Positive-type non-return valve and method of using valve

A positive-type, non-return valve (10) primarily for use with an injection molding machine utilizes a frame (12) which surrounds a first bore (34) and a second bore (32). The second bore (32) is accessed by inlets (30). Outlet passages (40) are located downstream of said inlet (30) and connect second bore (32) with an accumulation area (6). A piston (60) is dimensioned to slidably fit within the first bore (34) and extend into the second bore (32). In a downstream position, the piston (60) allows positive flow of material from the inlet (30) to the outlet (40). In an upstream position, the piston (60) positively blocks flow of material from the inlet (30) to the outlet (40). The piston (60) can be forced to its upstream position at the beginning of the injection step or can be preclosed by maintaining back pressure on the screw (2) prior to injection.

TECHNICAL FIELD OF THE INVENTION 
The present invention relates to a positive-type non-return valve. In 
particular, the valve is used to positively stop the reverse flow of 
material therethrough in a more consistent and repeatable manner than 
valves of common usage. 
BACKGROUND OF THE INVENTION 
Injection molding is one of the dominant forms of manufacture of plastic 
articles in the world today. However, a problem with uniformity of product 
plagues this process of injecting a quantity, or shot, of melted plastic 
into a mold. Uniformity is sacrificed due to the inability to perfectly 
control the quantity of material injected into the mold. This imperfection 
is due primarily to failure of a non-return valve, found on most injection 
molding machines, to close in a consistent, repeatable manner and then 
properly seal against back flow of material during the injection step. 
A review of the prior art illustrates two primary methods of sealing 
against this back flow of material during the injection step: a ring-type 
shut off valve or a ball-type check valve. With either method, as the 
injection ram strokes forward, a ball or piston is forced against a seat 
or a tapered ring is forced against another ring with a complementary 
taper. There are various alterations on these devices that either control 
the length of the stroke or the shape of the valve parts and the plastic 
flow passages. However, in either case, plastic leakage flow over the ball 
or piston, or under the ring, creates a pressure drop across this moving 
member of the valve. This pressure drop becomes the primary force to close 
the valve. Any variation in the leakage flow before the valve closes 
results in a variation in the quantity of plastic trapped in front of the 
closed valve. Leakage flow variations occur with these valves resulting in 
product variations of major or minor significance depending on the product 
being made, operating conditions, and plastic material characteristics. 
The sealing surfaces in either a ring-type or ball-type valve can become 
contaminated with particles which prevent a perfect seal. This allows for 
material migration back through the valve rather than forward into the 
mold. Variations in this lost quantity of material will cause an imperfect 
product from the mold which may be unacceptable for its intended use. 
Detection of these unacceptable products requires significant inspection 
costs or inconvenience for the ultimate user of the product. To improve 
quality, many manufacturers have implemented statistical process control 
(SPC) which attempts to define and control process variables so that all 
supplied product is sufficiently identical, eliminating the need for 
inspection. 
Therefore, a need exists for a non-return valve which never fails to 
furnish the same shot size regardless of plastic, fillers, contamination, 
product produced, or wear. This valve should be designed to allow its 
incorporation into existing injection molding machines or any other device 
which utilizes a non-return valve. This valve should not be dependent on 
leakage through the valve to generate the force necessary to move the 
valve to its closed position. Furthermore, this valve should be designed 
so that the seal can never be impaired by particles. Thus, each time the 
valve seals, the sealing action should shear and clear away any such 
particles. 
SUMMARY OF THE INVENTION 
The present invention relates to a positive-type non-return valve. The 
valve is designed for use in an injection molding device with a screw type 
injection plunger but may be used in any application requiring one-way 
flow of a liquid material. In its simplest form, the valve is comprised of 
a frame surrounding a primary chamber. The primary chamber is comprised of 
a first bore and a second bore. The frame is generally cylindrical and 
typically has a tapered, conical downstream end. The proximal end can be 
threaded to allow for connection to a screw located in a barrel; however, 
the valve could also be an integral part of the screw. The outer diameter 
of the valve must closely approximate the inner diameter of this barrel. 
The first bore can extend to an axially-centered opening in the distal end 
of the valve. Because material is flowing through the valve, its proximal 
end is also referred to as upstream and its distal end is also referred to 
as downstream. 
The second bore is accessed by at least one inlet port. This inlet port 
leads from the screw's material flow path to the second bore which is 
upstream of and connected to the first bore. Both of the first and second 
bores are cylindrical and typically concentric; with the diameter of the 
second bore less than that of the first bore. At least one outlet passage 
leads from this second bore to at least one port located on the downstream 
end of the valve. The upstream end of said outlet passage is located 
downstream from the downstream end of the inlet port. 
A piston is dimensioned to fit inside the first bore and to extend into the 
second bore. The piston has a main body dimensioned to slidably engage the 
first bore, and a downstream portion body which slidably engages the 
second bore. The downstream travel of the piston is limited by a retainer 
such as a flow-through cap removably attached to the downstream end of the 
valve. The upstream travel of the piston is limited by a stop means, 
typically a ridge formed where the diameter of the first bore reduces at 
its intersection with the second bore. The reduced diameter portion of the 
piston is dimensioned to block the upstream opening to the outlet passage 
when said piston is in an upstream position, and to uncover said opening 
when in a downstream position. 
Material, typically melted plastic, is fed into the inlets by the screw. 
This material floods the second bore and forces the piston towards its 
downstream position opening the outlet passages. The opening will be only 
partial to obtain equal forces on opposite ends of the piston. A pressure 
drop must occur across this partial opening so that the distal end area of 
the piston times its pressure equals the proximal end area times its 
pressure. The material then proceeds into the outlet passages, out of the 
valve and into an accumulation area. When this area has collected a 
selected amount of plastic, the screw stops rotating and typically the 
back pressure, used to assist the screw in melting plastic, is reduced to 
zero. Pullback is then frequently used to assure zero pressure in the 
accumulated plastic (to avoid leakage into the mold when the previously 
produced part is removed) and to improve the repeatability performance of 
ring or ball-check valves. After the previously produced part has cooled 
sufficiently, the mold is opened, the part is removed, and the mold is 
again closed. The screw ram then moves forward in the barrel producing 
pressure in the accumulation area to fill the mold. This pressure also 
tends to push material back into the outlet passage and back through the 
inlet However, the pressure also pushes the piston to its rearward closed 
position which blocks the flow of material from the outlet to the inlet. 
Closing action is caused by pressure alone. No undesired leakage flow 
across the valve, as required in slider ring and ball check valves, is 
required to accomplish the closing action. The piston will encounter 
pressure against its reduced diameter upstream surface equal to the 
pressure against its downstream face. However, the force against its 
downstream face is greater and will overcome the force on its upstream 
face due to the differences in their respective surface areas. Thus, the 
piston will close quickly and repeatably at the start of the injection 
step. The minimal partial opening of the valve described above assists in 
the quickness and repeatability of its action. The piston will also deter 
clogging and leakage after closure because it will tend to shear away any 
contaminants in its way. 
Since valve closing results from only pressure in the accumulation area 
without a source of overpowering pressure (typically screw rotation) on 
the proximal end of the piston, the valve can be preclosed before 
injection. A spring such as that shown in the Eichlseder U.S. Pat. No. 
4,512,733 is not required. The valve is designed to have no material flow 
around and over the movable member valve.

DETAILED DESCRIPTION 
The present invention relates to a positive-type non-return valve that 
overcomes many of the disadvantages found in the prior art. Referring to 
FIG. 1, a positive-type non-return valve embodying the present invention 
is disclosed. Valve 10 is typically made of steel and used as part of an 
injection molding machine unit having a barrel, with an injection nozzle 
on one end of the barrel and a screw movable in the barrel. The valve 10 
allows material to pass therethrough when screw is rotating but closes 
when the screw translated forward with no screw rotation. All dimensions 
provided below are for a valve 10 attached to a two and one-half (21/2") 
inch feed screw. Other dimensions may be used to suit the situation. 
Referring to FIGURES 1 to 5 simultaneously, valve 10 comprises a generally 
cylindrical frame 12 with an inclined or tapered surface 14 on the 
downstream end and an attachment surface 18 on the upstream end. A cap 16 
or other retaining means with a flow through central passage 17 is 
attached to frame 12, typically by annular threads. A piston 60 is located 
in a first bore 34 and second bore 32 of frame 12. The non-return valve 10 
is attached to or is a part of screw 2, as shown in FIG. 4, both of which 
are located in a barrel 8, shown in FIG. 3, with an accumulation area 6, 
shown in FIG. 3, located downstream of said valve 10. Both the screw 2 and 
valve 10 slidably fit within said barrel. Material is fed by the rotating 
screw 2 into inlet 30. As discussed, frame 12 contains a centrally located 
primary chamber comprising a first bore 34 and a second bore 32 which are 
accessed by several channels or passages. Inlets 30, located on ridge 20 
lead to the second bore 32, best seen in FIG. 3, which is typically 
coaxial with the barrel, screw and valve 10. Inlets 30 can be oppositely 
located on ridge 20 and radially extend to the axis 4 of frame 12. The 
inlets are typically about one-quarter (1/4) to three-eighths (3/8) inch 
in diameter The second bore 32 extends from the inlets 30 to the first 
bore 34. The second bore 32 is preferably several inches long, as space 
permits, and approximately one half (1/2) inch in diameter. 
FIG. 2 is a sectional view of valve 12 showing a typical relationship of 
inlet 30, outlet 40, first bore 34, and second bore 32. Although two 
inlets and two outlets are preferred, valve 10 would only require at least 
one of each. The first bore 34 is usually coaxially located in the frame 
12 immediately downstream of the second bore 32. The first bore 34 extends 
to the front opening 15 of the frame 12. The first bore 34 is defined by 
wall 36 and ridge 36a. The wall 36 immediately adjacent to the front 
opening 15 is typically threaded as at 38. The first bore 34 is preferably 
about two (2) inches long and three-quarters (3/4) inches in diameter. 
The downstream ends of outlets 40, best shown in FIG. 4, are located on the 
inclined surface 14 of frame 12. Outlets 40 can be positioned one hundred 
and eighty (180) degrees apart and are typically one-quarter (1/4) inch in 
diameter. The upstream end of outlets 40 is initiated in the second bore 
32. The outlet 40 may take any path from the second bore 32 to surface 14, 
but is preferably a smooth path without any sharp angles involved. 
The piston 60 is dimensioned to fit closely but slidably inside first bore 
34 and second bore 32. The piston 60 has a stepped outer surface creating 
at least two portions 60a, 60b. Piston portion 60a has a diameter slightly 
less than three-quarters (3/4) of an inch and a length of approximately 
three-quarters (3/4) of an inch. The piston portion 60b has a diameter of 
slightly less than one-half (1/2) inch and a length of approximately one 
and one-half (11/2) inch. Thus, there is at most a clearance of a few 
thousandths of an inch between piston portion 60a and wall of first bore 
34, and between piston portion 60b and second bore 32. The travel of 
piston 60 is limited by cap 16 at one end and by flange surface 36a at the 
other end. When in a forward position against cap 16, plastic may flow 
from inlet 30 into entrance 46 and through outlet 40. In a closed 
position, piston portion 60b blocks entrance 46 between inlet 30 and 
passage 44. 
FIG. 5 illustrates a preferred embodiment of the non-return valve 10. This 
embodiment differs from those previously described in two respects. First, 
the outlets 40 are initially angled off of the second bore 32. The 
downstream exits of outlets 40 are located near the periphery of the 
downstream face of valve 10. Second, the piston 60 includes a third 
portion 60f extending downstream from the main piston body 60a. This third 
portion 60f extends through the flow-through portion of cap 16, forming a 
more flow dynamic downstream surface. 
FIGS. 6 through 10 illustrate the various steps in a method of injection 
molding utilizing the present non-return valve 10. Two methods of 
injection are disclosed. The first method involves the steps of recovery, 
pullback, and injection. The second method involves the steps of recovery, 
preclosure, pullback after preclosure and injection. FIG. 6 illustrates 
the "recovery" step which occurs after a shot has been injected and the 
accumulation area is empty. The valve is open, allowing recovery of a new 
shot of material. The screw 2 and valve 10 are shown in a retracted 
position relative to the barrel 8. The screw 2 is rotating and plastic is 
flowing as indicated by the arrow A and feeding material through the 
inlets 30. Material next passes into the second bore 32 and encounters 
piston 60. The pressure exerted by the material due to the screw rotation 
pushes the piston to its downstream position within the valve 10. The 
material begins to fill accumulation area 6. As the accumulation area 
fills, the piston will experience back pressure on its downstream end. At 
some point, a constant pressure differential will be established across 
the piston 60, and the piston 60 will move to an intermediate position 
within the valve 10. This intermediate position typically has the piston 
portion 60b partially covering the upstream end of outlet 40. For example, 
if the pressure on the distal end of the piston is set at 1000 psi and its 
area is two units and the proximal end of the piston has an area of one 
unit, then the pressure upstream must be 2000 psi and a 1000 psi pressure 
differential exists. The pressure loss occurs primarily across the 
upstream entrance to the outlet 40 which is only partially open. A back 
pressure is applied to the screw, as shown by arrow B, to prevent the 
screw from unscrewing through the material and to set the 1000 psi 
pressure in chamber 6. In other words, a back pressure is utilized to keep 
the screw in a fixed position at the end of the recover step. 
FIG. 7 illustrates "pullback" which may occur after "recovery". During 
pullback, the screw 2 stops its rotation, and the screw 2 and valve 10 are 
pulled back a small distance as indicated by arrow C. Pullback places a 
slight negative pressure on the downstream face of the piston and 
minimizes any leakage of material from the accumulation area into the mold 
when the mold is opened. The slight negative pressure will tend to pull 
the piston slightly downstream but motion is limited by cap 16. 
FIG. 9 illustrates the valve 10 during "injection". Once the shot of 
material is present in the accumulation area and the valve has pulled 
back, the screw and valve translate forward, as indicated by arrow D, to 
inject the shot into a mold. The piston will close automatically due to 
the high material pressure generated in the accumulation area and the area 
difference between the two ends of the piston. The piston is in an 
upstream position against flange surface 36. In other words, the piston 
overstrokes the material flowpath. The shot is expelled as shown by the 
arrows E. 
FIG. 8 illustrates the step of "preclosure" which can occur between 
recovery and pullback. The goal of preclosure is to close the piston 60 
back over the upstream end of outlet 40 before the shot of material is 
injected. The piston precloses when pressure is maintained in the 
accumulation area without screw rotation to keep the valve open. This 
occurs when a back pressure is maintained on the screw as indicated by 
arrow B. Due to the area differential between the downstream face and 
upstream face of the piston 60, a force differential exists. Typically the 
ratio between the downstream face area to the upstream face area is 
between 2.0:1 and 1.5:1. This force differential precloses the piston 
prior to injection. 
FIG. 10 illustrates the piston during the "pullback after preclose" stage 
of the injection method. After preclose, the screw 2 and valve 10 are 
pulled back in preparation for injection, in similar fashion to the step 
disclosed in FIG. 7. However, despite the negative pressure experienced by 
the downstream face of the piston, the valve remains closed due to the 
length of the reduced diameter piston portion 60b extending somewhat 
beyond the entrance to outlet 40 and the short duration of pullback. 
FIGS. 11 and 12 illustrate alternate embodiments of the non-return valve 
designed to ensure that the valve 
In FIGS. 11 and 12 illustrate alternate embodiments of the non-return valve 
designed to ensure that the valve remains closed during pullback after 
preclose. In FIG. 11, the last portion of the opening stroke of the piston 
60 is slowed by a smaller hole 16a in the cap or piston retainer 16. After 
the piston closes outlet passage(s) 16d, during opening, material can be 
rejected from the piston cavity only through hole 16a, thus slowing its 
motion. Also, valve closings at both the start of injection and preclosure 
will be slowed. Material can enter the piston cavity during valve closing 
action only through small hole 16a until outlet passage(s) 16d has been 
opened. In FIG. 12, a small check valve 16b with a small hole 16a and 
retainer pin 16c is added downstream of the piston to avoid this slowing 
of closure. 
FIG. 13 illustrates a non-return valve 10 which is integral with the screw 
2. 
FIG. 14 illustrates a version of the valve with a ring functioning as the 
piston. In this version, the ring 61 has an area on its distal end larger 
than that on its proximal end. It is functionally identical to piston 60 
even though it closes inlets 30 instead of outlets 40. Retainer pin 62 
performs the same function as cap 16 performs on the piston design. The 
valve body is shown as an integral part of screw 2, but it can 
alternatively be threaded into the screw as shown in FIG. 1. 
In sum, a preferred embodiment of the valve 10 fits into the same area as a 
prior art non-return valve. The material proceeds downstream between the 
screw flights due to the rotation of the screw until it encounters the 
valve 10. As it reaches the valve, the material enters the two inlet holes 
30 on either side of the valve 10, and proceeds to the second bore 32. The 
material forces the piston 60 into a downstream position, exposing a 
portion of the upstream entrance to outlet 40. The material follows the 
outlet passage 40 until it discharges from the valve 10 and into 
accumulation area 6 which communicates with the downstream end of piston 
60. After the accumulation area 6 is filled to selected volume, the screw 
2 stops its rotation. After the previously molded part is removed from the 
mold, the forward stroke begins and the piston 60 is moved to an upstream 
position blocking the material flowpath. The piston 60 can be preclosed 
prior to injection if back pressure is maintained on the screw for a 
moment after screw rotation is stopped. The force that is seen on the 
downstream piston surface 60d is greater than the force imparted to the 
smaller upstream piston surface 60a although the pressures are the same. 
Therefore, the piston 60 will move upstream closing off the entrance 46. 
As the piston moves upstream and covers the entrance to outlets 40 there 
is a positive shut off through the sliding and covering of this radial 
hole. 
Although preferred embodiments of the invention have been described in the 
foregoing Detailed Description and illustrated in the accompanying 
drawings, it will be understood that the invention is not limited to the 
embodiments disclosed, but is capable of numerous rearrangements, 
modifications and substitutions of parts and elements without departing 
from the spirit of the invention. Accordingly, the present invention is 
intended to encompass such rearrangements, modifications, and 
substitutions of parts and elements as fall within the spirit of the scope 
of the invention.