NEWS

Glass felt reinforced thermoplastic (GMT) and GMT fiber composites have high flowability, enabling low-cost production of thin-walled automotive components. The impact resistance, mechanical strength, and hardness of these components have been improved.

In Europe, glass felt reinforced thermoplastic (GMT) made of polypropylene (PP) is mainly used in components with large surface areas in the automotive industry. In 1999, the processing volumes in Europe and globally were approximately 34000 tons and 60000 tons respectively. More than half of the GMT material components are noise shielding, front and interior structures.

In the future, cars will become lighter and cheaper, while also meeting stricter safety, comfort, and design requirements. This also requires more use of polymers and composite materials in high-strength structural components. GMT materials are an important technology in this regard. The seats, dashboard, etc. are now made by GMT. They must be able to absorb a considerable amount of load and a portion of the impact energy generated during collisions. GMT not only provides excellent performance (modulus, strength, impact resistance, density, etc.), but also relies on the flow molding process to easily machine complex parts, which is one of its advantages and makes it possible to produce large surface area structural parts at low cost and in large quantities. In the past few years, GMT has made many developments and progress, which can meet current and future needs.

Improvement of Traditional GMT

The economic production of GMT large parts through the flow molding process requires semi-finished materials with excellent flowability. The flowability of GMT is determined by the properties of the polymer matrix (viscosity, crystallization temperature, thermal conductivity, etc.), and on the other hand, it is determined by the properties of the fiber felt (fiber content, fiber length, etc.). Changing these parameters to improve the fluidity of GMT often brings other characteristics. For example, significantly reducing the average molecular weight or fiber length of PP can have adverse effects on mechanical properties.

In addition, when using non optimal formulations, it can also have adverse effects on other important properties, such as the suitability of bonding and painting, heat resistance, and long-term stability.

Liquidity is measured using squeezing energy. It is possible to reduce the extrusion energy from 100% to 53% by improving the PP matrix. The material used here is PP copolymer, which has adjusted molecular weight and molecular weight distribution. An appropriate additive system is also used to improve fluidity. By improving the glass fiber felt, the extrusion energy can be further reduced to 30% of the initial value.

The significant improvement in the fluidity of the material did not cause any significant decrease in mechanical properties. The tensile modulus, tensile strength, and impact strength remain the same as standard GMT. With these new high flow materials, it is possible to manufacture thinner and lighter parts. Correspondingly, the same wall thickness can also be produced, requiring less mold pressure. The first trial production of noise shielding parts has shown that the weight of the parts can be reduced by 30% without the need for production process parameters for this new GMT product.

Intelligent composite material system

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

Taking the bumper of Peugeot 806 as an example, we can believe 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 both 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-fashioned ones.

Another application that combines GMT and fabric technology to achieve mass production is the Audi A4 soundproofing device, made of polyester reinforced GMT. This composite is extremely easy to form and exhibits excellent impact performance. This material is particularly suitable for absorbing vibrations and impacts generated on extremely poor road surfaces. And when encountering collision situations, the fabric will hold the components together, so there will be no debris present. As described in the material concept, the fabric structure will produce stripes on the surface of the parts. Although this is acceptable in some design cases, post processing is necessary in most applications.