FR2568817A1 - METHOD AND APPARATUS FOR THERMOFORMING PNEUMATICALLY A THIN-FILM CONTAINER IN BIORIENTEE THERMOPLASTIC MATERIAL - Google Patents

Abstract

THE PROCESS FOR MAKING A THIN WALL CONTAINER FROM A PLASTIC RESIN SHEET INCLUDES THE STEPS OF HEATING SHEET 14, PLACING THE SHEET BETWEEN A PARAISON MOLD 64 AND AN IMPRESSION 34, AND CLOSING THESE LATTERS ON THE LEAF. AN OVERPRESSION IS THEN APPLIED TO FORCE THE SHEET IN THE MOLD 64, THEN A MOVABLE PISTON 30 IS MOVED IN THE MOLD 64 WHICH FORCES THE SHEET IN THE IMPRESSION 34. BY APPLYING AN OVERPRESSION TO THE INSIDE OF THE PRODUCT IN FORMATION, THE CONTAINER ADOPTS THE DESIRED SHAPE WHILE REINFORCING BY BIAXIAL ORIENTATION. AN APPARATUS FOR CARRYING OUT THE PROCESS OF THE INVENTION IS ALSO DESCRIBED.

Description

J. The present invention relates to a method and apparatus for

  make thin-walled containers from thermoplastic resin sheets. More particularly, but not exclusively, the invention relates to a method and apparatus for molding a thin-walled container by

  blowing from a sheet of thermoplastic resin.

  Various methods and apparatus have already been used to form containers or the like from thermoplastics. A number of these methods use a parison which is placed in a mold and then heated and expanded, usually by blowing, until it adoote the shape of the mold. Sometimes the apparatus for dilating the parison comprises a rod or a piston that is moved.

  axially to ensure correct elongation of the parison.

  As long as the temperature conditions are correct, the containers formed in this manner are bi-oriented, that is to say that the molecules are aligned axially and radially, which makes the finished product significantly more resistant than identical container mounted by blowing at a temperature different from the orientation temperature. These methods and apparatus are described in

US Patent 3,900,120.

  US Patent 3,814,784 to Wolf discloses a method of blow molding with the aid of a plug, wherein a sheet of a thermoplastic material is placed on the opening of the mold, and a low pressure is applied on one side of the leaf, causing a hemispherical lifting of the leaf. The plug is then brought against the sheet, forcing it into the mold. Then, a pressure is applied in the mold to give the final shape plastic of the container. The first expansion of the sheet is carried out at a temperature at which the resin is in the plastic state, and by applying pressure from above. The plastic is then cooled, and the plug brought into contact with the plastic. The temperature of the resin, when pushed upwards by the stopper and when it is expanded to give the final shape of the product, is in the range of the orientation temperatures of the resin. a deep bi-directional container with

relatively thin walls.

  Another patent of Wolf, US 3,737,494, discloses a very similar method and apparatus for making thin-walled products. Wolf's apparatus and the method used in his patents relate to open mold configurations. When the piston has been pushed to the bottom of the mold, the periphery of the mold carrying the piston comes into hermetic contact with the clamping ring of the part

  top of the mold, to close the mold.

  Obviously, it is highly desirable to make containers from a sheet, since it is possible to make a sheet comprising several layers of materials, including materials which provide the product with good properties at high and low temperatures, or providing a gas barrier to prevent either the ingress of oxygen into the product contained in the container or the loss of gas such as CO2 from carbonated beverages. It is also desirable that the container be produced with a closed mold configuration, because the temperatures are better controlled and the pressures necessary for the formation and orientation of the product are -3-

more easily reached.

  The present invention relates to an improved thermoforming method and apparatus from a thermoplastic resin sheet, whereby a relatively thin, biaxially enhanced daroi product is formed and formed in a closed mold where and control the

  temperature and desired pressures.

  The method of the present invention for forming a thin-walled container from a thermoplastic resin sheet comprises the steps of: - heating the sheet; positioning the sheet between a parison mold and an article mold aligned with the parison mold; close these molds on the sheet; introducing a gas pressure into the article mold which moves the sheet into the parison mold; and introducing gas into the parison mold for moving the sheet, and moving a Drum-responsive piston in the article mold to form the container while orienting the resin bi-axially and maintaining a thin wall to the container. The present invention also relates to an apparatus for thermoforming an article from a sheet of thermoplastic resin, this apparatus comprising means

  heating the sheet and a source of gas under pressure.

  The apparatus comprises: - a first plate comprising at least one cavity; a second plate, displaceable in the direction of the first plate and in both directions, and comprising at least one cavity located opposite that of the first plate; gas pressure regulating means for connecting the cavities to the source of pressure and for letting the gas escape from the cavities; an impression, located in the cavity of the second plate, defining the shape of the article and comprising several orifices through which the outside of the article can be exposed to the gas pressure and ventilated; a parison mold located in the cavity of the first

  plate comprising a wall defining an annular cavity

  several apertures through the wall, and a piston located in the parison mold, moved between upper and lower limits by a pressure of qaz, which penetrates into the cavity of the article mold to help loosen the

  resin sheet in this mold to form the article.

  The present invention is also described with the aid of the accompanying drawings in which:

  - Figure 1 is a schematic elevational view of the

  apparatus for carrying out the process of the invention; Figure 2 is a fragmentary and enlarged sectional view showing the detail of the parison mold and article imprint construction constructed in accordance with the present invention; Figure 3 is an enlarged fragmentary view showing the structure of a one-way valve used in the apparatus of Figure 2; Figures 4 and 5 show the apparatus of Figure 2 at later stages of the process; and - Figure 6 shows the apparatus of Figure 2 in step

  final production of a thin-walled container.

  The drawings, particularly Figure 1, show the apparatus 10 for producing containers from a thermoplastic resin sheet according to the method of the invention. The apparatus 10 comprises a base 12 carrying at one end a roll 14 of thermoplastic resin sheet, and having a transport system 16 for moving the resin sheet along the base 12. A heating system 18 is located above the transport system 16, and a second heating system 20 is shown dashed below the transport system 16. The heating systems 18 and 20 serve to bring the thermo-elastic resin sheet 14 to the desired thermoforming temperature before that the leaf

  does not reach an assembly 22 of molds.

  The assembly 22 of molds comprises a plate 24 with the parison mold and a plate 26 with the impression of the article. The plates 24 and 26 may be moved toward each other in both directions for the purpose described below. To the right of the mold assembly 22, as shown in FIG. 1, the cutting device 28 receives the sheet 16 after it has been transformed into the mold assembly 22, and cuts out the containers which have been formed in the assembly 22 from the sheet of

resin 14.

  In Figure 2, the resin sheet 14 is placed between the plate 26 bearing the impression of the article and the plate 24 carrying the parison mold. The plates 24 and 26 are closed, that is to say they have been brought closer to one another until they touch each other and hold the resin sheet 14 together. The sheet 14, previously heated by the heating systems 18 and 20, is deformed by the piston head 30 protruding into the plate 26. Depending on the type of article, the head 30 may not penetrate the plate 26

in its lowest position.

  Still in Figure 2, the plate 26 has a cavity 32 in which there is a wall 34 whose inner wall 36 has a shape such that the finished article will have the desired shape. The impression 34 is held in the cavity 32 by a locking ring 38 of any suitable type. A space 40 has been provided between the wall of the cavity 32 and the outside of the cavity 34. Several orifices 42 in the wall of the cavity 34 ensure communication between the space 40 and the interior of the cavity. 34. The plate 26 has several channels that allow the circulation of hot water, air or other suitable fluid to control the temperature of the plate 26 and the footprint

256881 7

  34. The plate 26 is also provided with a conduit 46 of a suitable gas, communicating with the space 40 to allow pressurization or ventilation of the cavity 34 according to the needs of the molding process. The pipes 46 are connected to a control system 48, which can be of any type capable of ensuring pressurization or ventilation at appropriate times. The control system 48 is connected to a source of pressurized gas (no

  shown) and provided with an exhaust for ventilation.

  The control system is also connected to a pipe 52 which leads to an annular space 54 surrounding a recessed cylinder 56. This built-in room 56 is located

  in a cavity 58 provided in the plate 24. Several openings

  tures 60 in the wall of the part 56 provide a connection with the annular space 54 and thus allow the entry of gas under pressure into the cavity 58, or the gas outlet of this cavity

during ventilation.

  The plate 24 carrying the parison mold is also provided with a plurality of pipes 62 suitably positioned for heating a parison mold 64 placed in the cavity 58. The lower part of the mold 64 is in contact with the upper part of the cavity 58. cylinder 56 which has a plurality of slots 66 extending radially. This parison mold 64 is held in the plate 24 by a

locking ring 68 suitable.

  The parison mold 64 comprises an axial orifice 70 which receives a sliding rod 72 carrying the aforementioned piston head 30 and extending to a valve 74 mounted in the recessed cylinder 56. The valve 74 will be described below,

in league with Figure 3.

  The parison mold 64 has an inner wall 76 which has a W-shaped cross-section. Several orifices 78 pass through the wall 76 of the parison mold 64 and communicate with an annular space 80, which is also connected with the cavity 7- 58 by slots 82 in the lower part of the mold 64 and slots 66 in the recessed cylinder 56. Several openings 84 also provide direct communication between

  the interior of the parison mold 64 and the cavity 58.

  Figure 3 shows the detail of the valve 74 which is attached to the lower end of the head 72 of the piston. The

  mounting of the valve 74 includes a disk 86 traversed by a plurality of

  The disc 86 is secured to the piston rod 72 by threaded nuts 90. An annular seal 92 surrounds the disc 86 and provides sealing during friction.

in the recessed cylinder 56.

  To open and close the openings 88, a valve 94 surrounds the rod 72 and is slidable thereon to the dashed position to prevent passage through the valve openings 88. The movement of the valve 94 is limited by several fasteners 96 screwed into the disc 86. To maintain the valve 94 in the closed position (shown in dotted lines), that is to say in the position where the passage through the openings 88 is closed, a second threaded nut 98 is placed on the rod 72 and a compressed spring 100 is placed between the nut 98 and the

valve 94.

  During operation of the apparatus 10 and the production of deep drawn thin walled containers, the resin sheet 14 is moved by the transport system 16 between the heating systems 18 and 20, where its temperature is raised above the glass transition temperature, but below the crystalline melting temperature, the plastic used. When the resin sheet 14 reaches the mold assembly 22, the transport system 16 is stopped and the plates 24 and 26 are brought into contact as shown in FIG. 2. The piston head 30 is then in contact with the sheet 14, forcing it upwards in the cavity 32 of the plate 26 bearing the impression of the article. At this moment, there is no pressure either in the cavity 32 of the plate 26 bearing the impression of the article or in the cavity

  58 of the plate 24 carrying the parison mold.

  The control system 48 sends gas under pressure through the pipe 46 into the space 40 of the plate 26, from where it penetrates inside the cavity 34 through the openings 42 that it comprises, forcing the sheet 14 in the cavity 32, as shown in Figure 4. The sheet 14 thins slightly when it deforms coming into contact with

  the inner wall 76 of the parison mold 64.

  While the sheet 14 is pushed into the mold

  64, the gas it contains leaves through openings 78 and 84.

  The gas coming from the openings 78 enters the space 30 outside the mold 64, passes into the slots 82 which are formed therein and, through the slots 66, into the cylinder 56. The gas passing through the openings 84 penetrates directly inside the cylinder 56 and, joined to that passed through the slots 66, can cause a downward movement of the valve 84 which, deviating from the disk 86, allows gas to pass through the openings 60 and the pipe 52 up to 1 '

  50. The wide variety of thermoplastic materials

  ticks and thicknesses that can be used makes that the pressure in the cavity 32 of the imprint of the article can

vary from 207 kPa to 3.1 MPa.

  During this process step, water or other suitable fluid flows through the pipes 44 of the plate 26 and through the pipes 62 of the plate 24 to control the temperature of the resin. It is usually necessary to heat to prevent the resin from cooling too much. Usually, gas, which is usually air, is also heated to a

  temperature between 2150C and 280 C before being in-

  produced in the cavities of the molds 84 and 64. The molds 84 and 64 are generally heated to a temperature ranging from 75 to 115 C. The initial stage of the process is completed when the sheet 14 is brought into the position of the mold.

Figure 4

  While the temperature of the sheet 14 in the assembly 22 is maintained at the orientation temperature of the resin used, the control system 48 opens the line 46 to ventilate the cavity 32 and the interior of the imprint 34 through openings 42 therein provided. The control system 48 also sends gas under pressure, through the pipe 52, the annular space 54 in the cylinder 56 and the openings 60, in the cavity 58 under the valve 74. There is therefore an overpressure under the valve 74, which closes the valve 74 with the help of the spring 100. The pressure difference is sufficient to drive the rod 72 and the piston head upwards, as in FIG. 5. As above, because of the different resins, thicknesses, etc ... that we

  can use, the pressure can range from 0.207 to 2.07 MPa.

  Each resin has its temperature range in

  which an orientation is possible. The temperature domains

  Orientation for several thermoplastic resins commonly used are: for polyethylene terephthalate, 90 to 100 C; for polypropylene, from 100 to

   C; and for polyethylene from 50 to 130 ° C.

  As the position head 30 moves upward, gas contained in the cavity 58 above the valve 74 is also displaced by the latter into the interior of the mold 64, so that the sheet 14 adopts Figure 5. It is noted that while the piston head 30 is moving upwards, the resin sheet 14 is stretched axially, whereas, due to the pressure prevailing at

  inside the mold 64, it is also stretched radially.

  The sheet] .4 is therefore subjected simultaneously to an axial orientation and to a radial orientation, which increases

  significantly the strength of the finished product.

  The movement described above continues until the seal 92 on the valve 74 exceeds the lower edge of the slots 66 in the cylinder 56. The upward movement of the piston head 30 is interrupted, and the pressurized gas in the cavity 58 penetrates, through the openings 84 and 78, and

- 10 -

  the inside of the mold 64 and the deformed sheet 14. This forces the sheet 14 against the inner wall 36 of the impression 34 and gives it the final shape of the product. The sheet 14 is now stretched to the maximum allowed by the impression 34, and has therefore received the molecular orientation

  biaxial maximum that can be given to the product.

  When the sheet 14 has reached the configuration of Figure 6, and the pressures in the cavity 58 and within the product formed by the sheet 14 have equalized, the piston head 30, the rod 72 and the valve 74 may be lower than the slots 66. When this is done, it is necessary to ventilate the cavity 58 through the pipe 52 to depressurize the interior of the product formed in the cavity 34 by the opening of the valve 74, which prevents the rapid release of pressurized gas when separating the plates 24 and 26 to remove the product. After the separation of the plates 24 and 26, the sheet 14 carrying the thin-walled product which has been formed is fed to the apparatus

  28 cutting that crops the product of leaves 14.

  The method and apparatus described above provide thin-walled, deeply-drawn products, such as containers, which have been bi-axially oriented to reinforce them. The process and apparatus efficiently and accurately produce the products, while maintaining the necessary pressure and temperature conditions to perform the desired operations in a controlled manner.

closed mold configuration.

  Although the present invention has been described using preferred embodiments, it is understood that they are not a limitation thereof, and that certain modifications made by those skilled in the art make them

completely part.

- 11 -

Claims (10)

R E V E N D I C A T T O N S   1) Improved process for making a thin-walled recipe   from a sheet of thermoplastic resin, characterized   in that it comprises the steps of: - heating the sheet;   - place the sheet between a parison mold and an   preinte aligned with the mold at Daraison; -   - bring the mold closer to the impression until you enclose the leaf; introducing gas into the impression to move the sheet into the parison mold; and - introducing gas into the parison mold to move a piston by pressure in the impression of the article To form the article while bi-axially orienting the   resin and now a thin-walled article.   2) Process according to claim 1, characterized in that the resin sheet is heated to its orientation temperature which is between its temperature of   glass transition and its crystal melting temperature   line before introducing gas into the parison mold, to allow the orientation of the resin by deformation in the footprint of the article.   3) Method according to any one of claims 1 and 2   characterized in that the mold and the impression are heated   between 75 and 115 C inclusive to prevent   cool below a desired temperature.   4) Method according to any one of claims 1 to 3,   characterized in that the gas is heated to a temperature of ture between 215 C and 280 C.   5) Process according to any one of claims 1 to 4, - 12 -   characterized in that the pressure is released after having   formed the article and before separating the plates.   6) Process according to any one of claims 1 to 5,   characterized in that the shaped article is removed from the mold.   7) Improved apparatus (10) for thermoforming an article at   firing a sheet (14) of thermoplastic resin, comprising   means (18 and 20) for heating the sheet (14) and a source of gas under pressure, characterized in that it comprises: i0 - two plates (24 and 26), each having at least one cavity, movable in the direction of one another and in both directions, and of which at least one cavity in one plate is opposite a cavity in the other plate:   a gas control system (48) for connecting the   exposed to the source of gas under pressure and to ventilate these cavities: an impression (34) for the article, located in the cavity   (32) of the plate (26), having a wall (36) defined   defining the shape of the article and passing through several orifices (42) to expose the outside of the article to an overpressure or to ventilate it; - a parison mold (64), located in the cavity (58) of the plate (24), having a wall forming an annular cavity, a plurality of openings in this wall, and a piston, controlled by gas pressure, located in the mold (64) and movable between an upper limit and a lower limit Dar ce qaz in the cavity (32) of the cavity (34) to help push the sheet (14) of   resin in the impression (34) to form the article.   8) Apparatus (10) according to claim 7, characterized in that the system (48) for controlling the gas comprises a system - 13 -   control system (48) for introducing gas at pre-pollinated pressure into the cavity (34) to depressurize   this impression (34), for introducing gas to a pressure   predetermined pressure in the mold (64), and for depressurizing   these molds before separating the plates (24 and 26) for remove the formed article.   9) Apparatus according to claim 7, characterized in that: - the parison mold (64) comprises a cavity (76) and a cylinder (56), and - the piston comprises - a piston head (30) displaceable in the impression (34) of the article, - a piston rod (72) moving alternately in the mold (64), and one end of which is attached to the piston head (30) and the second end is located at the cylinder (56), and a one-way valve (74) attached to the second end of the rod (72), sealingly slidable within the cylinder (56), allowing a gas flow of   cavity (76) towards the inside of the cylinder (56) and   fishing the gas flow in the opposite direction on a part of the piston path.   Apparatus according to claim 9, characterized in that the parison mold (64) comprises at least one slot connecting the inside of the cylinder (56) with the cavity (76) when the piston is close to its upper position, which allows at the gas pressure to stretch the resin (14) to form the article.   11) Apparatus according to claim 10, characterized in that the valve (74) comprises: - at least one opening therethrough, - a piece (94) closing moving along the - 14 -   rod (72) between positions for which the passage is open and closed, -a spring assembly (100) urging the workpiece (94) towards the position closing the passage of gas.