GB2282985A - Production of plastics sheet material - Google Patents

Abstract

In a process for the production of plastics panels, plastics material is extruded in screw 3, filtered at filter 4 and pumped by gear pump 5 through a flexible lip die 6 to form a sheet which is cooled by passage over three successive calender rolls 7, 8 and 9 before being edge trimmed at trim-station 12 and cut to desired length at cutter 13. A protective plastics film may be applied to one or both surfaces of the sheet before the edge trimming stage. <IMAGE>

Description

Production of Plastics Sheet Material" The invention relates to a process for producing plastics sheet material, particularly plastics panels.

There are many processes for producing plastics panels, however, there is an increasing need for improvements in such processes so that the panels can be formed in a cost and time efficient manner to desired specifications.

This invention is therefore directed particularly towards providing improvements in processes for producing plastics panels.

According to the invention there is provided a process for producing plastics panels comprising the steps of: mixing virgin plastics material; feeding the plastics material into an extruder screw; extruding the plastics material to a molten condition: filterina the extrudate; pumping the extrudate; adjusting the width of a flexible die; passing the extrudate through the flexible lip die to form a deposit of molten plastics material of desired width; passing the molten plastics material over a first calender roll for a first cooling of the material; passing the plastics material over a second calender roll for a second cooling; passing the plastics material over a third calender roll for a third cooling; passing the cooled plastics material along a conveyor; trimming the side edges of the plastics material; and cutting the material to a desired length to form a plurality of sheets of plastics material.

In a particularly preferred embodiment of the invention, the process comprises the steps of passing a separate stream of water through each of the calender rollers, sensing the temperature of each water stream exiting from the rollers and heat no or cooling each water stream to obtain water at a desired temperature for recycling to the respective calender rollers. Preferably the water is cooled by passing it through the tubes of a shell and tube heat exchanger and passing cooling water through the shell of the heat exchanger. Preferably the water is heated by passing it through an electrically powered immersion heater, the immersion heater being switched on in response to a lower than target temperature to heat the water for the calender roller.

In a particularly preferred embodiment of the invention, the process includes the step of applying a protective film of plastics material to at least one face of the plastics sheet material.

Preferably, the plastics film is passed over a guide and tensioning roller and applied to the plastics sheet material prior to trimming by passing the film and plastics sheet material between a pair of spaced-apart nip rollers. Typically, prior to trimming excess film is trimmed from the side edges of the sheet material and collected for recycling.

In a particularly preferred arrangement, a protected plastics film is applied to both faces of the plastics sheet material. In this case preferably excess film at the side edges is removed from the upper anc lower faces of the sheet material and collected separately for recycling.

In a preferred arrangement, there are a plurality of rolls of plastics film material mounted adjacent to the plastics sheet material conveyor and a film of a desired width is led from the associated film roll and applied to the sheet material.

In a particularly preferred embodiment of the invention, the process includes the step of regrinding the trimmings from the sheet material, delivering the regrind material to a regrind material hopper, delivering the regrind material to a mixing screw and proportionately mixing the regrind material with the virgin plastics material.

Preferably the process includes the step of delivering waste sheet material to a below-floor regrinder, regrinding the waste material and delivering the reground material to the regrind hopper.

In one embodiment of the invention, the process includes the step of removing toxic gasses from the extruder screw by a vacuum pump, condensing the gases and collecting the concentrated condensate in a collection drum.

In a preferred arrangement, the process includes the steps of: cutting a sample strip of sheet material from a formed sheet; immersing the sample strip in a bath of hot oil; measuring the deviation in the dimensions of the sample strip from a desired range; and adjusting the process parameters to correct any deviation in performance from the desired range.

In a particularly preferred embodiment of the invention, the width of the flexible lip die is adjusted by closing off the side edges of the lip for a desired amount by sliding seals partially across both sides of the die.

Typically, the seals ~comprise strips of heat resistant plastics material such as PTFE carried on sealing heads which are slidably mounted to the die for movement to desired positions to set the width of the extrudate material exiting from the flexible lip die.

In one embodiment of the invention, the process includes the step of extruding a second plastics material and injecting the extrudate into the die.

In a preferred arrangement, the virgin plastics material is formed by dry blending a plurality of different plastics material.

The invention also provides plastics panels or sheet material whenever produced by the process of the invention.

The invention will be more clearly understood from the following description thereof, given by way of example only with reference to the accompanying drawings in which: Fig. 1 is a side elevational view of apparatus used in a process for producing plastics panels according to the invention, Fig. 2 is an end view of a feed part of the apparatus of Fig. 1, Fig. 3 is a perspective view of portion of a flexible lip die forming part of the apparatus of Fig. 1, ig. 4 is a -ront iew of the die of Fig. 3, Fig. 5 is a cross sectional view of part of the die, Fig. 6 is a diagrammatic view of a calender roll temperature controlling apparatus used in the method of the invention, Fig. 7 is a perspective view of protective film applying apparatus used in the method of the invention, Fig. 8 is a side view of the apparatus of Fig. 7, Fig. 9 is a side view of another protective film applying apparatus used in the method of the invention, Fig. 10 is a perspective view of the apparatus of Fig. 9; Fig. 11 is a plan view of a cutting knife used in the method of the invention, and Fig. 12 is a side, partially cross sectional view of the cutting knife of Fig. 11.

Referring to the drawings, and initially to Fig. 1 thereof, thare is illustrated a process for producing plastics panels according to the invention comprising the steps of mixing virgin plastics material, feeding the plastics material through a hopper 2 into an extruder screw 3 in which the material is extruded to a molten plastic condition. The extrudate is filtered in step 4 and pumped by a gear pump in step 5 to pass the extrudate under pressure to a flexible lip die 6 in which the molten plastics material s formed into S sheet of molten material of desired width and thickness. The plastics material is then passed between first and second calender rolls 7, 8 and then between second and third calender rolls 8, 9, the material being cooled as it passes over the rollers as will be described in more detail below.

The partially cooled plastics sheet material is then passed along a conveyor 10 to a protective plastics film applying station 11 at which a film of plastics, typically polyethylene material, is applied to either one or both faces of the sheet material as will be described in more detail below. In the process of the invention, the side edges of the plastics sheet material are trimmed at a trimming station 12 and the continuous sheet is cut into panels of a desired length at a cutting station 13. The cut sheets are tested, collected, stacked and packaged.

Referring particularly to Fig. 2, raw plastics material for the feed hopper 2 to the extruder 3 is provided from a virgin plastics material feed hopper 25 which is fed from a main hopper 26 through an intermediate hopper 27.

The virgin plastics material is delivered to the hopper 2 by a virgin plastics feed screw 28 in a feed pipe 29 having an outlet 30. Regrind plastics material is delivered to the hopper 2 from a regrind intermediate hopper 30 to a regrind feed hopper 31, from which the regrind material is delivered by a regrind screw 33 mounted in a regrind feed pipe 32 having a regrind outlet 34. Similarly- masterbatch material is delivered through an intermediate hopper 36 to a masterbatch hopper 37 from which it is delivered by means of an extruder screw through an outlet to the main feed hopper 2 of the extruder 3. The feed screws 28, 33 for the virgin, regrind and masterbatch materials are driven by electric motors and the proportions of the virgin, regrind and masterbatch materials introduced into the hopper 2 is adjusted by varyina the speeds of the feed screws. In this way, the optimum proportions of virgin, masterbatch and regrind material may be attained for desired feed materials.

The extruder screw 3 is typically a three stage single screw, the temperature in each of the stages being closely controlled. Volatile gases which are generated in the extruder screw 3 are led away from a top outlet 40 by a vacuum pump 41 along a pipeline 42 to a condenser 43.

Condensate from the condenser 43 is collected in a drum 45. Gases passing through the condenser are also collected and further condensed, the condensate being returned to the drum 45. In this way, substantially all of the toxic volatiles generated in the extruder 3 are collected in the drum 45 for disposal of the concentrated material in a controlled manner.

The flexible lip die 6 used in the method of the invention is diagrammatically illustrated in more detail in Figs. 3 to 5. The die 6 includes upper and lower jaws 50, 51 which are spaced-apart to define therebetween a gap 52, the thickness of which is adjusted across the length of the die. To control the width of the molten plastics material exiting through the gap 52, a gap width adjustment means is provided on both sides of the gap.

Each width adjustment means comprises a sealing strip 59 of heat resistant material, typically PTFE, mounted on a sealing head 60 carried on a mounting bracket 61 which is slidably mounted on a slide 62 fixed to the die as will be particularly apparent from Fig. 5. The bracket 61 is moved along the slide 62 by a screw 64 which is operated by a handle 65 to adjust the width of the gap 52 to a desired level.

The temperature of the calender rolls 7, 8, 9 is closely controlled across the width of the rolls for maximum temperature control of the sheet and hence the optimum physical properties of the final panels formed using the process of the invention. The temperature of each of the rollers 7, 8, 9 is separately controlled using heating and cooling apparatus which is diagrammatically illustrated in Fig. 6. The heating and cooling apparatus for the calender rolls each comprises a circulating pump 70 which extracts water along an outlet line 71 from a calender roll 7, 8, 9 and delivers it along an outlet line 72 to a shell and tube heat exchanger 73.- The water from the calender roll is circulated through the tubes of the heat exchanger 73 and cooling water is delivered along a line 74 through a solenoid valve 76 to the shell of the heat exchanger 73. The outlet cooling water is delivered along a line 77 for recycling. The water from the roller is also passed through two immersion heaters 78 connected in series and the cooled water is delivered along a line 79 to the roller 7, 8, 9. A thermocouple 80 is provided in the line 71 upstream of the pump 70 to sense the inlet temperature of the water exiting from the calender roll 7, 8, 9. If the temperature is below a desired level, a control unit (not shown) closes the cooling water valve 76 and operates the immersion heaters 78 to heat the water to a desired level prior to delivery along the line 79 back to the roller. Alternatively, if the water from the calender roller is above a desired temperature, the immersion heaters 78 are de-activated and the cooling water valve 76 is cpened allowing cooling water to be delivered to the shell and tube heat exchanger 73 for cooling the calender roller cooling water to a desired temperature prior to delivery back along the line 79 to the calender roller. Separate heat exchangers are provided for each of the calender rollers 7, 8, 9 for optimum heat transfer control.

To protect he sheets of plastics material in transportation and storage, a film, which is typically of a polyethylene film, is applied over at least one face of the sheet. Referring particularly to Figs. 7 and 8, plastics film 90 is led from a roll 91 over a guide and tensioning roller 92 and applied over the sheet 93 between a pair of spaced-apart nip rollers 94, 95. The upper nip roller 94 is pressed downwardly by means of rams 96 to apply the film 90 to the sheet 93. Using cutting knives 95, the side edges of the sheet 93 with the film 90 applied are trimmed, waste plastics-film being wound onto a roll 97 for recycling and sheet trim 98 being collected and reground prior to recycling the regrind hopper 30.

Referring to Figs. 9 and 10, in some cases, for example, for translucent sheet material, plastics film material is applied to both faces of the sheet 100. The film is stripped using knives on both faces and rewound at the trimmed edge for recycling.

Referring particularly to Figs. 11 and 12, the cutting knives 95 used for trimming and removing plastics film are adjustable to a desired width of sheet. The cutting edge is typically provided by a blade 120 mounted to a blade carrying arm 121 which in turn is mounted to a collar 122 carried by a sleeve 123 which is non-rotatably mounted on a support bar 124 of hexagonal cross section. By operating main adjustment screws 125, the position of the cutting edge relative to the hexagonal bar 124 can be coarsely set. Using further adjustment screws 126, a finer positioning of the knife can be attained. The final very close tolerance position can be obtained by operating a fine adjustment screw 127 on the knife arm 121.

To check the performance characteristics of sheets or panels manufactured using the process of the invention, ample strips which are typically 500 mm wide and 100 mm long are cut from the sheets. The strips are then immersed in a bath of hot oil for a predetermined period to monitor the dimensional stability of the sheet. After a preset period which is typically 5 to 10 minutes in the hot oil, the dimensions of the strip are checked and if they are not within desired limits, the processing parameters such as the extruder temperature and/or speed of take-up rollers is adjusted to maximise the performance characteristics of the finished sheet of material.

To minimise waste, sheet material which does not meet required specifications is delivered to an underground grinder which grinds the waste material into granules which are then delivered to regrind storage for feeding to a delivery grind hopper 30 and recycling through the extruder 3. Because a gear pump is used at the extruder outlet, the maximum throughput of regrind material may be obtained.

The reground material may be further processed by preliminary extruding of the reground material and forming it into long narrow strips which are cooled in a water bath prior to cutting into granules for recycling through the extruder.

In some cases, the virgin plastics material comprises a blend of different virgin plastics materials which are dry blended prior to delivery to the virgin plastics hopper 26.

For certain materials, a co-extrusion of a different material may be carried out in parallel to the main extrusion and the second material introduced into the die along with the first material to produce a co-extruded sheet material.

The Invention provides a process for producing plastics sheet panels of high quality in a cost efficient manner.

The invention is not limited to the embodiments hereinbefore described which may be varied in both construction and detail.

Claims (20)

1. A process for producing plastics panels comprising the steps of: mixing virgin plastics material; feeding the plastics material into an extruder screw; extruding the plastics material to a molten condition; filtering the extrudate; pumping the extrudate; adjusting the width of a flexible die; passing the extrudate through the flexible ip die to form a deposit of molten plastics material of desired width; passing the molten plastics material over a first calender roll for a first cooling of the material; passing the plastics material over a second calender roll for a second cooling; passing the plastics material over a third calender roll for a third cooling; passing the cooled plastics material along a conveyor; trimming the side edges of the plastics material; and cutting the material to a desired length to form a plurality of sheets of plastics material.

2. A process as claimed in claim 1 comprising the steps of passing a separate stream of water through each of the calender rollers, sensing the temperature of each water stream exiting from the rollers and heating or cooling each water stream to obtain water at a desired temperature for recycling to the respective calender rollers.

3. A process as claimed in claim 2 wherein the water is cooled by passing it through the tubes of a shell and tube heat exchanger and passing cooling water through the shell of the heat exchanger.

s, A process as claimed in claim 2 or 3 wherein the water is heated by passing it through an electrically powered immersion heater, the immersion heater being switched on in response to a lower than target temperature to heat the water for the calender roller.

5. A process as claimed in any preceding claim including the step of applying a protective film of plastics material to at least one face of the plastics sheet material.

6. A process as claimed in claim 5 wherein the plastics film is passed over a guide and tensioning roller and applied to the plastics sheet material prior to trimming by passing the film and plastics sheet material between a pair of spaced-apart nip rollers.

7. A process as claimed in claim 6 wherein prior to trimming excess film is trimmed from the side edges of the sheet material and collected for recycling.

8. A process as claimed in any of claims 5 to 7 wherein a protective plastics film is applied to both faces of the plastics sheet material.

9. A process as claimed in claim 8 wherein excess film at the side edges is removed from the upper and lower faces of the sheet material ad collected separately for recycling.

10. A process as claimed in any of claims 5 to 9 wherein there are a plurality of rolls of plastics film material mounted adjacent to the plastics sheet material conveyor and a film of a desired width is led from the associated film roll and applied to the sheet material.

11. A process as claimed in any preceding claim including the step of regrinding the trimmings from the sheet material, delivering the regrind material to a regrind material hopper, delivering the regrind material to a mixing screw and proportionately mixing the regrind material with the virgin plastics material.

12. A process as claimed in claim -11 including the step of delivering waste sheet material to a below-floor regrinder, regrinding the waste material and delivering the reground material to the regrind hopper.

13. A process as claimed in any preceding claim including the step of removing toxic gasses from the extruder screw by a vacuum pump, condensing the gases and collecting the concentrated condensate in a collection drum.

14. A process as claimed in any preceding claim including the steps of: cutting a sample strip of sheet material from a formed sheet; immersing the sample strip in a bath of hot oil; measuring the deviation in the dimensions of the sample strip from a desired range; and adjusting the process parameters to correct any deviation in performance from the desired range.

15. A process as claimed in any preceding claim wherein the width of the flexible lip die is adjusted by closing off the side edges of the lip for a desired amount by sliding seals partially across both sides of the die.

16. A process as claimed in claim 15 wherein the seals comprise strips of heat resistant plastics material such as PTFE carried on sealing heads which are slidably mounted to the die for movement to desired positions to set the width of the extrudate material exiting from the flexible lip die.

17. A process as claimed in any preceding claim including the step of extruding a second plastics material and injecting the extrudate into the die.

18. A process as claimed in any preceding claim wherein the virgin plastics material is formed by dry blending a plurality of different plastics material.

19. A process substantially as hereinbefore described with reference to the accompanying drawings.

20. Plastics sheet material whenever produced by a process as claimed in any preceding claim.