Start-up procedure for underwater pelletizer
A method for starting up or stopping and restarting an underwater pelletizer is described. The method involves inducing partial freeze-off of the polymer melt in the holes of an internally insulated die plate to prevent polymer drool by passing full water flow across the face of the die plate. The method advantageously does not require using hot oil to heat the die plate or decreasing and/or increasing the heating provided to the die plate.
1 . A method for starting up a thermoplastic resin pelletizer, comprising:
(a) providing a thermoplastic resin pelletizer having an internally insulated die plate;
(b) heating a thermoplastic resin melt in the pelletizer to a temperature of Tm+30° C. to Tm+200° C., where Tm is the melt transition temperature of the thermoplastic resin as measured by ASTM D-3418;
(c) heating the die plate to a temperature of Tm+50° C. to Tm+250° C.;
(d) passing water having a temperature between Tm−20° C. to Tm−50° C. across the face of the die plate for an amount of time sufficient to solidify at least a portion of the thermoplastic resin melt in the holes of the die plate before extruding the thermoplastic resin melt through the die plate; and
(e) extruding the thermoplastic resin melt through the die plate.
2 . The method according to claim 1 , wherein the thermoplastic resin is low density polyethylene.
3 . The method according to claim 1 , wherein the die plate is heated with steam.
4 . The method according to claim 3 , wherein the steam has a pressure of about 100 psi to about 600 psi.
5 . The method according to claim 1 , wherein the water is passed across the face of the die plate for 1 minute to 30 minutes before extruding the thermoplastic resin melt through the die plate.
6 . A method for starting up a pelletizer, comprising:
(a) providing a pelletizer having an internally insulated die plate;
(b) heating a low density polyethylene melt in the pelletizer to a temperature between about 230 and 300° C.;
(c) heating the die plate with steam to a temperature between about 225 and 260° C.;
(d) passing water having a temperature between about 60 and 90° C. across the face of the die plate for 1 minute to 30 minutes before extruding the polyethylene melt through the die plate; and
(e) extruding the polyethylene melt through the die plate.
7 . The method according to claim 5 , wherein the die plate temperature is about 225° C., the polyethylene melt temperature is about 250° C., and the water temperature is about 75° C.
8 . A method for stopping and restarting a thermoplastic resin pelletizer having an internally insulated die plate, comprising:
(a) stopping extruding a thermoplastic resin melt through the die plate;
(b) maintaining water flowing across the face of the die plate at a temperature used during extrusion for an amount of time sufficient to solidify at least a portion of the thermoplastic resin melt in the holes of the die plate;
(c) maintaining the thermoplastic resin melt in the pelletizer at a temperature used during extrusion;
(d) maintaining the die plate at a temperature used during extrusion; and
(e) restarting extruding the thermoplastic resin melt through the die plate.
9 . The method according to claim 8 , wherein the water flowing across the face of the die plate is maintained at a temperature of Tm−20° C. to Tm−50° C., where Tm is the melt transition temperature of the thermoplastic resin as measured by ASTM D-3418.
10 . The method according to claim 8 , wherein the thermoplastic resin melt in the pelletizer is maintained at a temperature of Tm+30° C. to Tm+200° C.
11 . The method according to claim 8 , wherein the die plate is maintained at a temperature of Tm+50° C. to Tm+250° C.
12 . The method according to claim 8 , wherein the thermoplastic resin is low density polyethylene.
13 . The method according to claim 8 , wherein the die plate is heated with steam.
14 . The method according to claim 13 , wherein the steam has a pressure of about 100 psi to about 600 psi.
15 . The method according to claim 8 , wherein water flowing across the face of the die plate is maintained for 1 minute to 30 minutes before restarting extruding the thermoplastic resin melt through the die plate.
16 . The method according to claim 8 , wherein the pelletizer comprises rotatable blade(s) for cutting extruded thermoplastic resin into pellets and wherein the blade(s) are maintained rotating while the thermoplastic resin melt extruding through the die plate is stopped.