They are a challenge in processing but the ideal material for components that must remain dimensionally stable under demanding conditions in the long term: granular thermosets. High fiber content and batch fluctuations complicate shot weight consistency - therefore, KraussMaffei now offers the APC plus machine function for thermosets as well.
Brake pistons and components for oil pumps have to withstand high stress. Temperatures up to 400 degrees Celsius and corrosive media must not impede their proper function. Tolerance is partly in the range of 10 µm only - through the entire service life of the vehicle. Fiber-reinforced thermosets are the ideal material for highly complex technical parts, but they impose specific demands on processing. For instance, processing must take place without a check valve because curing partially takes place in the screw. Furthermore, batch fluctuations are more pronounced than in thermoplastics. Shot weight consistency becomes a challenge.
Therefore, KraussMaffei has further advanced the proven APC plus (APC = Adaptive Process Control) machine function and also offers it for thermosets. APC plus uniformly measures the viscosity of the melt, taking material properties such as the respective compression curve into account, and adjusts the change-over point and the holding pressure level in the current shot based on a predicted injection volume.
High filler contents of up to 80 percent
This is particularly important for high filler content of up to 80 percent. Due to these facts as well as the low processing temperatures, the molding compounds are barely compressible compared to thermoplastics. Minimal deviations of the start of filling, which are not-visible to the machine operators in the process, or of the residual melt cushion have an immediate impact on injected volumes. Without an automated correction option, one is powerless against this.
In contrast, APC plus keeps the volumetric filling of the cavity uniform and does so equally for free-flowing thermosets, BMC (Bulk Molding Compound) and liquid and solid silicones (LSR and HTV). For example, if the barrel temperature rises by two degrees Celsius in BMC processing, which can quickly happen with a drifting tempering unit or in summer heat, and if the plastic becomes less viscous due to that, APC plus switches from to injecting to holding pressure at an earlier point in time. Furthermore, the holding pressure level is lowered. Both are done to prevent component overflow. "Both in numerous lab tests and at the locations of our customers, we have been able to cut the variation coefficient of the component weight by half and thus achieve a significantly increased OK part yield," says Cordula Wieland, Manager Product and Technology at KraussMaffei.
Successful baptism of fire in large-scale thermoset production
APC plus for thermosets has gone through its baptism by fire: Baumgarten automotive technics GmbH, a KraussMaffei customer involved in its development, has been producing oil pump adjusting rings in continuous three-shift operation using it for one year and has been able to reduce scrap significantly.