New low pressure resin transfer molding injection machine

Resin transfer molding (RTM) is a promising automated molding method. In the past 10 years, China has introduced more than 70 RTM machines at prices ranging from 8,000 to 15,000 U.S. dollars. These devices are all piston-type structures. They use reciprocating cylinders as the pressure source, and the resin pump and the initiator pump are linked to each other. The dosage of the agent is injected into the injection gun and then mixed into the mold through the static mixer. Compared with the function of the new low pressure premix molding injection machine developed by Tongji University, there are many defects.

1. Six Defects of Piston Molding Machines During the course of use, researchers at Tongji University found that the piston RTM machines had the following six defects.

(1) The injection pressure is not smooth, there is a pulsating conveying phenomenon. The piston rod of the resin pump is directly connected with the piston rod of the cylinder and works with the piston rod of the cylinder moving up and down in a straight line. Therefore, the ratio of the ratio of the positive area of ​​the two pistons to the inverse area is not equal. When the piston moves up and down, the injection pressure of the resin varies. At the same time, the pressure output by the resin pump is also pulsating.

(2) According to the mixing ratio principle, sometimes the product appears a non-linear curing profile. Because the metering ratio of the resin to the initiator pump is relatively large (about 20:1), and there are pressure and flow fluctuations, the curing agent is inevitable. The ratio fluctuates, and the difference between the resin gel time and the cure time results in a nonlinear cured profile. The color of the cured resin can be seen by the eye to have a slight color difference.

(3) Filler should not be added in general resin, and it is easy to cause equipment failure. Filler will increase piston wear and shorten the life of the machine.

(4) The stall backpressure under normal injection pressure will cause the mold to overload. Usually open the gas source valve (gas source pressure is generally 0.8 ~ 1 MPa) before injection, and then open the pressure regulator to start the RTM machine, and The pressure required for injection is about 0.2-0.4 MPa waiting for injection. Herein, this article particularly points out that some technicians described the RTM machine as a low-pressure resin transfer molding system in introducing the related RTM article. Such a proposal is not appropriate and is not practical.

(5) The amount of material loss is large to make the piston injection machine to inject the product normally. Starting from the resin pump suction pipe, to the pump body cavity and the output pipe to the injection gun, they must be filled with resin before they can enter the product injection. If the equipment is not used for one week, the entire resin pump system should be flushed with solvent, requiring approximately 8 liters of solvent.

(6) Only for unsaturated polyester systems The piston RTM machine is designed specifically for unsaturated polyester systems. The ratio of the resin pump to the initiator pump is limited in the metering range, and the resin viscosity is not suitable for large viscosity and curing agents. Higher proportions of other resins such as epoxy.

In view of the above defects, and the high number of wearing parts of such RTM machines, and depending on the current situation of imports, it is very important to develop a new type of domestic low-pressure RTM injection molding machine. In recent years, researchers have concentrated on developing a new type of low pressure premix molding injection machine that has gained market recognition.

Second, the function of a new type of low pressure premixed molding injection machine The new type of low pressure premixed RTM injection machine is based on the design of a low pressure premixed single injection RTM machine. The injection system uses a pneumatic control circuit design to achieve semi-automatic control. The new RTM machine is designed according to the compound mold heating to 70 °C ~ 80 °C, premixed resin life (gel time) is 20 ~ 60min.

(1) Injection tank and control device The injection tank and control device are the core parts of this machine. During use, the system pressure status and the actual injection pressure display can be directly seen from the control panel, and the injection pressure can be adjusted at any time.

(2) Feed tank feed tank is a pressure vessel for delivering premixed resin to the injection tank. It can store 18 liters of resin for injection. The tank is also equipped with a manual deflation safety valve, which is used to check the exhaust before opening the lid and to protect the safety overflow from use.

(3) Solvent tank The function of the solvent tank is to provide the system with solvent (acetone) for cleaning. The solvent tank can store 8 liters of acetone. Add solvent once for multiple use. Similarly, it also has a manual deflation pressure relief valve and a simple can lid opening and closing lock bolt.

(4) Cantilever Swivel Frame The cantilever swivel frame consists of a column, a tripod, and a pendulum. It has the advantage of being flexible and convenient and does not occupy floor space.

Third, the use of a new type of low pressure premixed injection molding machine compared with the piston RTM machine, the new low pressure premixed RTM injection machine has a low injection pressure, stable pressure, can be added to the filler content of 100%, injectable epoxy, phenolic, etc. With other thermosetting resins, low stall pressures will not cause overloading of the mold, less material loss, easy maintenance, etc. The investment in equipment is only 1/3 to 1/5 of that of imports. The new RTM machines are all controlled by pneumatic components and meet explosion-proof requirements. The cantilever rotary rack makes the operation space large and flexible. It is ideally suited for the development of new products for research institutes and companies, or the production of small batches and varieties of RTM products, as well as mass production of RTM products with a weight of less than 5 kg.