GB2333989A - Testing extruded plastics - Google Patents

Abstract

A sample strip 80 of extruded plastics sheet is laid on a table 81 of a bed 83 having tracks 82 on which the table is mounted. The table 81 has a pipe network 90 supplied from heating/cooling fluid lines 91, 92 to provide a heat exchange surface. The table is moved into a heated position underneath two banks 84, 85 of infra red heaters. The sample 80 is then heated from below and on top for a preset period. After heating, the table 81 with the heated sample is removed and the dimensions of the sample 80 are compared with those of the sample prior to heating. A plastics sheet forming process may be adjusted accordingly.

Description

"Production of Plastics Sheet Material" Introduction The invention relates to the production of plastics material and in particular to a method for producing plastics sheet material, especially plastics panels. Such panels may be thermoformed into desired shapes.

In our UK-A-2282985A, we have described a process for producing such plastics sheet panels of high quality in a cost efficient manner.

In an increasingly competitive commercial situation, there is a need to produce such panels in an even more cost efficient manner while also maintaining the quality of the product.

Statements of Invention According to the invention there is provided a method of testing an extruded plastics panel for dimensional stability on thermoforming, the method comprising the steps of: cutting a defined sample of the plastics panel across the width of the panel; laying the sample on a table; moving the table with the sample in position into a heating position; heating the sample for a preset period corresponding to the thickness of the panel; removing the table with the heated sample in position from the heating position; and comparing the dimensions of the sample, after heating, with those of the sample prior to heating.

In one embodiment of the invention one face of the sample is heated in the heating position by heating the sample-receiving table.

Preferably the table is heated by circulating a heating fluid through the table.

In a preferred embodiment of the invention one face of the sample is heated in the heating position by applying radiant heat to the face of the panel. Preferably radiant heat is applied by infra red heating means located above the sample table in the heating position.

The invention also provides method for producing a plastics panel comprising the steps of: extruding a plastics material into a molten condition; forming the extrudate into a desired panel product; and testing the extruded plastics panel for dimensional stability by a method of testing as claimed in any preceding claim.

Preferably the method includes the step of adjusting the process parameters for forming plastics panels in accordance with the test results.

Reversion testing apparatus for reversion testing a strip of plastics panel material comprising: a table for receiving a strip of plastics panel material; the table being mounted for movement between a sample receiving position and a sample testing position; heating means for heating the sample for a preset time; and control means for controlling the operation of the table and/or the heating means.

Preferably the table has an integral sample heating means.

In a preferred embodiment the table includes dusting for circulating a heating fluid.

Preferably the heating means includes radiant heating means for heating a sample mounted on the table. Ideally, the radiant heating means comprises infra red heating means. Preferably the apparatus includes at least a pair of separately controllable infra red heating means.

Ideally the table is mounted on tracks for movement between a sample receiving and a sample heating position. Preferably the table is slidably movable along the tracks from the sample receiving to the heating positions.

BDeseritionoftheDrawin 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. A is a side elevational view of apparatus used in a prior art process for producing plastics panels; Fig. 1 is a side, partially cross sectional view of a gravimetric feeding apparatus used in the process of the invention; Fig. 2 is a plan view of a part of the apparatus of Fig 1; Fig. 3 is an end, partially cross sectional view of the apparatus of Fig. 1 in one position of use; Fig. 4 is a cross sectional view of part of the apparatus of Fig. 3 in another position of use; Fig. 5 is a side view of a multi-stage lamination apparatus used in the process of the invention; Fig. 6 is a plan view of the lamination apparatus of Fig. 5; Fig. 7 is a perspective, partially cut-away view of part of the apparatus of Figs. 5 and 6; Fig. 8 is a perspective view of one lamination station of the apparatus of Figs. 5 to 7; Fig. 9 is a plan view of a trimming station used in the process of the invention; Fig. 10 is a perspective view of the trimming station; Fig. 11 is a side view of the trimming station; Fig. 12 is a perspective view of a testing apparatus used in the process of the invention; and Fig. 13 is a side, partially cross sectional view of the testing apparatus, in use.

Detailed Description Referring initially to Fig. A, there is illustrated a process for producing plastics panels as described in our UK-A-2282985A 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 plastics 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 is formed into a sheet of molten material of desired width and thickness.

The plastics material is then passed between first and second calendar rolls 7, 8 and then between second and third calendar 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 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 to Figs. 1 to 4, in the preferred process preset quantities of virgin plastics material, regrind plastics material and masterbatch material are fed into an upper feed hopper 18 having an outlet door 19 operated by a ram 20. The material is delivered from the upper hopper 18 into an intermediate hopper 21 having rapid opening outlet doors 22, 23 which are closed as the material is delivered from the upper hopper 18. The outlet doors 22, 23 are clamshell doors which are rapidly opened and closed by fast acting rams 24. A blender/mixer 25 is provided in the intermediate hopper 21 for ensuring thorough mixing of the material and rapid delivery into a main feed hopper 26.

In use, predetermined quantities of virgin plastics material, regrind material, and masterbatch material are delivered into the upper feed hopper 18 from separate sources in a predetermined sequence. On demand, with the clamshell or bomb doors 22, 23 closed, the outlet door 19 from the upper hopper 18 is opened to deliver the material into the intermediate hopper 21. After a preset quantity of material has been delivered, the outlet of the upper hopper 18 is closed and the material in the intermediate hopper 21 is blended by the mixer blender 25. On demand, as determined by the quantity of material remaining in the lower main feed hopper 26, the rapid acting outlet bomb doors 22, 23 are opened by automatically actuating the rams 24 to rapidly deliver the blended material from the intermediate hopper 21 into the main feed hopper 26. The cycle is repeated to maintain a constant supply of predictable quality mixed raw material for extrusion. In this way, the process is optimised as there is always high quality feed material available for further processing by extrusion. Because of this feature very efficient high speed processing is achieved. In addition, particularly because of the mixing/blending in the intermediate hopper, thorough mixing of the material is ensured which is essential for high quality colour matching.

Referring particularly to Figs. 5 to 8, there is illustrated a multi-stage lamination apparatus 30. This is used to provide a film 31, typically of polyethylene material which is applied over at least one face of the formed plastics sheet material to protect the sheets in transportation and storage. The film 31 is led from one of a number of reels 33, 34, 35 over a guide and tensioning roller 32 and applied to a sheet 36 between a pair of spaced-apart nip rollers 37, 38. The upper nip roller 37 is pressed down by means of rams 39 to apply the film 31 to the sheet 36.

Each of the reels 33, 34, 35 of film are typically of different widths to optimally accommodate sheets of different size. The film delivery can be rapidly changed over from one reel to another so that production efficiency is optimised. The film is drawn from a reel at a substantially constant tension by applying a braking force through a brake controller 39 on each of the reels 33, 34, 35.

It will be noted that the reels 33, 34, 35 are mounted on a common carriage 40 having rollers 41 which run along corresponding transverse tracks 42 to accurately adjust the position of the carriage 40 and hence the film 31 to the sheet 36 travelling along a conveyor 45. Adjusting ratchet screws 46 are used to move the carriage 40 on the tracks 42. In this way, film application and usage is optimised.

Referring to Figs. 9 to 11, using cutting knives 50, 51, the side edges of the sheet 36 are trimmed and waste sheet trim 52 is delivered under the conveyor for regrinding in a grinder 53. If desired, particularly for larger sized waste, the waste may be pre-chopped in a chopper 54 prior to grinding.

To improve quality, the side edges of the cut sheet 34 are de-burred by passing the cut sheet 34 between de-burring rollers 55 on opposite sides of the sheet 34.

Referring particularly to Figs. 11 and 12, to check the dimensional stability of sheets or panels manufactured using the process of the invention, sample strips which are typically 500 mm wide and 100 mm long are first cut from the sheets.

The sample 80 is laid on a table 81 of a reversion testing apparatus 79. The testing apparatus 79 comprises a bed 83 having tracks 82 on which the table is mounted for movement inwards and outwards as illustrated particularly in Fig. 13. The apparatus 79 also includes a head part 88 housing two banks 84, 85 of infrared heaters in a carriage 86 which is height adjustable by a handle-operated screw 87, as illustrated. The heaters 84, 85 are positioned over the sample table 81 when the table is in the testing position illustrated in dotted outline in Fig. 13.

The table 81 has a heat exchange surface below the bed which is provided by a pipe network 90 in the bed. The pipes are supplied with heating/cooling fluid lines 91, 92.

In use, with the cut sample 80 in position, the table 81 is moved into the heated position underneath the heaters 84, 85. The sample is then heated from below and on top for a preset period corresponding to the thickness of the panel from which the sample was cut. After heating, the table 81 with the heated sample 80 in position is withdrawn to the position illustrated in Fig. 12, the sample 80 is removed and the dimensions of the sample 80, after heating, are compared to those of the sample prior to heating. In this way, the sample is rapidly reversion tested to a high level of accuracy. Depending on the results of the reversion test the panel production process may be adjusted.

The invention may be applied to any suitable plastics material, either a unitary material or one having a number of layers. One such material is a base of acrylonitride-butadiene-styrene with a coating of polymethyl methacrylate.

The invention provides a quick and effective means for reversion testing panels produced in a complex manufacturing process. The reversion testing system not only ensures quality control but also provides a rapid result which can be used in adjusting process operating parameters to optimum conditions.

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

Claims (18)

1. CLAIMS 1. A method of testing an extruded plastics panel for dimensional stability on thermoforming, the method comprising the steps of: cutting a defined sample of the plastics panel across the width of the panel; laying the sample on a table; moving the table with the sample in position into a heating position; heating the sample for a preset period corresponding to the thickness of the panel; removing the table with the heated sample in position from the heating position; and comparing the dimensions of the sample, after heating, with those of the sample prior to heating.
2. 2. A method as claimed in claim 1, wherein one face of the sample is heated in the heating position by heating the sample-receiving table.
3. 3. A method as claimed in claim 2, wherein the table is heated by circulating a heating fluid through the table.
4. 4. A method as claimed in any of claims 1 to 3, wherein one face of the sample is heated in the heating position by applying radiant heat to a face of the panel.
5. 5. A method as claimed in claim 4, wherein radiant heat is applied by infra red heating means located above the sample table in the heating position.
6. 6. A method of testing substantially as hereinbefore described with reference to the drawings.
7. 7. A method for producing a plastics panel comprising the steps of: extruding a plastics material into a molten condition; forming the extrudate into a desired panel product; and testing the extruded plastics panel for dimensional stability by a method of testing as claimed in any preceding claim.
8. 8. A method as claimed in claim 7 including the step of adjusting the process parameters for forming plastics panels in accordance with the test results.
9. 9. A method for producing a plastics panel substantially as hereinbefore described with reference to the accompanying drawings.
10. 10. Reversion testing apparatus for reversion testing a strip of plastics panel material comprising: a table for receiving a strip of plastics panel material; the table being mounted for movement between a sample receiving position and a sample testing position; heating means for heating the sample for a preset time; and control means for controlling the operation of the table and/or the heating means.
11. 11. Reversion testing apparatus as claimed in claim 10 wherein the table has an integral sample heating means.
12. 12. Reversion testing apparatus as claimed in claim 11 wherein the table includes dusting for circulating a heating fluid.
13. 13. Reversion testing apparatus as claimed in any of claims 10 to 12 wherein the heating means includes radiant heating means for heating a sample mounted on the table.
14. 14. Reversion testing apparatus as claimed in claim 13 wherein the radiant heating means comprises infra red heating means.
15. 15. Reversion testing apparatus as claimed in claim 14 including at least a pair of separately controllable infra red heating means.
16. 16. Reversion testing apparatus as claimed in any of claims 10 to 15 wherein the table is mounted on tracks for movement between a sample receiving and a sample heating position.
17. 17. Reversion testing apparatus as claimed in claim 16 wherein the table is slidably movable along the tracks from the sample receiving to the heating positions.
18. 18. Reversion testing apparatus substantially as hereinbefore described with reference to the accompanying drawings.