Lightweight Resin Transfer Molding Process

 

RTM LRTM Process Actual Production Application

 

Introduction to Light RTM Process

 

The traditional RTM process, as a closed mold process, has the advantages of reducing volatile organic compound (VOC) emissions (can reach below SPPm), expanding the range of available raw materials, reducing labor, environmental friendliness, 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 not to be damaged or deformed under the injection pressure.

Usually, sandwich composite materials with steel pipe support, or aluminum molds 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 RTM technology on large products, otherwise the mold will become very heavy. And the investment will also be very large.

The light resin transfer molding process (RTM-Light) is also called 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 a trend of surpassing RTM process.

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. A double-channel sealing belt constitutes 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 RTM-Light includes resin injection device, mold, vacuum device and other auxiliary tools. (Figure 1)

 

 

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. 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 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 and as large as an 8m long hull (about 25m2), 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 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 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 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 to 10,000 times.

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

 

Injection Pressure, Flow Rate and Equipment

The injection pressure of RTM process is generally 0.1-0.4MPa, while the injection pressure of RTM-Light 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, part size, fiber type and layer structure, and the usual injection rate is 1.3-2L/min.

To prevent mold deformation or punching open the upper mold (especially at the injection port), this requires a stricter control of pressure. Injection equipment used for RTM-Light process generally has a pressure feedback device to perform closed-loop control of 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-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 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 low mold cost, does not require complex clamping devices or pressure systems, has low investment, simple operation, and saves 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.

In the past, the production speed of the RTM-Light process was half that of the RTM process. For an 8-hour shift, for a process using a gel coat surface and a non-heated mold, the RTM process can produce 10 to 12 molds per shift, while the RTM-Light process can only produce 4 to 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, a heated mold, and 5 replaceable molds. The RTM-Light process in the same situation can produce 20 molds. However, there is no need for a hydraulic press, and the mold price is also half as low.

In recent years, through improvements in mold design and process control, the production speeds of the two have been close.

For example, after Xiraplas adopted the RTM-Light process to replace the open mold process, the workshop became orderly, and the production increased by 25% using the original 50 workers and 3,000m2 workshop area. According to the company, the production efficiency has increased by 90% compared with the original open mold process.

 

Runner Design

Generally, the runner design of the RTM process is to inject from the center and discharge from the periphery. However, 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 magnitude 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 ​​3M2 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 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 relative flow rate of the resin in the fabric is also slowed down, the back pressure is also reduced, the injection flow rate can be increased, and the injection time can be shortened.

Experiments show that for a 0.2m2 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. For example, Spectrayte’s 18m long lamp column uses a long flow channel. Brands’ 6m2 floor slab, 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 13m2 hull of the Royal Netherlands Navy was manufactured with two diagonally arranged resin inlets because the product was 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. 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 a suitable clamping device or clamping with a press is used, the dimensional accuracy repeatability of the part is very good, and the thickness deviation is no more than 0.OlOmm. The RTM-Light 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 RTM-Light 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 80Kg/m3, while the pressure of the RTM-Light and ZIP RTM processes is lower, and the density of the foam used can be as low as 37Kg/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 high professionalism of the process and the heavy daily maintenance tasks also affect 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 high requirements on resin viscosity, compatibility of resin and reinforcement materials, and the forward speed of the resin front.

 

Issues to be noted in the Light RTM process

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. The following is a brief explanation of the relevant issues.

 

Sealing

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 uses a flexible chloroprene rubber wing seal (Wing seal profile), the bottom width of which is 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 sealant, attention should also be paid to the mold assembly, the connection between the seal ring and the pipe, and the leakage caused by the mold cracks. In fact, any seal or joint at the resin inlet, including the outlet of the vacuum zone, should be strictly checked.

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 resin prepared with a catalyst to the outer surface of the mold before the vacuum is achieved inside the mold. 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 to balance the pressure of the cavity in the mold, make the resin penetrate evenly, and help improve the quality of the product. 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 the thinner area of ​​the inner cavity.

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 on the layering of fabrics. Unreasonable layering, 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) in the product.

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

Different fabrics and felts have a great impact 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 felt or “Hi-Flow” composite felt can be twice as fast as that of conventional chopped strand felt.

The appropriate resin system should be selected according to the different product requirements. Try to use low viscosity and low shrinkage resins. Its standards are equivalent to the requirements of vacuum resin diffusion process.

 

Surface Cracks

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 half molds do not fit well with each other, a margin exceeding the expected thickness will be generated. To solve this problem, in addition to correcting the half molds, additional glass fiber can be added to make up 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. If the injection speed is too fast, the inner cavity of the mold will 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 cracks will occur on the mold surface.

 

Resin Overflow

In order to receive the resin discharged before the end of molding, many manufacturers feel it is necessary to use a larger resin collector. This is the result of their inability to accurately control the molding filling. 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 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. But 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 operation, 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 overflow occurs in the vacuum zone at the edge of the mold, there may be several reasons:

 

Poor Sealing of the Sealant

When the sealant path 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. 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.

 

Improper 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 in the path of the sealant, which will directly lead to resin leakage, so the fiber layering should be carefully checked before each molding.

 

Excessive 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. The resin injection machine can generate more pressure than actually required. If there is no more sensitive and accurate pressure control, the resin will leak into the vacuum area, which will lead to the loss of vacuum around the mold and the failure of mold making.

 

Loss of Vacuum Seal

Whether it is long-term or temporary marginal vacuum loss, it will cause resin leakage. So don’t 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

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.