The next series of tests (Table 2), run on a new press at a major European machine builder, shows the advantage of being able to preclose the DNRV before injection.
In all tests, the DNRV gave much more consistent results than the ring valve.
They are interesting because, in three of the four comparisons, tests with the DNRV actually showed greater part-weight variation than with a ring valve.
An important clue to the poor results with the DNRV in the other tests in this series is the variability in recovery time, which was significantly greater for the DNRV tests (1B, 2B, 3B) than the ring-valve tests (1A, 2A, 3A).
The DNRV closes at the end of recovery when the pressure equalizes on both sides of the valve, and remains closed prior to inject.
What's more, it was evident that the screw actually surged more with the DNRV than with the ring valve.
With the DNRV, the screw could not even recover at a 250-psi backpressure setting, so the setting was dropped to 50 psi.
The DNRV was run in production here for six months on a 1978-model, 700-ton, U.S.-built "workhorse" machine without microprocessor controls.
But variation was reduced 90% with the substitution of the DNRV. After six months of 24-hr, seven-day production, no measurable wear was seen on this valve.
The injection stroke on the DNRV with preclose (tests 1A and 2A) was about 2.05 mm shorter than it was without preclose (tests 3A and 4A).
The DNRV closes faster than a ring valve, as shown by comparing injection strokes and recovery times of tests 1A through 4A with those of tests 1B through 4B (disregarding test 3).
That a DNRV closes more consistently than a ring valve is shown by the lower three-sigma range of recovery TABULAR DATA OMITTED time (tests 1A through 4A) than for the ring valve (tests 1B through 4B).