New check valve improves part uniformity.

A new injection check-valve design is providing greatly reduced part-weight variation due to elimination of variation in valve closing at the start of injection. Spirex Corp., Youngstown, Ohio, has applied for a patent on its solution to this perennial source of variability, which involves spring-loading a poppet member that seats against the valve body.

The spring holds the valve closed except during screw rotation, when the pressure of flowing melt forces the poppet off its seat, as shown in the diagram. As soon as the screw stops turning, the spring pulls the poppet back onto its seat. Thus the valve precloses before the start of injection, unlike conventional ball and ring-type valves. Because the poppet seats firmly against the valve body, no melt can be forced back through the valve during injection. (PLASTICS TECHNOLOGY recently reported on a different new approach to solving the same problem. See PT, Jan. '92, p. 17.)

Spirex has experimented with the new "Auto-Shut" valve for about a year, and has sold at least 15 valves to molders in the last three months. One molder compared the new valve with standard ring and ball valves in molding an 8-9 lb lawn-tractor hood out of a thermoplastic polyester alloy. Test runs of 30-35 parts were made on a 1500-ton press with a 5 1/4-in.-diam. screw. According to Spirex v.p. of sales Michael Durina, the standard deviation of part weight was reduced 89% when going from a ring-type to the Auto-Shut valve, and was reduced 66% in comparing a ball valve to the Auto-Shut. The accompanying graph shows the part-weight distribution with the ball and Auto-Shut valves.

LOW WEAR, TOO

Durina says the new valve is expected to provide long service life, because little wear should occur in the area between the front seat and poppet. The stroke of the poppet is generous, so as to create very little pressure drop. In addition, the volume of the flow channel between the poppet and valve body decreases slightly in the flow direction, so that the valve should be self-cleaning, according to Spirex. Durina adds that the valve should be "universal" in its applicability to different materials, because the spring closing action is independent of material viscosity.

The valve components are made of materials able to withstand 750 F continuous service, and even higher temperature versions are available. A thrust bearing at the base of valve accepts the load applied to the spring when the valve is opened by the pressure of flowing melt.

The valve comes in sizes from 35 mm diam. on up. It is premium priced, e.g., about 50% more than a similar-sized Spirex ring-type valve. (CIRCLE 1)