Underwater pelletizing machine and method of extruding foamed thermoplastic pellets

A pelletizing machine for forming pellets from extruded material includes an extruder having at least one exit port and a body defining a cutting chamber through which high-temperature liquid flows, flooding the cutting chamber. The exit port of the extruder opens into the cutting chamber, which includes a cutting section defining a flow path for liquid through the cutting section. A cutter in the cutting section of the cutting chamber is mounted for rotation about an axis generally perpendicular to the first direction and disposed for cutting the extruded material exiting the exit port into the pellets. The axis of rotation of the cutter is parallel to or coincident with the flow path of the cutting section. A method for forming pellets of thermoplastic material with a foaming agent is also disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an underwater pelletizing machine and method of extruding foamed thermoplastic pellets.

2. Background of the Invention

Conventionally, the foamed thermoplastic pellets are made in a batch process. First, the foaming agents and thermoplastic materials are put in a container under a high temperature and high pressure environment (e.g., an autoclave). Then the mixture is released into a foaming chamber to form foamed thermoplastic pellets. However, during the manufacturing process, a high pressure is necessary for the foaming agent. A high-pressure container necessary to manufacturing is expensive and greatly increases manufacturing costs. The volume of the thermoplastic materials expands tens of times into foamed thermoplastic pellets, therefore a very large container is necessary to collect the foamed thermoplastic pellets. In addition, the suspension and dispersion of the thermoplastic materials in a fluid further hinders the penetration of foaming agent, which is in gas phase, into the thermoplastic material. This reduces the production capacity of the equipment after long-term operation. The conventional production method not only has high production cost but also has low production efficiency. Therefore, the price of products from foamed thermoplastic material is relatively high. Due to the high cost, the applications of products from foamed thermoplastic materials are limited and can not be broadly utilized.

In view of the problems of the aforementioned prior art, a method of continuously manufacturing foamed pellets was developed, as described in Taiwan Patent No. I269698. The method of extruding foamed thermoplastic pellets includes a thermoplastic material, a foaming agent, an extruder, a high-pressure underwater pelletizing machine, a high-pressure tube, a foaming chamber, a separation device, and a storage tank. The thermoplastic material is first uniformly mixed with the foaming agent in the extruder. The molten thermoplastic material containing the foaming agent is extruded by the extruder to the high-pressure underwater pelletizing machine, which cuts the thermoplastic material into a plurality of small cylinders. A high-pressure tubes containing high-temperature liquid then transports the cylinders of the thermoplastic material away from the underwater pelletizing machine. In the transportation process, the thermoplastic cylinders, which are in molten state, gradually transform into small spheres because of the surface tension of the thermoplastic materials

The high-pressure tube is connected to a foaming chamber. The foaming chamber has a temperature-control device to regulate the temperature of the thermoplastic material entering the foaming chamber from the high-pressure tube. There is a nozzle (a “pressure drop device”) connecting the high-pressure tube to the foaming chamber. The molten thermoplastic material and high temperature transportation liquid are injected into the foaming chamber through the nozzle. A pressure drop occurs through the nozzle, which induces the foaming process. When the molten thermoplastic material is released into the foaming chamber, the thermoplastic material is cooled down by the lower temperature regulated by the temperature-control device of the foaming chamber to form thermoplastic pellets. The high temperature transportation liquid turns into steam when injected through the nozzle into the foaming chamber. The steam is condensed by a condenser installed with the temperature-controlled device of the foaming chamber. The steam includes unused foaming agent. When the steam is condensed, the unused foaming agent can be collected in the foaming agent recycle tank.

In addition, there is low-pressure tube connected to the foaming chamber. The low-pressure tube is connected back to the underwater pelletizing machine. A temperature-controlled device regulates the temperature in the low-pressure tube. The low-pressure tube moves the low temperature liquid from the foaming chamber back to the underwater pelletizing machine of the extruder. There is another duct connected to the foaming chamber. The thermoplastic pellets are carried by the flow of the low-temperature liquid to the separating unit out of the foaming chamber, which separates and moves the thermoplastic pellets into a foamed thermoplastic pellets storage tank. The low-temperature carrying liquid separated from the thermoplastic pellets in the separating unit is later transported back to foaming chamber through a transportation tube.

The underwater pelletizing machine includes a cutter driven by a motor that cuts extruded strands of thermoplastic material (including the foaming agent) into pellets (i.e., the cutter pelletizes the extruded thermoplastic. The cutter is located in a chamber into which the thermoplastic material is extruded. However, the rotation of the cutter, which is oriented generally perpendicular to the flow of fluid through the chamber, causes substantial turbulence that can interfere with the granulation process.

SUMMARY OF THE INVENTION

In one aspect of the present invention, a pelletizing machine for forming pellets from extruded material generally comprises an extruder having at least one exit port, and a body defining a cutting chamber. The exit port of the extruder opens into the cutting chamber, which includes an inlet for passage of liquid into the cutting chamber and an outlet for passage of liquid and pellets out of the cutting chamber. The cutting chamber further includes a cutting section defining a flow path for liquid through the cutting section. A cutter in the cutting section of the cutting chamber is mounted for rotation about an axis generally perpendicular to the first direction and disposed for cutting the extruded material exiting the exit port into the pellets. The axis of rotation of the cutter is parallel to or coincident with the flow path of the cutting section.

In another aspect of the present invention, a method of extruding foamed thermoplastic plastic pellets generally comprises a step of uniformly mixing thermoplastic material and foaming agent. The thermoplastic material mixed with the foaming agent is extruded to a pelletizing machine. High temperature liquid is forced over a cutter rotatable about an axis of rotation parallel to or coincident with a flow path of the liquid over the cutter. Extruded thermoplastic material mixed with the foaming agent is cut into pellets by rotation of the cutter; and the thermoplastic pellets are transported out of the pelletizing machine using the high-temperature liquid.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1shows a prior art system for extruding foamed thermoplastic pellets. Referring toFIG. 3, the method of extruding foamed thermoplastic pellets of the present invention comprises the following steps.

Step I,101: Mix uniformly thermoplastic material and blowing agents in an extruder11. Extrude the molten thermoplastic material containing the blowing agent to a high-pressure underwater pelletizing machine12that is connected to a high-pressure tube13. The high-pressure underwater pelletizing machine12(described in more detail hereinafter) continuously converts the thermoplastic material into a plurality of small thermoplastic cylinders, which are transported to the high-pressure tube13containing high temperature liquid. The high-pressure tube13can sustain the high temperature liquid. The liquid may be water or other suitable liquid.

Step II,102: The high-pressure tube13connects the high-pressure underwater pelletizing machine12to a foaming chamber14(broadly, “a container”). A pressure drop device or nozzle141in the end of high-pressure tube13connects to foaming chamber14. The molten thermoplastic material is released to the foaming chamber14through the nozzle141and cooled down by the first temperature control device142externally connected to the foaming chamber14to form thermoplastic pellets. The thermoplastic pellets manufactured are first-staged foamed materials. The high temperature liquid is released to the foaming chamber14along with the thermoplastic pellets carried by the liquid, and phase changes to steam. The steam of the high-temperature liquid is cooled down and condensed by the first temperature control device142. Some of the foaming agent from the pellets will be released into the steam. Unused foaming agent in the steam can be collected in the recycling tank143externally attached to the first temperature control device142. The first temperature control device142can condense the steam into liquid and partially recycle the foaming agent by collecting in the recycling tank143. If the temperature is lower than the softening point of the thermoplastic material, non-foamed pellets, which have the capability to be foamed, could be obtained. The bottom of the foaming chamber14has a low-pressure tube15, which is connected back to the high-pressure underwater pelletizing machine12in order to transport the low temperature liquid in foaming chamber14back to high-pressure underwater pelletizing machine12. A pump152and back pressure regulator153in line with the tube15are used to increase and control the pressure of the water heading back to the pelletizing machine12. A heating device151downstream from the pump152in the low-pressure tube15heats up the liquid before entering high-pressure underwater pelletizing machine12.

Step III,103: A transportation duct16is connected to the top of foaming chamber14. The thermoplastic pellets in the foaming chamber14are continuously affected by the flow of the low-temperature liquid and are transported from the foaming chamber14to the separation unit17via transportation duct16. These are second-staged thermoplastic pellets, which internally contain some foaming agent that can increase the internal pressure of thermoplastic pellets to help the subsequent product forming. The thermoplastic pellets separated in separation unit17are moved to storage tank18. A tube171connected to separation unit17transports the low temperature liquid in separation unit17to foaming chamber14. A blower172is externally connected to the separation unit17to help transport the foamed pellets into the storage tank18. Normally the pressure of the foaming chamber14is set to ambient. In another embodiment, the pressure of the foaming chamber14may be increased by a pressure control device (not shown), or by placing a back pressure regulator (not shown) between first temperature control device142and recycling tank143. The amount of foaming agent in the thermoplastic pellets can be controlled by controlling the pressure drop across the nozzle141. In that regard, pressure in the high pressure tube13can be regulated by a back pressure valve13afor controlling the pressure drop across the nozzle141.

Referring toFIG. 2, another embodiment of a prior art system for extruding foamed thermoplastic plastic pellets is shown. A third temperature control device131is installed on the high temperature tube13connecting the high pressure underwater pelletizing machine12and foaming chamber14. The third temperature control device131controls the liquid temperature inside high-temperature tube13to manipulate the size of the pellets in coordination with the pressure inside the high-temperature tube13. Due to the better control of the liquid temperature in the high-temperature tube13, the bubbles inside the foamed thermoplastic pellets are dense and small. In addition, the high temperature liquid can be nano liquid or other liquid, which is more viscous. The regular high-temperature liquid more readily blocks the nozzle, so by utilizing the higher viscosity of nano liquid, the liquid flow speed could be controlled and could eliminate the blockage phenomena at the nozzle.

Referring now toFIGS. 4-7, an extruder11and a high pressure underwater pelletizing machine12according to the present invention are shown. The extruder11includes an extrusion die1that contains the flow channel10in a body of the die that provides a path for the thermoplastic material to enter the pelletizing machine. The body is formed by upper and lower body members2and3, respectively, which have semi-cylindrical cavities21and31. There is at least one row of extruding holes311(broadly, “exit openings”) inside the semi-circular cavity31of the lower cover body3, which are directly attached to the extrusion die1. The die also has channels connecting the extruding holes311with the flow channel10of the extrusion die1. When the upper and lower body members2and3are connected together using suitable fasteners such as screws9, a cylindrical space is formed to accommodate a cutter4. The upper and lower body members2and3are attached to the extrusion die1.

The cutter4has a series of grooves or flutes, each extending along a path that is a segment of a spiral and generally lengthwise of the cutter. Edges of the grooves cut the strands of thermoplastic material extruded from the respective extruding holes311of the lower body member3into pellets. The cutter4is located in the cylindrical space formed by the semi-circular cavities of upper21,31and lower body members2and3. The two, reduced diameter ends of the shaft of the cutter4are journalled in bearings51and61of the two holding structures5and6. The holding structures5and6mount the bearings51and61in the centers of holding structures to hold ends of the cutter4. The bearings51,61having ribs51a,61athat engage the interior of the holding structures5,6to locate the bearings and to provide passage for the high temperature liquid. Two cones52and62are mounted on outer ends of the bearings51and61to direct the water flow radially outwardly to the ribs51a,61aand around the cutter4. The holding structures5and6are fixed to the sides of the upper and lower body members2and3with screws9. The water-guiding structures7and8connect tube71for entering water and tube81for exiting water to the holding structures5,6. The two water-guiding structures7and8are attached the outer ends of holding structures5and6with screws9.

The high temperature liquid is directed from the heating device151to the underwater pelletizing machine through tube71of the water-guiding structure7. The cone52of the holding structure5directs the high temperature liquid flowing through the space in between the ribs51aof the support frame51. The rapidly flowing high temperature liquid enters the grooves of the cutter4, causing the cutter to rotate at high speed in the bearings51and61to perform the cutting process. Referring toFIGS. 6 and 7, when the thermoplastic material is extruded through the flow channel10of the extrusion die1to the multiple extruding holes311of the lower body member3, the thermoplastic material is cut into pellets by the fast rotating cutter4. The same high temperature liquid that causes rapid rotation of the cutter4also carries the thermoplastic pellets out of the pelletizing machine. The thermoplastic pellets flow through the space in between the ribs61aof the support frame61of the holding structure6and exit the underwater pelletizing machine through tube81of the water-guiding structure8. An external pressurizing system can be installed in the present invention to regulate the flow speed of the high temperature liquid in order to control the rotational speed of the cutter4.

The cutter4in the improved underwater pelletizing machine of the present invention has the feature of cutting the pellets in a longitudinal direction. The rotation of the cutter4is driven by the flow of the high temperature liquid, which smoothes the cutting processes and does not generate turbulent flow. In addition, the rotation of the cutter is driven by the water flow, there is no need for electrical power, which reduces the electricity expense.

Referring now toFIGS. 8-14, a pelletizing machine of a second embodiment is shown to comprise a body201(including an extrusion die) having an internal flow channel210, only portions of which are illustrated in the drawings (see,FIG. 9). The body201is connected to an extruder212, gear pump or other suitable device (not shown). A pressure sensor211communicates with the flow channel210to read the pressure of the thermoplastic material. The body201includes multiple body members that are joined together in a suitable manner such as by fasteners209. First and second body members, designated222and223, respectively, sandwich a manifold holding member224between them. The second body member223defines a portion of the flow channel210including an annular flow channel section225that opens toward the manifold holding member224(see,FIGS. 9 and 14). The manifold holding member224holds an annular injection manifold226having an axially projecting flange227having injection ports228(broadly, “exit ports”) extending radially through the flange at equal circumferentially spaced locations around the flange (see,FIGS. 9 and 13). Different spacings of the injection ports are possible within the scope of the present invention. Thermoplastic material mixed with foaming agent is formed through the flow channel210and through the injection ports228. Heating tubes229and230are received in recesses in the second body member223and the manifold holding member224(respectively) heat the second body member and manifold holding member to maintain the molten, flowable state of the thermoplastic material in the body201at the location where it is extruded.

Referring still toFIG. 9, the body201further includes holding members231,232located on opposite sides of the first and second body members222,223and are connected to the body members by the fasteners209. The holding members231,232mount bearings233,234supporting a cutter204for rotation about a longitudinal axis of the cutter. The bearings233,234are preferably bearings, such as ceramic or stainless steel bearings, suitable for operation in a high temperature and pressure environment. The bearing233receives a reduced-diameter end of the cutter204, and the bearing234receives the cutter entirely through the bearing on its way out of the body. A gland235is received around the cutter204and seals against the holding member232. An outlet member236is attached to the holding member231opposite the first body member222.

The members222,223,224,231,232,236of the body201collectively define a liquid flow path along which the high temperature liquid flows through the body. The liquid flow path includes a cutting chamber237in which the extruded thermoplastic material is cut into pellets. An inlet238of the liquid flow path is formed in the holding member232and receives heated liquid under pressure into the body201. The inlet238extending in a generally radial direction of the holding member232and intersects a generally axial opening239athrough the second body member. The gland235seals the axial opening239aon the outside of the holding member232through which the cutter204extends out of the body201. The axial opening239ais aligned with corresponding axial openings239b-239fin the second body member223, manifold holding member224and injection manifold226, first body member222, holding member231and outlet member236. The axial opening239cincludes coaxial openings in both the holding member224and the injection manifold226. The axial openings239a-239fextend through the body201to define a flow path for the liquid that is parallel to or coincident with an axis of rotation of the cutter204. A pressure sensor240mounted in an opening in the holding member232is able to detect the pressure of the liquid coming into the body201.

The axial openings239b,239c,239ddefine the cutting chamber237in the illustrated embodiment. These axial openings239b-239dreceive parts of a cutting portion of the cutter204having flutes241formed by spiral grooves in the cutter. The axial opening239d(also referred to as “a diverging portion”) in the first body member222has an outwardly flaring diameter, and the cutter204has a corresponding outwardly flaring diameter portion243in registration with the axial opening. The surfaces defined by the axial opening239dand the cutter portion243guide the liquid and pellets carried by the liquid radially outwardly in addition to transporting in through the body201so that the liquid may smoothly pass around the bearing233rotatably mounting the end of the cutter204. The bearing233is held in a central portion231aof the holding member231that is connected to radially outer portions of the holding member by circumferentially space vanes245(see,FIG. 11). The vanes split the axial opening239einto several passages around the bearing233for flow of liquid and pellets. Downstream of the bearing233, the flow path converges due to the decreasing diameter of part of the axial opening239fin the outlet member236in the downstream direction. The axial opening239fconverges to a smaller diameter portion suitable for connection to a conduit (not shown). A cone247mounted on the central portion231aof the holding member231provides a surface generally parallel to the decreasing diameter portion of the axial opening239fThe shapes of the cone247and the decreasing diameter portion of the axial opening239ffacilitate smooth flow of liquid through the radial transition and inhibits turbulence in the flow.

Referring now toFIG. 12, the flutes241of the cutter204are generally curved so that they extend along a segment of the spiral path. The radially outer edge of the flutes241also function as cutting edges to cut strands of extruded thermoplastic material coming out of the injector ports228. The annular arrangement of the injector ports228allows the portion of the cutter204containing the flutes241to be confined to a relatively short axial extent of the cutter. It is to be understood that although the flutes241extend along a spiral path, they may extend in other configurations, including straight axially within the scope of the present invention. Moreover, while the configuration of the flutes241provides at last some rotation of the cutter204in the illustrated embodiment, the flutes need not interact with the flowing liquid to provide rotation. Still further, structure (not shown) capable of interacting with the flowing liquid to provide rotation of the cutter204could be separate from structure (not shown) that cuts the extruded thermoplastic.

The right end of the cutter204(as oriented inFIG. 12) is formed to have a reduced diameter and a key249. The key allows the cutter to be connected to a shaft251of a motor253external to the body201(FIG. 8). The motor253drives rotation of the cutter204in the body201, which is supplemented by the flow of the liquid over the flutes241. The gland235seals with the portion of the cutter204passing out of the holding member232to allow rotation of the shaft while sealing the interior of the body201against leakage of the liquid.