Light Resin Transfer Molding (L-RTM) for Close Mold Process

 

1. Introduction to L-RTM process

 

The traditional RTM process, because it is a closed-mold process, has the advantages of reducing volatile organic compounds (VOC) emissions (up to SPPm or less), 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.

 

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. Currently, its application in the fields of ships, automobiles, industrial and medical composite materials has surpassed the RTM process.

 

The RTM-Light process retains the mold matching process of the RTM process, thereby retaining almost all the advantages of the RTM process. However, the upper mold is a semi-rigid fiberglass mold with a thickness of generally 6 to 8 mm. It usually does not need to be reinforced with steel pipes. The mold has a rigid periphery about 100 mm wide and consists of a double sealing tape to form an independent sealing area. The vacuum mold is closed immediately, which is very convenient and fast. Then the mold cavity is evacuated, 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 can penetrate into the pre-laid reinforced fibers or preforms. RTM-Light’s mold cost 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 the mold from the open mold process.

 

The main equipment of RTM-Light includes resin injection device, mold, vacuum device and other auxiliary tools.

 

2 Comparison Between Light RTM 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 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 in terms of how to deal with 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 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, production efficiency is greatly improved because the gel coat can be directly applied and heated on the exchangeable mold skin outside the mold.

 

 (2) Injection Pressure, Flow Rate and Equipment

 

The injection pressure of the RTM process is generally 0.1-0.4MPa, while the injection pressure of the RTM-Light process is generally no 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 1.3-2 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 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 multiple uses. Plastech’s SSB injection equipment uses a patented piston-modified precision metering pump, with a catalyst minimum ratio of 0.5%. With industrial MPG (Mould pressureguard), the machine can control the pump speed automatically. In 12 to 15 seconds, 1m2 of reinforcement material can be impregnated, and the impregnation speed can be precisely controlled. Other options can also be used for hand lay-up process glue preparation and glue brushing.

 

 (3) Production Efficiency and Cost

 

RTM-Light process is a low-cost production technology. Compared with traditional RTM technology, it has low mold cost, no need for complex clamping devices or pressure systems, small 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. Based on 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, heating the mold, and having 5 replaceable molds. The RTM-Light 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.

 

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 the RTM-Light process to replace the mold opening process, the workshop became orderly, and the production increased by 25% with the original 50 workers and 3000m2 workshop area. According to the company, compared with the original mold opening process, the production efficiency has increased by 90%.

 

(4) 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 the injection is 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 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 Royal Netherlands Navy’s 13m2 hull manufacturing used two diagonally arranged resin inlets due to the large size of the product.