Dead Head Testing
In dead head tests—where a plate is placed in front of the sprue bushing that accepts the nozzle tip and seals off flow during injection—APV forward movement was 0 for 20 seconds at 20,000 psi, using 25 melt-flow index (MFI) propylene. In this test the ring-to-barrel clearance was .001 inch on all rings tested. In the three- and four-piece valves tested, the cushion was continuously penetrated, displaying continuous internal leakage. Ring valves (three- and four-piece) cannot reduce the distance to close without increasing recovery time and melt temperature. This is due to the increased resistance, or pressure drop. Ring valves must have a sealing surface that is not point-to-point contact to minimize leakage during injection. The ring and rear retainer must be properly aligned to seal and the ring thickness must be adequate to resist the inject pressure. If the ring thickness is reduced, the hoop strength can be exceeded, causing ring breakage. The APV design has corners with little or no land length. With less distance to close in the APV the pressure drop is less.
There have been numerous performance trials conducted on the APV in the lab as well as in production. The data presented is from a series of independent trials in its original form.
The laboratory tests were all with 50 parts; the machine setup was identical for all tests. The valve size was 50 mm and the four-piece valves had an OD of 1.9668 inches (barrel ID 1.968 inches), leaving a barrel ID to ring OD clearance of .0012 inch. The Dray APV barrel ID to ring OD clearance was .006/.007 inch.
The rear seats of the four-piece valves were changed to provide different distances to close; these distances are shown at the top of the graphs. The Dray APV valves had different ring lengths and these are also shown on the graphs.
In these trials the APV part weight variation was approximately half that of the four-piece valve. Short stroking the four-piece valve to .059 inch did not improve the performance, as the seat-type closure requirements are the same. It may be noted that the APV performance was similar in all distances to close due to the sliding closure.
The APV d3 test provided the best results, although all of the APV tests were similar. The results are as follows: minimum part weight 100.67g, maximum part weight 100.88g, with a difference in part weight of .21g.
The d2 test provided virtually the same results as the d3 test. The results: minimum 100.62g, maximum 100.86g, for a difference of .24g. The d1 test provided virtually the same results as the other tests.
As expected in rotation time tests, the minimum distance-to-close (.009 inch) APVs had the longest recovery times. The times were longer on the more viscous resins, notably PE and PC, and less on PS and nylon. The recovery times for the .039-inch APV tests were virtually the same as the .120-inch four-piece and the .098-inch three-piece.
In melt-temperature trials the APV valves were lower in average melt temperature in 14 of 16 runs. It should also be noted that the .009-inch distance-to-close APV actually scored first in 10 of the 16 runs. This is a result of the APV’s ability to provide dispersive and distributive mixing of the resins.
These results indicate that the APV minimizes closure leakage, reduces component wear, improves mixing, and reduces part weight variation as compared with ring valves. Improved part weight is the most critical component in Non-Return Valve performance. Additional benefits are less wear and improved dispersive and distributive mixing. These results highlight the importance of modification. The plastics industry cannot depend on a "one size fits all" approach. At R. Dray Mfg. our expert design and production team will tailor your valve to maximize your production potential.
If you would like a compilation of all comprehensive independent trial data, contact us! We are proud to be the leader of innovation in the plastics industry and stand by the performance of our products.