Non-return valve for plastic injection molding

A non-return valve for use in plastic injection molding is disclosed comprising an elongated body having a threaded end for connection to one end of a reciprocating feed screw. A first removable valve seat insert is received over a cylindrical portion on the valve body and is in abutment with a shoulder formed on the body. A second removable valve seat insert is also received over the cylindrical body portion and is axially spaced from the first valve seat by a tubular spacer. A ring-shaped valve member having valve surfaces on each end face thereof is disposed between the first and second valve seat inserts. An anti-rotation pin is mounted in the valve body and extends into a keyway type notch formed into the first valve insert. The axial clamping force developed by the threaded connection between the valve body and the feed screw functions to clamp the valve seat inserts and the tubular spacer into assembly.

BACKGROUND OF THE INVENTION 
This invention relates generally to non-return or one-way flow valves for 
valving the flow of molten plastic during the injection molding process 
and is particularly suited for use in processing highly abrasive types of 
plastic materials and those materials in which the input of frictional 
heat due to flow through the valve must be minimized or eliminated. 
DESCRIPTION OF THE PRIOR ART 
The machinery most commonly used in the injection molding process generally 
incorporates a reciprocating auger type feed screw mounted in a heated 
barrel for plasticizing or transforming into a molten state pelletized or 
granular thermoplastic materials which are fed into one end of the barrel. 
As is well known in the art, the injection molding process proceeds in two 
stages, first a plasticizing stage, and second an injection stage. 
During the plasticizing stage the feed screw rotates and forces the 
pelletized polymer material to the forward end of the screw. As the 
pelletized material flows toward the front of the barrel and screw, it is 
transformed to a molten state by a combination of frictional heat 
generated by movement of the plastic against the screw and barrel surfaces 
and conductive heating transferred through the wall of the barrel from 
electrical resistance heaters mounted on the exterior of the barrel. 
Continued rotation of the feed screw results in the molten material or 
"melt" flowing to the end of the screw where it then enters the non-return 
valve. As the material is deposited in front of the screw and valve, a 
differential pressure is developed across the ends of the screw which 
causes the screw to move toward the feed end of the barrel as it rotates. 
After a predetermined volume or "shot" of molten plastic is present in 
front of the screw, the feed screw rotation is stopped by associated 
controls. At this point the injection stage proceeds by ramming the feed 
screw forward toward the front end of the barrel, thereby forcing or 
injecting the shot from the barrel, where it then passes through a nozzle, 
and into a mold. During injection the non-return valve functions to shut 
off flow of the shot back toward the screw. 
The service conditions imposed upon a one-way valve during injection are 
extremely severe, being a combination of high pressures in the range of 
10,000 to 20,000 lbs. per square inch and temperatures in the range of 300 
to 600 degrees F. In addition many of the polymer materials produce 
corrosive by-products and contain abrasive fillers which attack the 
one-way valve. 
A non-return valve presently in widespread use is a type known in the art 
as a "check ring valve". A typical prior art check ring valve is shown by 
FIG. 1 of the drawings. This prior art valve includes a valve body 1, a 
sliding ring 2, and a valve seat 3. The valve is shown as connected to one 
end of the feed screw 4 and wherein both the valve and screw are received 
in a conventional barrel 5. Valve body 1 comprises a stud portion 6 which 
is received in a threaded bore in one end of the feed screw, a tapered 
ring seating surface 7, and a plurality of flow passages 8 formed adjacent 
one end of the valve body. As assembled, valve seat 3 abuts against a 
shoulder on valve body 1 and also against the end face of screw 4. A 
tapered seating surface 9 is formed on seat 3. Sliding ring 2 has a 
tapered seating surface 9 formed on the left or downstream face thereof in 
a tapered seating surface 10 formed on the right side, or upstream, face 
thereof. The outer diameter of ring 2 fits closely within barrel 5 and the 
diametral clearance is sized to substantially prevent the flow of molten 
plastic material therepast, thus functioning as a sealing surface. 
During the plasticizing stage of operation, valve body 1 and seat 3 rotate 
with screw 4 during which molten plastic is forced between the internal 
diameter of ring 2 and the valve body and through passages 8 formed in the 
tip of valve body 1. During this plasticizing stage, ring 2 does not 
rotate along with valve body 1, but is pulled to the right by the motion 
of the screw, resulting in tapered surface 9 of the ring rubbing against 
seating surface 7 on the valve body. During the injection stage of 
operation the valve screw assembly moves to the left resulting in ring 2 
seating against seat 3. Since the screw is not rotating during the 
injection phase, no sliding wear occurs between tapered surface 10 on the 
ring in seat 3. However, the sliding of the valve body against the ring 
during plasticizing results in excessive wear of the ring and the tapered 
surface 7 of the valve body. After a period of time, the erosion of the 
valve body around surface 7 is sufficient to allow the ring to slide off 
the end of the valve. In those prior art valve body configurations having 
"dead ended" passageways, the flow passages on the end of the body wear 
away to the extent that material can no longer escape from the end of the 
valve. 
When valve wear has reached this point, both the valve body and the ring 
must be replaced at considerable expense. 
It should be noted that the wear problems described above are especially 
severe when processing those materials containing fillers such as glass 
fibers and abrasive mineral compounds. 
A requirement in the processing of some plastic materials calls for 
minimizing or eliminating the amount of frictional heat transferred to the 
melt due to flow through the valve. It is known tht valves having 
restrictive cross-sectional flow passageways tend to add heat to the melt 
due to excessive viscous shear and pressure drops. The amount of 
restriction through the valve is expressed as the ratio in percentage of 
the minimum flow area through the valve divided by the flow area at the 
end of the feed screw times 100%. When this ratio, known as the 
compression ratio, can be established close to 100%, then frictional heat 
is minimized. However, even though some prior art valves might have 
compression ratios approaching 100%, frictional heat inputs still exist, 
especially in those valves which have a circuitous flow path. A need 
exists for a valve having a high percentage compression ratio and a 
relatively unobstructed flow path. 
There has also arisen a need for an improved ring type non-return valve 
assembly which minimizes the cost of valve replacement and which has 
improved wear life. 
SUMMARY OF THE INVENTION 
In the present invention a check ring type valve assembly is provided 
having a novel replaceable downstream seat carried on the valve body. In 
the preferred form of the invention, the replaceable seat is formed of a 
suitable high wear resistant material, thus allowing the valve body to be 
fabricated from a more ductile, tougher material. This enables the valve 
to more effectively resist twist off of the valve body threaded end from 
the screw due to high fiscous shear loads from the melt. 
The replaceable seat has flow passages formed therethrough and is slidably 
received over the valve body. A shoulder on the valve body axially locates 
the replaceable seat. In one embodiment, a radially extending locking pin 
mounted in the valve body extends into a slot formed into the internal 
diameter of the replaceable seat for preventing seat rotation. A spacer 
ring is received over the valve body and abuts against the replaceable 
seat. A rear valve seat member is also received over the the valve body 
and is maintained in axial position by abutment against the spacer ring. A 
sliding ring is positioned between tapered seating surfaces on the 
replaceable seat and the rear seat. 
When the expected life of the replaceable front seat, ring, and/ or rear 
seat have been reached, the valve can be removed from the screw, 
disassembled, and a new front seat, ring, and rear seat can be installed 
on the valve body. The valve body and spacer are reuseable indefinitely, 
thus saving considerable expense since the body generally represents over 
half the cost of a standard ring valve assembly. 
A further feature of the invention is that the replaceable seat can now be 
fabricated from a suitable highly wear resistant material. Since most 
highly wear resistant materials are often brittle, they are unsuitable for 
use in fabricating a standard valve body due to the need for the material 
possessing good shock resistance. Another drawback associated with the use 
of special materials, in a standard valve configuration as shown by FIG. 1 
is the almost prohibitively high cost. 
It is therefore an object of the invention to provide a non-return valve 
which is highly resistant to wear. 
It is another object of the invention to provide a valve assembly which has 
a highly wear resistant, replaceable front valve seat. 
It is another object of the invention to provide a non-return valve which 
has a free flowing path for molten plastic passing therethrough. 
It is a still further object of the invention to provide a non-return valve 
which has a reuseable valve body. 
These and other objects, features, and advantages of the present invention 
will be understood in greater detail from the following description and 
the associated drawings wherein reference numerals are utilized in 
designating a preferred embodiment.

DETAILED DESCRIPTION 
Referring now to FIGS. 2 and 3, there is illustrated a non-return valve 
assembly, indicated generally at 10, embodying the principles of the 
invention comprising a valve body 12, a replaceable downstream valve seat 
14, a spacer ring 16, a rear valve seat 18, a sliding ring 20, and an 
anti-rotation pin 22. Body 12 includes a threaded end portion 24 and pilot 
diameter portion 26 which mounts valve 10 to one end of a feed screw 28. 
Valve 10 and feed screw 28 are shown as mounted within a conventional 
injection unit comprising a bore 30 defined by a barrel 32. A barrel end 
cap 34 is mounted on the left end of barrel 32. A nozzle 36 (shown 
partially) is connected to the end of end cap 34 and functions to transfer 
processed molten plastic to a mold, not shown. A plurality of heater bands 
38 are mounted around the outer surface of barrel 32 and end cap 34. The 
barrel and screw arrangement which forms part of a standard plastic 
injection molding machine is well known in the art and is shown merely for 
illustrative purposes and, as such, forms no part of the invention. 
Barrel body 12 includes an elongated cylindrical portion 40 which 
terminates toward the left end of the valve in a shoulder 42. Pin 22 
extends downwardly into cylindrical portion 40 at a position closely 
adjacent shoulder 42. The left end of valve body 12 is tapered to conform 
generally to the tapered contour of end cap 34. 
Replaceable front valve seat 14 includes a plurality of passageways 44 
defined by the space between adjacent radially extending portions 46. A 
bore 48 extends through seat 14 and is sized for closely fitting sliding 
relationship with the outer surface of cylindrical portion 40. 
Spacer 16 has a bore 50 formed therethrough which is also in closely 
fitting sliding relationship to cylindrical portion 40. The outer diameter 
of spacer 16 is preferably sized to blend into the root diameter of 
passageway 44 to allow for free flow of plastic material through the 
valve. The functional significance of the flow pattern achieved with this 
novel configuration will be described in detail below. 
Rear seat 18 has a bore 52 formed therethrough which is in closely fitting 
sliding relationship to the outer diameter of cylindrical section 40. 
Ring 20 has tapered valve seating surfaces 54 and 56 formed on the left and 
right side surfaces thereof which are engageable with corresponding 
tapered surfaces 58 and 60 formed on front seat 14 and rear seat 18, 
respectively. 
A slot 62 is formed into the surface of bore 48 to a depth and width 
sufficient to slide over retaining pin 22. Flat surfaces 64 are formed 
into the front end of valve body 12 and spaced 180 degrees apart for 
facilitating assembly of valve 10 and disassembly to screw 28. Alternate 
flat patterns can be used, for example hexagonal or octagonal, to 
accommodate wrench sockets for valve removal from screw 28. 
In the preferred form of the invention, the perpendicular end faces of 
seats 14 and 18 and spacer 16 are ground square to insure proper part 
alignment without gaps or sufface discontinuities. 
As best shown by FIG. 3, valve 10 is assembled by first sliding downstream 
seat 14 over the right end of valve body 12 and aligning slot 62 with pin 
22 such that seat 14 abuts against shoulder 42. Spacer 16 is then slid 
over cylindrical section 40 followed by insertion of ring 20 against seat 
14. The assembly is completed by sliding rear seat 18 over cylindrical 
portion 40 and into abutment with spacer 16 thereby capturing sliding ring 
20 between tapered surfaces 58 and 60. The valve assembly 10 is then 
connected to screw 28 by threading stud portion 24 into a mating female 
thread in the end of screw 28 and tightened snugly until shoulder 42 
clamps seat 14, spacer 16, and seat 18 against the end face of screw 28, 
thus completing assembly of the valve to the screw. 
In operation, plasticized molten plastic flows between the space between 
face 56 on ring 20 and face 60 on seat 18, between the outer diameter of 
spacer 16 and the inner diameter of ring 20, and through passageways 44 
and then to the space in front of valve 10. During this phase of 
operation, valve 10 is in the open position. 
During injection, screw 28 and valve 10 are moved to the left relative to 
FIG. 2, whereupon tapered surface 56 on ring 20 sealingly engages with 
tapered surface 60 on front seat 18, thereby preventing a return flow of 
molten material past valve 10 and back into screw 28. 
When seat 14 requires replacement, valve 10 is removed from screw 28 using 
a wrench applied to flats 64. Seat 14, spacer 16 and rear seat 18 are then 
pressed from cylindrical portion 40 and a new replacement seat 14 and/or 
ring 20 are connected to valve body 12 in the manner described above. 
In determining the flow area through the valve at any particular point 
along the valve, the cross-sectional area at the end of screw 28 is first 
determined. The cross-sectional flow areas through the valve are then 
adjusted to suit the particular molding application. It should be noted 
that the relatively straight flow path through the valve provides for a 
minimization of frictional heat. Material flowing between ring 20 and 
spacer 16 directly enters the spaces or fluted areas of the front valve 
seat 14 where it then flows directly over the tapered left end of the 
valve body. 
The embodiment of the invention as shown and described above is 
representative of the inventive principles stated therein. It is to be 
understood that variations and departures can be made from the embodiment 
as shown without, however, departing from the scope of the appended 
claims.