APPARATUS AND PROCESS FOR SEALING THERMOPLASTIC MATERIALS

TECHNICAL FIELD This invention relates to an apparatus and process for sealing thermoplastic materials. More particularly, it relates to sealing of thermoplastics utilizing a temperature gradient encompassing the melting temperature or melting temperature range for the thermoplastic materials.

BACKGROUND ART

There are a number of different methods of sealing thermoplastic materials to ensure water or air-tightness such as using a heating rod to melt the materials to create a seal. However, there is the danger that the heating element used to seal the materials is heated to a temperature so high that the materials are melted through and do not form an adequate seal, as well as the possibility that the melted material will stick to the heating rod, thereby lessening the efficiency of the heating rod for sealing subsequent materials and creating problems in automating the sealing process. Other methods include creating multiple seals to ensure seal integrity by having redundancy in the seals. It is preferable, for reasons of simplicity and economy, to create a single, strong seal as opposed to multiple seals.

Thermoplastic materials by definition are capable of softening and melting when heated to a characteristic temperature. When so softened and melted, they can be fused to other compatible thermoplastics or other materials, to form a seal. There are instances, for example in sealing in sealing thermoplastic containers of food, beverage, medicine etc. where the formation of a complete, strong, hermetic seal is necessary. Temperature control it is an important factor in forming thermoplastic seals. Application of too high a temperature risks damaging the thermoplastic material in areas adjacent to the seal. Application of too low a temperature risks the formation of an incomplete or insufficiently strong seal. The problem is further complicated by the fact that many thermoplastic materials, being macro

molecules and commonly having regions and domains of diverse composition, have a melting temperature range over which they soften and melt, as opposed to a sharp melting point characteristic of homogeneous compounds. The present invention is directed to the problems inherent in sealing thermoplastic materials in an efficient and cost-effective manner. It is an object of the present invention to provide a novel process and/or device for sealing thermoplastic materials.

SUMMARY OF THE INVENTION

The present invention is based on the concept that a continuous effective thermoplastic seal can be formed between two compatible thermoplastic materials by the application to the thermoplastic materials in contact, over the area where the seal is to be formed, of a temperature gradient embracing the melting temperature or melting temperature range of the materials, under appropriately applied pressure. It is not necessary to exercise tight control of the applied temperature to the precise melting temperature, as long as a temperature range embracing the melting temperature is applied, along with appropriate pressure, over the area to be sealed. The area may be relatively large, and the seal may not be formed in a linear manner, but an effective seal is formed rapidly and simply.

In one of its aspects, the invention provides an apparatus for sealing or fusing thermoplastic material to a complementary substrate. The term "substrate" includes any suitable material for sealing with the thermoplastic material, which can include the same thermoplastic material or different thermoplastic materials. The apparatus includes a sealing means in the form of a sealing unit and having first and second ends, and temperature control means to establish a thermal or temperature gradient between the first and second ends of the sealing unit, the gradient which encompassing the melting temperature or melting temperature range of at least one of the thermoplastic material and the substrate.

The apparatus includes a mounting unit to hold the materials to be sealed and the sealing unit heated to a temperature. The sealing unit, having a first and a second end, can be heated to temperatures above the melting temperature of at least one of the thermoplastic material and the substrate by any suitable means, including the use of a heating cartridge.

A cooling unit is associated with the sealing unit. The first end of the heated sealing unit may be treated with coolant such that the first end is cooled to a temperature below the melting temperature of at least one of the thermoplastic material and the substrate. The apparatus has a sealing unit which can be heated to a temperature equal to or above the high end of the melting temperature range of at least one of the thermoplastic material and substrate. The first end of the sealing unit can be cooled to a temperature equal to or below the low end of the melting temperature range of at least one of the thermoplastic material and substrate, thus creating a temperature gradient across the sealing unit between the first and second ends encompassing the melting temperature range for at least one of the thermoplastic material and substrate.

The apparatus also includes a control system to automatically control at least one of the heating of the sealing unit, the cooling of the sealing unit, and the sealing of the materials.

In a preferred embodiment the sealing unit presents a surface of revolution extending between the first and second ends thereof, such as a cylindrical surface or a frustoconical surface. The mounting unit and the sealing unit are both motorized to permit concurrent rotational movement of the units when in operation. The mounting unit can be motorized to permit rotational movement, with the sealing unit being rotatable in response thereto. Alternatively the sealing unit is motorized to permit rotational movement and the mounting unit is rotatable in response thereto. Suitably the sealing unit is heated by an electrical resistive cartridge.

The sealing unit may include a plurality of different materials (whether shaped like rings, wafers, or other shapes) of different electrical resistivities

sandwiched together such that a temperature gradient is created when an electric current is applied to the sandwiched materials.

In another embodiment of the invention described above, a pressure control means is associated with the sealing means to establish a pressure gradient between the first and second ends of the sealing means where more pressure is applied at one end and less pressure is applied at the other end of the sealing means.

The invention also provides a process for sealing thermoplastic material to a substrate. The process includes/comprises the steps of bringing the thermoplastic material and the substrate together and applying a load or pressure to the substrate and the thermoplastic material to define a localized sealing region or area to be sealed, where the sealing region has a first end and a second end spaced from each other. Further, the pressure is applied as a gradient between the first end to the second end of the localized sealing region, wherein more pressure is applied to the first end than the second end, or vice versa, of the sealing region. In applying a pressure gradient to the localized sealing area, the likelihood of creating a line or area of weakness in the seal is minimized. If a large amount of pressure is applied to the localized sealing region, the sealing region, and especially the ends of the sealing region, will be subject to high stresses whereby the seal will be much thinner and consequently weaker. In addition, high stresses at the ends of the seal regions during the sealing process will create lines of weaknesses in the sealing region at the ends. Applying pressure in a high-to-low gradient from one end of the sealing region to the other end reduces the stresses in the seal region and reduces the likelihood of creating lines of weakness or weak seals.

In another aspect, the sealing region is heated and the first end is maintained at a temperature above the melting temperature or melting temperature range of at least one of the thermoplastic material and the substrate. The second end is maintained at a temperature below the melting temperature or melting temperature range of at least one of the thermoplastic material and the substrate by cooling the second end to the

desired temperature. Thus, a temperature gradient is created between the first and second ends of the sealing region encompassing the melting temperature or melting temperature range of at least one of the thermoplastic material and the substrate. The thermoplastic material and substrate are kept in contact while maintaining the temperature gradient and the pressure gradient until a heat seal is formed and then the application of pressure to, and heating of the sealing region is discontinued.

At least one of the heating, establishment and/or maintenance of the temperature gradient, applying pressure, establishment and/or maintenance of the pressure gradient, and discontinuance of heating and/or application of pressure is preferably automatically controlled. The heating and/or maintenance of the temperature gradient may suitably take place concurrently with the application of pressure and/or maintenance of the pressure gradient. An advantage of the preferred embodiment of the invention is that a strong seal is made without need to perform multiple sealings. A further advantage is that it is not necessary to form a straight line seal since a seal will be formed as long as the materials are exposed to the melting temperature range at any point in the localized region as a result of the temperature gradient, as well as using the pressure gradient. In addition, the process of sealing thermoplastic materials is simplified and with lower cost, thus having benefits for manufacturing, especially manufacturing materials for use in medicine such as bags for fluids, or in the heat sealing of thermoplastic material to continuous surfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a perspective view of the sealing apparatus of the specific preferred embodiment of the invention;

Figure 2 is a cross-sectional view along line 2-2 of Figure 1 ; Figure 3 is a perspective view of the mounting unit and the sealing unit of the apparatus of Fig. 1 ;

Figure 4 is an enlarged cross-sectional view of the element labeled 4 in Figure 2;

Figure 5 is a top view of another embodiment of a sealer for use in the apparatus of Figure 1 ; Figure 6 is a cross-sectional view along line A-A of Figure 5; and

Figure 7 is an enlarged cross-sectional view of the detail portion B of Figure 6.

DESCRIPTION OF THE BEST MODE As seen in Figure 1 , sealing apparatus 10 has a mounting unit 12 and sealing unit 14. Mounting unit 12 is cylindrical in shape and has a plurality of vertically disposed slots 16. Mounting unit 12 is rotatable by way of a drive means, in this case motor 17 (drawn schematically).

Sealing unit 14 comprises sealer 18 that is cylindrical in shape. Sealer 18 is rotatable about its central axis as shown in 18a (Fig. 2) and is coupled to a drive means (not shown) for concurrent movement with mounting unit 12. Guard 20 is mounted above sealer 18 and serves to prevent inadvertent sealing of thermoplastic materials beyond the sealing zone. Sealer 18 is heated by an electrical resistive cartridge (not shown) that heats sealer 18 to a temperature above the melting temperature of at least one of the thermoplastic material and substrate to be sealed.

As seen in Figure 1 , disposed adjacent to top end 22 of sealer 18 is a cooling unit 24 comprising nozzle 26 that is connected to a gas source 28 (shown schematically). Gas source 28 contains coolant which is dispensed through nozzle 26 (in this case, a gas such as nitrogen or oxygen is shown being dispensed by arrows) to cool top end 22 (see Figures 2 and 4) to a temperature below the melting temperature of at least one of the thermoplastic material and substrate. When top end 22 is cooled, a temperature gradient is created across the outer peripheral contact surface of sealer 18 in the axial direction of sealer 18 from a temperature below the melting temperature to a temperature above the melting temperature, which encompasses the melting temperature of at least one of the thermoplastic

material and substrate to be sealed. Sealer 18 of the preferred embodiment is shown as a solid ring with a circumferential, axially extending flange 30.

Sealing unit 14 can be moved in a lateral direction towards mounting unit 12 by means of a motor (shown schematically at 31 , Fig. 1 ). Sealing unit 14 moves along tracks 32 in the direction of the arrows shown in Fig. 1 so that sealer 18 can abut against thermoplastic materials mounted on mounting unit 12. Sealer 18 is associated with springs 19 or other positional control means which permit a variable pressure to be applied to the area to be sealed by regulation of the amount of compression of springs 19, thus permitting optimal sealing.

Control unit 34 (shown schematically) controls the rotation of mounting unit 12 and the travel of sealer 18 along tracks 32, abutment of sealer 18 against the thermoplastic materials, heating of sealer 18, and operation of cooling unit 24 for automated optimum sealing of the thermoplastic material, though one or more of the above functions may be manually actuated as desired.

In use of the apparatus to seal a thermoplastic bag 36 to a bag holder 38 (see Figure 1 ), bag 36 is mounted about holder 38. Holder 38 has axially extending fins 40 which are dimensioned to fit into slots 16 on mounting unit 12. Holder 38 has a circumferential axially extending lip 39 over which the end of bag 36 is fitted. Holder 38, along with bag 36, is placed onto mounting unit 12 with fins 40 fitting into slots 16 and mounting unit 12 is rotated under control of control unit 34. Sealer 18 is heated to a temperature above the melting temperature of one of bag 36 and holder 38 and coolant is dispensed through nozzle 24, both by control unit 34, such that a temperature gradient from below the melting temperature to above the melting temperature of at least one of bag 36 and holder 38 is created in an axial direction on sealer 18, relative to axis 18a.

Sealing unit 14 is moved towards mounting unit 12 such that sealer 18 abuts and melts bag 36 at a localized area 42 (see Figure 4) to create a seal. Mounting unit 12 and sealer 18 rotate concurrently such that a continuous seal 44 (shown schematically at Figure 3 as a stippled area) is

formed circumferentially. After sealing the circumference of bag 36, sealing unit 14 is moved away from mounting unit 12 to remove sealer 18 from contact with bag 36. The sealed bag and holder assembly (not shown) is then removed from mounting unit 12 and another bag 36 and holder 38 can then be placed on mounting unit 12 for sealing.

Figure 5 illustrates another embodiment of the sealer that can be used in the invention described above. Sealer 46 is shaped similarly to sealer 18 shown in figures 1-4, is cylindrical in shape, and has a central aperture 48. Sealer 46 is rotatable about its central axis 50 (see figure 6). An upstanding annular wall 52 borders central aperture 48. An upstanding, circumferential, annular lip 54 protrudes from the periphery of sealer 46.

As seen in figure 7, annular lip 54 has a lower vertical surface 56 and an upper radiused surface 58 that is radiused away from the surface to be sealed. When a load is applied to sealer 46 when sealer 46 is in contact with bag 36 and holder 38, pressure is applied to bag 36 and holder 38. However the pressure applied to bag 36 is not uniform as a pressure gradient is created between lower vertical surface 56 and upper radiused surface 58 since less pressure is applied at upper radiused surface 58 than at lower vertical surface 56 because sealer 46 has gradually less contact with bag 36 between lower vertical surface 56 and upper radiused surface 58. Accordingly, the amount of pressure imparted on bag 36 by sealer 46 is controlled by the upper radiused surface 58 when a load is applied. The use of the pressure gradient in conjunction with heat, causes less mechanical stresses to be placed on the localized sealing region where sealer 48 contacts bag 36, thus lessening the likelihood of creating areas of weakness in the localized sealing region.

It is to be appreciated that elements of the invention can be varied. The mounting unit can have different configurations such as a flat plate, and the configuration of the mounting unit can be changed to correspond to the shape of the thermoplastic materials to be sealed. As well, other suitable heating sources can be used. The heating source can comprise layers of materials of different electrical resistivities sandwiched together, whereby a

temperature gradient is created when an electrical current is applied because of the different electrical resistivities of the materials. These materials can be ring shaped to create a cylindrical sealer or can be made into different shapes depending on the materials to be sealed. Suitable thermoplastic materials include polyethylene and polyvinyl chloride. It is to be appreciated that the person skilled in the art will be knowledgeable of the melting temperature ranges of substrates, thermoplastic materials, or polymers of the same.

In another aspect, the mounting means and sealing means have opposing surfaces on opposite sides of the thermoplastic material and the substrate, wherein one of the surfaces is arcuate.

In another embodiment of the invention, more than one sealing unit can be used. For example, two or more sealing units can be used with one mounting unit. Further, more than one mounting unit can be used. With respect to movement of the sealing unit to abut the thermoplastic materials to be sealed, any suitable motorized or manual means of moving the sealing unit can be used.

Cooling can be accomplished through direct thermal conduction to a heat sink. Examples include using a second non-heated wheel contacting one side of the sealer, using a sealer with fins on one side to convectively cool one side of the sealer, or a thermoelectric cooler in contact with one side of the sealer. In addition, other coolants that can be used include liquids such as water.

Other drive means can be used such as belt drives, servo motors, stepper motors, CD motors, and hand rotation. An alternative embodiment of the drive means associated with the mounting unit further includes a plurality of circumferentially mounted teeth associated with the mounting unit which intermesh with a plurality of complementary, circumferentially mounted teeth associated with the sealer. When the mounting unit is rotated by the drive means, the intermeshing of the teeth causes concurrent rotation of the freely rotatable sealer.

A further embodiment of the variable load application means is a single spring. A further embodiment of the variable load application means is two or more springs.

The control unit 34 can be a computer or any other suitable device that can control the activities of the various parts of sealing apparatus 10 for automated or manually controlled sealing of materials.

Other means can be used to control the pressure being applied to a sealing region to create a pressure gradient. More than one pressure control means can be associated with the sealing means. The sealing means can have at least one end being arcuate, where the arcuate portion is arced away from the sealing region surface. In addition, at least one pressure gradient can be established by pressure control means associated with the sealing where at least one part of sealer 46 applies a higher pressure when a load is applied, than the portions of the sealing means adjacent to the part. The sealing means can have arcuate portions, arcing away from the sealing region, at both ends such that a pressure gradient is established where the ends of the sealing means apply less pressure than the middle of the sealing means. In addition, the sealing means can have at least two convex protuberances with the arcuate portions of the protuberances being proximal to the area to be heat sealed.