BE1003741A6 - A forming method. - Google Patents

Abstract

Thin-walled trays are formed by extruding a mixture of a regrind synthetic material, a synthetic raw material and a coloring material into a sheet, which is then thermoformed to a described shape. The components of the mixture are loaded into a loading hopper from where they are supplied by a conveyor screw 15 to an extruder screw 5. The extruder screw 5 has a primary propeller 41 and, in the transition zone 35, a secondary propeller 46 , which defines a channel 47 where the material is thoroughly mixed. The diameter of the secondary propeller 46 is approximately 99.17% of the diameter of the primary propeller 41 which leads to an efficient material transfer in the channel 47 with reduced tearing and improved evacuation action. The combination of the reduced diameter of the secondary propeller 46 and the conveyor screw 15 allows efficient treatment of materials showing a high level of regrind material and the transition from one mixture to another takes place quickly with minimal loss of material.

Description

       

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  A forming process The invention relates to a process for forming thin-walled plastic containers. The invention also relates to an apparatus for use in the method according to this invention.



  Conventional thin-walled containers are produced by a thermoforming process from a sheet of synthetic material. It is essential that the sheets of synthetic material are produced as efficiently as possible and that they are available on request for ther. moforming. This is particularly important where a wide range of containers of different colors will have to be produced.



  In some cases, manufacturers buy the preformed sheets in rolls, but this is not only expensive, but the quantity and quality of such material can vary. In addition, the thermoforming process produces a considerable amount of plastic waste, which is not only ineffective, but which causes storage and disposal problems.



  In other cases, manufacturers have tried to operate an integrated process, where they extrude on the rolls of synthetic material used in thermoforming processes. This reduces costs, but it is difficult to operate such an extrusion process effectively, When a wide range of plastics

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 for sheets is required. More particularly, the transition from the extrusion of a sheet of a certain color to the extrusion of a sheet of another color leads to considerable downtime and loss of material. Another problem is the recycling of the large quantities of lost synthetic material generated in the thermoforming process.



  This invention aims to provide an integrated process for the production of thin-walled plastic containers, overcoming at least some of these problems.



  According to the invention a method is provided for forming thin-walled plastic packaging containers, comprising the following phases: extruding a plastic sheet, reheating the sheet, forming the sheet reheated to the desired shape, and cutting off the excess material to provide the desired container, the material for the extrusion being supplied with a first hopper of synthetic raw material, possibly a second hopper of material re-ground synthetic, and possibly a third hopper of coloring material, by corresponding screw conveyors supplying the material to the loading hopper, mixing the material in the loading hopper,

   the additional mixing and providing the

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 material mixed with a screw extruder by means of an auxiliary mixer, which supplies the material from the loading hopper to the cylinder of a screw extruder having a single screw, provided with a primary propeller with a diameter dl, the passage of the mixed material through a feed zone of the extruder, the passage of the material through a transition zone of the extruder screw, the screw having a secondary helix in the transition zone, which is spaced from the primary propeller to define a mixing channel between the two where the material is mixed passing through the transition zone, the secondary propeller having a diameter d2 is
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 between 99.0% and 99.3% of the diameter d, of 1 the primary propeller,

   the passage of the extruded material through a calibration zone of the extruder screw, the passage of the extruded material through a matrix, forming the extruded material to a sheet, the cooling of the extruded sheet by passing it through consecutive calendering rollers, and winding the chilled sheet to a roll providing the sheet for thermoforming into a desired container, The invention also provides thin-walled packaging containers of synthetic material each time they are prepared by the method of invention.

   

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 The characteristics and advantages of the method according to the invention will emerge more clearly from the following description of some preferred embodiments, given exclusively by way of example with reference to the appended drawings, in which; Figure 1 is a flowchart of the forming method according to the invention; Figure 2 is a schematic side elevation view of part of the apparatus used in the process; Figure 3 is a side view, partially in section of an extruder screw forming part of the apparatus; and FIG. 4 is a perspective view partially in section on a larger scale of part of the screw of FIG. 3.



  Thin-walled plastic packaging containers, such as trays, used for packaging food products and the like, are formed by the method of the invention by first extruding the plastic material into a sheet in a process indicated generally by the reference number 1 in FIG. 1 followed by the thermoforming of the sheets of synthetic material of the container by a process indicated generally by the reference number 2 in FIG. 1.



   The material for the extrusion of the sheets is supplied to an extruder screw 5 from a first hopper 6 of synthetic raw material, a second hopper 7 of coloring material and a third hopper 8 for the synthetic recovery material.

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  The synthetic raw material is supplied to the loading hopper 6 from a large outside silo A feeding an inside silo B by means of a vacuum collector C. The synthetic raw material from the inside silo B is supplied to the loading hopper 6 at by means of a vacuum collector / loading hopper D. The hopper 7 is supplied with coloring matter manually, as required.



   Waste plastic material from the thermoforming process is recovered and supplied to the hopper 8 from a silo E placed above. 4 The material of the hoppers 6,7, 8 is supplied by means of corresponding conveyor screws to a direct loading hopper 10 where the material is mixed by a mixer of the paddle type (not shown). The recovery material is supplied to the hopper 10 at a constant speed and the material to be mixed is varied according to the requirements by modifying the speed of the motors controlling the conveyor screws for supplying the synthetic raw material and the coloring material.



  An auxiliary mixer, in this case provided by a conveyor screw 15, continues the mixing and supplies the mixed material from the loading hopper 10 to the screw extruder 5. The auxiliary conveyor screw and constituting the mixer 15, which is shown in more detail in Figure 3 is of essential importance to avoid obstruction of the outlet of the hopper 10 and to ensure that the mixture of synthetic material supplied from the loading hopper 10 to the extruder 5 is as complete as possible. Such a positive supply of the material mixed with the screw extruder ensures a continuous flow of material through the extruder screw.

   By providing such an additional conveyor and mixing screw, the extruder can be operated with a synthetic feed material having a relatively large proportion of synthetic material.

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 recovery.



  With particular reference to Figures 3 and 4, the extruder screw 5 comprises a cylinder 20 with a bore with a diameter of 635 mm and a single screw which extends therein. The screw consists of a shaft 25 having a base diameter of 375 mm and whose longitudinal axis is coaxial with the axis of the cylinder 20. The extruder screw 5 is divided into three zones, namely a supply zone 30, where the material to be extruded is completely mixed, reheated and plasticized, a transition zone 35, where the material is melted and fully mixed and a calibration zone 40, completing the extrusion process and positively pumping the beaded material of the extruder screw 5.



  The screw comprises a primary propeller 41 having a diameter of 615.94 mm with a radial clearance in the bore of 1.016 mm.



  The propeller 41 has a pitch of 635 mm and a width of 63.5 mm. The depth of the channel defined by the shaft 25 and the primary propeller 41 is 120 mm in the supply zone 30 and decreases by passing the transition zone 35 to a depth of 53 mm in the calibration zone of the opinion.



  A secondary propeller 46 esL provided in the transition zone 35 of the screw. The secondary propeller has a diameter of 610.9 mm and a width of 31.75 mm. in the transition zone, the secondary propeller 46 is spaced from the primary propeller 41 and defines a second channel 47 having a width of 152.4 mm. The depth of channel 47 increases by passing through the transition zone with an initial depth of 35 mm,
In the supply zone 30, the configuration of the primary propeller 41, relative to the shaft 25 and the bore of the screw, defines a channel having a relatively large volume for heating and plasticizing the synthetic material which is introduced. in the screw.

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  In the transition zone 35, the configuration of the primary channel tears the material to homogenize the molten material. Passing through the transition zone, the material passes through the secondary helix 46 to the secondary channel 47, where the material is thoroughly mixed and where the coloring material is then dispersed.



  In the calibration zone 40 the mixed and coiled material is removed from the screw in a positive manner.



  Because of the reduced diameter of the secondary propeller 46, a good transfer of material to the secondary channel is obtained with a reduced tearing action and an improved evacuation action. This is important in case the feed material contains a large percentage of recovery material having a high mass density.



  The configuration of the feed mixer and the extruder screw contributes significantly to the efficient forming and minimal losses of plastic sheet material. In particular, as mentioned above, the material containing a high percentage of recovery material can be easily processed. In addition, because of the configuration of the extruder screw, particularly because of the reduced diameter of the secondary propeller in the transition zone, the material is quickly treated and the time to pass from one mixture of raw materials to another is significantly smaller than so far. Thus, material losses are minimal when switching from one material to another, for example, when changing from one color to another, and the passage is carried out quickly.



  The material leaving the extruder screw 30 is passed through a die 48 having a slot in the form of a hanger (not shown) which transforms the synthetic material into a continuous sheet which is then passed

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 by a series of calenders, namely a first, a second and a third calender 50, 51 and 52. The first calender 50 is maintained at a temperature of approximately 70 ° C., the second calender (51) is maintained at a temperature d 'about 60 ° C and the third calender (52) is maintained at a temperature of about 50 ° C, so that the sheet is refrigerated gradually through the set of calenders.

   When the sheet leaves the third calender 52 it passes along a conveyor, where it is further cooled by ambient air and drawn by means of drive rollers 55 which tension the sheet before winding it on a coil 57.



  The thickness of the material formed by the method according to the invention is typically between 250 micrometers and 1000 micrometers and the width of the sheet is between 350 mm and 860 mm.



  The plastic sheets formed by the extrusion process and the apparatus described above are then passed to the thermoforming process 2, where the sheets are first reheated in an oven 70, thermoformed to the desired shape at a station forming 71. The formed sheets are then passed to a cutting station 72 where the excess material is removed and the formed containers are stacked and packed for transport. The waste from the cutting phase 72 is collected in a box 75, it is reground in a regrinding phase 76 and recycled to the hopper of regrind material 8.



  Many variations of the specific embodiment of the invention described are obvious and therefore the invention is not limited to the embodiments described above which can be varied either in construction or in detail.

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    Legend Extrusion process Extrusion process Regrind (waste) Re-grinding (waste) Virgin Raw material Pigment Dye Mixer Mixer Transition (solid to liquid Transition (from solid to liquid Calender stack Set of calenders Film / sheet / foil Sheet Feed area Zone d ' food Metering Calibration Cdoling area Refrigeration zone Thermoforming Process Thermoforming process Oven Four Forming Forming Cutting Cutting Stacking Stacking Product Product Skeletal waste Waste Regrind Recovery


    

Claims (1)

CLAIMS 1.- A process for forming packaging containers to  EMI10.1  i thin wall of synthetic material comprising the phases: extruding a plastic sheet, reheating the sheet, forming the reheated sheet to the desired shape, and cutting off the excess material to supply the desired container, the material for the extrusion being supplied from a first hopper of synthetic raw material, optionally from a second hopper of regrind synthetic material, and optionally from a third hopper of coloring material, by conveyors with corresponding screws supplying the material to the loading hopper, mixing the material in the loading hopper,  the additional mixing and the supply of the mixed material to a screw extruder by means of an auxiliary mixer, which supplies the material from the loading hopper to the cylinder of a single screw screw extruder provided with a propeller primary with a diameter d ,, the passage of the mixed material through a feed zone of the extruder,  <Desc / Clms Page number 11>  the passage of the material through a transition zone of the extruder screw, the screw having a secondary helix in the transition zone, which is spaced from the primary helix to define a kneading channel between the two where the material is mixed in passing through the transition zone, the secondary propeller having a diameter d lying between 99.0% and 99.3% of the diameter d, of the primary propeller,  the passage of the extruded material through a calibration zone of the extruder screw, the passage of the extruded material through a matrix, forming the extruded material to a sheet, the cooling of the extruded sheet by passing it through consecutive calendering rollers, and the winding of the refrigerated sheet to a roller supplying the sheet for thermoforming into a desired container, 2.-A process according to the resale Lion 1, or the diameter d 2 of a secondary propeller is between 99, 1% and 99 0.2%, most preferably about 99.17% of the diameter d, of the primary propeller.   3. A method according to claim 1 or 2, wherein the width of the secondary channel between the primary and secondary propellers in the transition zone is between 140 and 160 mm, most preferably about 152 mm.   4.-A method according to either of the preceding claims, wherein the depth of the secondary channel between the primary and propellers. secondary at the start of the  <Desc / Clms Page number 12>  transition is between 30 and 40 mm, most preferably about 35 mm.   5. A method according to either of the preceding claims, wherein the material supplied by an auxiliary screw of the loading hopper to the cylinder of the extruder screw.   6. A process according to either of the preceding claims, in which the sheet is refrigerated by passing the sheet through a first calender roll which is maintained at a temperature between 65. C and 7S. C and then through a second calendering roller which is maintained at a temperature between 55'C and 65'C, and preferably the partially refrigerated sheet is passed through a third calendering roller which is maintained at a temperature between 45. C and 55 C . 7. An apparatus for forming a sheet of synthetic material comprising: a first hopper for the raw material, a second hopper for the regrind material, a third hopper for the coloring material, a loading hopper, a first, second and third screw conveyor for supplying material from the first, second and third hopper respectively to the loading hopper, a main mixer in the loading hopper for mixing the materials, and an auxiliary mixer at an outlet from the loading hopper for providing the material in the screw extruder,  <Desc / Clms Page number 13>  the screw extruder comprising a cylinder having a cylindrical bore and a screw which extends into the bore, the extruder comprises a feed, transition and calibration zone,  the extruder screw having a primary propeller and a secondary propeller in the transition region of the screw, the secondary propeller being spaced from the primary propeller in the transition region to define a mixing channel between the two, the secondary propeller having a diameter d, which is between 99.0% and 99.3% of the diameter d of the primary propeller. a die at the exit of the extruder cylinder through which the coiled material has passed to be formed into a sheet, calender rolls through which the formed sheet has passed to cool gradually, tension rollers for tensioning the sheet, and a roller reel to which the sheet formed of synthetic material has passed.  EMI13.1   8. Apparatus according to claim 14, wherein the diameter d2 of the secondary propeller is between 99.1% and 99.2%, most preferably about 99.17% of the diameter d of the primary propeller.   9. A method according to either of claims 14 to 16, where the width of the secondary channel between the primary and secondary propellers in the transition zone is between 140 and 160, most preferably about 152 mm, preferably the depth of the secondary channel between the primary and secondary propellers at the start of the  <Desc / Clms Page number 14>  transmission is between 30 and 40 mm, most preferably about 35 mm.   10. A coil of synthetic material, each time it has been formed by a method according to either of claims 1 to 6 or by using an apparatus according to claim 8 or 9.