Method

Abstract

An extrusion method 30 for producing a film comprises calculating a throughput of molten material through a die based on a desired film width 32. The method comprises providing the molten material to

Description

[0001] The present invention relates to an extrusion method and system.

[0002] Films can be produced using extrusion processes. For example, solid thermoplastic feed materials are converted into a molten state, and the molten material is pushed through an apparatus with an internal cross-sectional profile called a die, through a die gap, onto a carrier, forming a film on the carrier.

[0003] Typically, excess molten material is pushed to the edges of the cast film, and this excess is then trimmed off as waste. The thickness of the waste trims can be up to five times thicker than the film thickness set by the die gap and film profile.

[0004] According to the present invention there is provided an apparatus and method as set forth in the appended claims. Other features of the invention will be apparent from the dependent claims, and the description which follows.

[0005] According to an aspect, there is provided an extrusion method for producing a film, the method comprising: calculating a throughput of molten material through a die based on a desired film width; providing the molten material to the die according to the calculated throughput; passing the molten material through a gap of the die to form a film having a central portion and edge portions that are thicker than the central portion; and trimming the edge portions to provide a film of desired width.

[0006] Calculating the throughput of molten material based on the desired film width may reduce the amount of excess molten material that is pushed to the edges of the film and then trimmed away as waste. Calculating the throughput of molten material based on the desired film width may also improve the uniformity of thickness across the width of the film, thereby reducing the amount of material that is wasted due to variations in thickness across the width of the film.

[0007] The throughput may be an amount of molten material emitted from the extruder barrel to the die in a given amount of time.

[0008] Passing the molten material through the gap of the die to form the film may comprise applying the molten material that passes through the die onto a carrier. The carrier may be a moving carrier onto which the film is formed. The carrier may move relative to the die to form the film along the length of the carrier.

[0009] The throughput may be calculated based on the speed of the carrier, the desired film width and the desired film weight, wherein the film weight is the mass per unit area of the film.

[0010] For example, the throughput may be calculated using the equation: carrier speed x desired film width x film weight x time.

[0011] For example, the throughput may be calculated as a weight in kg of molten material to be output to the die in an hour, and may be the product of the carrier speed in metres per minute, the desired film width in metres, the film weight in kilograms per square metre, multiplied by 60 minutes.

[0012] Providing the molten material to the die according to the calculated throughput may comprise controlling the throughput. Controlling the throughput may comprise controlling a rate at which molten material is emitted from an extruder barrel. Controlling the rate at which the molten material is emitted from the extruder barrel may comprise controlling the speed at which molten material moves through the extruder barrel. Controlling the speed at which molten material moves through the extruder barrel may comprise controlling a speed of revolution of a screw in the barrel. Controlling the throughput may thereby comprise controlling the speed of the screw.

[0013] The method may further comprise monitoring the thickness of the central portion of the film in the width direction and adjusting a height of the die according to the monitored thickness. Monitoring the thickness may comprise determining a thickness profile of the film across the width of the film. Adjusting the height of the die may comprise adjusting the height of the gap of the die at a lip of the die. The height of the gap of the die may be adjusted by mechanical means such as hydraulic means, shimming, clamping or adjusting one or more die bolts of a plurality of die bolts. The method may comprise identifying a deviation of the thickness of the film from a desired flat profile and adjusting the gap of the die at a position corresponding to a location of the deviation. Adjusting the gap of the die may comprise adjusting one or more die bolts of a plurality of die bolts positioned along the width of the die.

[0014] The molten material may be any suitable material. Typically, the molten material may be comprise a thermoplastic resin, preferably a thermoplastic polyurethane resin. The molten material may also comprise a stabilising agent and / or additives. In other examples, the molten material may comprise a polyethylene resin, such as HDPE resin or LDPE resin.

[0015] According to another aspect, there is provided an extrusion system for forming a film, the system comprising an extruder barrel, a die and a controller; wherein the extruder barrel is configured to provide molten material to the die, and wherein the controller is configured to calculate a throughput of molten material to the die based on a desired film width and is configured to control the rate at which molten material is provided to the die based on the calculated throughput; wherein the system is configured to pass the molten material through the die to form a film having a central portion and edge portions that are thicker than the central portion.

[0016] The controller may be configured to control a revolution speed of a screw in the extruder barrel to control the rate at which molten material is provided to the die.

[0017] The controller may be configured to calculate the throughput based on the speed of a carrier onto which material that has passed through the die is applied, the desired film width and the desired film weight, wherein the film weight is the mass per unit area of the film.

[0018] The throughput may be calculated using the equation: carrier speed x desired film width x film weight x time.

[0019] The extrusion system may further comprise a scanner configured to monitor the thickness of the central portion of the film. The controller may be configured to adjust the height of die bolts controlling the gap of the die at the die lip based on the monitored thickness being greater or less than the desired thickness.

[0020] According to another aspect there is provided a non-transitory machine-readable storage medium encoded with instructions executable by a processor, the machine-readable storage medium comprising: instructions to calculate a throughput of molten material through a die based on a desired film width; instructions to provide the molten material to the die according to the calculated throughput; instructions to pass the molten material through the die to form a film having a central portion and edge portions that are thicker than the central portion; and instructions to trim the edge portions to provide a film of desired width.

[0021] The non-transitory machine-readable storage medium may comprise instructions to calculate the throughput of the molten material based on the speed of a carrier onto which material that has passed through the die is applied, the desired film width and the desired film weight, wherein the film weight is the mass per unit area of the film.

[0022] The non-transitory machine-readable storage medium may comprise instructions to calculate the throughput of the molten material using the equation: carrier speed x desired film width x film weight x time.

[0023] The non-transitory machine-readable storage may comprise instructions to monitor the thickness of the central portion of the film, and instructions to adjust the height of die bolts controlling a gap of the die based on the monitored thickness being greater or less than the desired thickness.

[0024] According to another aspect there is provided an extrusion method for producing a film, the method comprising: calculating a throughput value of molten material to a die based on achieving a desired film width; providing the molten material to the die at a rate corresponding to the calculated throughput value; passing the molten material through a gap of the die to form a film having a central portion with a first thickness, having a width corresponding to the desired film width and edge portions that are thicker than the first thickness of the central portion; and trimming the edge portions to provide a film having the desired film width.

[0025] The method may include trimming the edge portions to provide a film having the desired film width and the first thickness. The thickness may be a depth of the film.

[0026] The method may include adjusting the size of the gap of the die at one or more positions along a length of the die to obtain a desired film thickness.

[0027] According to another aspect there is provided a film, the film being produced by calculating a throughput value of molten material to a die based on achieving a desired film width; providing the molten material to the die at a rate corresponding to the calculated throughput value; passing the molten material through a gap of the die to form a film having a central portion with a first thickness, having a width corresponding to the desired film width and edge portions that are thicker than the first thickness of the central portion.

[0028] The film is advantageously considerably closer in width to the desired film width when produced by the method discussed above. The combination of film of the desired width and the edge portions are advantageously narrower than a film produced by prior art methods. Thus, great savings are produced because of reduced wastage in the trimmed portions.

[0029] All of the features described herein may be combined with any of the above aspects in any combination.

[0030] Although a few preferred embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims.

[0031] For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example only, to the accompanying diagrammatic drawings in which:

[0032] Figure 1 shows a schematic of an extrusion system;

[0033] Figure 2 shows a profile view of a cast film;

[0034] Figure 3 shows an example extrusion process;

[0035] Figure 4 shows a graph illustrating measured weights of a plurality of films.

[0036] As shown in figure 1, an extrusion system comprises a feeder 6, a barrel 8, a screw 10, and a die 12. The feeder 6 provides solid feed material to the barrel. The solid feed material is converted into a molten state in the barrel. The screw 10 rotates along its axis, corresponding to a longitudinal direction of the barrel 16, and pushes the feed material further along the length of the barrel, where under pressure, friction and heat melt the feed material. The screw 10 mixes the molten material to provide a homogenized mix, and the screw 10 acts as a pump to provide the molten material to the die under pressure. The molten material (extrudate) passes through a gap provided at a die lip of the die 12 (also referred to herein as the die gap) and is applied (dropped or placed) onto a moving carrier and is cooled.

[0037] The film cast through the die has a central portion 18 with a width wi corresponding to the width of the die gap, and side edge portions 20, 22. The thickness ti of the film is set by the die gap and the relationship of the speed of the moving carrier and the throughput of molten material.

[0038] When the molten material is pushed through the die gap, the excess material is pushed to the sides, so that the central portion 18 of the film has a thickness ti. The edges 20, 22 of the cast film have a thickness t2, ta that is thicker than the central portion 18, as shown in figure 2, due to the amount of excess material being pushed to the edges as the material passes through the die 12.

[0039] The edges 20, 22 are then trimmed to provide a film of the desired width wi and thickness ti, and the carrier and film is wound into a roll.

[0040] The pressure applied to pump the film depends on the throughput of molten material delivered to the die 12 and is controlled by the speed of the revolutions of the screw 10 in the extruder barrel 8. Too little can affect the film width whilst too much will affect the thickness control. The throughput is adjusted by adjusting the speed of revolutions of the screw 10. A given number of revolutions of the screw 10 will provide a given amount of molten material to the die, and adjusting the speed of the screw 10 will adjust the rate at which the molten material is provided to the die.

[0041] The speed of movement of the carrier also affects the thickness of the film. When the film is applied from the die onto a carrier moving more slowly, the film will be thicker than when the film is applied to a carrier moving more quickly, because the speed of movement of the carrier affects how much of the material is applied to a given area of the carrier. The speed of movement of the carrier is controlled by a machine-based motor and speed encoder.

[0042] Whilst adjusting the speed of the movement of the carrier would adjust the thickness of the film, setting the carrier speed as a fixed amount for a given film and adjusting the screw speed has been found to be a more efficient method of controlling the thickness of the film.

[0043] The feeder 6 provides the solid feed material into the barrel 8. The feeder 6 may be a gravimetric feeder, which is configured to continually adjust the amount of feed material provided to the barrel 8 based on weight of material in the barrel 8. Adjusting the throughput changes the rate at which extrudate is pumped out of the extruder and hence will result in adjustments of the amount of feed material provided to the barrel by the gravimetric feeder.

[0044] The feed material may comprise multiple different components. For example, the feed material may comprise two resins at a ratio of 95% main resin and 5% additive. The feeder 6 may adjust the amount of feed material provided to the barrel whilst maintaining a set ratio of components of the feed material.

[0045] A controller 24 is configured to calculate the throughput of molten material based on the desired film thickness, carrier speed and desired width wi and is configured to control the rate at which the molten material is provided to the die based on the calculated throughput. The controller 24 is configured to control the speed of revolution of the screw 10 based on the calculated throughput.

[0046] The system may further comprise a scanner configured to monitor the thickness of the central portion of the film.

[0047] According to an example, the scanner comprises two sensors. The first sensor is configured to determine a thickness of the carrier prior to the film being applied to the carrier. The second sensor is configured to determine a thickness of the carrier and film together, when the film has been applied to the carrier. From the values of thickness determined from the two sensors, the thickness of the film can be determined.

[0048] The scanner may be a radioactive scanner, comprising a radiation source that emits radiation to the carrier or film and carrier, and a detector that measures the amount of radiation that passes through the materials, and a processor that determines the thickness of the carrier based on the measured amount of radiation. The detector may be a Geiger counter and the radiation source may be a chambered gas such as Krypton-85.

[0049] The scanner is configured to determine the thickness of the film at a plurality of points along the width of a film and is synchronized to measure the same points. The system may thereby determine a variation in film thickness along the width of the film, relative to a desired flat profile to provide constant thickness. The controller is configured to automatically adjust the heights of the die bolts provided across the width of the die gap according to the determined variation in film thickness to reduce the variation in film thickness across the width of the film. The heights of the die bolts are individually adjustable, such that local variations in the film thickness can be controlled. The heights of the die bolts may be adjusted by applying heat to one or more of the bolts, which will expand the bolts and in turn reduce the die gap of the die lip, thereby restricting material flow through the die gap at the region of the bolt. Similarly, heat applied to the bolts may be reduced to allow the bolts to contract and thereby increase the die gap of the die lip to increase flowthrough the die gap of the die in the region of the bolt. The controller may control the amount of heat applied to the individual bolts based on feedback from the scanner. Controlling the amount of heat applied to the bolts comprises controlling electrical power supplied to an internal heater coil that heats a bolt. In other examples, the height of the gap of the die may be adjusted by other mechanical means such as hydraulic means, shimming, or clamping.

[0050] An extrusion method 30, shown in figure 3, comprises calculating a throughput of molten material to a die based on a desired film width 32. The throughput is a mass of the molten material provided to the die per unit time. The throughput is calculated as a product of the speed of the carrier onto which the molten material is applied from the die, the desired film width and the film weight, wherein the film weight is the film mass per unit area.

[0051] Conventionally, the full width of the film cast onto the carrier is measured and used to calculate the throughput of the material to the die. However, the applicant has identified that by instead using the desired width wi of the film to calculate the throughput of the material to the die, an improved quality of film has been obtained, with reduced waste. In particular, the applicant has found that the thickness of the film has not been reduced by calculating the throughput based on the desired film width rather than full cast width but only the outer edges have been reduced in both width W2, W3 and thickness t2, t3, thereby reducing waste.

[0052] The method may comprise setting the die height, reducing the overall total film width according to the desired film thickness and control. Setting the die height may comprise adjusting die bolts.

[0053] The method 30 further comprises providing the molten material to the die according to the calculated throughput 34. For example, a speed of the screw in the extruder barrel may be controlled to provide the molten material according to the calculated throughput.

[0054] The method 30 comprises passing the molten material through the die to produce a product 36 having a central portion and edge portions, wherein the edge portions are thicker than the central portion, due to the pressure required to guarantee uniformity and consistent control.

[0055] The method 30 may comprise scanning the thickness of the film in the width direction of the film 38. Scanning the thickness of the film may comprise determining the thickness of the film at a plurality of locations along the width of the film and determining a thickness profile of the film based on the determined thicknesses. The method may comprise identifying a position on the width of the film in which the thickness is greater or less than a desired thickness.

[0056] The method may comprise adjusting the die gap based on the determined thickness of the film 40. Adjusting the die gap may comprise reducing or increasing the die bolt at a location corresponding to the position on the width of the film where the thickness is greater than or less than the desired thickness. Adjusting the die gap may comprise adjusting a die bolt that controls the die gap of the die lip at the location. After the die gap is adjusted, step 36 is repeated and the product output from the die onto the carrier may have an improved uniformity of thickness. The film thus produced is beneficially narrower than a film made by previous methods because less waste results in parts of the full cast width outside the film of the desired width.

[0057] The method 30 further comprises trimming away the edge portions 42 to provide a film of desired width.

[0058] The applicant has found that by calculating the throughput based on the film width rather than the full cast width, the quality of the film can be improved. According to an example, film thickness across the width of the film was measured for a plurality of thermoplastic polyurethane films generated using a throughput calculated based on the full cast width and film thickness was also measured for a plurality of thermoplastic polyurethane films generated using a throughput calculated based on the desired film width, according to the method of the invention. Figure 4 shows a graph of film weight for the conventional method in which throughput is calculated based on full cast width (solid line 44) and for the method wherein throughput is calculated based on the desired film width (dashed line 46). As shown in figure 4, using the method of the invention, a mean weight closer to the target of 30 gsm has been achieved, and the amount of variation from the mean weight (standard deviation) has been reduced. Thus, using the method described above the amount of trimming can be substantially reduced.

[0059] Various elements and methods described herein may be implemented through the execution of machine-readable instructions by a processor. Figure 5 shows a processing system comprising a processor 50 in association with a non-transitory machine-readable storage medium 52. The machine-readable storage medium 52 may be a tangible storage medium, such as a removable storage unit or a hard disk installed in a hard disk drive. The machine-readable storage medium comprises instructions to calculate the throughput of molten material to the die the based on a desired film width. The machine-readable storage medium further comprises instructions to provide molten material to the die at a rate according to the calculated throughput. The machine-readable storage medium further comprises instructions to pass the molten material through the die to form a film having a central portion and edge portions that are thicker than the central portion. The machine-readable storage medium further comprises instructions to trim the edge portions to provide a film of desired width.

[0060] Although the present disclosure includes certain embodiments, examples and applications, it will be understood by those skilled in the art that the present disclosure extends beyond the specifically disclosed embodiments to other alternative embodiments or uses and obvious modifications and equivalents thereof, including embodiments which do not provide all of the features and advantages set forth herein. Accordingly, the scope of the present disclosure is not intended to be limited by the specific disclosures of preferred embodiments herein, and may be defined by claims as presented herein or as presented in the future.

[0061] Conditional language, such as “can,” “could,” “might,” or “may,” unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements, or steps. Thus, such conditional language is not generally intended to imply that features, elements, or steps are in any way required for one or more embodiments or that one or more embodiments necessarily include logic for deciding, with or without user input or prompting, whether these features, elements, or steps are included or are to be performed in any particular embodiment. The terms “comprising,” “including,” “having,” and the like are synonymous and are used inclusively, in an open-ended fashion, and do not exclude additional elements, features, acts, operations, and so forth. Also, the term “or” is used in its inclusive sense (and not in its exclusive sense) so that when used, for example, to connect a list of elements, the term “or” means one, some, or all of the elements in the list. Further, the term “each,” as used herein, in addition to having its ordinary meaning, can mean any subset of a set of elements to which the term “each” is applied.

[0062] Conjunctive language such as the phrase “at least one of X, Y, and Z,” unless specifically stated otherwise, is otherwise understood with the context as used in general to convey that an item, term, etc. may be either X, Y, orZ. Thus, such conjunctive language is not generally intended to imply that certain embodiments require the presence of at least one of X, at least one of Y, and at least one of Z.

[0063] Language of degree used herein, such as the terms “approximately,” “about,” “generally,” and “substantially” as used herein represent a value, amount, or characteristic close to the stated value, amount, or characteristic that still performs a desired function or achieves a desired result. For example, the terms “approximately”, “about”, “generally,” and 5 “substantially” may refer to an amount that is within less than 10% of, within less than 5% of, within less than 1% of, within less than 0.1% of, and within less than 0.01% of the stated amount. As another example, in certain embodiments, the terms “generally parallel” and “substantially parallel” refer to a value, amount, or characteristic that departs from exactly parallel by less than or equal to 15 degrees, 10 degrees, 5 degrees, 3 degrees, 1 degree, or 10 0.1 degree.

[0064] The scope of the present disclosure is not intended to be limited by the specific disclosures of preferred embodiments in this section or elsewhere in this specification, and may be defined by claims as presented in this section or elsewhere in this specification or as presented in the future. The language of the claims is to be interpreted broadly based on the 15 language employed in the claims and not limited to the examples described in the present specification or during the prosecution of the application, which examples are to be construed as non-exclusive.

Claims

1. An extrusion method for producing a film, the method comprising:calculating a throughput value of molten material to a die based on a desired film width;providing the molten material to the die at a rate corresponding to the calculated throughput value;passing the molten material through a gap of the die to form an output product having a central portion and edge portions, the central portion having a first thickness and a width corresponding to the desired film width and the edge portions being thicker than the first thickness of the central portion;monitoring a thickness profile of the central portion of the film in the width direction and adjusting a height of the gap of the die according to the monitored thickness profile; andtrimming the edge portions to provide a film having the desired film width,wherein the method comprises applying the material that has passed through the die onto a carrier, and wherein throughput value is calculated based on the speed of movement of the carrier, the desired film width and the desired film weight, wherein the film weight is the mass per unit area of the film, andwherein providing the molten material to the die according to the calculated throughput value comprises controlling a speed of revolution of a screw in an extruder barrel.

2. The method according to claim 1, wherein the throughput value in a given time is calculated using the equation:carrier speed x desired film width x film weight x time.

3. The method of claim 1, wherein the height of the gap of the die is adjusted by applying heat to one or more die bolts of a plurality of die bolts.

4. The method of claim 1, wherein the height of the gap of the die is adjusted by mechanical means such as hydraulic means, shimming, clamping or adjusting one or more die bolts of a plurality of die bolts.

5. The method according to claim 4, wherein adjusting the height of the gap of the die comprises adjusting one or more die bolts of a plurality of die bolts positioned along the width of the die.

6. The method according to any preceding claim, wherein the molten material is a molten thermoplastic resin, preferably a thermoplastic polyurethane resin7. An extrusion system for forming a film, the system comprising an extruder screw, an extruder barrel, a die and a controller;wherein the extruder barrel is configured to provide molten material to the die, and wherein the controller is configured to calculate a throughput value of molten material to the die based on a desired film width and is configured to control the rate at which molten material is provided to the die based on the calculated throughput value;wherein the system is configured to pass the molten material through a gap in the die to form an output product having a central portion and edge portions, the central portion having a first thickness and a width corresponding to the desired film width and the edge portions being thicker than the first thickness of the central portion,wherein the controller is configured to monitor a thickness profile of the central portion of the film in the width direction and adjust a height of the gap of the die according to the monitored thickness profile,wherein the extrusion system comprises a carrier, wherein the system is configured to apply the molten material onto the carrier when it has passed through the die, wherein the controller is configured to calculate the throughput value based on the speed of movement of the carrier, the desired film width and the desired film weight, wherein the film weight is the mass per unit area of the film, andwherein the controller being configured to control the rate at which molten material is provided comprises wherein the controller being configured to the speed of revolution of the extruder screw.

8. The extrusion system according to claim 7, wherein the controller is configured to calculate the throughput value in a given time using the equation:carrier x desired film width x film weight x time.

9. The extrusion system according to any of claims 7 or 8, further comprising a scanner configured to monitor the thickness of the central portion of the film along a width of the film, wherein the controller is configured to adjust the gap of the die based on the monitored thickness being greater or less than the desired thickness.

10. The extrusion system according to claim 9, wherein the scanner comprises a first sensor and a second sensor, wherein the first sensor is configured to determine a thickness of the carrier prior to the film being applied to the carrier and the second sensor is configured to determine a thickness of the carrier and film together, when the film has been applied to the carrier, wherein the controller is configured to calculate the thickness of the film based on the determined thicknesses.

11. The extrusion system according to claim 9 or claim 10, wherein the scanner is a radioactive scanner, the scanner comprising a radiation source configured to emit radiation to the carrier or film and carrier, and a detector configured to measure the amount of radiation that passes through the materials, wherein the scanner further comprises a processor configured to determine the thickness of the carrier and the film based on the measured amount of radiation.

12. The extrusion system according to any of claims 9 to 11, wherein the die comprises a plurality of die bolts provided along the width of the gap of the die, wherein the scanner is configured to determine the thickness of the film at a plurality of points along the width of a film and determine a variation in film thickness along the width of the film, and wherein the controller is configured to automatically adjust the heights of one or more die bolts of the plurality of die bolts according to the determined variation in film thickness to reduce the variation in film thickness across the width of the film.

13. The extrusion system according to claim 12, wherein the heights of the die bolts are adjustable by applying heat to one or more of the die bolts or reducing heat applied to the bolts, wherein applying heat to one or more of the die bolts causes said one or more bolts to expand and reduce the gap of the die, and wherein reducing heat applied to the one or more die bolts causes said one or more die bolts to contract and increase the gap of the die.

14. A non-transitory machine-readable storage medium encoded with instructions executable by a processor, the machine-readable storage medium comprising:instructions to calculate a throughput value of molten material through a die based on a desired film width;instructions to provide the molten material to the die according to the calculated throughput value;instructions to pass the molten material through the die to form an output product having a central portion and edge portions, the central portion having a first thickness and a width corresponding to the desired film width and the edge portions being thicker than the first thickness of the central portion;instructions to monitor a thickness profile of the central portion of the film in the width direction and adjust a height of the gap of the die according to the monitored thickness profile; andinstructions to trim the edge portions to provide a film of desired width,wherein the instructions to calculate the throughput value comprise instructions to calculate the throughput value of the molten material based on the speed of passing the material through the die, the desired film width and the desired film weight, wherein the film weight is the mass per unit area of the film, andwherein the instructions to provide the molten material to the die according to the calculated throughput value comprise instructions to control a speed of revolution of a screw in an extruder barrel.

515. The non-transitory machine-readable storage medium according to claim 14, comprising instructions to calculate the throughput value of the molten material using the equation:carrier speed x desired film width x film weight x time.10 16. The non-transitory machine-readable storage medium according to any of claims 14 or15, further comprising:instructions to monitor the weight of the central portion of the film along a width direction, andinstructions to adjust a height of the gap of the die and / or the throughput value based on 15 the monitored thickness being greater or less than the desired weight and / or thickness.

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