At German injection moulding specialist, neo-plastic Dr. Doetsch Diespeck GmbH, monitoring the quality of large-scale production of injection moulded parts is not a matter of chance. Using cavity pressure to determine the switchover to holding pressure for process optimization and cavity pressure-based monitoring from Kistler Instruments for quality assurance using both direct or indirect cavity pressure measuring, ensures minimum rejects. The medium-size German company focuses on producing high quality technical components mostly for manufacturers of ball bearings, linear guides and the automotive industry.
Injection moulding of hinge covers is a typical example of seamless in-line quality assurance. These flat, palm-size SEBS parts protect the sensitive electric seat adjustment systems during the production of foamed car seat systems. The seat manufacturer inserts the injection moulded covers into a mould, where they form a very tight bond with the seat during foaming. Although these inserts are installed in a concealed place, they need to be precisely moulded to ensure that they are fully functional.
The injection moulding machine for this project, acquired in 2008, was equipped with a machine control system that provided outputs for pressure signals and integrated cavity pressure monitoring. Each cavity of the 2+2-cavity hot-runner family mould for the production of right-side and left-side hinge covers is equipped with Kistler 2.5 mm pressure sensors.
For other projects, the company also deploys Kistler’s CoMo Injection system. “CoMo is fully configured for analysing and monitoring injection moulding processes. When it comes to direct comparison, machine control systems provide rather limited analysis options,” managing director, Patrick Freiherr von Twinkle reports.
With new, medium-term projects with six or seven-digit annual output rates, neo-plastic operates with cavity pressure technology right from the start. This applies to the production of a small technical PA46 breaker plate with a shot weight of only 3.5 grams. The brand new 8-cavity mould, made by the company’s in-house mould engineering department, is equipped with eight direct 1 mm pressure sensors. Again, the CoMo Injection process monitoring system will control the process by means of cavity pressure-dependent switchover and guarantees the quality of the moulded parts by monitoring the pressure curves. “Without sensors, this project would generate massive problems due to underfed parts. Automatic switchover makes the process significantly more stable.”
Faster setup changes and restarts
How long does it take before the investment in a cavity pressure monitoring system pays off? Von Twickel: “This is hard to pin down. There are many positive influences. Just think of the cost of complaints and the subsequent sorting effort. With the new system, we have removed that risk completely. Cavity pressure dependent switchover also facilitates and speeds up any setup changes: after ten shots with the new mould, the quality is perfect again. During the active production process, lot variations or changes of flow are registered immediately and can be remedied directly. Assuming an out-put rate of 200,000 parts per year, I would expect the system to have paid off after 18 months.”
At neo-plastic, the CoMo Injection monitoring system is not operated in fixed connection with one single machine, but, like the moulds, is flexibly used on several machines of similar size. Everywhere the system is applied, the process achieves stable conditions, no matter whether the machines are electric or hydraulic, and independent of their age.
After several years of experience, von Twickel is able to sum up the benefits of cavity pressure measuring and the integration of Kistler sensors and systems: “I can look into the cavity. That constitutes an unbeatable advantage. I have not encountered any other method that would deliver similar information”, he sums up. “Today, we are working in the mould, not in the machine.”