Electric heating – Metal heating – By arc
Reexamination Certificate
1999-11-23
2002-11-26
Elve, M. Alexandra (Department: 1725)
Electric heating
Metal heating
By arc
C219S121640, C219S121850
Reexamination Certificate
active
06486432
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to the field of plasticating components, such as screws and barrels, used for extruding plastic. In particular, the present invention is directed to a structure for a more abrasion-resistant and corrosion-resistant plasticating barrel, and a technique for manufacturing the lining of the improved barrel.
BACKGROUND ART
Extruders and tubers (rubber extruders) have been in use at least since the beginning of the twentieth century. With the advent of plastics, the demand for such extruders has become greater and the processing conditions have become more severe. Originally such devices were essentially a simple screw rotating inside a single-material barrel without a lining. This is no longer the case due to the newer and more difficult to process materials.
Both of these components are subject to wear from metal-to- metal contact, and from abrasive and corrosive fillers in the plastic or rubber compounds. The original barrels had an internal surface that was nitrided to give improved abrasive wear resistance. In the later 1950's bimetallic barrels were developed using a centrifugal casting process, as briefly described in the Spirex publication, entitled
Plasticating Components Technology,
1997, incorporated herein by reference. Also, such improved barrels were adapted for use with injection molding machines, in addition to conventional extruders.
Centrifugal casting of plasticating barrels is a process used to line the internal surface of a barrel with an abrasion and/or corrosion resistant liner that is different from the barrel backing material or substrate. The process involves installing a lining material, such as a powder, inside the heavy-walled barrel cylinder at room temperature. The ends of the barrel are capped (usually welded) and the barrel and unmelted powder are placed in a casting oven. The barrel is then rotated and heated until the liner material metals are melted uniformly distributed on the internal surface of the barrel. Early liner materials were iron/boron materials that created some metal carbides and were very much more wear resistant that the nitrited barrels. In 1968 improved liners became more abrasion resistant by the addition of very small, discrete metal carbides particles like tungsten carbide and equivalent materials.
Most rotational casting ovens are gas heated but some are induction heated. In either case, the inside of the barrel must be heated to a point where the liner powder melts, but the thick-walled barrel material or substrate does not melt. After melting is accomplished the barrel is slowly cooled so that stresses are not induced, and so that the liner material does not crack. After cooling, the barrel is honed, straightened and machined to it's final dimensions. Often this requires installation of a high-pressure sleeve at the discharge end of the barrel.
There are a number of disadvantages to this technique. The gas fired or induction furnaces with rotating equipment are very expensive, and require extensive maintenance. This includes periodic and prolonged shutdowns to reline the refractory surfaces of the oven. Further, even when the furnaces are functioning properly, set up for the coating of each barrel is an awkward and time, consuming process.
Also, the process of centrifugal coating requires that the liner material or material matrix melt at a lower temperature than the backing or substrate material. This creates severe limitations on the liner materials than can be used. As a result, abrasion-resistant and corrosion-resistant materials are limited to formulas that melt at a lower temperature than the barrel substrate. In many cases the optimum barrel substrate and under materials cannot be used for the materials to be handled.
There is also the requirement of raising the backing or substrate material to a temperature close to the melting point of the substrate material followed by a slow cooling to anneal the backing material. This lowers the strength of the annealed backing material. Unfortunately, very high strengths are now required because such barrels can be subject to internal pressures of 40,000 psi or higher, and temperatures up to 700 deg. F. These conditions require the installation of a high pressure sleeve at considerable expense. Some newer, higher priced alloys can reduce this effect somewhat by reducing the loss of strength. However, greater expense is incurred.
During the rotational casting process the heavier metal carbide particles tend to be thrown outward by centrifugal force. This moves these particles away from the inside surface where they are needed for abrasion resistance. As a result, the resulting lining is far more susceptible to wear caused by abrasion than if the metal carbide particles are properly located on the inner surface of the lining or evenly distributed throughout the lining or cladding.
The high barrel temperatures that are reached during casting make it difficult to maintain the straightness which is critical to the plastic processing operation. Straightening of the barrel cannot be done by conventional straightening presses because reverse bending cracks the relatively brittle liner. The rotational casting process requires a long time to heat up the liner and barrel substrate. Additional time is required for slow cooling after the lining operation. This causes added expense in labor and electrical costs.
Because the lining process can only be successful in a very narrow range of time and temperatures, often the results are not satisfactory. High temperatures and long time periods spent at these temperatures cause dilution by migration of the substrate material into the barrel lining material. This causes poor hardness and poor abrasion resistance. Also substrate migration of the base iron material can cause poor corrosion resistance in certain applications. Extended periods at high temperatures also cause the metal carbide particles coating the liner inner surface to melt into solution in the matrix matter (constituting the liner) rendering them useless.
When temperatures are too low and the time periods at properly elevated temperatures are too short, an inadequate metallic bond can result. Such an inadequate metallic bond means that the liner may become separated from the barrel substrate or backing material. This condition could render the entire barrel useless. Further, in some cases portions of the liner may come dislodged, corrupting the molten plastic and/or fouling the screw pushing the molten plastic through the barrel. In either case, the barrel is subject to catastrophic failure, and the plastic processed therein ruined.
A totally different method to produce barrel liners is constituted by laser welding or cladding. Laser cladding is laser welding of a different surface onto a base or substrate metal. This new process diminishes or eliminates all of the disadvantages listed above.
The more conventional MIG or TIG welding of the inside diameter (ID) of barrels can be done, but it is more difficult to get into smaller diameter barrels. The zone affected by heat is much greater, and the welded surface is poorer, causing much greater expense in finishing compared to the “near-net shape” of laser-welding.
Laser welding of the ID of barrels involves the depositing of the liner material prior to welding in the form of paste or a separate liner tube, or during welding with a powder or continuous wire. The laser welder usually includes a laser beam delivered from a remote source via fiber optics and optical systems, or by direct laser beams.
This technique has a number of advantages. For example, devices have been made that will allow laser welding into diameters as small as ¾ inch. Laser cladding also has a very shallow heat-affected depth which gives much less dilution of the liner material into the barrel substrate. This technique also creates much less stress in the substrate, reducing the tendency to bend or warp.
Laser cladding is a relatively robust process that allows a wide latitude of materials to
Bodnar Shawn P.
Colby Paul N.
Colby Paul T.
Gatti Joeo M.
Lev Robert G.
Spirex
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