Robert Dray Sr. of R.F. Dray Mfg. Inc. designed a new non-return valve, 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 valve technology.

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

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 hang-up can cause degradation and color streaking. It is difficult to eliminate dead spots in ball check valves due to the configuration necessary to make it operate. Ball check valves were not included in these tests.

m 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.

m Valve and barrel wear. Friction creates wear on the upstream side of the retainer and the downstream side of the ring and also the barrel ID and the ring OD. Another version of 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. As the ring is required to rotate with the screw barrel wear is increased, as a minimum ring to barrel clearance is still required to close the valve.

As the barrel wears 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, as the wear decreases as the valve is moved rearward in the barrel.

m 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 due to the rearward movement of the screw creating a negative pressure by attempting to pull back the resin downstream of the screw. As the screw flights create a helical component to the resin flow path, the 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 due to reduced pressure on the upstream side of the ring.

m Velocity and non return 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 on the upstream side 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.

m Sliding closure APV. In hydraulics "spool valves" (Fig. 2) 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 very positive and if designed properly will not leak.

The All Purpose Valve initially uses a sliding closure. This sliding closure allows for minimal distance to close. The distance to close determines the time to close, or less distance to close means less time required 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 continues upstream over a rear land, and achieves final seal as the ring encounters the rear retainer.

m Dead head testing. In dead head tests, that is, placement of a plate in front of the sprue bushing that accepts the nozzle tip and seals off flow during injection, with 25 melt-flow index(MFI) propylene the APV forward movement was 0 for 20 sec at 20,000 psi. In this test the ring to barrel clearance was 0.001 on all rings tested. In the three- and four- piece valves tested the cushion was continuously penetrated, displaying continuous internal leakage. (Figure 3).

Ring valves (three- and four-piece) cannot reduce the distance to close without increasing recovery time and increasing 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 attempts are made to reduce the ring thickness the hoop strength can be exceeded, causing ring breakage. The APV design has corners with little or no land length (Fig.4). Therefore the pressure drop is less with less distance to close in the APV. See Examples 1 and 2.

Dispersive and Distributive Mixing
APV distance to close (Figure 4), 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 is attempting to go through the orifice. Conductive heat transfer from the metal surfaces and the adjacent material occur as the melt flows by. These influences, plus the laminar flow, melt the agglomerate and bring the temperature to unity with the adjacent melt.

The APV incorporates distributive mixing due to the communication of the circumferential groove with 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 of .009 inch, APVs had the longest recovery times. The times were longer on the more viscous resins , notably PE and PC, and less on the PS and nylon. The recovery times for the 0.039 inch APV tests were virtually the same as the 0.120 inch four-piece and the 0.098 inch three-piece.

These testing results indicated 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, improved dispersive and distributive mixing.

Demag has named their version of the valve the CloserNRV. Xaloy, Pulaski, VA , also become a licensee of the APV valve technology. The complete run data, showing machine setup and operating functions, is available in SPC format upon request to either Demag , R.F. Dray, or Xaloy.

Xaloy Inc., Pulaski, VA
Günther Hoyt
(540) 994-2243; www.xaloy.com
Infolink 000

Sidebar: Lab trials

There have been numerous performance trials conducted on the APV in the lab as well as in production. The following is a series of trials conducted by personnel at Van Dorn Demag. Mr. Mark Blevins conducted the first in the Van Dorn Demag customer demonstration laboratory and the second by Mr. Scott Knoop in the Van Dorn Demag engineering laboratory. The data shown was collected by them and is presented in this paper in the original form.

The customer demonstration lab trials by Mr. Blevins were conducted at the Demag lab. The tests were all with fifty parts; the machine setup was identical for all tests. The valve size was 50mm with the four-piece valves OD’s 1.9668 (barrel ID 1.968) leaving barrel ID to ring OD clearance of 0.0012. The Dray APV barrel ID to ring OD clearance was 0.006/0.007.

The four-piece valves rear seats 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 0.059 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 .120-to-close test (d3) provided the best results, although all of the APV tests were similar. The results are: minimum part weight 100.67g, maximum part weight 100.88g, with difference in part weight at 0.21g.

The d2 test provided virtually the same results as test d3. The results are; minimum 100.62g, maximum 100.86g, for a difference of. 0.24g. The d1 test provided virtually the same results as the d2 test

Knoop conducted shot weight, rotate time, and melt temperature trials at Demag’s engineering laboratory. A 45-mm valve was tested. All tests were with the same injection unit setup. The four-piece valve distance to close was 0.120 inch and the three-piece valve distance to close was 0.098 inch. The APV valves tested had ring lengths of 1.175 and 1.205, this establishes distances to close of 0.039 and 0.009 respectively.

The 4-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.