FIELD OF THE INVENTION

The present technology relates to an extruder screw, including an extruder screw configured to maximize heat transfer to particles of solid material extruded thereby, improve mixing and minimize solids wedging.

BACKGROUND OF THE INVENTION

This section provides background information related to the present disclosure which is not necessarily prior art.

Extruder screws are used for transforming solid materials into a molten mixture for subsequent use in a mold or die via extrusion or injection. Materials that are used for this purpose include a range of plastic materials, as well as various metal or thixotropic materials. An extruder screw can have a generally cylindrical body or root with at least one helical thread or flight formed thereon. The outer diameter of the helical thread can be referred to as the main diameter of the screw, the area of the body between revolutions of the helical thread can be referred to as a channel of the screw, where a channel depth at given point can be defined as a difference between a diameter of the body at the given point and the main diameter. For a given revolution of the helical thread, the thread can define a pushing surface and a trailing surface bounding the channel, where revolution of the screw advances material in a direction from the pushing surface toward the trailing surface. In some screw designs, two channels of different depths can be formed between one or more revolutions of the primary helical thread defining the main diameter of the screw, where a secondary or barrier thread can form a barrier between the two channels. The screw can be disposed in a cylindrical barrel and a small clearance can exist between the main diameter of the screw and the interior wall forming the barrel. The barrel can be heated in order to conduct heat to the solid material contacting the barrel as the solid material progresses through the channels to promote melting of the solid material.

Extruder screws can include a feed section, a transition or compression section, an optional mixing section, and a metering section. The feed section can be formed at a feed end of the screw where the solid material is first introduced into the channels of the screw. The feed section of the screw can have a substantially constant channel depth that is relatively larger than the depth of the channels formed in the transition section or the metering section. The transition section of the screw can have the one or more channels formed between each turn of the primary helical thread gradually decreasing in depth as the screw extends in a direction from the feed end thereof to a discharge end thereof. Accordingly, the transition section can have an increasing outer diameter of the body of the screw to give the transition section of the screw a substantially conical or tapered appearance. The mixing section can include multiple channels of different depths separated by one or more barrier threads, where the channels can be formed between one or more revolutions of the primary helical thread. The metering section of the screw can have a substantially constant channel depth that is relatively smaller than the depth of the channel formed in the feed section.

Since the late 1950s, due to work by Bruce Maddock of Union Carbide, the solid bed theory of melting has dominated extruder screw design. Subsequent improvements in screw design have been based upon this theory. Although the theory is now taken as fact, as proven by numerous experiments and by melting equations developed by Tadmore and Klein in the 1960s, the solid bed theory may not be the only way of melting plastic in an extruder screw. The natural behavior of the unmelted plastic, entering the extruder screw from the feed hopper and conveyed forward by the helical thread, can be formed by channel depth reduction into a solid mass of unmelted plastic no matter the original solid material feed form (e.g., pellet, powder, or regrind). This solid mass of material is referred to as a solid bed. As this solid bed is forwarded down the screw channel transition section, the channel depth reduction causes the solid bed to be forced into pressurized contact with the extruder barrel, forcing melting to occur at the barrel/solid bed interface due to the energy of shear stress in the melt film. By the forwarding action of the helical thread, the melt can then be scrapped off of the barrel surface by the trailing surface of the thread forcing it into a developing melt pool at the rear of the solid bed. Since the solid bed itself can take the form of a tightly compacted mass, no mixing of the plastic in the solid bed can occur. In addition, at the end of the channel depth taper, the remaining solid bed can exert very high pressures in the channels. This pressure can result in high screw thread wear—a phenomena known as “solids wedging.” Further downstream, since the channel depth remains constant and thus no further compression can occur, the solid bed can break up into solid particles that float within the melt stream.

Most screw design developments to date have been aimed at completing the melting and mixing of the remaining solids to ensure a uniform melt exiting the extruder. Some designers, such as Maillefer and Barr, have controlled the solid bed by using a barrier thread or flight in the screw channel. This can be at a different lead than the main helical thread thus forcing the solid bed to be reduced in width as it moves down the screw channel, as per the Maillefer design (U.S. Pat. No. 3,358,327), or by using a same lead of the barrier thread between the solids and melt channels but reducing the solids channel depth to force the solid bed onto the barrel surface, as per the Barr design (U.S. Pat. No. 3,487,503). Variations of these concepts have been created over the last almost 60 years, but all still rely on the solid bed theory of melting the solid material.

Barr's later patent (U.S. Pat. No. 4,405,239), called the ET screw, proposed improved melting of the remaining solid bed, at the end of the tapered or transition section of the screw, by shuffling the melt/solid mix back and forth between adjacent channels over a barrier thread to improve conductive melting of the remaining solids. This design has proven successful as has its successor, the VBET screw (U.S. Pat. No. 6,599,004 also to Barr). Such designs ensure that no stationary melt film can exist around the remaining solid particles, thus improving the heat transfer from melt to solid.

One continuing problem is that the solid bed mass, in solid bed melting, is only exposed to heat input from its four sides within the channel; i.e., barrel surface, melt pool, screw body surface, and forward thread surface. Accordingly, it would be advantageous to provide a way to ensure that the solid material (e.g., pellet, powder, regrind) is surrounded by melted polymer. This would allow for maximizing heat transfer into the relatively small solid material particles. If such melting can be initiated as soon as there is a melt film on the barrel, then the solid bed may not be able to form. Screw thread wear could therefore be minimized as any “solids wedging” is thereby reduced or substantially eliminated.

BRIEF SUMMARY OF THE INVENTION

The present technology includes articles of manufacture, systems, and processes that relate to an extruder screw and uses thereof that improve melting of extruded material.

Extruder screws having a feed end and a discharge end are provided that can be used in a single screw extruder, which houses the extruder screw rotatably in a cylindrical barrel for processing a material to a molten state. Such extruder screws include a body, a helical thread formed on the body, and a channel for receiving the material to be processed. The body extends longitudinally between the feed end and the discharge end. The helical thread has a pushing surface and a trailing surface. The channel is defined between the outer surface of the body and the outer diameter of the helical thread bounded by the pushing surface and the trailing surface of the helical thread. In a transition or melting section of the screw, just after the feed section, a volume of the channel decreases over a plurality of revolutions of the helical thread in a direction from the feed end to the discharge end. Each revolution of the thread defines a separate channel portion. In one embodiment, starting at the feed end, a first part of successive channel portions is increased in depth and then decreased in depth, while a second part is decreased in depth and then increased in depth. In another embodiment, starting at the feed end, channel portions having a decreasing inclined depth, a constant depth and an increasing inclined depth are provided.

Objects of the present technology include the use of a changing screw channel depth profile to keep solid particles of processed material moving back and forth in the channel. This can be accomplished by having two, or more, channel depth profiles between the helical threads of the screw. A volume of the screw channel can be gradually reduced from the feed end of the screw transition section to the end of melting determined by a chosen compression ratio. With approximately each revolution of the helical thread or turn of the screw from the inlet, the forward portion of the channel depth, near the tailing surface of the helical thread, can be reduced while the rear portion of the channel depth, near the pushing surface of the helical thread, can be increased. The feed material can therefore be shuffled from the front to the back of the channel and then in a subsequent revolution of the helical thread, the action can be reversed forcing the particles to move into the forward portion of the channel. This shuffling action minimizes the formation of a solid bed of material and also allows the melt from the barrel surface to be mixed into any remaining solid particles, further promoting melting. Absence of a barrier thread in the transition section where melting occurs allows free movement of any solid particles from the forward channel portion to the rear and vice versa. The channel profile can also be configured to taper the channel depth from one side of the channel to the other.

Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

