BACKGROUND OF THE INVENTION

The invention relates to an apparatus for conditioning plastic pellets, that is granular synthetic plastic resins, for use in plastic molding or extrusion machines.

Prior to injection molding or extruding plastics, it is important to ensure that the raw material, which commonly comprises plastic material in pelletized or granular form, is essentially free of any moisture, so as to eliminate flaws in the finished product. Many resins used in plastic forming techniques are hygroscopic, and prior to injection or extrusion molding, the pellets are conditioned in conventional conditioning machines which may require many hours to remove moisture trapped within the plastic. There are many patents disclosing granular plastic conditioning apparatus, for example U.S. Patent 3,875,683 (Waters), 4,773,168 (Lamous et al) and 5,033,208 (Ohno et al) . All of the patents referred to above require a relatively large flow of heated air through a mass of pellets, which requires considerable energy input, not only for heating air, but also for moving the air through the mass of pellets. In addition, heat transfer between air and solids is relatively inefficient, thus requiring more energy input.

An improved apparatus for conditioning plastic material is shown in U.S. Patent 4,531,308 (Neilson et al) , in which one of the co-inventors is the inventor of the present invention herein. This improved apparatus reduces the time and energy requirements for drying the plastic pellets considerably from earlier prior art machines, as well as providing a far simpler device than some prior art machines. The improved apparatus relies primarily on thermal conductivity and radiation to heat the pellets, and

does not require a large flow of heated air, thus reducing energy requirements. It has also been found that time to process a batch of similar material in the improved apparatus is usually much shorter than the time required in the earlier patented machines described briefly above. In the improved apparatus, the pellets are heated by direct contact with a heated heat transfer surface, by radiation and/or by heat transferred by contact with adjacent warmed pellets. However, one difficulty experienced with the improved device relates to maintaining accurate surface temperature so as to prevent overheating of the heat transfer surfaces which could unintentionally melt the resin and foul the interior of the device. The improved device of the patent discloses use of insulating spacers to reduce chances of overheating due to contact between the heater and structure which could result in some areas having undesirably much higher temperatures than other areas, termed "localized hot spots".

To optimize extraction of moisture from pelletized plastic, the pellets should be rapidly heated to a temperature as close as possible to the thermal deflection temperature of the plastic, commonly called "sticking point", and yet the thermal deflection temperature must not be attained otherwise surface melting will occur and fouling of the device will result. By heating the pellets quickly, problems associated with long term heating or "soaking" of the plastic material are reduced. Optimally, the pellets should be maintained at approximately 5 degrees F (about 2.7 degrees C) below the "sticking point", but because most heaters can only maintain uniform surface temperatures within plus or minus 5 degrees F, to avoid overheating the temperature control must be set at a temperature well below optimum temperatures for extracting moisture, thus increasing processing time. One of the problems associated with accurate temperature control relates to different rates of heating of items having different heat capacities,

for example for a given heat input, a particular item made of aluminum will heat up much faster than the same item made of steel.

SUMMARY OF THE INVENTION

The invention reduces the difficulties and disadvantages of the prior art by providing a conditioner for pellets which heats surfaces to transfer heat to the pellets in such a manner that temperature of the surfaces can be accurately and uniformly controlled to prevent overheating of the pellets. In addition, air is drawn through the heated pellets by an air mover located at an exhaust of the apparatus, and an intake of the air is restricted thus subjecting the chamber containing the pellets to sub- atmospheric pressure, so as to enhance drying. The chamber containing the pellets is provided with a plurality of heated fins to provide a relatively large surface area to transfer the heat to the pellets. The fins are subjected to a generally uniform temperature along their length which can be relatively close to the "sticking point" so as to optimize the heating of the pellets.

A conditioner according to the invention comprises a main body, a plurality of fins, a heater and an air conduit.

The main body has a main wall defining in part a main chamber within the body, the main chamber having a generally vertical, main longitudinal axis. The chamber has an inlet to receive pellets and a discharge to discharge the pellets. The fins extend longitudinally of the main body and each fin has an outer fin portion adjacent the main wall, and an inner fin portion located inwardly of the main wall. The heater is located within the fins to heat the fins generally uniformly. The air conduit has an inlet portion to receive air, a pre-heating portion communicating with the inlet portion, an extraction portion communicating with the pre-heating portion and the

main chamber, and an outlet portion communicating with the extraction portion to discharge air from the conduit.

Preferably, the extraction portion of the air conduit has a perforated side wall, and the pre-heating portion of the air conduit comprises a longitudinally extending passage provided within each fin and exposed to heat from the heater. Preferably, the heater comprises a primary heat source and an axially extending heat tube. The heat tube is disposed generally vertically and contains a working fluid to accept heat from the primary heat source, and to emit heat selectively along the axis of the tube to maintain a generally uniform temperature. Also, an air mover cooperates with the air conduit to move air from the inlet portion to the outlet portion of the air conduit. Preferably, the inlet portion of the air conduit has a restriction, and the air mover is a vacuum fan located adjacent the outlet portion of the air conduit so as to draw air through the conduit and to expose the main chamber to sub-atmospheric pressure so as to enhance drying of the pellets.

An alternative condition according to the invention comprises a main body having a main wall defining in part a main chamber within the body, the main chamber having an inlet to receive pellets, and a discharge to discharge pellets. The conditioner also comprises a heater extending into the chamber, the heater comprising a primary heat source and a heat tube, the heat tube containing a working fluid to accept heat from the respective primary heat source and to emit heat selectively along the heat tube to maintain a generally uniform temperature along the tube.

A detailed disclosure following, related to drawings, describes a preferred embodiment of the invention which is capable of expression in the structure other than that particularly described and illustrated.

DESCRIPTION OF THE DRAWINGS

Figure 1 is a simplified, partly fragmented longitudinal section of a conditioning apparatus according to the invention using vertically disposed heaters, plane of the section being generally on line 1-1 of Figure 3,

Figure 2 is a simplified top plan view of the apparatus, and

Figure 3 is a simplified transverse cross section, generally on line 3-3 of Figure 1,

Figure 4 is a simplified diagrammatic section through an alternative conditioning apparatus according to the invention using horizontally disposed heaters,

Figure 5 is a simplified transverse cross section on line 5-5 of Figure 4,

Figure 6 is a simplified diagram of an alternative eductor vacuum pump, and

Figure 7 is a simplified fragmented section through a portion of an apparatus showing an alternative electrical resistance heater.

DETAILED DESCRIPTION

Figures 1-3

Referring mainly to Figure 1, a conditioning apparatus 10 according to the invention has a main body 12 having a main cylindrical wall 14 defining in part a main chamber 16 within the body. The main chamber 16 has a generally

vertical, main longitudinal axis 18, an inlet 19 to receive pellets at an upper end of the axis, and a discharge 20 to discharge pellets at a lower end of the axis. An insulated sleeve 21 surrounds the wall 14 to reduce heat loss therethrough. The inlet 19 has an upper flange portion 22 secured to a hopper 24, only a portion of which is shown, the hopper receiving pelletized plastic resin as is well known. The apparatus 10 has a base portion 26 which has an upper rim portion 23 cooperating with the wall 14, and a downwardly tapering funnel portion 27 defining the discharge 20. The portion 26 has a mounting flange secured to an inlet of a conventional plastic forming machine 25, the machine having a feed screw for conveying pellets to a heater for later forming by extrusion or injection molding.

The apparatus 10 further comprises a plurality of fins 28 extending longitudinally of the main body. The fins 28 have upper portions 29 generally adjacent the inlet 19 of the main chamber, and lower portions 35 adjacent the discharge 20 of the main chamber. In the present embodiment, as seen in Figure 3, six generally similar fins 28 are shown extending radially from the axis 18, and thus only one fin will be described. The fin 28 has an outer fin portion 30 located adjacent the main wall 14, and an inner fin portion 32 located inwardly of the main wall but spaced from the axis 18 for reasons to be described.

As seen in Figure 3, the outer fin portion 30 of the fin 28 has a generally triangular cross section defined by a curved base portion 45, and straight side portions 47 and 48 extending inwardly therefrom to converge relatively steeply to a vertex portion 31. The fin 28 further includes an intermediate fin portion 33 which tapers relatively slowly inwardly from the vertex portion 31 towards the inner fin portion 32. The portions 45, 47 and 48 are relatively thin walls, whereas the intermediate portion 33 is solid. The base portion 45, side portions 47

and 48 and vertex portion 31 define a conduit 50 which extends longitudinally through the outer fin portion generally adjacent a maximum width thereof and serves as a portion of an air conduit 34 as will be described. Clearly, the outer fin portion has a cross-sectional area greater than the inner fin portion to provide adequate space for the conduit 50.

As seen in Figure 1, the air conduit 34 transports air through the apparatus through a convoluted route to pre¬ heat the air prior to passing around the pellets to remove moisture driven off from the pellets by heating. The air conduit 34 has six parallel routes, one route passing initially through each fin 28, and these routes unite into a single route adjacent lower portions of the fins. Each of the six routes has an inlet portion 36 to receive air, and each inlet portion has a restriction to restrict flow of air therethrough. The air conduit further includes the conduit 50 of each fin which serves as a pre-heating portion communicating with the respective inlet portion 36. The conduit 34 also has an extraction portion 42 which receives air from the six conduits 50 and communicates with the main chamber and with an air mover housing 38 through a connector portion 40. The housing 38 has a rotatable vacuum fan 39 to draw air through the air conduit prior to discharge through an outlet portion 37. Undesignated arrows show direction of air flowing downwardly through the inlet portions 36, downwardly through the pre-heating portions 50, upwardly through the extraction portion 42, downwardly through the connector portion 40 and into the air mover housing 38 and then upwardly through the outlet portion 37.

The extraction portion 42 of the air conduit is a generally cylindrical tube 52 having a perforated side wall extending concentrically about the axis 18 and disposed closely adjacent the inner fin portions 32. Thus the tube 52 and

the fins 28 divide the main chamber 16 into six equally shaped minor chambers defined by opposed faces of adjacent fins 28, and opposed faces of portions of the main wall 14 and the cylindrical tube 52 extending between the adjacent fins. Each minor chamber is filled with pellets, many of which contact the sides of the minor chamber.

As seen in Figure 1, the conditioning apparatus 10 further comprises a heater 55 located within the fins to heat the fins generally uniformly by conduction, and thus the fins are preferably fabricated from material having a high thermal conductivity such as aluminum or copper. The heater 55 comprises a pair of annular electrical resistance heating elements 57 located adjacent the lower portions 35 of the fins, the heater having a control system to maintain relatively accurate temperature control of the elements, i.e. within 3 degrees F. The heating elements extend around and are in intimate thermal contact with a cup-like holder 59 which locates lower ends of the fins to hold them in spaced apart relationship. As seen in Figure 3, the holder 59 has complementary recesses to receive the lower ends of the fins, which are insert cast therein so as to be in intimate thermal contact therewith to ensure efficient heat transfer to the fins. The lower portions of the outer fin portions 30 have a tapering portion 61 to provide an outlet for air leaving the pre-heating portion or conduit 50 to pass between the heating element 57 and an opposing wall of the base portion. The holder 59 and the base portion 26 define a narrow annular channel 60 adjacent the funnel portion 27 and the discharge 20 of the chamber to transfer air flow from the pre-heating portion 50 to the extraction portion 52.

The heater 55 further comprises a heat tube 63 disposed generally vertically within the vertex portion 31 of each fin. The heat tube is a known heating device for maintaining highly accurate temperatures along a relatively

long length and is a sealed tube containing a working fluid selected for a particular condensing temperature. The fluid accepts heat from and is evaporated by a primary heat source, which in this instance is the electrical resistance element 57, and condenses at specific locations along the tube, which locations initially are at a temperature below the condensing temperature. Clearly, latent heat of condensation is emitted when the vaporized working fluid condenses at a particular location, thus raising the temperature of that location. Vaporized working fluid will continue to condense at that particular location until the temperature reaches the temperature of the vaporized fluid, and thus the temperature of the location cannot exceed the condensing temperature. The particular working fluid and operating pressure of the heat tube depends upon the specific application, and in the apparatus 10 the working fluid is water as the operating temperature of the heat tube is selected to be between 175 and 500 degrees F (80 and 260 degrees C) .

Suitable heat tubes obtained directly from manufacturers usually have relatively thin walls to enhance heat transfer, and such heat tubes can be inserted within complementary bores within the vertex portion 31 of each fin 28 to be in intimate contact with the fin for efficient heat transfer. Preferably, the fin itself should have a bore to serve as the heat tube itself so as to simplify manufacturing and to further enhance heat transfer into the fin. Thus the heat tube passes longitudinally along each fin and is located inwardly of the pre-heating portion or conduit 50 of the air conduit. For one example of the apparatus 10, for a typical fin having a length of 24 inches (600 mms) power input to the heating element 57 is approximately 200-250 watts, and temperature along the length of heat pipe can be controlled to be within about 2- 3 degrees F (1 to 1.6 degrees C) of an operating temperatures of 400 degrees F (204 degrees C) . Clearly,

temperature of the heating elements 57 must not exceed the sticking point of the plastic so as to avoid contamination of the lower portions of the fins.

Thus the working fluid accepts heat from the heating element 57, and then emits heat selectively along the axis of the tube to maintain a generally uniform temperature along the fin. Because each fin has a heat tube extending longitudinally therethrough, heat from the respective heat tube passes outwardly from the tube along the side portions 47 and 48, as well as along the intermediate fin portion 33 to the inner fin portion 32 adjacent the cylindrical tube 52. It can be seen that the primary heat source, namely at least one of the annular electrical resistance heating elements 57, is essentially contiguous with the lower portions 35 of the fins to ensure adequate heat transfer from the heating element into the lower portion of the fins and eventually the heat pipe. The heater 55 is located within the fins to heat the fins generally uniformly and to provide a surface temperature dependent on condensation temperature of the fluid and thus is accurately controlled. The temperature can be selected to be slightly below the sticking point of the plastic resin, so as to optimize venting of moisture from the plastic without risk of approaching the sticking point which could otherwise cause fouling of the apparatus.

The funnel portion 27 has a downwardly extending annular lip 66 spaced inwardly from an inner wall of the lower mounting base portion 26 to define an annular extraction opening 68 therebetween adjacent the discharge 20 of the main chamber. An extraction conduit 69 communicates the opening 68 with the air mover housing 38 so as to expose the annular extraction opening 68 to sub-atmospheric pressure within the air conduit 34, i.e. the pressure within the housing 38. The extraction conduit 69 thus draws any vapor generated in the forming machine 25

outwardly from the inlet opening of the forming machine, thus further reducing any tendency of moisture to be trapped within the melted plastic.

OPERATION

A batch of plastic pellets is fed into the hopper 24, and the pellets pass through the inlet 19 and fall under gravity down the six minor chambers within the chamber 16 (defined by the six fins 28) to accumulate on the funnel portion 27 adjacent the inlet of the forming machine 25. When the main chamber 16 is filled with pellets, the hopper 24 is filled to provide additional pellets to replace those that are removed from the discharge 20 of the apparatus. The two heating elements 57 and the motor of the vacuum fan 39 are supplied with power so that the heat pipes rapidly attain operating temperature, and the main chamber is subjected to sub-atmospheric pressure as air is drawn through the restriction adjacent the inlet portion 36 and then discharged through the outlet portion 37. When both the heating elements are operating, the heat pipes attain operating temperature relatively quickly and thus heat the fins 28 rapidly to the same temperature.

The fins are heated generally uniformly because any "cold spots" in the fins cause condensation of the fluid at an adjacent location within the heat tube, thus receiving more heat of vaporization from the condensing fluid. Clearly, air passing down the pre-heating portions or conduit 50 of the fins similarly becomes heated, which heat is further augmented as the air passes around the resistance heating elements 57 and into the extraction portion 42. As the fins attain operating temperature, pellets in contact with or immediately adjacent the fins become heated first and vent off any entrapped moisture. Pellets remote from the fins receive radiant heat from the fin, or heat by conduction from adjacent heated pellets. Air passing

upwardly through the mass of pellets contained within the chamber picks up moisture from surfaces of the heated pellets, and the moisture-containing air is extracted either directly from below the bottom of the cylindrical tube 52, or indirectly through the perforations in the side wall of the tube 52 adjacent upper portions of the fins.

It can be seen that the inlet 19 of the main chamber is generally adjacent a downstream portion of the extraction portion 42 of the air conduit, i.e. a portion closest to the outlet portion 37 of the air conduit. Thus, it can be seen that the invention effectively provides a counter-flow heat exchanger in which atmospheric air leaving the pre¬ heating portion 50 is driest and at its highest temperature, and as the air passes upwardly through the extraction portion 42 its humidity is increased to a maximum which is achieved within the tube 52. In this example, the heat pipes are always disposed generally vertically with the primary heat source located at a lowermost portion thereof but this is not necessary as will be explained. The primary heat source heats the working fluid which emits heat selectively along the axis of the tube to maintain a generally uniform temperature along the heat tube. Heated moist air is drawn from the top of the extraction porion 42 into the connecting portion 40 and out to atmosphere through the outlet portion 37.

Simultaneously, any additional water vapor given off by the pellets located adjacent the inlet of the plastic forming machine 25 is similarly removed through the extraction conduit 69 cooperating with the annular extraction opening 68.

ALTERNATIVES

While a specific embodiment of the invention has been described and illustrated, such embodiment should be

considered illustrative of the invention only and not as limiting the invention as construed in accordance with the accompanying claims. For example, other directions of flow of pellets and extraction air can be arranged but gravity feed of the pellets and counterflow of the air are considered to be the most effective. While the main body is shown having a circular cross section for engineering efficiency, clearly a body of square or rectangular cross section could be substituted, with the fins arranged in a suitable manner, e.g. a rectangular grid, to provide intimate contact with the pellets.

Figures 4 and 5

An alternative conditioning apparatus 75 has a main body 77 having a main wall 79 which can be rectangular in cross- section, or it can be a surface of revolution centred on a main longitudinal axis 80. The axis 80 passes through the centre of a main chamber 81 defined by the wall 79, the main chamber having an inlet 82 to receive pellets, and a discharge 84 to discharge pellets into a plastic molding machine 86, shown fragmented.

The conditioning apparatus 75 further includes a plurality of generally horizontally disposed heaters 89 extending generally transversely into the chamber, and the main wall

79 supports ends of the heaters. The heaters are arranged in a staggered array, in which upper and lower rows 91 and

92 of heaters are vertically aligned with each other, whereas a middle row 93 is staggered with respect to the upper and lower rows so as to increase the chances of contact between the pellets and the heater. As seen in

Figure 5, the heaters are contained within a generally triangular cross-sectioned fin 90 and similarly to the heater 55 of Figure 1, the heaters 89 comprise a primary heat source 96 and the elongated heat tube 97. The heat tube located within the fin 90 contains a working fluid to

accept heat from the primary heat source and to emit heat selectively along the axis of the tube and fin and as in the previous embodiment. Clearly, other arrangements of tubes and fins can be devised to increase chances of contact with a gravity fed stream of pellets. As in the first embodiment, the heat tubes provide the advantage of maintaining generally uniform temperature along the heat tube, thus enabling the heat tube to operate at a temperature optimally close to the sticking point of the plastic.

Figure 6

While a rotary vacuum fan 39 is disclosed in Figure 1 to subject the air conduit to sub-atmospheric pressure, an alternative eductor vacuum pump 100 can be substituted. The vacuum pump has an annular discharge nozzle 102 extending around a pump conduit portion 101 located with an alternative air mover housing 103 which in turn communicates with the air conduit 40 and outlet portion 37. The pump further comprises a pressurized driving fluid duct 105 which extends around the nozzle 102, and receives high pressure fluid, e.g. air, from a supply input 106 communicating with the duct 105. When pressurized driving fluid is discharged from the nozzle 102 into the portion 101, the connector portion 40 and associated conduit portions are subjected to sub-atmospheric pressure as before. Thus, the ejector vacuum pump functions generally equivalently to the rotary vacuum fan 39, but does not require any moving parts, and can be operated on normal shop air to provide sub-atmospheric pressure within the chamber. Other types of discharge nozzles or fluid driven pumps can be substituted, providing the advantage of eliminating moving parts and corresponding wear and maintenance problems.

Fiqure 7

Fins are preferably heated by heat pipes which simplify temperature control and essentially eliminate any chance of "localized hot spots" on the surface of the fins, thus essentially preventing fouling of the fins or chamber. In some circumstances, elongated electrical resistance heating elements 110 can be substituted for the heat pipes, provided very accurate temperature control can be attained along the complete length of the fin so as to prevent localized overheating and fouling of the fins. The alternative electrical heating elements 110 are fitted within longitudinal complementary bores 112 within the fins, as used for the heat tubes. The elements 110 are controlled to provide essentially uniform temperatures along the length of the fins, thus reducing or avoiding "localized hot spots".