Brief Introduction of GMT Processing Technology

Glass Mat Reinforced Thermoplastic (GMT) refers to a novel, energy-saving and lightweight composite material that uses thermoplastic resin as a matrix and glass fiber mat as a reinforced skeleton. It is currently an extremely active composite material in the world. The development of materials is regarded as one of the new materials of the century and is widely used in automobiles, building construction, ships, electrical and electronic fields.

The reinforcing fiber of GMT can be chopped glass fiber or continuous glass fiber mat, and the thermoplastic resin matrix can be general plastics, engineering plastics or high-performance plastics. GMT can generally produce semi-finished sheet products, and then directly process them into products of the desired shape.

Similar to thermoset SMC, GMT is a semi-finished product of sheet-shaped thermoplastic glass fiber composite material, and its manufacturing methods include melt dipping and suspension dipping.

The melt impregnation method is a GMT production method developed by the American company PPG in early 1970s. The main process is to sandwich two layers of glass fiber mats between three layers of PP resin. The middle layer is molten PP, and the top layer and the bottom layer can be either PP film or melted PP. Put this sandwich structure below the melting point of PP, compress it with a crawler press, then cool it, and then cut it into different specifications.

When using this manufacturing process to produce GMT materials, the types of glass fibers are different, and so are the properties of GMT materials. If the properties of the sheet need to be isotropic, the fiber orientation in the glass fiber mat can be completely arbitrary; if the properties of the sheet need anisotropy, the fibers in the fiber mat are arranged in the desired direction. If only low fluidity is required for the product, then either normal chopped strand mat or continuous fiber mat can be used. If higher fluidity is required, special needled felt made of glass fiber non-twisted roving should be used.

This method is represented by the American company Azdel of PPG, whose glass fiber is generally 20% to 45% (weight ratio).

The suspension dipping method was developed by the French company Arjomari and the British company Wiggins Teape. The GMT material produced by this method is characterized by its good fluidity and is particularly suitable for the manufacture of components with complex geometric shapes and thin-walled structures.

Process flow: Mix the 6~25mm long chopped glass fiber strands with thermoplastic powder, latex, flocculants etc. in water. The raw materials are suspended and fall onto the conveyor belt to filter out water; the dried sheet is heated and pressurized at a temperature higher than the melting point of the thermoplastic resin, then is cooled and consolidated and is cut into a certain length.

The characteristic of GMT material by papermaking method is that the glass fiber is evenly dispersed in the whole sheet. According to the performance requirements of the end product, sheets of different density and thickness can be made. The flexibility of the production process and the diversification of products are the main advantage of this process.

In order to make the product with higher rigidity or hardness, better mechanical properties, surface appearance, flame retardancy or EMI shielding performance that meets automotive requirements, any kind of fiber and additives can be mixed. To produce EMI/FRI shielding materials for office equipment, adding conductive fibers can achieve the best performance-price ratio. In addition, the excellent flow properties of this GMT make it an ideal material for the production of complex geometric shapes and thin-walled structures.

This method is represented by Azdel, Arjomari, Nippon Steel Group, and domestic Harbin FRP Research Institute. The weight ratio of glass fiber is 20% to 70%.

GMT sheets are usually formed into products using high-speed presses or punching machines. The GMT sheet is firstly cut into pieces according to the size of the product. Preheat it at a temperature 20°C higher than the melting point of the thermoplastic matrix, put it into the mold, quickly close the mold and pressurize, then cool and release and finally it is trimmed into products. The pressure is generally 10-20 MPa, the pressing speed is 30-100mm/s, and the mold temperature is normally room temperature or 60-80°C.

The characteristics of this process are: short cycle of production, low pressure and high automation.

GMT has the function for complex design and excellent impact resistance. The products are easy to assemble and reprocess, and are highly praised for their strength and lightness, making it an ideal structural component to replace steel and reduce mass.