WO1999037460A1

WO1999037460A1 - Vented method for injection molding

Info

Publication number: WO1999037460A1
Application number: PCT/US1999/001082
Authority: WO
Inventors: Makoto Yamaguchi
Original assignee: Makoto Yamaguchi
Priority date: 1998-01-27
Filing date: 1999-01-19
Publication date: 1999-07-29
Also published as: AU2459399A

Abstract

The vented method for injection molding is practiced as follows: the flow of a plasticated material from an injection cylinder (76) to a mold (90) is stopped. An injection piston (72) in the cylinder pushes out gas from a discharging valve (60) on the cylinder. After the gas is discharged, the valve (60) is shut off.

Description

VENTED METHOD FOR INJECTION MOLDING

FIELD OF THE INVENTION

This invention relates to plastic injection molding, specifically to a material charging process for injection molding.

BACKGROUND OF THE INVENTION

An injection molding machine makes plastic products. The machine plasticates a solid plastic material with a plasticating unit of the machine. The plasticating unit accumulates the plasticated material in an accumulator, which is also used as an injection cylinder. After enough plasticated material is accumulated in the cylinder, the unit injects the material into a cavity of a mold. The machine has a clamping unit where the mold is placed, and the injected material is solidified in the cavity of the mold. Cavities of injection molds have various shapes, and one can repeatedly make products with the identical shape. The clamping unit keeps the mold closed while the material is being solidified inside the cavity. After the material is solidified, the mold is opened by the clamping unit. Then, the solidified material is ejected from the cavity as a product.

The machine is operated either pneumatically, hydraulically, electrically, or a combination of the three. They are chosen in depending on the preference of users or the circumstances. All types of the machines have advantages and disadvantages. The hydraulic machines are the most popular regardless of the size and circumstance. The pneumatic machines are mostly used in laboratories and occasionally for production. The electric machines have the advantage of energy efficiency over the other types. The electric machines excel at the speed and position controls of their plasticating screws. The electric machines, however, have a problem in pressure control. They will need a further improvement in that particular area. 2

All types of the injection molding machines use either in-line screw or plunger-type injection units for material injection. Both types accumulate the materials in their accumulators and inject them into cavities. The in-line screw machine is the most broadly used. The in-line type injection unit uses an in-line screw. The screw is used as a plasticating screw as well as an injection piston. First, the screw plasticates a solid material and sends it forward inside the cylinder. As the material is plasticated, the screw moves backward and accumulates the material in front of it. Then the screw moves forward to inject the material into a cavity as an injection piston. The screw consists of a plasticating screw and piston tip. The piston tip has a check valve that prevents the material from flowing backward. When the material is injected by the piston, the valve is slid back and shuts off the back-flow of the material. The in-line screw machines require only one set of a screw and cylinder, because the screw works as a plasticating screw and an injection piston. The plunger- type injection unit uses a plasticating screw and plunger piston. The plasticating screw of the plunger machine plasticates a solid material and sends it to the plunger unit. The plunger unit accumulates the material and the piston of the unit injects it into a cavity. The plunger machines normally excel at injection accuracy compared to the in-line screw machines. The plunger machines require two sets of cylinders, and are usually more expensive than the in-line machines of the equivalent sizes.

There are some problems in injection molding process, such as the existence of gas inside an injection cylinder, the inconsistent check valve movement of a piston tip, and the inconsistencies that derive from hydraulic, pneumatic, electric circuits, or a combination of the three. The third problem is somehow uncontrollable, because it thoroughly depends on the quality of parts that the machines use and how the machines are built. One must simply choose better parts and build the machines well.

Gas, unlike liquid and solid matters, can easily contract by pressure. When an injection piston presses a plasticated material to inject it into a cavity, the majority of gas contracts and remains in the material. Since the amount of the gas in the material cannot be controlled, the volume of the gas is slightly different each time the screw plasticates a solid material. This remaining gas causes the fluctuation of injection volume.

The problem of the check valve is another uncontrollable factor. The valve is mounted on an injection piston tip, and slides back and forth to control the flow of a plasticated material. The valve is normally at either end, and there is no way to control the accuracy of its movement. The movement of the valve depends on the condition of the material. Since the temperature of the 3 injection cylinder perpetually changes, the condition of the material inside the cylinder also changes.

These two problems mentioned above are normally considered unrelated. They have been independently attacked over the years, but have never been solved concurrently until now. As has been mentioned, there is a pressure controlling problem involving electric molding machines. The problem is only particular to the electric machines. The electric machines are, however, considered to be the machines of the future. If the electric machines overcome the particular problem, they have the potential to become dominant players in the injection molding industry.

One can remove most moisture, one of different types of gas emitted from plastic materials, by using dryers. The dryers are considered to be the most effective and commonly used way of removing the moisture. However, many types of plastic materials require hours of drying time and close attention to the process. Injection molders spend a lot of time and money simply on drying the materials. Even after the materials are dried, the other types of gas are emitted from them. The only way to remove such gas from the injection cylinder is to apply back-pressure to the materials. The back-pressure is applied by the screw. When the screw plasticates a solid material, the screw sends it forward while applying pressure to it. This pressure pushes back some gas through the check valve. This way is, however, hardly thorough.

One can remove most gas by using a vented cylinder. It has a vent on the injection cylinder and can remove most gas, including moisture, emitted from plasticated materials. By using the vented cylinder, one can reduce or totally eliminate the need for material drying with the dryers. However, there are several disadvantages of the vented cylinder. The first is that the residence time of a plasticated material with the vented cylinder is longer than that of the standard cylinder. The "residence time" is a period that a material stays inside an injection cylinder. Generally, the shorter the residence time is the better the product becomes. If the time is too long, the material is degraded. The second is that the vented cylinder can only get 50-80 percent of injection volume of the standard cylinder of the equivalent size machine. This limits a variety of products the injection molding machine can make. The third is that the vented cylinder machine requires a special vent cylinder and screw. They are complicated and more expensive than ones used by the standard machine. The fourth is that a material change is very time-consuming. When one changes material with the standard machine, one cleans the surfaces of the screw and cylinder only by purging a new material. The vented cylinder, however, requires purging and also special cleaning around the vented area. And, the fifth is that the vented cylinder sometimes causes a "plugging". The 4 plugging is a problem that a material is solidified around the vented area and blocks gas from being vented. When the plugging happens, gas cannot escape from the cylinder.

Inconsistent check valve movement has always been a problem with the in-line screw machines. The check valve shuts off the back-flow of the material when the piston moves forward. The nature of the particular mechanism is such that there is always a slight material back-flow while the valve is in motion. Since the valve is not mechanically controlled, one cannot expect it to slide back and forth consistently. Some effective ways to reduce or totally eliminate the problem have been introduced over the years. The first is to close the check valve mechanically by using an arm mounted inside a plasticating screw. This method can be effective but extremely expensive to implement. The second is to close the check valve by using a spring mounted inside a piston tip. It is cheap and relatively effective. It, however, doesn't completely eliminate the problem. U.S. patent 5,266,247 by Yokota (JP) is the third way to efficiently eliminate the problem, but it is also very expensive. It requires a shut-off nozzle, a pressure transducer, an arithmetical unit, and a specially programmed controller. This system is almost impossible to be retrofitted with existing injection molding machines. These examples mentioned above are only used to control the movement of the check valves, and they do not have an ability to eliminate gas from the injection cylinders.

The plunger machine does not have a check valve on its injection piston. Since there is no check valve, there is no material back-flow in the plunger unit. The material injection of the plunger machines are generally more accurate than that of the in-line machines. There are, however, several disadvantages of the plunger machine. The first is that the plunger machines are generally more expensive than the in-line machines. The plunger machine must have one set of a cylinder for plasticating and another for injection. It also must have a material shut-off mechanism between the plasticating and plunger cylinders. The second disadvantage is that old material always remains inside the plunger. The plunger machine does not have a "first-in, first-out" material injection mechanism. The "first-in, first-out" mechanism is a type of injection system where a material that is plasticated first goes out first. An injection mechanism of the plunger machine always involves a possibility that an old material is peeled off from the surface of the plunger and goes into a product along with a new material. This is a great disadvantage, because one cannot expect when this will happen. Because of the injection accuracy, it is hard to separate the products contaminated with the old material just by weighing them. If they are contaminated with the old material which is normally degraded, the strength of the products are weaker. The third disadvantage of the plunger machine is that material change is very time-consuming. Like the previous disadvantage, an old 5 material sticks to the surfaces of the plunger and cylinder and one cannot change material in the plunger machine just by purging a new material. One has to open the plunger cylinder, and thoroughly clean inside the cylinder.

Several years ago, an electric injection molding machine was introduced to the market. It uses several electric servo motors to very accurately control the speed and position of an injection piston. Furthermore, it consumes less energy than the hydraulically operated machines. It is also easy to create a clean environment in the work place since the electric machine does not use hydraulic oil.

The electric machine, however, has a weakness in pressure control. It must use a pressure transducer to detect material pressure inside the cylinder. The hydraulic machines only use pressure relief valves to control the pressure of their hydraulic circuits. The valves are very simple, effective, and cheap, and they do not require any logic control. The electric machine, on the other hand, has a very complicated pressure controlling mechanism, and it is not as effective as those of the hydraulic machines.

A controller of the electric machine must receive signals from the pressure transducer which is installed somewhere on a material passage in the cylinder. The controller then calculates the amount of pressure the screw must apply to the plasticated material. The signals that are sent from the transducer to the controller are intermittent. The electric machines normally use the fastest microprocessors for their controllers. Even when the charging time is very short, the controllers can process the signals from the transducers 100-500 times per second. However, large metal screws and other mechanical parts certainly cannot handle this many signals in such a short period of time. If the charging time is one second, for example, the screw can probably react once or twice to all the signals during that period. This process slows down the entire molding cycle of the electric machine. The concept of pressure is particular to a fluid controlling mechanism and it is extremely hard to control any fluid in the absence of another one. Because of the awkwardness of the electric machine's pressure controlling mechanism, the machines are normally very complicated and expensive.

U.S. patent 5,266,247 by Yokota (JP) is an invention that can shut off the back-flow of a plasticated material and adjust injection start positions. The invention can eliminate the problem of the inconsistent check valve movement. As having been mentioned, this system requires a special arithmetic control unit and a pressure transducer. It is very expensive to be retrofitted with existing injection molding machines. Although the invention can solve the inconsistent check valve movement, it does not solve the problem of gas inside the cylinder. As long as the gas exists inside 6 the cylinder, an injection volume for each injection will not be consistent. The invention also does not solve the pressure controlling problem that the electric machines have.

U.S. patent 4,822,269 by Kamiyama and Fujita (JP) is an invention to remove gas from mold cavities, injection nozzles, and injection cylinders. However, the invention does not have any device to enable accurate material injection. The invention requires a vent port and vacuum pump to remove gas from an injection cylinder. The intention of the invention is to remove all gas from the cavity of the mold and the inside of the nozzle and cylinder. The invention might be able to help make the cavity in vacuum condition, however it cannot effectively remove gas from the nozzle and cylinder. Typically with the in-line type injection unit, most gas stays near the piston tip and the top inside-surface of the cylinder. If the vacuum pump is activated in this condition, the plasticated material will be removed before the gas. Even if the gas can be accumulated in the front of the cylinder, the pump will certainly remove the material at the same time. The material has been already measured for the next injection by this point, so this operation will disrupt accurate injection. The inventor of this invention assumes that there is no material inside the nozzle before the pump is activated. This is hardly the case. The material is, in fact, always there from the previous injection, and it will prevent the vacuum pump from removing the gas in the cylinder.

U.S. patent 4,060,226 by Schweller (US) is an invention that removes gas from a vented cylinder while prohibiting the entrance of oxygen into the cylinder. The pressure of a plasticated material below a vent on the vented cylinder is normally very low. This is the reason that the material normally stays under the vent and does not rise up to the vented hole. If any pressure is applied to the hole from above, some gas cannot escape from the cylinder and moves forward with the material. This system does not guarantee that all the gas can escape from the vented hole. The invention is only used to remove the gas and cannot be used to control injection piston positions.

U.S. patent 5,350,288 by Kimoto and Shihota (JP) is an invention that has a relief valve to discharge a plasticated material. The valve cannot remove gas nor control the accuracy of injection piston positions but control a peak pressure during the injection. One cannot use the invention to remove gas from the injection cylinder even before the injection. The relief valve is set to control pressure overload during injection. The valve will not be activated unless the pressure hits a set pressure, that is the maximum injection pressure. The gas can be pushed out after the completion of the injection. But it is practically too late. The valve keeps the peak pressure of the material at the pre-determined level during the injection. If one uses the valve before the injection, one cannot obtain accurate injection volumes. The pressure inside the cylinder before the injection is normally much lower than the injection pressure. So, if the valve is set to control the material pressure 7 before the injection, a large portion of the material will be discharged from the valve during the injection period.

U.S. patent 4,342,717 by Gardner is an invention to control the pressure of an injected material. The relief valve on the nozzle is used to control the pressure, and the shutoff mechanism on the nozzle prevents the material back-flow to the plasticating unit. This cannot control the accuracy of injection volumes before the injection nor remove gas from the cylinder.

Here are the problems related to injection molding process that have been presented in the previous paragraphs.

(a) The existence of gas inside an injection cylinder

(b) The inconsistency of check valve movement of an in-line type injection piston

(c) The pressure control of an electric injection molding machine

SUMMARY OF THE INVENTION

Accordingly, several objects and advantages of my method are:

(a) to provide my vented method with a discharging valve with a shutting device to remove gas from an injection cylinder;

(b) to provide my vented method with a discharging valve with a shutting device to adjust injection starting positions and reduce the inconsistency of check valve movement by discharging some plasticated material;

(c) to provide my vented method with a discharging valve with a shutting device to easily enable the pressure control of an electric injection molding machine.

Further objects and advantages are to provide my vented method with a discharging valve with a shutting device, which can be easily retrofitted with most injection molding machines, which can easily change material, and which does not permit the entrance of oxygen into an injection cylinder. Still further objects and advantages will become apparent from a consideration of the drawings and ensuing description.

In accordance with my vented method for injection molding, the flow of a plasticated material from an injection cylinder to a mold is stopped, and an injection piston pushes out gas from a discharging valve on the cylinder. The valve is shut off before the injection. 8 BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, closely related figures have the same number but different alphabetic suffixes.

Fig. 1 is the isometric exploded view of the discharging valve with a shutting device, and shows the proper assembly.

Fig. 2 shows the side view of the discharging valve with a shutting device being assembled with the plasticating unit of an injection molding machine.

Fig. 3 A only shows the side view of an manifold drain that is located in the outer part of the main body of the discharging valve with a shutting device.

Fig. 3B only shows the front view of the manifold drain that is located in an outer part of the main body.

Fig. 4 shows the side view of an entire plasticating unit with the discharging valve with a shutting device.

Fig.5A to 5D show an entire injection molding process with the discharging valve with a shutting device in order.

Fig. 6 shows the side view of the discharging valve with a shutting device actuated by a cylinder.

Fig. 7 shows the side view of the discharging valve with a shutting device without a shutoff nozzle.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The main body of my method is illustrated in Fig. 1 and Fig. 2. A nozzle 10 is a piston-like object. Nozzle 10 is inserted in a shut-off nozzle body 17 and slides back and forth in body 17. Since nozzle 10 works like a piston, the surface that touches body 17 must be well polished. It is preferably less than 6.3 micron of roughness. Nozzle 10 has an nozzle orifice 11. The angular surface on the tip of nozzle 10 must match with the surface of a sprue bushing 82 on a fixed side mold 86. The diameter of orifice 11 should be a little smaller than that of bushing 82. Care must be taken when determining the diameter. The inside passage nearby the orifice is called "land." The land is an area that prevents the material from drooling. It is the narrowest passage, right before the material is injected from the nozzle into a cavity. The tip of nozzle 10 has an umbrellalike shape, and disk springs 46 are inserted from the other end of nozzle 10. Springs 46 are placed between the edge of body 17 and right behind the umbrella part of nozzle 10, so the end of the umbrella part is always pushed against springs 46 (Fig. 2). There is a gap between a nozzle end 9 surface 15 (Fig. 2) and a nozzle sealing surface 12 (Fig. 2). The inside of the other end of nozzle 10 has a threaded surface and a sliding nozzle 36 is threaded into nozzle 10.

The material of nozzle 10 must be carefully chosen, because each end of nozzle 10 repeatedly hits bushing 82 or surface 12 and must support most of the nozzle touch force from an injection unit. The material of nozzle 10 should be less rigid than that of body 17. It is simply because body 17 is more expensive and should not wear faster than nozzle 10. By the same token, nozzle 10 should be more rigid than bushing 82. The structure of nozzle 10 is more complicated than that of bushing 82, therefore it is harder to make and more expensive.

Sliding nozzle 36 is just like the one used by the conventional sliding nozzle unit. Nozzle 36 (Fig. 1) has a material passage inside and a passage hole 38 on the one end. The diameter of passage 38 (Fig. 2) should be the same as the hole of nozzle 10 on nozzle 36 side so that the two holes match. This end of nozzle 36 is inserted from an injection cylinder 76 side of body 17 (Fig. 1) and threaded together with nozzle 10. Nozzle 10 and nozzle 36 are connected by the threads, so they always move back and forth together. There are sliding nozzle orifices 40 on injection cylinder 76 side of nozzle 36 (Fig. 1 and Fig. 2). There must be one orifice or more. There should preferably be three or four orifices 40. The size of all orifices 40 should be identical, and so should spaces between the orifices. In this way, all the material flow the same distance from injection cylinder 76 to orifice 11. For example, if there are three orifices, a space between the centers of the two orifices should be made at 120° apart. If there are four orifices, a space between the centers of the two adjacent orifices should be at 90° apart. The size of orifices 40 (Fig. 2) should be the same or a little smaller than the gap between nozzle 10 and body 17. A sliding nozzle stopper 44 of nozzle 36 (Fig. 2) has a spear shape. The angle of the spear-shape part should be determined to minimize shear caused by the plasticated material. The surface of stopper 44, which is touching body 17, should be polished so that nozzle 36 smoothly moves in body 17. The surface should be polished less than 6.3 micron of roughness.

The material of nozzle 36 should be less rigid than that of body 17. Nozzle 36 constantly moves back and forth, and stopper 44 of nozzle 36 hits the inside surface of body 17. Since body 17 will be more expensive than nozzle 36, body 17 should be harder and protected from wearing down. Nozzle 36 does not get as much force as nozzle 10, so nozzle 36 does not have to be as rigid as nozzle 10. Though, anti-wear steel is recommended for nozzle 36. Stopper 44 will get high degrees of shear and wear on injection cylinder 76 side from some types of plasticated materials and also from glass-filled materials used by many injection molders. 10

A cylinder head 18 (Fig. 1) consists of three main bodies: shut-off nozzle body 17, a discharging valve body 26, and a manifold drain 32 (Fig. 3A and Fig. 3B). Cylinder head 18 is designed so that it can be retrofitted with most types of injection cylinders for injection molding machines. Cylinder head 18 has bolting holes 30 (Fig. 1 and Fig. 2), and it is bolted down on injection cylinder 76 by hexagonal socket head screws 31. The number of bolting holes 30 depends on the size of injection cylinder 76. Generally, the larger the injection cylinder the more the bolting holes.

Cylinder head 18 should be one piece. It is not easy to machine such a complicated piece. And, if it is damaged, the entire piece must be replaced. However, there are several reasons that it should be so. The first is that temperature conductivity inside cylinder head 18 is better. The temperature of a nozzle and cylinder head is considered to be the most important among all the temperature zones in an injection cylinder. If there are some voids in the parts, they cannot uniformly heat up plastic material. And, there will be the randomness of viscosity within the plasticated material. The second is that its mechanical strength is better. As one can imagine, the threaded part of a metal part is generally weaker than the rest of the part. Since body 17 and body 26 have to endure strong force from inside and outside, they must be firmly fixed with cylinder head 18. And, the third is that the one-piece design has fewer possibilities of material leakage. If the material leaks from cylinder head 18, one cannot obtain accurate injection volumes.

The first main body of cylinder head 18 is shut-off nozzle body 17, which is round and cylindrical. The length and thickness of body 17 depend on the size of an injection unit. The larger the injection unit the larger body 17 is. Nozzle 10 and nozzle 36 are inserted inside the cylindrical part and move back and forth (Fig. 1 and Fig. 2). Springs 46 are placed against the edge of body 17. A band heater for shut-off nozzle body 68 (Fig. 2) is tightly fixed around body 17. A drain 13 is drilled from surface 12 and the outside of body 17. The size of drain 13 also depends on the volume of leakage from injection cylinder 76. A stress reliever 14 (Fig. 2) is made around body 17. The depth and width of reliever 14 depend on the size of an injection unit. A material shut-off sealing surface 16 (Fig. 2) is a place where the material is actually shut off. The angle of the surfaces of stopper 44 and surface 16 must be the same. The angle should be somewhere around 45° in relation to the direction of the movement of nozzle 36. A gas leader 20 (Fig. 2) is in the top inside part of cylinder head 18. Leader 20 is made like a ditch by partially being drilled on the top inside surface. A discharging line 22 should be led from leader 20 to a discharging valve sealing surface 24.

The second main body of cylinder head 18 is discharging valve body 26 (Fig. 1 and Fig. 2), which is placed in the upper part of cylinder head 18. Body 26 does not have to be completely in 11 the top of cylinder head 18 as long as gas can escape from line 22. Body 26, however, cannot be placed in the lower part of cylinder head 18. Body 26 and body 17 should stick out and be placed in parallel, because a locking rod 48 (Fig. 1 and Fig. 2) of the discharging valve unit must hit a shock absorber 84 perpendicularly on a locate ring 80. Locate rings are placed on fixed side molds and they have to fit in a hole in fixed side platens. The locate rings are used literally to "locate" a place to mount on the fixed side platens. Locate ring 80 is fixed on mold 86 which is mounted on platen 78. Although, locate ring 80, bushing 82, platen 78, fixed side mold 86, and movable side 88 are used in my method, these are the conventional parts of injection molding machines and are not unique to my method. Inside body 26 are several parts that make up a relief valve mechanism.

The third main body of cylinder head 18 is illustrated clearly in Fig. 3 A and Fig. 3B. A gas and material drain 28 is in the top part of body 26. Drain 28 is drilled from the surface of cylinder head 18 to surface 24 (Fig. 2). Drain 28 is covered by a drain cover 62. A manifold drain 32 is a longitudinal groove on one side of the surface of cylinder head 18. Drain 32 is covered by a manifold cover 66. A band heater 70 fastens over and tightly covers manifold cover 66. If the material leaks from drain 32, cover 66 can be bolted down on cylinder head 18. The gas and material are discharged from a drain orifice 34.

The material for cylinder head 18 should be the most rigid among all the parts. Anti-wear bimetallic steel is recommendable for cylinder head 18. The material should be heat-treated, and all sliding and sealing surfaces of the parts and plasticated material passages should be micro-polished. The material passages are preferably polished less than 0.8 micron of roughness. The sliding and sealing surfaces should be polished less than 6.3 micron of roughness. All the inside surfaces should be hand-polished.

A relief valve unit that goes inside body 26 is illustrated in Fig. 1 and Fig. 2. The relief valve unit consists of six parts, a spool 60, a relief valve spring 58, a guide tube 54, a spool lock 52, a spring for locking 50, and a locking rod 48. They should be assembled as illustrated in Fig. 1.

Spool 60 (Fig. 2) has a cone shape on the one end and a cylindrical shape on the other. Spool 60 is placed inside body 26 and guide tube 54. At the other end of spool 60 is spring 58. Spool 60 is always pushed toward surface 24 by spring 58. Since spool 60 should not wear fast, the material of spool 60 should be anti-wear steel.

Spring 58 keeps the pressure inside injection cylinder 76 at a predetermined level. The pressure for spring 58 should be somewhere 150 - 300 kgf/cm² depending on pressures required by plastic materials. The pressure inside injection cylinder 76 before the injection is normally not very critical, and one does not have to change the set pressure as long as the pressure is not too high. 12 Even if one wants to change the set pressure for spring 58, all one has to do is just change the load of spring 58. Spring 58 can be a different type of spring, such as a disk spring like the one used for spring 46.

Guide tube 54 is a main body of the relief valve unit. Guide tube 54 is a cylindrical part and has two chambers, the one for spool 60 and spring 58 and the other for lock 52, spring 50, and rod 48. The two chambers are connected with a lock hole 56 where lock 52 is inserted. Guide tube 54 has an outside thread (Fig. 1) on one side and is threaded into body 26. Although guide tube 54 does not directly receive the high degree of force, the material of guide tube 54 is preferably as rigid as cylinder head 18.

Spool lock 52 (Fig. 2) is a cylindrical part. The diameter of lock 52 on the one end where it locks spool 60 is smaller than the other end. The smaller end of lock 52 is inserted in hole 56. The other end of lock 52 is pushed by spring 50, when the injection unit moves forward and rod 48 touches a shock absorber 84. A distance between hole 56 and the larger end of lock 52 should depend on the size of an injection unit. The distance should be long enough for lock 52 to apply force to spool 60. The material of lock 52 must be carefully chosen. Lock 52 must endure the nozzle touch force applied by the injection unit. The force will be reduced by spring 50, so lock 52 actually does not get the entire force. The material should still be strong enough to support itself and spool 60.

Spring 50 is inserted between lock 52 and rod 48 inside guide tube 54. The length and strength of spring 50 is critical and should carefully be considered. Spring 50 can also be made of several disk springs like the ones used for springs 46.

Locking rod 48 is a piston-like part. The one side of rod 48 is flat, and the other side can have either a flat or round surface. The cylindrical surface of rod 48 that is touching guide tube 54 should be polished less than 6.3 micron of roughness. The material of rod 48 does not have to be anti-wear steel.

Cover 62 (Fig. 1 and Fig. 3A & 3B) is a flat-cubical piece. Cover 62 is bolted down by screws for cover 64 on drain 28. Drain 28 is drilled from the surface of cylinder head 18, and is connected to drain 32 (Fig. 3 A & 3B). Drain 32 is a longitudinal groove created on the one side of cylinder head 18. Cover 66 (Fig. 1) is a thin and bending metal piece. Cover 66 covers the one side of cylinder head 18 and fits in the surface of cylinder head 18. Cover 66 is tightly fastened by band heater 70 (Fig. 3A & 3B). 13

Shock absorber 84 (Fig. 2) can be a square or round shape. It should be flat and thick enough to absorb a shock from rod 48. Absorber 84 should be placed on a locate ring 80 where rod 48 hits. Absorber 84 should be made of a hard rubber, or a hard material with some elasticity.

An injection piston tip 72 (Fig. 2 and Fig. 4) of an plasticating screw 75 has a flat and round shape. Unlike most in-line injection piston tips, tip 72 should not have a sharp-pointed tip. Tip 72 should look similar to a piston of plunger-type injection unit.

A check valve 74 has a cylindrical shape. The mechanism of valve 74 is the same as those of the standard check rings. Distinctive features of valve 74 are that it should be heavier and longer than the normal check valves. Valve 74 should look similar to the tips of plunger-type injection units.

In the following, the functions of all the parts described in the previous section will be explained with Fig. 1, 2, 3 A & 3B , and 4 first, and the working procedure of my method with Fig. 5 A, 5B, 5C, and 5D second. Some of the descriptions of the parts will be repeated so that the reader can easily follow physical orientations of the parts.

Nozzle 10 has orifice 11 to inject a plasticated material into cavity 90 (Fig. 2). The tip of nozzle 10 hits against bushing 82 and supports most of the nozzle touch force applied by the injection unit. The end of the umbrella part of nozzle 10 is always pushed against springs 46. Nozzle 10 works like a piston and can slide back and forth inside body 17. Nozzle 10 moves back and forth in a gap between surface 15 and surface 12 while being pushed by springs 46.

Nozzle 36 (Fig. 2) is just like the one used by the conventional sliding nozzle unit. Nozzle 36 moves back and forth with nozzle 10. Stopper 44 stops the movement of nozzle 10 and nozzle 36. Stopper 44 also stops a material flow by sliding forward. Nozzle 10 and nozzle 36 are slid forward by spring 46 when the injection unit moves backward. And, they are slid backward by the nozzle touch force when nozzle 10 touches bushing 82. When nozzle 36 moves backward, orifices 40 are opened and permit the material flow from injection cylinder 76 to orifice 11. When nozzle 36 moves forward, orifices 40 are closed shutting off the material flow.

Body 17 (Fig. 2) consists of nozzle 10, nozzle 36, springs 46, and heater 68. Heater 68 heats up a plastic material that goes through inside nozzle 10 and nozzle 36. Body 17 shuts off the material flow at surface 16 when nozzle 10 is separated from bushing 82. Drain 13 drains the material that has leaked between nozzle 36 and body 17. Reliever 14 absorbs a shock when nozzle 10 touches bushing 82.

Much material leak is not expected from drain 13 because surface 15 shuts off the leak completely while nozzle 10 is touching bushing 82. Nonetheless, there will be a little leakage. When nozzle 10 is touching bushing 82, some heat of nozzle 10 is transferred to bushing 82. This 14 will create a slight temperature difference between nozzle 36 and body 17. This temperature difference, in turn, creates a difference in heat expansion rates of the two steel parts. There is actually no perfect way to seal fluid material and the difference of the heat expansion rates will create more leakage. If there is no leak drain from body 17, the material will build up inside the gap between nozzle 10 and body 17. The material will eventually fill the gap and prevent nozzle 36 from moving back and forth.

Reliever 14 (Fig. 2) is a device that protects the outside surface of nozzle 10. Since nozzle 10 and nozzle 36 are not fixed at any position, there is a possibility that the surface of nozzle 10 will be damaged when nozzle 10 touches bushing 82 from a wrong angle. Sometimes, injection cylinders are not well-fixed, meaning the centers of nozzles move off the centers of sprue bushings of injection molds. Some cylinders can still inject the material by adjusting the centers by their nozzle touch force. This, however, creates strain on the surface of nozzle 10 and the edge of body 17. If the machine is repeatedly used in this condition, the strain will eventually damage either nozzle 10 or the edge of body 17. Reliever 14 eases this strain.

Surface 16 (Fig. 2) is a place where a plasticated material is actually shut off. The angle of stopper 44 and surface 16 must be the same so that the surface between stopper 44 and surface 16 seals well by surface, not by point.

Leader 20 (Fig. 2) is a device to help accumulate gas inside injection cylinder 76. Leader 20 also smoothens a process for the gas to rise up through line 22. Before the gas is pushed out, it is accumulated in line 22.

Body 26 (Fig. 2) discharges gas and some plasticated material. Body 26 does not have to be completely in the top of cylinder head 18. Body 26, however, cannot be placed in the lower part of the cylinder head. Gas is lighter than liquid, so gas is accumulated in the top inside part of cylinder head 18. If body 26 is placed in the lower part, only the plasticated material will be discharged. Body 26 and body 17 should stick out and be placed in parallel, because rod 48 of the discharging valve unit must perpendicularly hit absorber 84 on locate ring 80. Inside body 26 are several parts that create a relief valve mechanism.

Spool 60 (Fig. 2) is the one that actually shuts off the flow of gas and plasticated material in body 26. Spool 60 also keeps the pressure inside injection cylinder 76 at a predetermined level. When nozzle 10 is separated from bushing 82, nozzle 36 shuts off the material flow at surface 16. At the same time, rod 48 is also separated from absorber 84, and spool 60 will only be pushed by spring 50 and be able to control the pressure inside injection cylinder 76. When the pressure passes the set-pressure of spring 58, spool 60 slides back and discharges the gas and material that have 15 come up from line 22. The gas and material are discharged to drain 28. Spool 60 gets approximately the maximum pressure of 2000 kgf/cm² from line 22.

Spring 58 (Fig. 2) keeps the pressure inside injection cylinder 76 at a predetermined level.

Lock 52 (Fig. 2) locks spool 60, when rod 48 touches absorber 84 and is pressed against it. Lock 52 must endure some of the nozzle touch force applied by the injection unit. By the force, lock 52 locks spool 60 and stops the plasticated material coming from line 22.

Spring 50 works to protect spool 60 and lock 52 from getting damaged (Fig. 2). Spring 50 absorbs the force from rod 48 so that the force applied to lock 52 does not exceed a predetermined level. If there is no spring between rod 48 and lock 52, there is a great possibility that either surface 24 or spool 60, or both, will be damaged. Spring 50 also works to prevent nozzle 10 from drooling the plasticated material. After the unit is repeatedly used, there is a possibility that surface 12 will be worn down. As a result, the gap between nozzle 10 and body 17 will be widened. If there is no spring between rod 48 and lock 52, body 26 will get the entire nozzle touch force and surface 12 will not be completely sealed. If this happens, nozzle 10 will be closed only by the pressure applied by springs 46. Since the injection pressure of an injection molding machine is generally much higher than a pressure that spring 46 can support, a large amount of the material will leak between orifice 11 and bushing 82.

Rod 48 (Fig. 2) moves inside guide tube 54 like a piston. Rod 48 is pushed by the nozzle touch force against absorber 84 and presses spring 50. When the injection unit moves backward, rod 48 is separated from absorber 84 and releases lock 52.

Absorber 84 absorbs a shock from rod 48. Absorber 84 also protects locate ring 80.

The manifold part of cylinder head 76 is illustrated clearly in Fig. 3 A and Fig. 3B. Drain 28 and drain 32 drain gas and plasticated material to orifice 34. An advantage of having drain 32 within cylinder head 76 is that one does not have to have another heater element to drain the plasticated material. Since some plasticated material is also discharged from injection cylinder 18, the material must be somehow disposed. One can use a separate drain with a heater element and drop the material down. This may, however, be very costly. A controller used in an injection molding machine normally has a limited number of temperature controlling zones. If one wants to add another temperature zone, one may have to add another electric card to the controller. Drain 32 utilizes an existing heating zone of an injection molding machine controller. The gas and material have virtually no pressure after they are released from injection cylinder 76, so they will not go back into injection cylinder 76. 16

Valve 74 (Fig. 4) is a part of piston head 72. The mechanism of valve 74 is actually just like the one used on the standard check valve. Valve 74 and piston head 72 move together with screw 75. When screw 75 plasticates a solid material fed from a hopper 77 and sends it forward, screw 75 moves backward from the pressure of the accumulated material. When screw 75 moves backward, valve 74 slides forward and creates a path inside valve 74 for the material to go forward. When screw 75 moves forward to inject the material into cavity 90, valve 74 slides back and shuts off the material path. By this mechanism, the majority of the material will be injected. The process explained above is not unique to my method, and is used in most injection molding machines.

Valve 74 (Fig. 4) for my method is longer and heavier than the normal check valves. They are normally designed small and light so that they move quickly and consistently. Valve 74 of my method shuts off a material flow before screw 75 starts injecting the material into cavity 90. Therefore, valve 74 does not cause any back-flow at the time when screw 75 starts moving forward for injection. Since consistent movement is not expected from valve 74, it does not have to be light and small. The long check valves actually have an advantage over the small ones. When screw 75 is supported by a check valve with a large surface area, the movement of screw 75 is more straight than the one with a small surface. As a result, the leak caused at the surface between valve 74 and cylinder head 76 is less than that with the short check valve. This advantage may seem insignificant, but it is important when one does precise injection molding.

Tip 72 is much more flat than most in-line type piston tips that are sharply pointed. The reason is that a flat surface of tip 72 can come much closer to nozzle 36, which is pointed toward tip 72, and that can easily control the amount of cushion. The "cushion" is a term used among injection molders. It is the amount of a plasticated material left inside the injection cylinder right after the injection. It is measured by a distance between the maximum forward-position of a piston tip, which does not change, and an injection-end position of the tip after each shot. It is considered to be a good measurement to compare repeating accuracy of injection molding machines. The more the cushion amount fluctuates, the worse the machine. The flat surface of tip 72 actually does not guarantee a better screw position control. However, if tip 72 is pointed like a cone shape, the piston always has to leave a large amount of the cushion. Since fluid plasticated material has some elasticity, it will not be very good for precise injection.

An actual operational procedure of my method is illustrated in Fig. 5A, 5B, 5C, and 5D in order. The figures show how my method with an plastic injection molding machine should be operated in one cycle. The cycle is repeated many times when the machine is in operation. 17

(1) End of injection, compression, and holding pressure stages

Fig. 5 A shows the conditions of the end of injection, compression stage, and holding pressure stage. The compression stage comes right after the end of injection. Although cavity 90 is mostly filled with a plasticated material after the injection, the material does not totally stick to the surface of the cavity. During the compression stage, tip 72 further presses the material into cavity 90 and enhances the quality of the surface appearance. This is the period when the material pressure generally reaches the highest. The holding pressure comes right after the compression stage. During the holding pressure stage, tip 72 remains at a fixed position with some pressure, and prevents the material in cavity 90 from flowing backward until the material is solidified.

In these conditions, tip 72 is at an injection-end position. Nozzle 10 touches bushing 82, and nozzle 10 and nozzle 36 are slid back. Orifices 40 are opened so that the material passage from injection cylinder 76 to cavity 90 is open. Rod 48 touches absorber 84 and locks spool 60 to shut off the material flow from line 22.

Fig. 5 A also shows the condition when screw 75 charges plasticated material for the next injection. The material charging stage is normally done while an injection nozzle is touching the sprue bushing of a mold. Most injection molding machines do not have a nozzle orifice shut-off device. When the injection nozzle is touching the bushing and the cavity is filled with the material from the previous injection, the plasticated material in the cylinder cannot flow to the cavity. Since a plasticated material for the next injection is blocked by the material of the previous injection, the plasticating screw can plasticate a new material and accumulate it in the cylinder while the injected material is cooling down. Cooling times needed for most molded products are normally longer than their material charging times. Therefore, this is not considered a loss of time during the cycle. This is the reason that most injection molding machines do not need a nozzle orifice shut-off device.

In my method, whether one keeps nozzle 10 touched with bushing 82 during the charging stage is optional. If one wants to keep the temperature of nozzle 10 more stable, it is better to separate nozzle 10 from bushing 82 while screw 75 is charging the material. Since my method has a device to shut off the material flow, one does not have to keep nozzle 10 with bushing 82.

(2) Injection unit retract and clamping unit open

Fig. 5B shows the conditions when injection cylinder 76 is retracted and a movable side mold 88 is opened to eject the solidified material as a product.

Nozzle 10 is separated from bushing 82, and nozzle 10 and nozzle 36 slide forward to shut off the material flow. As having been explained, the nozzle unit slides forward by the force of spring 46 when nozzle 10 is separated from bushing 82. Spool 60 is released from lock 52, because rod 48 18 is separated from absorber 84 at the same time when nozzle 10 separated from bushing 82. At this point, spool 60 is only pressed by spring 58 from behind. The material inside nozzle 10 and nozzle 36 does not usually drool from orifice 11 because of the land — the inside narrow space right before orifice 11.

Tip 72 is at a charging-end position. At this point, injection cylinder 76 still contains an excessive material and gas.

(3) Gas discharge stage

Fig. 5C shows a stage when tip 72 presses the charged material and pushes out gas from drain 28. When the material is plasticated and charged in front of tip 72, the material emits gas. Some gas is pushed back through a passage inside of valve 74 and the rest of the gas remains with the charged material. The remaining gas rises up to the top surface of injection cylinder 76. This process is enhanced especially when the material is decompressed by tip 72. Tip 72 moves forward and the discharged gas rises up through line 22. When a pressure inside injection cylinder 76 passes over the set pressure of spring 58, spool 60 slides back and discharges all the gas and some material from drain 28. After injection cylinder 76 becomes free of gas, tip 72 moves a little farther and stops at an exact injection starting position to obtain a final injection volume. Since there is no way of knowing how much gas is in the plasticated material, screw 75 (not shown in Fig. 5C) always charges slightly more material than the product requires. So, one always should discharge a little material to make sure that gas is completely out of injection cylinder 76. Some material being discharged from injection cylinder 76 indicates that the gas has been removed from the cylinder. By this point, valve 74 has slid back and shut off the material passage inside the check valve.

This stage is actually the reason that my method is very effective for electric injection molding machines. When they charge a material, they do not have to apply back-pressure to the material. The screw of the machine can just plasticate the material and move backward. The screw roughly charges the material then adjusts a piston position later. Since the piston does not apply backpressure to the material while the screw is charging it, there will be a lot of gas in the cylinder. However, no matter how much gas is in the cylinder, the gas will be discharged from drain 28 in my method. And, one can still obtain an accurate injection volume by adjusting the piston position. My method will enable the electric machines to be as fast and accurate as the hydraulic machines. By incorporating my method, the pressure control of the electric machines will be much simpler.

(4) Injection cylinder forward & injection

Fig. 5D shows when tip 72 injects a plasticated material into cavity 90. After gas is discharged and an injection volume is accurately measured, injection cylinder 76 moves forward. Rod 48 19 touches absorber 84, presses lock 52, and locks spool 60 to shut off the material flow. Nozzle 10 touches bushing 82, and orifices 40 on nozzle 36 are opened. A signal to indicate that nozzle 10 is at an injection position is normally sent from a limit switch (not shown anywhere in the drawings) placed somewhere on injection cylinder 76. The limit switch is a feature that most injection molding machines have, and a position that the switch is installed to send the signal to the machine's controller is normally changeable. As soon as the limit switch sends the signal to the controller, tip 72 starts moving forward to inject the material. The entire cycle described from Fig. 5 A to Fig. 5D above is repeated many times.

Fig. 6 shows one of the examples that the discharging valve with a shutting device operates with an outside actuator. After screw 75 pushes out gas and some material, spool 60 stops the flow of the material by a force applied by a clamp 92. Clamp 92 is actuated by a cylinder 94. Cylinder 94 replaces the process done by the injection cylinder reciprocation described in the previous paragraphs.

Fig. 7 shows the discharging valve with a shutting device without a shutoff nozzle. Before screw 75 and valve 26 remove gas, a material flow from cylinder 76 to cavity 90 is stopped by a sprue runner, a part of the material injected on the previous injection. When screw 75 discharges gas and some material, the runner blocks the material flow to a mold. After the discharging stage, mold 88 is opened and the runner is ejected with the solidified material in cavity 90. When a shutoff nozzle is not used, the injection cylinder does not have to reciprocate to open and close the material flow. In this case, valve 26 cannot have spring 58. Spring 58 works to control a pressure inside the cylinder. If the pressure after the runner is ejected is more than the atmospheric pressure, there is a great possibility that the material goes out from the nozzle. Valve 26 must be a simple "open-and-close" type valve so that it equalizes the material pressure to the atmosphere. When these pressures are equal, the material will not go out even if the nozzle is open. Timings to open and close valve 26 must be carefully controlled.

Accordingly, the reader will see that, my vented method for injection molding with a discharging valve with a shutting device, can remove gas from an injection cylinder, can adjust injection starting positions and eliminate the inconsistency of check valve movement by discharging some plasticated material, and can easily enable the pressure control of an electric injection molding machine. Furthermore, the vented method for injection molding with a discharging valve with a shutting device has additional advantages in that

• it does not call for a special vented screw and injection cylinder and can be retrofitted with most injection molding machines; 20

• it does not permit the entrance of oxygen into an injection cylinder so that one can prevent several oxygen-sensitive materials from decomposition and degradation;

• it does not require special cleaning for the injection unit when one changes material. Although the descriptions above contain many specificities, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. For example;

1. my vented method with a discharging valve with a shutting device, can be used for a variety of materials such as engineered plastics, plastics with additives, thermoset plastics, rubbers, silicones, cement, ceramics, powder metal, or a mixture or mixtures of above;

2. my method can be used for plunger-type injection molding machines or different applications such as reaction injection molding, transfer molding, or injection blow molding;

3. my method can be used with different configurations of injection molding machines such as horizontal, vertical, or diagonal;

4. my method can be larger, smaller or have a different shape;

5. my method can be used by employing a different power source for the shutting device for the discharging valve such as a pneumatically, hydraulically, electrically, or mechanically actuated device;

6. my method can be used with a different check valve of an injection piston or even without a check valve;

7. my method can be used with a different type of shut-off nozzle such as a needle-type or rotary- type shut-off nozzle or even without a shutoff nozzle;

8. my method can be used with a discharging valve with a different mechanism;

9. my method can be used with the locking rod of my invention placed on the clamping unit side of an injection molding machine, not the injection side;

10. my method can be used with an additional piston to remove gas;

11. my method can be designed for the discharged material to go back into the injection cylinder for recycle.

Although No. 9 above is not mentioned in the previous description section, it is recommendable for the actual operation of my method. It is a little costly to attach a locking rod on the mold side. However, the locking rod, in addition to its original purpose, can keep the injection unit from moving off the center of a sprue bushing.

Thus the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.

Claims

21CLAIMSI claim:

1. In combination with an injection molding machine, the injection molding machine having a piston acting in a cylinder and having a mold a method to discharge gas comprising the steps of: providing a valve with a valve shutting device; stopping the flow of material from the cylinder to the mold; actuating the piston in the cylinder; opening the valve; stopping the piston; and shutting the valve.

2. In combination with an injection molding machine, the injection molding machine having a piston acting in a cylinder and having a mold a device to discharge gas, comprising of a valve with a valve shutting device on the cylinder and the piston to push out the gas.