Lightweight Resin Transfer Molding (RTM-LIGHT) Technology

 

Introduction to Light RTM Process

The traditional RTM process, as a closed mold process, has the advantages of reducing volatile organic compound (VOC) emissions (up to 5PPm or less), expanding the range of available raw materials, reducing labor, being environmentally friendly, and obtaining products with smooth surfaces on both sides.

However, in the RTM process, the injection of resin is carried out at a higher pressure and flow rate, so we must make the structural strength and rigidity of the mold large enough to not be damaged or deformed under the injection pressure. Usually, sandwich composite materials with steel pipe support or aluminum or steel molds processed by CNC machine tools are used, which increases the manufacturing cost.

Only for products with large enough output can the mold cost be offset. In addition, in order to close the mold, the surrounding area must have sufficient clamping capacity or a closed mold pressure system must be used. The above factors limit the application of the RTM process on large products, otherwise the mold will become very heavy and the investment will be very large.

Lightweight resin transfer molding (RTM-Light), also known as LRTM, 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 a trend of surpassing RTM technology.

The RTM-Light 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 with a width of about 100mm, 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. The main equipment of the RTM-light process includes resin injection device, mold, vacuum device and other auxiliary tools.

 

Comparison between Light RTM and Conventional RTM

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 production of more than 2,000 pieces.

The biggest advantage of RTM-Light is its low mold production cost, which is about half of the cost of conventional RTM molds, but the mold service life is also lower than that of RTM molds, suitable for products with an annual production of about 1,000 pieces.

The size of the product produced by the RTM-Light process can be larger than that of traditional RTM. Usually, the product is as small as a basketball cap or as large as an 8m long hull – but this is not the ultimate size limit. 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 RTM-Light and RTM processes must be of high quality and have precise cross-sections. In composite molding processes, the cost of the surface requirements of the final product can reach 60% of the final product price.

Composite molds can be used 500 times to achieve automotive surface quality, 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 RTM-Light 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 8,000-10,000 times. When several exchangeable mold skins are used at the same time, the gel coat can be directly applied and heated on the exchangeable mold skin outside the mold, which greatly improves production efficiency.

 

Injection pressure, flow rate and equipment

The injection pressure of the RTM process is generally 0.1-0.4 MPa, while the injection pressure of the RTM-Light process is generally not more than 0.1 MPa, usually 0.03-0.07 MPa.

The resin injection rate is affected by many factors, such as resin viscosity, part size, fiber type and layer structure. The usual injection rate is 1.3-2.0 liters/minute.

In order to prevent mold deformation or punching open the upper mold (especially at the injection port), this requires a stricter control of the pressure. The injection equipment used for the RTM-Light 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 (VMPC mold protection) system and a POD electronic closed-loop system used in conjunction with VMPC 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-purpose. 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 press guard), the pump speed can be controlled by the machine itself, and 1㎡ of reinforcement material can be impregnated within 12-15 seconds. The impregnation speed can be precisely controlled. The equipment is equipped with other options and can also be used for hand lay-up process glue preparation and glue brushing.

 

Production efficiency and cost

RTM-Light process is a low-cost production technology. Compared with traditional RTM technology, it has the advantages of low mold cost, no need for complex clamping devices or pressure systems, small investment, simple operation, and saving human capital. Compared with the open mold process, it has the advantages of high product dimensional accuracy, low void ratio, low styrene volatilization during production, environmental friendliness, low material waste (resin utilization rate can reach 95%), low scrap rate, and high production efficiency.

Due to the low injection pressure, the flow rate of the resin cannot be accelerated to the optimal flow rate. The production speed of RTM-Light process is half that of RTM process. For 8 hours per shift, for the process with gel coat surface and non-heating mold, RTM process can produce 10-12 molds per shift, while RTM-Light process can only produce 4-6 molds.

For a 34 square foot product that needs to be heated and cured, RTM process can produce 40 molds per shift when using hydraulic press, heated mold, and 5 replaceable molds. The RTM-Light process in the same situation can produce 20 molds. But no hydraulic press is required, and the mold price is also half of that.

In recent years, through the improvement of mold design and process control, the production speed of the two has been close. For example, after Xiraplas adopted RTM-Linght process to replace the open mold process, the workshop became orderly, using the original 50 workers and 3000 square meters of workshop area. The output increased by 25%. According to the company, compared with the original open mold process, the production efficiency has increased by 90%.

 

Runner Design

Generally, the runner design of the RTM process is to inject from the center and discharge from the periphery, but the RTM-Light 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 and meets the fabric, the fabric will generate a back pressure (resistance) on the resin. The size of the back pressure is related to the permeability of the fabric, the viscosity of the resin and the flow rate of the resin. When the fabric and resin are selected, it is proportional to the flow rate of the resin.

Taking a product with an area of ​​3㎡ 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 back pressure 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 injection from the center often takes more than 6 minutes. 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 flow rate of the resin in the fabric also slows down, the back pressure also decreases, the injection flow rate can be increased, and the injection time can be shortened.

Experiments show that for a 0.2㎡ product, the injection time from the periphery is 2.1 minutes, while the injection from the center is 9 minutes, a speed difference of 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’ 6㎡ 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 13㎡ hull manufacturing of the Royal Netherlands Navy uses two diagonally arranged resin inlets because the product is large.

 

Product Accuracy, Structure and Others

The dimensional viscosity and repeatability of RTM and RTM-Light products are affected by the resin used for molding, process control and product curing conditions. The cross-sectional accuracy of the product is also affected by the resin flow rate and injection pressure during the process.

For the RTM process, when the mold is manufactured according to the standard, no bending occurs, and the clamping device is suitable or clamped by a press, the dimensional accuracy repeatability of the part is very good, and the thickness deviation is no more than 0.01mm. The RTM-Light process usually has a certain deformation of the upper mold, but the product dimensional accuracy can also reach ±0.02mm, and in some places it is ±0.03mm.

Both RTM and RTM-Light processes can press sandwich materials, and the core material can be balsa ood 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 80Kg/m³, while the pressure of the RTM-Light process is lower, and the foam density used 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 RTM-Light processes can also use preforms and inserts. When preforms are used, products with high fiber volume content can be obtained.

RTM-Light process products do not need to be coated with gel coats. As long as general demoulding wax is used, the product can be demoulded. However, if RTM products do not use gel coats, demoulding is more difficult.

Compared with the open mold process, the investment in RTM-Light is still relatively high, and the rationality of the cost required for configuring the mold must be considered. In addition, the process is highly professional and the daily maintenance tasks are heavy, which also affects the use of RTM-Light by some composite material manufacturers with manual layering.

 

Compared with RTM, RTM-Light has the advantages of low energy consumption and not too high requirements on mold rigidity and other indicators, but it has very high requirements on the viscosity of the resin, the compatibility of the resin and the reinforcing material, and the forward speed of the resin front.

 

Issues to be Noted in the Light RTM Process

The RTM-Light process is a highly professional process, and operators must be properly trained. Without reasonable fiber laying, good airtightness and precise mold installation, and consistent resin flow control, the product will have problems such as messy dry spots, radial bubbles, and resin enrichment.

 

Sealing

The RTM-Light 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 around 15mm/Hg. The vacuum sealing groove for peripheral clamping uses a wing seal profile of flexible neoprene rubber, with a bottom width of 20mm. The seal joint should be cut vertically and glued with a flexible adhesive to ensure its elasticity. The outer ring is then sealed with a 6mm wide silicone rubber edge.

During the molding process, in addition to paying attention to the sealing of the mold vacuum sealing cavity, 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 is usually not discovered. The solution to this problem is to apply a catalyst-modulated resin 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 solvent cleaning should be avoided as much as possible. The mold release agent should preferably be a semi-permanent mold release agent that does not require cleaning.

 

Precision Matching of the Upper and Lower Molds

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

If white spots appear continuously at the same position of the product, this may be due to inaccurate mold closure, resulting in inaccuracy in the inner cavity, which directly leads to uneven thickness of the inner cavity. In this case, assuming that the glass fiber 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 cavities.

Poor mold positioning is an important reason for poor mold fit. 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 injection characteristics will be changed.

 

Reasonable Layering and Raw Material Selection

Due to the low molding pressure, the RTM-Light process has more stringent requirements for the layering of fabrics. Unreasonable layering, especially the treatment of lap joints, will seriously affect the consistency of the resin flow channel. This will cause resin enrichment or cavity missing (dry spots) in the product.

During the layering, the fabric can be fixed with glue spraying to make the layer smoother. However, the glue spraying must be compatible with the resin used. Excessive glue spraying will still have a certain impact on the final performance of the product.

Different fabrics and felts have a great impact on the process. Try to use reinforcing materials with good permeability, such as closed mold felt, whose resin flow rate can be twice as fast as that of ordinary chopped felt.

The appropriate resin system should be selected according to different product requirements. Try to use low viscosity, low shrinkage resins that are comparable to the requirements of the vacuum resin diffusion process.

 

Surface Cracks

Surface cracks are often observed in the corners. This is a common problem in resin-rich areas and can be traced back to mold manufacturing. If the two mold halves do not fit well together, an excess thickness that exceeds the expected thickness will be produced. To solve this problem, in addition to correcting the mold halves, additional glass fiber can be added to compensate for the thickness in these thicker areas to prevent cracks in the product.

In large flat areas, cracks on the mold caused by excessive thickness of the product will be found. This is caused by the operator arbitrarily increasing the injection speed of the resin. Injection speed is too fast, which 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 too much resin will cause cracks in the mold. In extreme cases, irreparable mold surface cracks will occur.

 

Resin Overflow

Many manufacturers feel the need to use a larger resin collector to receive the resin that is discharged before the end of the molding process. This is the result of their inability to accurately control the mold filling. If the resin filling process proceeds too quickly, 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 it with a larger resin collector to prevent resin overflow.

The simple way to solve this problem is to calculate how much resin to use in advance. However, when molding large products, it is difficult to know exactly how much resin to use. Another way 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, avoiding 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 resin waste and ensures increased profits.

If resin overflows in the vacuum zone at the edge of the mold, there may be several reasons:

 

①Poor Sealing of the Sealant

When the path of the sealing cavity is not in a horizontal position, resin leakage will occur. If there is a raised part on the panel of the mold, it is necessary to ensure that the sealant in this part has enough pressure to completely seal it. A better method is to install a dynamic rubber strip, which can adapt to the sealing surface on the complex edge path. When the mold is closed, this rubber strip can expand slightly to achieve sealing.

 

②Improper Placement of Fibers in the Mold

When the fibers are placed in the mold, they are partially too thick or not trimmed sufficiently, so that the fibers cannot be completely placed in the mold, and some fibers are on the path of the sealant. These will directly lead to resin leakage, so the fiber layering must be carefully checked before each molding.

 

③Excessive Injection Pressure

Another common reason for the leakage of resin from the sealant strip is that the pressure in the injection pipe is too high during the injection process. The mold closure of RTM-Light is only based on the atmospheric pressure, and the resin injection machine can generate more pressure than actually needed. Without sensitive and accurate pressure control, resin will leak into the vacuum area, which will lead to the loss of vacuum around the mold and the failure of mold making.

 

④ Loss of Vacuum Seal

Whether it is long-term or temporary edge vacuum loss, it will cause resin leakage, so do not connect several molds under the same vacuum system, because this will lead to a decrease in the vacuum degree of the main vacuum area. In fact, a simple automatic vacuum lock valve can help reduce the possibility of this vacuum loss

 

Application Fields of Light RTM

There are not many reports on RTM-Light technology and products in China, but its application in foreign countries is developing rapidly and has a tendency to exceed the application of RTM technology. At present, 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 and upper decks in the shipbuilding field, etc.

 

Such as: 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 and upper decks in the shipbuilding field.