periphery of about 100mm wide, and a double-channel sealing belt forms an independent sealing area.

As long as the vacuum is drawn, the mold is closed, which is very convenient and fast. Then the mold cavity is vacuumed, and the negative pressure in the mold and the lower injection pressure are used to inject the resin into the mold, so that the resin penetrates into the pre-laid reinforcing fibers or prefabricated parts.

The mold cost of RTM-Light is low, and because the pressure in the mold is reduced, its mold is similar to the open mold, and it is easy to transform from the mold of the open mold process.

 

 

Comparison Between LRTM and Conventional RTM

 

1. Mold

 

The mold is the biggest difference between the two processes. In RTM investment, due to the high injection pressure, a considerable part of the cost is spent on the mold and clamping device. This is not suitable for products with low production volume in terms of price. The service life of the RTM process mold can reach more than 5,000 pieces, with high production efficiency, suitable for products with an annual output of more than 2,000 pieces.

 

The biggest advantage of LRTM is the low production cost of its mold. The cost is about half of that of conventional RTM molds, but the mold service life is also lower than that of RTM molds, suitable for products with an annual output of about 1,000 pieces. The size of the products produced by the LRTM process can be larger than that of traditional RTM. Usually, the product is as small as a basketball cap and as large as an 8m long hull (about 25 square meters), but this is not the ultimate limit of size. The difficulty of products smaller than basketball caps is to lay fibers, while products larger than 8m are difficult to handle on the upper mold.

 

The disadvantage of FRP molds is that the service life of the mold surface is short. In order to obtain excellent mold life and product repeatability and dimensional accuracy, molds for both LRTM and RTM processes must be of high quality and have precise cross-sections. In composite molding processes, the cost of the final product surface requirements can reach 60% of the final product price.

Composite molds can achieve automotive surface quality for 500 uses, and then the mold surface treatment is required.

One way to increase life is to use exchangeable mold skins, such as JHm Technologies’ patented ZIP RTM technology, which can be used for RTM and LRTM processes.

By using exchangeable mold skins to replace vulnerable mold surfaces, mold life is extended and mold quality is improved, and the mold service life can reach 8,000 to 10,000 times.

When several exchangeable mold skins are used at the same time, production efficiency is greatly improved because the gel coat can be directly applied and heated on the exchangeable mold skins outside the mold.

 

 

2. Injection Pressure, Flow Rate and Equipment

 

The injection pressure of RTM process is generally 0.1-0.4MPa, while the injection pressure of LRTM process is generally not more than 0.1MPa, usually 0.03-0.07MPa. The resin injection rate is affected by many factors, such as resin viscosity, component size, fiber type and layer structure, and the usual injection rate is 1.3-2 liters/minute.

 

In order to prevent the mold from deforming or punching the upper mold (especially at the injection port), this requires a stricter control of the pressure. The injection equipment used for LRTM process is generally equipped with a pressure feedback device to perform closed-loop control of the pressure.

It is also possible to design a simple air pressure control system and a matching electronic closed-loop system on the RTM standard equipment line, so that the original production equipment can be used to obtain the best productivity without causing mold deformation and damage.

 

Equipment research is also developing towards low prices and multi-purposes. Plastech’s SSB injection equipment uses a patented piston-modified precision metering pump, with a minimum catalyst ratio of 0.5%. With industrial MPG (Mould pressureguard), the machine can control the pump speed by itself.

In 12-15 seconds, 1m2 of reinforcement material can be impregnated, and the impregnation speed can be precisely controlled. The equipment is equipped with other options and can also be used for hand-layup process glue preparation and glue brushing.

 

3. Production Efficiency and Cost

 

Due to the low injection pressure, the flow rate of the resin cannot be accelerated to the optimal flow rate. The production speed of the LRTM process is half that of the RTM process. Based on an 8-hour shift, for processes using gel coat surfaces and non-heated molds, the RTM process can produce 10-12 molds per shift, while the LRTM process can only produce 4-6 molds.

For a 34-square-foot product that requires heating and curing, the RTM process can produce 40 molds per shift when using a hydraulic press, heated molds, and 5 replaceable molds. The LRTM process in the same situation can produce 20 molds. But it does not require a hydraulic press, and the mold price is half as low.

 

 

4. Runner Design

 

Generally, the runner design of the RTM process is to inject from the center and discharge from the periphery. However, the LRTM process usually flows in from the periphery and discharges from the center. We know that when the resin enters the mold cavity from the resin pipe, it meets the fabric, and the fabric will produce a resistance to the resin. The size of the resistance is related to the permeability of the fabric, the viscosity of the resin and the flow rate of the resin. When the fabric and the resin are selected, it is proportional to the flow rate of the resin.

 

Taking a product with an area of ​​3 square meters and a thickness of 3mm as an example, the general injection pressure is 0.05MPa, the injection time is 6min, and the injection flow rate is 1.33L/min.

If injected from the center and the flow rate is kept unchanged, the resistance can increase to more than 0.1MPa, resulting in the opening or expansion of the mold, and leading to problems such as the loss of control of the resin flow front and the formation of dry spots on the product.

To this end, the flow rate must be reduced, but this in turn prolongs the injection time, so it often takes more than 6 minutes to inject from the center. When injected from the periphery, the resin first enters a peripheral flow channel with a gap of about 1mm and almost no resistance, and then enters the fiber.

As the passage for the resin to enter the fiber increases (from a point to a periphery), the relative flow rate of the resin in the fabric is also slowed down, the resistance is also reduced, the injection flow rate can be increased, and the injection time can be shortened.

 

Experiments show that for a 0.2 square meter product, the injection time from the periphery is 2.1 minutes, while the injection from the center is 9 minutes, and the speed difference is four times. Of course, the RTM process can also inject resin from the periphery, and the pressure gradient of the inner cavity remains unchanged, but the highest point of pressure moves from the original center point to the periphery, which is beneficial to controlling the deformation of the mold, because the rigidity of the periphery of the mold is better than that of the central area, but at the same time, the sealing requirements for the periphery are also increased.

 

The flow channel design varies with the product, such as Spectrayte’s 18m long lamp column, which uses a long flow channel. Brands’ 6m2 floor, due to its asymmetric structure, uses two outlets, and a resin collector (Catchpot) is placed in different structural centers, and the injection time is 15 minutes. The Royal Netherlands Navy’s 13m2 hull manufacturing used two diagonally arranged resin inlets due to the large size of the product.

 

 

5. Product Accuracy,Structure and Others

 

The dimensional viscosity and repeatability of RTM and LRTM products are affected by the resin used for molding, process control and product curing. The cross-sectional accuracy of the product is also affected by the resin flow rate and injection pressure in the process. For the RTM process, when the mold is manufactured according to the standard, no bending occurs, and the appropriate clamping device or clamping with a press is used, the dimensional accuracy repeatability of the part is very good, and the thickness deviation is not greater than 0.OlOmm. The LRTM process usually has a certain deformation of the upper mold, but the product dimensional accuracy can also reach ±0.020mm. In some places, it is 4 ±0.030mm.

 

Both RTM and LRTM processes can press sandwich materials. The core material can be balsa wood and foam. However, the RTM process has a high injection pressure, which limits the use of low-density foam materials. The minimum density of the foam is not less than 80 kg/m3, while the pressure of the LRTM and ZIP RTM processes is lower, and the density of the foam he uses can be as low as 37 kg/m3. However, it should be pointed out that when manufacturing sandwich materials, the dimensional accuracy of the core material must meet the mold requirements to ensure the repeatability of the molding process and product quality.

 

RTM and LRTM processes can also use preforms and inserts. When using preforms, products with high fiber volume content can be obtained. LRTM process products do not need to be coated with gel coats. As long as general demoulding wax is used, the products can be demoulded. However, if RTM products do not use gel coats, demoulding is more difficult.

 

Compared with the open mold process, the investment of LRTM is still relatively high, and the rationality of the cost required for the configuration of the mold must be considered. In addition, the strong professionalism of the process and the heavy daily maintenance tasks also affect the use of LRTM by some composite material manufacturers that lay up manually.

 

Compared with RTM, LRTM has the advantages of low energy consumption and not too high requirements for mold rigidity and other indicators, but it has high requirements for resin viscosity, compatibility of resin and reinforcement materials, and the forward speed of the resin front.

 

LRTM process is a highly professional process, and operators must be properly trained. Without proper fiber laying, good air tightness, precise mold installation, and consistent resin flow control, the product will have problems such as messy dry spots, radial bubbles, and resin enrichment. The following is a brief explanation of the relevant issues:

 

 

Issues to be Noted in the Lightweight RTM Process　

LRTM process is a highly professional process, and operators must be properly trained. Without reasonable fiber laying, good air tightness, precise mold installation, and consistent resin flow control, the product will have problems such as messy dry spots, radial bubbles, and resin enrichment. The following is a brief explanation of the relevant issues.

 

 

1. Sealing

 

The RTM-Ligh process has high requirements for details, especially the sealing of the mold. The lower the vacuum degree of the vacuum groove for peripheral clamping, the better.

The vacuum degree of the inner cavity is generally controlled at about 15mm-Hg. The vacuum sealing groove for peripheral clamping adopts a wing seal profile of flexible neoprene, with a bottom width of 20mm.

The joint of the seal ring should be cut vertically and glued with a flexible adhesive to ensure its elasticity. The outer ring is then sealed with 6mm wide silicone rubber.

 

During the molding process, in addition to paying attention to the sealing of the mold vacuum sealant, attention should also be paid to the assembly of the mold, the connection between the seal ring and the pipe, and the leakage caused by the cracks in the mold.

In fact, any seal or joint at the resin inlet, including the outlet of the vacuum zone, should be strictly inspected. A more hidden cause of air leakage is cracks on the surface of the mold panel, which are usually not discovered.

The solution to this problem is to apply resin prepared with a catalyst to the outer surface of the mold before the mold reaches a vacuum. This is a very effective method.

 

In addition, the sealing surface should be kept clean and solvents should not be used for cleaning. It is best to choose a semi-permanent release agent that does not require cleaning.

 

 

2. Precision Matching of Upper and Lower Molds

 

Precision matching of upper and lower molds helps balance the pressure of the cavity in the mold, making the resin penetrate evenly and helping to improve product quality. Since the upper mold is a semi-rigid mold, each mold closing must be carefully corrected.

 

If white spots appear continuously at the same position of the product, it may be because the mold closure is inaccurate, resulting in inaccuracy of the inner cavity, which directly leads to uneven thickness of the inner cavity. In this case, assuming that the fiberglass layer is uniform, the flow of resin during injection will be selective, and it will choose places with larger thickness (gap), so white spots will appear in areas with thinner inner cavity.

 

Poor mold positioning is an important reason for poor mold matching. When the side pins of the mold are installed, the X-axis and Y-axis of the mold are naturally determined. If the side pins are not positioned properly, unpredictable errors will occur and the characteristics of the injection will be changed.

 

3. Reasonable ply and Raw Material Selection

Due to the low molding pressure, the LRTM process has stricter requirements on the ply of fabrics. Unreasonable ply, especially the treatment of lap joints, will seriously affect the consistency of the resin flow channel, resulting in resin enrichment or lack of glue (dry spots) of the product.

 

During plying, the fabric can be fixed with the help of spray glue to make the ply smoother. However, the spray glue must have good compatibility with the resin used. Excessive spray glue still has a certain impact on the final performance of the product.

 

Different fabrics and mats have a great influence on the process. It is necessary to use reinforcing materials with good permeability as much as possible. The resin flow rate of current O.C. closed mold mat or “Hi-Flow” composite mat can be twice as fast as that of ordinary chopped mat. The appropriate resin system should be selected according to different product requirements. Try to use resins with low viscosity and low shrinkage. Its standards are equivalent to the requirements of vacuum resin diffusion process.

4. Surface Cracks

 

The cracks on the surface of the product are often observed in the corner area. This is a common problem in resin-rich areas. This can also be traced back to the manufacture of the mold.

If the two mold halves do not fit well with each other, an excess thickness that exceeds the expected thickness will be generated.

To solve this problem, in addition to the correct mold halves, additional glass fiber can be added to make up the thickness in these thicker areas to prevent cracks in the part.

 

Cracks in the mold caused by excessive thickness of the part can be found in large flat areas.

This is caused by the operator arbitrarily increasing the injection speed of the resin. Injecting too fast will cause the inner cavity of the mold to expand.

If the injection process is completed in a very short time, the cavity will not have time to recover, so the excess resin will cause cracks in the mold. In extreme cases, irreparable cracks on the mold surface will occur.

 

 

5. Resin Overflow

 

Many manufacturers feel the need to use a larger resin collector to receive the resin that is discharged before the end of molding. This is the result of their inability to accurately control the filling of the molding.

If the resin filling process is too fast, it is difficult to correctly judge when to stop the injection. Because if you stop the injection when you see the resin reaching the resin collector, then you will see too much resin flowing into the resin collector because the over-expanded mold returns to its original size. To overcome this possibility, you can only replace a larger resin collector to prevent the resin from overflowing.

 

The simple way to solve this problem is to calculate in advance how much resin to use.

However, when molding large products, it is difficult to know exactly how much resin to use.

Another method is to provide information to the operator through precise air pressure control to avoid blind judgment by the operator.

The air pressure reading in the cavity provides more accurate mold filling information for the operator and avoids the need for a larger resin receiver.

Such a system ensures that there is enough 10 to 100 ml of surplus after each injection process, which minimizes the waste of resin and ensures increased profits.

 

Application Fields of Light RTM

 

Currently, common application fields include aerospace, military, transportation, construction, shipbuilding and energy. For example: hatches, fan blades, nose radar covers, aircraft engine covers, etc. in the aerospace field; torpedo shells, fuel tanks, launch tubes, etc. in the military field; light rail doors, bus side panels, car chassis, bumpers, truck top baffles, etc. in the transportation field; tubular lamp poles of street lamps, wind turbine covers, decorative doors, chairs and tables, helmets, etc. in the construction field; small rowing boat hulls, upper decks, etc. in the shipbuilding field, etc.

 

 

Prepreg can be formed by different methods, and the more appropriate forming method can be selected according to different applications.

 

The main prepreg forming processes include vacuum bag process, autoclave process, molding process, tube rolling process, pultrusion process, roll forming process, winding process, pressure bag process, etc.

 

Among them, the vacuum bag process is mainly used for interior decoration of the shipbuilding industry and railway system, the autoclave process is mainly used for high-quality composite materials, the molding process is mainly used for flat panels, sporting goods, sleds, and industrial products, the tube rolling process is mainly used for fishing rods, ski rods, golf clubs, and pipe fittings, and the pressure bag process is mainly used for masts, antenna poles, and various pipe fittings.

 

 

1. Vacuum Bag Process

 

Definition: The vacuum bag molding process is to seal the product between the mold and the vacuum bag, and pressurize the product by vacuuming to make the product more compact and have better mechanical properties.

 

Advantages:

 

1. High fiber content, better mechanical properties of the product;

 

2. Uniform pressurization, uniform performance of the product;

 

3. Effective control of product thickness and glue content;

 

4. Reduce bubbles in the product;

 

5. Can form complex and large parts;

 

1. Reduce the damage of volatile components to personnel.

 

Vacuum Bag Wet Process:

 

1. Mold preparation, apply release agent

 

2. Product layering (hand lay-up, spraying, prepreg)

 

3. Lay release cloth

 

4. Lay isolation film or perforated isolation film (optional)

 

5. Lay breathable mat

 

6. Paste sealing strips (can be done in advance)

 

7. Seal the vacuum bag film

 

8. Install vacuum valve, quick connector and vacuum tube

 

9. Connect the air source and check the vacuum degree

 

10. Vacuum, product curing

 

11. Product demoulding

 

 

2. Hot-Pressing Tank Forming Technology

 

Features: uniform pressure inside the tank, fast, uniform air temperature inside the tank, high precision, high quality, wide adaptability, suitable for batch, reliable forming process; Disadvantages: high cost, large initial investment.

 

3. Roller Forming

 

Roller forming is mainly based on metal forming methods. The equipment consists of a series (one or more groups) of hot pressing rollers and cold pressing rollers. After the prepreg is heated, it is first deformed by a group of hot rollers, and then formed by a group of cold rollers with gradually decreasing spacing.

 

Advantages:

(1) The blank is dense, strong and not easy to deform.

(2) Low moisture content, smooth and flat products, and consistent product specifications.

(3) High productivity and low labor intensity.

(4) Suitable for female and male mold forming, single machine connection.

(5) The operating technology does not need to be very high, which is convenient for forming an automated production line.

 

According to the forming process, it is divided into male mold forming and female mold forming.

 

⑴ Male mold forming: Use a punch, the blank is facing the model, and the roller head determines the outer surface of the blank. It is suitable for flat plate products (shallow products).

 

Advantages:

It is not easy to deform when drying with a mold (supported), the use surface can be engraved with patterns and curved edges according to the model, and it is relatively simple to take the blank and return to the mold in the production line.

 

Disadvantages:

It is easy to deform and crack the blank when taking it out.

 

⑵ Female mold forming: Use a concave mold, the non-use surface of the blank faces the model, and the roller determines the inner surface of the blank. It is suitable for bowls, cups, plates and small-sized plate products (deep products).

 

Advantages:

When taking the blank, there is no contact with the blank, and the deformation is small. The spindle speed can be higher, and it is not easy to fly mud.

 

Disadvantages:

It is more complicated to return the mold in the production line (flip 180°).

 

 

4. Winding Filament Molding

 

Twist molding is a process method in which the prepreg is wound on the core mold in a certain way by controlling the tension and winding angle of the winding machine, and then cured to form a composite material product. The winding molding process is only applicable to rotating tubular products.

 

The reinforcing materials used for winding molding are mainly various fiber yarns:

such as alkali-free glass fiber yarn, medium-alkali glass fiber yarn, carbon fiber yarn, high-strength glass fiber yarn, aramid fiber yarn and surface mat, etc.

 

For general civilian products such as pipes and tanks, unsaturated polyester resins are mostly used.

For winding products with high requirements for mechanical properties of compressive strength and interlaminar shear strength, epoxy resin can be used. The inner mold of the molded hollow product is called the core mold. Under normal circumstances, after the winding product is cured, the core mold must be removed from the product.

 

The winding machine is the main equipment for realizing the winding molding process. The requirements for the winding machine are:

 

①, it can realize the winding law and accurate yarn arrangement of the product design;

 

②, easy to operate;

 

③, high production efficiency;

 

④, low equipment cost.

 

Winding molding can be divided into dry winding, wet winding and semi-dry winding.

 

Dry winding is to use pre-impregnated yarn or tape, which is heated and softened to a viscous state on the winding machine and then wound onto the core mold.

 

Wet winding is to dip the fiber bundle (yarn tape) into glue and then directly wind it onto the core mold under tension control.

 

Semi-dry winding is to add a set of drying equipment to remove the solvent in the impregnated yarn on the way from the fiber dipped into glue to the winding onto the core mold.

 

Compared with the dry method, the pre-gluing process and equipment are omitted; compared with the wet method, the bubble content in the product can be reduced.

 

Among the three winding methods, wet winding is the most commonly used; dry winding is only used in high-performance, high-precision cutting-edge technology fields.

 

Advantages of Fiber Winding:

① The winding rules can be designed according to the stress conditions of the product, so that the strength of the fiber can be fully utilized;

② High specific strength: Generally speaking, the weight of fiber-wound pressure vessels can be reduced by 40-60% compared with steel vessels of the same volume and pressure;

③ High reliability: Fiber-wound products are easy to achieve mechanized and automated production. After the process conditions are determined, the quality of the wound products is stable and accurate;

④ High production efficiency: Mechanized or automated production requires fewer operators and fast winding speed (240m/min), so the labor production efficiency is high;

⑤ Low cost: On the same product, several materials (including resin, fiber and lining) can be reasonably selected and compounded to achieve the best technical and economic effect.

 

Disadvantages of Winding Filament Molding:

 

① Winding molding has low adaptability and cannot be used to wind products of any structural form, especially products with concave surfaces, because when winding, the fiber cannot be close to the surface of the core mold and is suspended;

 

② Winding molding requires a winding machine, core mold, curing heating furnace, demoulding machine and skilled technical workers, which requires large investment and high technical requirements. Therefore, only mass production can reduce costs and obtain greater technical and economic benefits. The fiber content of the winding molding process can reach 60%-80%, the product thickness is 2-25mm, the maximum diameter of the product is 4m, the length is 12m, the curing temperature is 80-130℃, the molding cycle is determined by the size of the product, the molding pressure is determined by the tension of the winding machine, the mold is a metal or gypsum core mold, and the production is continuous.

 

5. Compression Molding

 

Advantages of compression molding: High production efficiency, easy to realize specialized and automated production;

High product size accuracy and good repeatability;

Smooth surface, no secondary modification required;

Can form complex products in one go;

Mass production, low price.

 

However, the compression molding process has complex mold manufacturing, high mold quality requirements, high molding pressure, large equipment investment, and limited by the press. It is most suitable for mass production of small and medium-sized composite materials. With the continuous improvement and development of metal processing technology, press manufacturing level and synthetic resin process performance, the tonnage and table size of the press are constantly increasing, and the molding temperature and pressure of the molded material are relatively reduced, making the size of the molded process products gradually develop towards large-scale. At present, it can produce large automotive parts, bathtubs, and integral bathroom parts.

 

The fiber content of the molded products is between 25% and 60%, the thickness of the products is 1-10mm, the curing temperature is 40-50℃ for cold pressing and 100-170℃ for hot pressing, the molding cycle is short, 5-60min, the molding pressure is 10-40MPa, the mold is generally a steel mold, and the cold mold can be made of fiberglass. The product size is limited by the mold size and the tonnage of the press. The production requires hydraulic presses, heating molds, cold molds and other equipment, which is suitable for medium-volume production of 100-20000T.

 

 

3. Pultrusion Process

 

Pultrusion process is to form and solidify the prepreg through an extrusion die under the action of traction, and continuously produce FRP profiles of unlimited length. This process is most suitable for producing FRP profiles of various cross-sectional shapes, such as rods, tubes, solid profiles (I-shaped, grooved, square profiles) and hollow profiles (door and window profiles, blades, etc.). The pultrusion process consists of yarn feeding, dipping, preforming, curing and shaping, traction, cutting and other processes.

 

Its advantages are:

 

① The production process is fully automated and has high production efficiency;

 

② The fiber content in the pultruded product can be as high as 80%, and the dipping is carried out under tension, which can give full play to the role of the reinforcing material and the product has high strength;

 

③ The longitudinal and transverse strength of the product can be adjusted arbitrarily to meet the use requirements of products with different mechanical properties;

 

④ There is no scrap in the production process, and the product does not need post-processing, so it saves labor, raw materials and energy consumption compared with other processes;

 

⑤ The product quality is stable, the repeatability is good, and the length can be cut arbitrarily.

 

The disadvantages of the pultrusion process are large equipment investment, monotonous product shape, only line profiles can be produced, and the lateral strength is not high.

 

Among the raw materials of the pultrusion process, the most widely used resin matrix is ​​unsaturated polyester resin,

 

The reinforcing material is mainly glass fiber and its products, such as untwisted roving, continuous fiber mat, etc.

 

The mold is an important tool for pultrusion, generally consisting of a preforming mold and a forming mold.

 

① Preforming mold: During the pultrusion process, the prepreg must pass through a preforming mold composed of a group of yarn guide elements before entering the forming mold. The function of the preforming mold is to gradually form a preformed body with a shape and size similar to the forming mold control shape and size according to the cross-sectional configuration of the profile after the impregnation, and then enter the forming mold, so as to ensure that the yarn content of the product section is uniform.

 

② Forming mold: The ratio of the cross-sectional area of ​​the forming mold to the cross-sectional area of ​​the product should generally be greater than or equal to 10 to ensure that the mold has sufficient strength and rigidity, and the heat distribution is uniform and stable after heating. The length of the pultrusion die is determined by the traction speed and the curing speed of the resin gel during the molding process to ensure that the product reaches the demoulding curing degree when it is pulled out.

 

Generally, steel chrome plating is used, and the cavity surface is required to be smooth and wear-resistant to reduce the friction resistance during pultrusion molding and increase the service life of the mold. The fiber content of the pultrusion molding process can reach 60%-75%, the profile thickness can reach 12mm, the rod diameter is 40mm, the curing temperature is 100-160℃, and it can be produced continuously. The maximum traction force is 40 tons. The mold is a pultrusion unit mold. The cross-sectional size of the product depends on the unit mold, and the length is not limited.