Advantages of vacuum forming:

by:Mengxing     2021-04-09

① Use glass fiber roving instead of fabric, which can reduce material cost;

②The production efficiency is 2 to 4 times higher than that of hand lay-up;

③Good product integrity, no joints, high interlayer shear strength, high resin content, good corrosion resistance and leakage resistance;

④It can reduce the consumption of flashing, cutting scraps and remaining glue;

⑤The product size and shape are not restricted.

Its disadvantages are: ① high resin content, low product strength;

②The product can only be smooth on one side;

③ Pollution of the environment, harmful to the health of workers.

The injection molding efficiency is 15kg/min, so it is suitable for large-scale hull manufacturing. It has been widely used in processing bathtubs, machine covers, integral toilets, automobile body parts and large relief products.

(2) Production preparation

Site In addition to meeting the requirements of hand lay-up, the spray molding site must pay special attention to environmental exhaust. According to the size of the product, the operation room can be built into a closed type to save energy.

Material preparation

The raw materials are mainly resin (mainly unsaturated polyester resin) and untwisted glass fiber roving.

Mold preparation

Preparations include cleaning, assembly and application of release agent.

(3) Injection molding process control

Selection of spraying process parameters:

① Resin content In injection molded products, the resin content is controlled at about 60%.

②Spray pressure When the resin viscosity is 0.2Pa・s and the pressure of the resin tank is 0.05~0.15MPa, the atomization pressure is 0.3~0.55MPa to ensure that the components are evenly mixed.

③Included angle of the spray gun The mixing distance of the resin sprayed from different included angles is different. Generally, the included angle of 20° is selected, and the distance between the spray gun and the mold is 350-400mm. To change the distance, the angle of the high-speed spray gun is required to ensure that the components are mixed and mixed near the surface of the mold to prevent the glue from flying away.

Matters needing attention in injection molding:

① The ambient temperature should be controlled at (25±5)℃, too high, it is easy to cause blockage of the spray gun; too low, uneven mixing, slow curing;

②No water is allowed in the jet system, otherwise it will affect the product quality;

③Before molding, spray a layer of resin on the mold, and then spray a resin fiber mixed layer;

④ Before injection molding, adjust the air pressure to control the resin and glass fiber content;

⑤The spray gun must be moved evenly to prevent leakage of spray, and no arcs. The overlap between the two rows is less than 1/3, and the coverage and thickness must be uniform;

⑥After spraying a layer, immediately compact it with a roller, pay attention to the edges and uneven surfaces, ensure that each layer is flat, discharge air bubbles, and prevent burrs from being brought up by fibers;

⑦After each layer is sprayed, check it, and spray the next layer after it is qualified;

⑧The last layer should be sprayed thinly to make the surface smooth;

⑨ Clean the sprayer immediately after use to prevent the resin from curing and damaging the equipment.

Resin Transfer Molding Resin Transfer Molding is abbreviated as RTM (Resin Transfer Molding). RTM started in the 1950s and is a closed-mold molding technology improved by hand lay-up, which can produce double-sided products. In foreign countries, there are also resin injection technology (Resin Injection) and pressure injection technology (Pressure Infection). The basic principle of RTM is to place the glass fiber reinforced material in the cavity of the closed mold, inject the resin glue into the cavity with pressure, soak the glass fiber reinforced material, and then solidify and demold the molded product.

(1) RTM process and equipment

Molding process The entire production process of RTM is divided into 11 processes (as shown in the figure). The operators, tools, and equipment of each process are in fixed positions. The mold is transported by a trolley and goes through each process in turn to achieve streamlined operation. The cycle time of the mold on the assembly line basically reflects the production cycle of the product. Small products generally only take more than ten minutes, and the production cycle of large products can be controlled within 1h.

Molding equipment RTM molding equipment is mainly resin injection molding machine and mold.

①Resin injection molding machine The resin injection molding machine is composed of a resin pump and an injection gun. The resin pump is a set of piston-type reciprocating pumps, with an aerodynamic pump at the top.

②Moulds RTM molds are divided into three types: glass steel molds, glass steel surface-plated metal molds and metal molds. FRP molds are easy to manufacture, and the price is low. Polyester FRP molds can be used 2000 times, and epoxy FRP molds can be used 4000 times. The metal-plated glass fiber reinforced plastic mold can be used more than 10,000 times. Metal molds are rarely used in the RTM process. Generally speaking, the mold cost of RTM is only 2% to 16% of SMC.

(2) RTM raw materials

The raw materials used in RTM are resin systems, reinforcing materials and fillers.

Resin system The resin used in the RTM process is mainly unsaturated polyester resin. Reinforcing materials Generally, the reinforcing material of RTM is mainly glass fiber, and its content is 25%-45% (weight ratio); commonly used reinforcing materials are glass fiber continuous felt, composite felt and checkered cloth.

Filler Filler is very important to the RTM process. It can not only reduce costs and improve performance, but also absorb heat during the exothermic phase of resin curing. Commonly used fillers are aluminum hydroxide, glass beads, calcium carbonate, mica and so on. The dosage is 20%-40%.

Bag pressing method, autoclave method, hydraulic kettle method and thermal expansion molding method are collectively referred to as low pressure molding process.

(1) Bag pressing method

Bag compression molding is the uncured product formed by hand lay-up, and gas or liquid pressure is applied to it through a rubber bag or other elastic material to make the product compact and solidify under pressure.

(2) Autoclave and hydraulic autoclave method Both autoclave and hydraulic autoclave method are a process in which uncured hand lay-up products are heated and pressurized by compressed gas or liquid in a metal container to solidify and shape. (3) Thermal expansion molding method

Thermal expansion molding is a process used to produce fasting, thin-walled high-performance composite products. Its working principle is to use mold materials with different expansion coefficients, and use the squeezing force generated by different heated volume expansions to apply pressure to the product. The male mold of the thermal expansion molding method is silicone rubber with a large expansion coefficient, and the female mold is a metal material with a small expansion coefficient. The uncured product is placed between the male mold and the female mold. When heating, due to the different expansion coefficients of the male and female molds, there is a huge difference in deformation, which makes the product solidify under hot pressure.
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