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

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, front and interior structures.

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 material is an important technology. The 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 and 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 large GMT parts through flow molding 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, greatly reducing the average molecular weight or fiber length of PP will adversely affect the mechanical properties.

In addition, when using a non-optimized formula, it will also have an adverse effect on other important properties, such as the suitability of bonding 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 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 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 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 mold loading pressure is smaller. The first trial production of noise shielding parts showed 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 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 used 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 the Peugeot 806 car 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's sound insulation device, which is made of polyester reinforced GMT. This composite is very easy to shape, while exhibiting 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 bring 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.