Overview of GMT materials, molding and molds

Overview of GMT materials, molding and molds

GMT material properties:

GMT (Glass Mat Thermoplasitcs), glass fiber mat reinforced thermoplastic. GMT is produced on huge laminators using thermoplastic resins and one or more reinforcement technologies. This process produces a sheet-like lightweight, partially consolidated, semi-finished composite material. Most GMT sheets have a width of 1300 mm to 1400 mm and a thickness of 2.5-5.4 mm. GMT's unique long-fiber technology ensures high energy absorption before fragmentation. The main purpose of this semi-finished product in the form of a sheet is to replace metal (steel, aluminum, etc.) and is used in the automotive industry. Typical products include passenger car back beams and chairs. The backrest and engine bottom cover.

GMT fiber composites have high flow properties, enabling the production of thin-walled automotive components to achieve low cost, and the impact resistance, mechanical strength and hardness of these components have been improved.

GMT can be recycled. But compared with SMC and BMC, only GMT material needs to be preheated. GMT compression molding is heated to about 170°C and the material is placed in the mold. Then, it is compressed by a press to form a shape. GMT can produce large parts with lower mold costs and shorter cycle times. The potential of part consolidation provides opportunities for further weight reduction and cost reduction.

GMT molding advantages:

lThe pressure is from 500 to 40,00 tons, which can work under very high pressure

lThe maximum pressing plate size is 3,000 X 2,500 mm

lOpening 3,000 mm

lStroke 2,000 mm

lQuick closing speed 600mm/s

l Working speed 20 mm/s

lQuick opening speed 600mm/s

lThe cycle time ranges from 45 seconds for structural parts to 120 seconds for the most technical parts.

GMT mold:

It is a cavity tool with a certain shape and size. It is used in conjunction with various systems or auxiliary mechanisms in the mold to fill various high-temperature liquid GMT materials into the mold cavity to produce a specific shape and size. , Function and quality of GMT parts.

In Europe, glass mat reinforced thermoplastics (GMT) made of polypropylene (PP) are mainly used in components with large surface areas in the automotive industry. In 1999, European and global processing volumes were approximately 34,000 tons and 60,000 tons, respectively. More than half of the GMT material parts are noise shielding, the front part of the car and the car interior structure.

Future cars will become lighter and cheaper, and at the same time they will have to withstand stricter safety, comfort and design requirements. This also requires more use of polymers and composite materials in high-strength structural parts. In this regard, GMT materials are an important technology.

Today's seats, dashboards, etc. are all made of GMT. They must be able to absorb considerable load and part of the impact energy generated in a collision. GMT not only provides excellent performance (modulus, strength, impact resistance, density, etc.), but also depends on the flow molding process, it can be easily processed into complex parts, which is one of its advantages, and it also enables low-cost, high-volume It is possible to produce large surface area structural parts. In the past few years, GMT has made a lot of development and progress to meet current and future needs.

Perfection of traditional GMT

The economical production of GMT large-scale parts through the flow molding process requires semi-finished materials with excellent fluidity. The fluidity of GMT is determined by the properties of the polymer matrix (viscosity, crystallization temperature, thermal conductivity, etc.). On the other hand, it is determined by the properties of the fiber mat (fiber content, fiber length, etc.). Changing these parameters to improve the liquidity of GMT often brings other characteristics. For example, if the average molecular weight or fiber length of PP is greatly reduced, the mechanical properties will be adversely affected.

In addition, when using a non-optimized formula, it will also have an adverse effect on other important properties, such as the suitability of adhesion and painting, heat resistance and long-term stability.

Fluidity is measured by squeezing energy. If the PP matrix is improved, it is possible to reduce the extrusion energy from 100% to 53%. The material used here is a PP copolymer, which has a well-adjusted molecular weight and molecular weight distribution. An appropriate additive system is also used to improve fluidity. By improving the glass fiber mat, the squeezing energy can be reduced even more to 30% of the initial value.

The significant improvement in the fluidity of the material did not cause any drastic reduction in mechanical properties. The tensile modulus, tensile strength and impact strength remain the same as those of the standard GMT. With these new high-flow materials, it is possible to make thinner and lighter parts. Correspondingly, the same wall thickness can also be manufactured, and the required molding pressure is smaller. The first trial production of noise shielding parts shows that the weight of the parts can be reduced by 30% without the need to adopt the production process parameters for this new GMT product.

Intelligent composite material system

GMT is provided to molding manufacturers in sheet form. This means that manufacturers can mix reinforcing fibers or non-woven reinforcements. The glass/PP fiber mixture is applied as the outer layer on the traditional random glass GMT. This semi-finished product can be processed on a standard GMT processing line, just like a standard GMT. By improving the hardness, strength and impact strength of the inlaid area, the properties of the fabric can be changed to suit the application. When using traditional random glass GMT, the mold is filled from the center of the semi-finished part.

Take the bumper of Peugeot 806 (parameter | picture) as an example. We can think that using this material can save costs and reduce weight, but the price of semi-finished products is slightly higher. The central part of the bumper is made of GMTex, while the two sides are made of traditional GMT. With this material concept, it is possible to eliminate steel energy absorbers, and the weight of the entire system is 2kg lighter than the old ones.

Another application that comprehensively utilizes GMT and fabric technology to achieve mass production is Audi A4 (parameter | picture) sound insulation device, which is made of polyester-reinforced GMT. This composite is extremely easy to shape, and at the same time exhibits excellent impact properties. This material is particularly suitable for absorbing shocks and shocks generated on extremely poor roads. And in the event of a collision, the fabric will hold the elements together, so no debris will appear. As mentioned in the material concept, the fabric structure will produce stripes on the surface of the part. Although this is acceptable in some design situations, post-processing is necessary in most applications.

Therefore, a different method is adopted, that is, the GMT with a special surface is laminated on the viewing surface. Using this type of semi-processed parts or composites can improve part performance, such as impact performance, hardness and strength, and adjust to application requirements. In the future, as an example, we can see that this material can meet much more stringent impact requirements.

Basic research shows that for future rear-seat doors, it is possible to significantly improve the mechanical performance while maintaining the same weight and cost of semi-finished parts, or it is possible to reduce the weight and cost while maintaining the original characteristics. More in-depth testing and research on the parts confirmed these results. All investigations have shown that this material can be used in traditional molds and processing lines, and has obvious and easy-to-adjust improvements in mechanical properties. Molds with smooth edges and ribs are also filled without any problems. The fabric is kept in the cross section and will not migrate to the surface.

Volvo FH12 truck dashboard uses aramid fabric as reinforcement at knee height. This ensures that the driver’s feet are protected in the event of an accident. The properties of this composite material, such as hardness, impact strength, etc., are specifically designed for this particular application. The current reinforcement is formed by sealing the fabric in PP through injection molding in a separate process, and then using GMT compression molding. A study showed that fabrics can be applied directly to GMT. Cost savings can be achieved by reducing process steps. When designing parts using these materials, it is necessary to leave a processing margin for performance anisotropy and thickness dependence. When processing this material, some of the random fibers are squeezed out of the middle layer and fill the part. This increases the fabric content in the inlaid area, leading to improved mechanical properties in this area.

Other applications

Building templates made of reinforced thermoplastics have considerable advantages over steel templates. In addition, GMT is also widely used as a container, trestle, anti-corrosion, and electrical material in transportation and other sectors.

Conclusion and outlook

The potential of GMT and plate-shaped semi-finished parts made of fiber-reinforced plastics is far from being discovered. New material concepts emerge in endlessly, making it possible to produce large-area structural parts with low cost and high yield. This means that a wide range of thermoplastic, fiber-reinforced semi-finished products with diverse properties are already available. At the same time, some special products, such as colored GMT and natural fiber reinforced plastics (NMT) are also available.

In the near future, a large number of traditional GMT applications will be replaced by low-cost technologies and materials, such as LFT (Long Fiber Reinforced Plastic). Compared with standard GMT, LFT has its shortcomings, especially in terms of impact strength. In order to provide potential LFT users with competitive semi-finished products, low-cost GMT products are currently being developed, which will help reduce the cost of parts. The development work that has been carried out includes special products like lightweight GMT and flame-retardant GMT, which opens up larger application areas for GMT.