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March 7, 2003

10 Min Read
Technology Notebook: Testing a new nonreturn valve design

Editor?s note: Robert Dray Sr. is owner of R. Dray Mfg. Inc.; Mark Blevins is manager of molding systems at Van Dorn Demag, Demag Plastics Group; and Scott Knoop, formerly at Van Dorn Demag, was Demag Product Specialist at the time of the experiments.

Robert Dray Sr. of R.F. Dray Mfg. Inc. designed a new nonreturn valve (NRV), designated the All Purpose Valve (APV), which seemed to function well. However, the valve initially lacked the extensive testing that an injection machine OEM could provide. Van Dorn Demag (now identified as Van Dorn Demag, Demag Plastics Group) became interested in this technology.

Van Dorn?s Blevins and Knoop undertook an extensive series of laboratory and field trials that led to Van Dorn?s licensing of the technology.

Valve Designs
To understand how the APV works it is important to know how pre-existing designs operate. A nonreturn valve?s function is to close after the screw plasticates a selected amount of resin (shot size) so that this plasticated resin can be forced (injected) into the mold.

  • Ball check valves. Nonreturn valves were originally of the ball check design. Closure in this valve design is accomplished by reverse resin flow during injection, created by forward movement of the screw and valve that moves the ball into a seat that is large enough to accept the recovery flow but small enough not to allow the ball to pass through. This design seals by contact of the ball?s outside diameter (OD) with the receiving surface of the retaining seat.

    As there is minimal contact surface, any loss of contact due to misalignment or contamination results in leakage. Very small clearances due to incomplete sealing can cause significant cushion penetration. Proper flow paths through valves are important. Any dead spots that allow for material hangup can cause degradation and color streaking. It is difficult to eliminate dead spots in ball check valves because of the configuration necessary to make it operate. Ball check valves were not included in these tests. technotebook1.jpg

  • Ring valve (sliding). The ring valve, in three-piece and four-piece versions, is also widely used in the injection molding industry. The four- piece design incorporates a replaceable downstream retainer. Closure is accomplished primarily by a minimal clearance from the ring OD to the barrel?s inside diameter (ID). This tight fit creates friction to hold the ring in place while the screw rotates and is forced rearward, as the screw develops the necessary pressure to overcome the backpressure resistance.

  • Valve and barrel wear. Friction creates wear on the upstream side of the retainer, the downstream side of the ring, and the barrel ID and the ring OD. Another version of a ring-type valve incorporates interlocking between the upstream side of the retainer and the downstream side of the ring. This design eliminates frictional wear on the retainer as the ring must rotate with the screw. This design does not help barrel wear and is more prone to breakage as the ring is required to rotate with the screw as do ball check valves. Because the ring is required to rotate with the screw, barrel wear increases as a minimum ring-to-barrel clearance is required to close the valve.

    Figure 2. A series of shot weight, rotate time, and melt temperature trials were conducted by Scott Knoop in the Van Dorn Demag engineering laboratory. A 45-mm valve was tested. All tests used the same injection unit setup. The four-piece valve distance to close was .120 inch and the three-piece valve distance to close was .098 inch. The APV valves tested had ring lengths of 1.175 and 1.205 inches; this establishes distances to close of .039 and .009 inch, respectively. The four-piece valve placed last in all but two trials of the weight tests, across all resins and conditions. The Dray APV valves scored first in 12 of the 16 trials. In the zero decompress trials the Dray APV was first in six of the eight trials.

    With barrel wear, ring valve types lose the friction required for sealing and perform erratically. This is seen in increased cushion variation. To help stabilize this situation, increasing decompression (pullback) is used. This helps because wear decreases as the valve moves rearward in the barrel.

  • Decompression or pullback. Decompression was intended to prevent mold drool. Normal ring valves require decompression or pullback to close consistently. Greater and greater distances are required, as the barrel wears, to maintain the normal ring-to-barrel clearance. In many cases splay is created on the molded part due to excessive pullback. Decompression also assists closure in new barrel and valve situations. This is a result of the screw?s rearward movement, creating negative pressure by attempting to pull back the resin downstream of the screw. As screw flights create a helical component to the resin flow path, decompression is greatest at the area from the end of the flight to the ring. This decompression or reduced pressure creates less resistance to closure as the screw moves forward because of reduced pressure on the upstream side of the ring.

  • Velocity and nonreturn valve closure. Resin velocity is also important to the closure of ring valves and ball check valves. The greater the velocity, the greater the initial sealing force. At low velocities the separating force created by the resin leaking upstream, and the increasing pressure upstream of the ring, can be great enough to resist complete closure and cause cushion variation or nonclosure. The injection unit may actually be used more to close the valves than to fill the mold.

  • Sliding closure APV. In hydraulics, ?spool valves? are widely used. They align entry and exit openings to attain flow. Closure is accomplished by sliding either the entry or exit opening to a misaligned position. This type of closure is positive and if designed properly will not leak.

    The All Purpose Valve initially uses a sliding closure. This sliding closure allows for minimal distance and time to close. Distance to close and time to close also are major factors in the amount of leakage prior to closure. Faster closure means less chance for cushion variation and therefore reduced part weight variation. It also allows for slower injection velocities. The APV seals as the ring slides over a circumferential groove, continues to seal as the ring moves upstream over a rear land, and achieves final seal as the ring encounters the rear retainer.

    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 (Figure 1, p. 27). With less distance to close in the APV the pressure drop is less.


    Performance trials

    There have been numerous performance trials conducted on the APV in the lab as well as in production. Data in Table 1 and Figure 2 (p. 27) are from a series of trials conducted by personnel at Van Dorn Demag. Mark Blevins conducted the first in the Van Dorn Demag customer demonstration laboratory and the second was conducted by Scott Knoop in the Van Dorn Demag engineering laboratory. The data shown was collected by them and is presented in the original form.

    Blevins? customer demonstration 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 at the top of 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.

    Dispersive and Distributive Mixing
    APV distance to close (Figure 1), normally .040 inch, provides dispersive mixing. Dispersive mixing is accomplished by forcing material through an orifice that has a clearance less than the size of the particle or agglomerate that is attempting to pass through. Shear stress deforms the agglomerate as it passes through the orifice. Conductive heat transfers from the metal surfaces and the adjacent material as the melt flows by. These influences, plus the laminar flow, melt the agglomerate and bring the temperature in line with the adjacent melt.

    These test results indicate that the APV minimizes closure leakage, reduces component wear, improves mixing, and reduces part weight variation as compared with ring valves.

    The APV incorporates distributive mixing through communication between the circumferential groove and the longitudinal grooves. Distributive mixing is dividing of flow and agitation of flow. This is accomplished without ?dead spots? and with flow that is self-cleaning. Color dispersion is improved and degradation is eliminated.

    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 NRV performance. Additional benefits are less wear and improved dispersive and distributive mixing.

    Demag has named its version of the All Purpose Valve the CloserNRV. Xaloy (Pulaski, VA) also became a licensee of the APV valve technology. The complete run data, showing machine setup and operating functions, is available in SPC format upon request from either Demag, R.F. Dray, or Xaloy.

    Contact information
    R. Dray Mfg. Inc., Dallas, TX
    R.F. Dray Sr.
    (214) 368-5424; www.rdray.com

    Van Dorn Demag, Demag Plastics Group, Strongsville, OH
    Bob Spreat or Mark Blevins
    (440) 876-6231

    Xaloy Inc., Pulaski, VA
    Günther Hoyt
    (540) 994-2243; www.xaloy.com

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