Victorinox of Ibach, Switzerland, the famous maker of the Swiss Army knife, and Kistler Instrumente AG of Winterthur, Switzerland have enjoyed a cooperative customer/supplier relationship that has lasted more than 20 years. The very first Kistler sensor and charge amplifier used at Victorinox are still in use.
Originally, Kistler's systems solved machine cycle consistency problems and subsequent part quality troubles in insert molding the nylon cores of the knives around the manually inserted knife blades at Victorinox. Based on its initial successes, Victorinox wanted to mold its knife shells, but the 6-mm diameter Kistler transducers available at the time were too big. As a result, Victorinox switched to indirect (behind-the-pin) force sensors using smaller, 2-mm diameter pins. Since then, Kistler has developed direct 2.5-mm sensors.
When using indirect sensors on the bottom molds in shuttle- or rotary-table insert molding machines, Victorinox noticed differences of up to 725.5 psi (50 bar) in pressure gradients between the molds even when molds with identical sensors were run under identical injection conditions using the same amplifier on the same machine.
By carefully investigating the mounting conditions of its indirect force sensors, Victorinox discovered four problematic sources of error that had never been previously encountered with its direct pressure transducers:
- Sensors were being exposed to pressure before injection when pressure-bearing ejector pins were fitted with too little clearance
- The mold structure was not stable enough, and the sensor fixturing gave way under peak pressure
- The measuring pin jammed in the hole due to deformation or resin residue contaminants
- Sensors were not fitted at the correct angle to the sensor hole
Victorinox was also experiencing unexplained pressure spikes and dips in a new eight-cavity mold equipped with a hot runner manifold. All eight cavities consisted of individual cores on the injection and ejection sides, four of which were bolted onto a common baseplate. Periodically, scrap parts were coming from one cavity or another. Victorinox attributed this to either hot runner system irregularities or mold structure deformations and shift.
So, Victorinox and Kistler developed a complex method to test inmold performance of direct and indirect cavity pressure sensors. One of the eight-cavity molds was equipped with sensors in all eight cavities in the configuration detailed in the accompanying chart. Cavity sensor assignments were selected so there were always two identical sensor systems positioned diagonally opposite each other in the mold. This configuration also ensured an identical measuring system was present in each of the runner manifolds.
In every test, Victorinox found the two direct pressure sensors fitted with mounting nuts produced the best results. The indirect force sensors produced large differences, particularly in holding pressure, with the larger 3.5-mm diameter pins worse than the smaller 2.0-mm ones. Victorinox now believes only direct pressure transducers with threaded mounting nuts should be used. However, its feelings may change within the next 20 years. If they do, we'll let you know.
Detailed Victorinox test results are now available from Kistler Instrument Corp. (Amherst, NY), including comparative pressure profiles. To get a copy, see contact information below.
Kistler Instrument Corp.
Phone: (716) 691-5100
Fax: (716) 691-5226