BARTLETT, Michael, Nicholas, Oldfeld (de Hilver 25, Goirle, NL-5052, NL)
| CLAIMS 1. -Method for molding molten plastic material into a finished part (15) in a mold cavity (14), defined by a pair of cooperating relatively movable mold halves (13,16), at least one of them (16) equipped with a moveable core (17), characterized in that an extruder (1) is used to convey the molten material into the mold cavity (14) through a feed pipe (12) and in that, starting from a closed and fully clamped mold position and a cavity size (14) in its standard finished part form (15), upon introduction of the molten material into the mold, the movement of the core (17), as well as the speed and timing of the extruder (1) are controlled by the melt flow behavior of the molten material in the mold. 2. -Method according to claim 1, characterized in that the extruder (1) is continuously running. 3.- Method according to claim 1, characterized in that as soon as the part weight of the molten material has been introduced into the mold cavity (14), the feed pipe (12) connecting the extruder (1) to the mold is shut off and the speed of extruder (1) is reduced. 4. -Method according to claim 1, characterized in that an extruder screw (3) and an extruder barrel (4) are used capable of accommodating the material conveyed by the extruder (1) during the time that the feed pipe (12) connecting the extruder to the mold is shut off and the extruder (1) is running at said reduced speed. 5.- Method according to claim 1, characterized in that an extruder screw 3 is used having an enlarged melt volume capacity resulting from the fact that the screw diameter 10 is slightly reduced as compared with a standard screw, thereby making the screw flights 11 deeper than normal increasing the individual flight depth, and from the fact that the distance between the flights 11 and the inner wall of the barrel 4 is increased. 6.- Method according to claim 1, characterized in that a positive displacement pump (24) is used between the extruder (1) and the mold. 7. -Method according to claim 1, characterized in that the molten material is extruded into the mold via melt channels (30) and runners (31) that are appropriately designed in diameter to cut all shear stress to a minimum without creating back pressure. 8.- Method according to claim 1, characterized in that the molten material is extruded into the mold via a feed pipe (12) of which the diameter is not less than 10 mm, preferably 12-15mm. 9. -Method according to claim 1, characterized in that the molten material enters the mold through a mold valve entry gate (29) that is suitably large to cut down the shear stress . 10. -Method according to claim 1, characterized in that a magnetic field is applied by placement of a metal coil around the outside of the extruder, creating a viscosity drop in the resin melt and improving the flow of the resin in the mold, without effecting the properties of the raw material . 11. -Method according to any of the previous claims, characterized in that it is used for the multicavity production of ultra thin wall polypropylene, PP, and polystyrene, PS, containers where the wall thickness can be as low as 150 microns. 12.- Method according to any of the previous claims, characterized in that it is used for the multicavity production of ultra thin wall transparent polyethylene terephtalate, PET, containers where the wall thickness can be a low as 200 microns. 13.- Method according to any of the previous claims, characterized in that it is used for the multicavity production of thin wall transparent and opaque PET containers that are suitable for use for oven reheating of frozen or chilled ready meals. 14.- Method according to any of the previous claims, characterized in that it is used for the multicavity production of thick wall PET preforms. 15.- Method according to any of the previous claims, characterized in that it is used for the multicavity production of transparent, high oxygen barrier, heat resistant, PET containers, suitable for use for processed foods that need to withstand the temperatures and pressures encountered in the pasteurisation ranging from 80 to 100 0C and sterilisation at 121 0C of shelf stable foods. 16.- Method according to any of the previous claims, characterized in that it is used for the multicavity production of transparent, high barrier, pressure resistant PET containers for the aerosol market. 17.- Method according to any of the previous claims, characterized in that it is used for the multicavity production of transparent and opaque PET squeezable tubes. 38- Method according to any of the previous claims, characterized in that clear thin wall trays are produced with a wall thickness of 200-500 micron, preferably 300 micron, using polyethylene terephtalate, PET, grade number 101AS from Pearl Engineering Polymers Ltd, or equivalent. 19.- Method according to any of the previous claims, characterized in that PET grade 101CJ from Pearl Engineering Polymers Ltd, or equivalent, is used for producing both opaque and transparent trays with a level of crystallinity between 20-30%, preferably 25%, and suitable for the ovenable tray market. 20.- Method according to any of the previous claims, characterized in that PET grade 105CHH from Pearl Engineering Polymers Ltd, or equivalent, is used for the production of heat processible food cans having a crystallinity above 30%, preferably 35%. 21.- Method according to any of the previous claims, characterized in that PET grade number IIOCHH from Pearl Engineering Polymers Ltd, or equivalent, is used for the production of transparent thermal resistant containers that could withstand sterilization at 121°C for at least one hour. 22.- Method according to any of the previous claims, characterized in that it is used for the production of PET cans for carbonated soft drinks and beer, that are filled on a metal can line and roll seamed closed with standard metal ends. 23.- Method according to any of the previous claims, characterized in that it is used for the production of transparent PET aerosol containers. 24.- Method according to any of the previous claims, characterized in that it is used for the production of polyethylene, PE, tubes, followed by forming a neck thread by reheating the neck in a thread mold. |
FIELD OF THE INVENTION
The present invention relates to a method and equipment for molding plastics.
More specifically the invention relates to a method and equipment for producing plastic parts by extrusion molding, in particular thick wall, thin wall and very thin wall plastic parts.
The process relates to the production of parts with significant economic benefits as well as quality improvements, with a very wide design window.
BACKGROUND OF THE INVENTION.
Many methods and techniques of injection molding, IM, and injection compression molding, ICM, have been proposed over the years in order to achieve higher production output and to produce plastic parts with lower stress and improved product quality, with thinner wall thicknesses.
Standard injection molding falls short in terms of product quality in areas where stress free parts, due to the high pressures involved, are required such as for optical lenses and optical disks where low birefringence and excellent optical properties are vital.
Further market sectors, where low part stress content is vital, are in the areas where flatness and low warpage are critical such as for flatscreen TV backplates and car window panes .
In-mold-decoration, IMD, is also a process that benefits very highly from low pressure and low stress in the mold so that the decoration material is not subject to stress and possible movement during the process.
This demand for a low stress process resulted in the introduction of the low stress injection compression molding process, ICM.
This process avoids the use of high injection pressures and imparted part stress by utilizing an expanded mold cavity for the injection phase, followed by a compression stroke that compresses the initially injected plaque form into the final part dimensions.
The standard process can be either sequential, where the compression phase follows the end of the injection phase, or simultaneous, where the compression phase starts already during the injection phase.
Prior to the start of the injection of the resin melt, the movable mold half is slightly withdrawn from the fixed mold half, by retraction of the movable machine platen, so that the cavity is still just closed whilst the mold halves are however maintained slightly apart from each other, with the distance between the molds equalling the compression stroke .
After completion of the resin injection, in the case of sequential ICM, into the expanded mold cavity, in the form of a cylindrical disk, the mold halves are joined together as a result of the forward movement of the moveable machine platen and locked at full clamp force causing the immediate reduction of the cavity size and compression molding of the part.
This process is disclosed in detail in many patents, for instance in US 5552094, "Method for producing a thin film" of General Electric US; US 4900242, "Apparatus for injection compression molding articles" of Maus Steven e.a., or US 5059364 , "Injection-compression molding machine and method of molding by using the machine" of Komatsu Seisakusha KK , Japan.
Whilst this process indeed has the advantage of low pressure injection and low part stress it is very difficult to control and hence many of the issued patents have been concerned with improvements in the control of the process.
For example US 4917840 of Toshiba Machine Co Ltd, Japan, "Mold compression control process for an injection molding machine and apparatus thereof", discloses the control of the compression by velocity rather than pressure or time.
EP 0.500.679 of Cincinatti Milacron Inc., US, "Mold clamping system", discloses an improved system for location of the mold using electric motors.
E. I. Du Pont De Nemours and Co, Delaware, US, discloses in WO 91/08890 a control part of the standard sequential injection compression molding cycle with no moving parts in the mould, just the normal two halves, neither of which contain separate moveable parts.
As a result of this the mould cannot be fully clamped when closed prior to injection, as the injection pressure has to be able to separate the mould halves, which are lightly touching.
This is the only way that the cavity can be enlarged , with the result that the mold is partially open, though the cavity is still closed off.
After the injection and mould opening, the concept is introduced of controlling the machine mold closing phase, which is the compression phase, using the cavity pressure as fixed reference to control the mold clamping and /or closing.
Further it is taught to control the clamp force in the final phase of mold closing at fixed volume. In practice however the inherent drawbacks of the process such as needing to control the compression phase by precision movement of the whole machine platen; needing to control the shape and formation and temperature of the injected disk in an open cavity; needing to maintain the exact temperature of the hydraulic fluid; needing to maintain the exact mold separation during the injection of the resin, led to major problems with the process control.
Antoine Baciu e.a. recognized the inherent problems associated with the process, namely that it lacked any control of the formation and geometry of the preformed disk as well as lack of control of the compression stroke and developed what is now termed selective injection compression molding, ICM, also called core stamping, disclosed for instance in US4522778, "Method and apparatus for the injection molding of plastic articles".
As opposed to using the conventional forward machine platen movement to close the mold and thereby compress the pre- injected molten disk, they used a moveable piston or core, operated by a hydraulic cylinder, that was located in a closed and fully clamped mold.
Initially the mold cavity is closed in its standard finished part form. Upon injection the moveable piston or core mold part is mechanically retracted to expand the mold cavity, offsetting any pressure increase.
Once the cavity is fully extended the piston is pushed back mechanically, under pressure of the hydraulic cylinder, into the mold reducing the cavity size and hence compressing the melt to form the final part.
This system is definitely an improvement on standard ICM techniques but still does not solve the fundamental issue of inherent process' instability which is due to the lack of control of the cavity expansion, during injection, and subsequent compression during cavity contraction.
SUMMARY OF THE INVENTION.
It is therefore an objective of the invention to provide a solution to one or more of the above mentioned disadvantages and short-comings.
This objective is met by using a method for molding molten plastic material into a finished part in a mold cavity, defined by a pair of cooperating relatively movable mold parts, at least one of them equipped with a moveable core, whereby an extruder is used to convey the molten material into the mold through a feed pipe and whereby, starting from a closed and fully clamped mold position and a cavity size in its standard finished part form, upon introduction of the molten material into the mold, the movement of the core, as well as the speed and timing of the extruder are controlled by the melt flow behavior of the molten material in the mold.
The invention involves indeed the incorporation of the mold based melt flow molding technology, FMT, into a melt process controlled extrusion molding system.
In the application PCT/IB2008/002406 of Ellenville NV, said flow molding process is described, which successfully solves the basic inherent issue of lack of process control in ICM which typically manifests itself in quality issues of part weight variance; wall thickness deviation; flow marks; low output efficiency; air venting; " flash generation; very large cavity to cavity part variation etc.
The simple key to this being that the process according to the inventions is entirely dictated to and controlled by the resin melt flow behavior only, meaning, in turn, that the process is inherently self controlled and self adjusting.
This is in complete contrast to the previous processes which were mechanically controlled, relying on specific external physical variables such as pressure, time, distance and velocity to control the process of injecting the preformed disk and subsequently compressing it.
Compression however is a low stress, low shear process which is much better suited to being combined with extrusion rather than injection, as extrusion, unlike injection, is similarly a low shear, low stress process.
However, until now, all extrusion compression processes involve the preformed billet or gob being directly extruded into a lower mold half that is fully open and completely exposed to the air, prior to the compression phase when the upper mold half is brought into contact with the lower mold half to compress the gob or billet into a finished part.
Said process is well known in the art for the continuous rotary production of plastic closures.
According to the invention however, the low shear extruded resin enters a closed mold and, owing to its preferred flow in the machine direction, as opposed to the lateral direction that is forced upon it by the mold cavity geometry, the resin melt pressurizes the opposite mold half with the resultant effect of pushing opening and expanding the flexible mold cavity through a retraction of the movable core to allow the resin melt to continue to flow in the preferred direction, as well as being able to expand in the now widened lateral direction.
This means that the resin can naturally dictate its own flow path without the need for any externally imposed control .
Immediately, once the melt flow slows in the machine direction, the pressure on the expanded cavity half drops sharply and the pre-pressurized part of the cavity half, i.e. the moveable core, naturally returns to its original position with the resultant compression of the melt.
This compression phase is again, as with the extrusion dosing phase, dictated and naturally controlled by the resin melt behavior. This factor explains why the process is inherently self regulated and controlled.
Furthermore the complete process is controlled by the melt flow behavior with the extrusion timing being governed by the melt flow in the mold.
An advantage is that it allows for the high output, low cost, multi-cavity production of both thick wall, thin wall and ultra thin wall parts using a low stress and low shear process .
Another major advantage is that the quality of the homogeneous resin melt that is produced by extrusion, as opposed to high pressure, high shear injection, has superior mechanical properties owing to the gentle low shear processing.
This also means that according to the invention parts can be produced from better quality raw materials.
It also means that a wide variety of raw materials can be processed including high viscosity materials like polyethylene terephthalate, PET, and composite long fiber filled thermoplastic resins, LFT.
Another advantage of the process according to the invention is that it can be particularly well, though in no way exclusively, be applied in the high volume packaging market where the process can be effectively used to compete on quality and cost with extrusion thermoforming in the ultra thin wall container market, where other known processes like injection molding cannot compete.
In a preferred embodiment the mold entry gates are also suitably large to cut down the shear stress as far as possible that is so extremely high at the very small diameter gates used in injection molding.
In a further preferred embodiment however of the extrusion compression process according to the invention, the resin is extruded into the mold via melt channels and runners that are appropriately designed in diameter to cut all shear stress to a minimum, as there is no back pressure requirement and resultant need for small channels that is integral in an injection molding process.
In another preferred embodiment the process and materials, according to the invention, are used for multicavity production of ultra thin wall polypropylene, PP, and polystyrene, PS, containers where the wall thickness can be as low as 150 microns.
In another preferred embodiment the process and materials, according to the invention, are used for multicavity production of ultra thin wall transparent polyethylene terephtalate, PET, containers where the wall thickness can be a low as 200 microns.
In a preferred embodiment the process and materials, according to the invention, are used for multicavity production of thin wall transparent and opaque PET containers for use for oven reheating of frozen or chilled ready meals.
In still another preferred embodiment the process and materials, according to the invention, are used for the multicavity production of thick wall PET preforms.
In a further preferred embodiment the process and materials, according to the invention, are used for the multicavity production of transparent, high oxygen barrier, heat resistant, PET containers for use for processed foods that need to withstand the temperatures and pressures encountered in the pasteurisation ranging from 80 to 100 0 C and sterilisation at 121 0 C of shelf stable foods.
In still another preferred embodiment the process and materials, according to the invention, are used for the multicavity production of transparent, high barrier, pressure resistant PET containers for the aerosol market.
In a further preferred embodiment the process and materials, according to the invention, are used for the multicavity production of transparent and opaque PET squeezable tubes.
In another preferred embodiment of the invention a magnetic field is applied by placement of a metal coil around the outside of the extruder, to effect a viscosity drop in the resin melt to improve the flow of the resin in the mold, without effecting the properties of the raw material. BRIEF DESCRIPTION OF THE DRAWINGS.
The invention may be more completely understood in consideration of the foregoing detailed description of various embodiments of the invention in connection with the accompanying drawings in which:
Figure 1 is a cross section of the extruder barrel and screw and mold feed set up for producing parts having a wall thickness in excess of 0.5mm;
Figure 2 is a cross section of the mold showing the mechanical system that sustains the moveable mold part, particularly the moveable core, in the position in mold;
Figure 3 is a drawing of a polypropylene beaker, showing the beaker in perspective view, in lateral view and in cross section;
Figure 4 is a side view of the extruder, gear pump and mold for producing parts with a wall thickness of 250 to 500 microns;
Figure 5 is a cross sectional view of a 0,5 liter preform for carbonated beverages;
Figure 6: is a drawing showing the dimensions of a shelf stable food container. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT.
The drawings, which are not necessarily to scale, depict illustrative embodiments and are not intended to limit the scope of the invention.
Figure 1 shows the equipment of the type used in the invention, in particular it shows a cross section of the extruder barrel and screw, and mold feed set up for producing thick wall parts, being parts with a wall thickness of greater than 0.5 mm.
The plasticizing extruder 1 is composed of a hopper 2, a helical screw 3, an extruder barrel 4, heaters 5, a screen and breaker plate 6, a nozzle 7 and a shut off valve 8, and a feed pipe 12 connecting the extruder 1 to the fixed and hot mold half 13 and further to the mold cavity 14.
The helical screw 3 itself has a diameter 10 that is variable over its length 9 and is equipped with flights 11.
In a typical non limiting embodiment of the invention, particularly for the production of thick wall parts, the plasticizing extruder 1 incorporates a helical screw 3 having a special length 9 to diameter 10 ratio of 24:1, with a 2:1 compression ratio, suitable for processing PET resin, as well as PS and PP, said screw having an enlarged melt volume capacity.
This melt volume capacity increase results from the fact that the screw diameter 10 is slightly reduced as compared with a standard screw, thereby making the screw flights 11 deeper than normal.
The screw 3 when rotating in the direction indicated by the arrow A, catches the material that is fed in the form of granules or pellets through the hopper 2, and melts and feeds it as flowable material, called the melt, in the conventional extrusion manner, into a nozzle 7, which, through a shut off valve 8 and a feed pipe 12 is connected to the fixed, hot, front mold half 13, and further to the mold cavity 14.
The nozzle 7 has a feed pipe 12 diameter of not less than 10 mm, preferably 12-15 mm to ensure both sufficient material dosing and to reduce any shear effects on the melt .
The feed pipe 12 is further equipped with a shut off valve 8 which can be closed to interrupt the flow from the extruder 1 to the fixed, hot, front mold half 13, and further into the mold cavity 14.
The operation is very simple and as follows.
At the beginning of a product cycle the extruder 1 is directly connected to the fixed, hot, front mold half 13 and the mold cavity 14 with the shut off valve 8 in the open position, and the melt flows e.g. under an extrusion pressure of e.g.30 bar and at a rotational speed of 250 RPM via the nozzle 7 and the feed pipe 12 into the front mold half 13, and from there into the mold cavity 14. As soon as the part weight has been extruded into the mold cavity 14, the feed pipe 12 is closed, via the closure of the shut off valve 8, for the subsequent part of the product cycle namely during forming, cooling and dispensing of the finished part 15.
During the time that the shut off valve 8 is in the closed position the extruder 1 continues to rotate but at a reduced speed of e.g. 30-50 RPM.
The low amount of melt that is generated at this part of the cycle, whilst the shut off valve 8 is in the off position, is capable of being contained in the extruder barrel 4 as the extruder melt capacity has been increased by increasing the distance between the flights 11 and the inner wall of the extruder barrel 4 as well as by increasing the individual flight depth, creating an extra 15% in melt volume that can be contained in the extruder 1.
The latter is sufficient to contain the additional melt that is produced during the time that the shut off valve 8 is in the closed position and hence the extruder 1 output must be contained within the barrel 4 to avoid having to shut the extruder off and on during the production cycle.
A more detailed practical example of a preferred operating procedure during the extrusion dosing phase according to the invention is illustrated as follows.
A thick wall PP tray, with a 1.25 mm wall thickness at a part weight of 35 gm, was produced in a mono cavity mold with a cycle time of 8 seconds using an extruder 1 with a special 35 mm screw 3 running at a speed of 250 RPM and at an output of 65kg/hour.
The delivery time for supply of the molten resin, a homopolymer with an melt flow index, MFI, of 10 gm/10 min, to the mold cavity 14 was 3 seconds, leaving the extruder 1 to run for a further 5 seconds with the shut off valve 8 in the closed position, cutting off any resin supply from the extruder 1 to the mold cavity 14.
During this 5 seconds time interval the extruder 1 runs at just 5% of its normal output and produces just 5 gm of resin that is stored in the extra melt space in the extruder 1 provided by the larger flights 11 and the larger gap between the flights 11 and the barrel wall.
Figure 2 shows a cross section of the mold, consisting of a fixed, hot, front mold part 13, and a moveable back mold part 16, and the mechanical system 18 that sustains the moveable mold part 18, particularly the moveable core 17, in the position in mold.
The operating procedure during the molding and compression phase according to the invention is e.g. as follows.
During extrusion of the low viscosity homopolymer resin melt at 260 0 C into the closed mold, as shown in figure 2, the mold cavity 14 is enlarged by pressure of the resin onto a moveable mold part located in the mold, being the moveable core 17.
This moveable core 17 is displaced backwards by the resin, to temporarily enlarge the cavity 14 and thereby allow the resin to flow more easily in the mold cavity 14 during dosage of the resin into the mold.
As soon as the expansion pressure of the resin melt drops, the moveable core 17 is no longer under pressure and immediately contracts to close the cavity 14 back to its original size with the resultant compression of the preformed resin disk into the final finished part geometry.
Figure 2 also shows how the moveable core 17 is maintained in its fixed position in the mold by a positive pressure which comes from a vertical hydraulic cylinder 19 that is connected to the rear of the moveable mold part 16 by a mechanical slide link 20.
This mechanical slide link 20 acts as the connection between the moveable core 17 and the hydraulic cylinder 19.
Upon resin dosing into the mold cavity the cavity pressure increases to the point at which the cavity pressure is greater than the holding pressure from the hydraulic cylinder 19 on the moveable core 17.
At this point the melt pressure pushes back the moveable core 17 to expand the cavity 14 and in turn pushes back the hydraulic cylinder 19 via the mechanical slide link 20. In a practical example the resin melt pressure forces the moveable core 17 back in the direction of the arrow C in a time of 0.02 seconds to a distance of 3 mm, which is as far as necessary to reduce the pressure on the melt flow in the cavity 14.
Immediately the necessary cavity expansion, to relieve the pressure on the melt, is reached, the resin melt pressure, in the forward machine direction, subsides with the corresponding direct release of pressure on the moveable core 17 which then retracts to close the mold cavity 14 with a compressive force on the resin melt.
This compressive force is approximately equal to the mold opening force and is sufficient to force the preformed resin in the mold cavity to flow into the remaining unfilled parts of the mold cavity 14 to produce the final full part geometry 15 (see figure 1).
The degree of resistance pressure on the moveable core 17 can be varied by varying the angle B of the mechanical slide link 20.
A smaller degree of the angle B clearly results in a greater force necessary on the core 17 to displace the hydraulic cylinder 19 as the cavity 14 is expanded in he direction indicated by the arrow C , with a correspondingly greater compressive force acting on the cavity 14 as it is closed.
Figure 3 shows a perspective view 23, a side view 22 and a cross section along the line DD' of view 22, of a typical thin wall polypropylene beaker, that is a beaker having a wall thickness of smaller than 250 micron, made following the method according to the invention, and at a part weight of 3.6 gm, with a production tolerance of +/-0.2 g.
The flow path length to wall thickness ratio of this part given as an example is 120/0.2 = 700.
The height 27 of the beaker is 80 mm, the outside diameter of the bottom 28 is 48.33 mm.
The wall thickness 30 is thereby 0,20 mm and the thickness of the bottom 29 is 0.35 mm.
Figure 4 is a side view of the extruder 1, gear pump 24 and fixed, hit, front mold part 13, used according to the invention for producing clear e.g. thin wall trays with a wall thickness of 250-500 micron.
In a practical operating procedure, e.g. a preferred embodiment, such trays weighing 15 gm with a 300 micron wall are made in modified polyethylene terephthalate, PET, grade number 101AS, supplied by Pearl Engineering Polymers Limited.
This material has high viscosity, compared to PP, and also the part weight is in this case relatively high.
Therefore, to generate sufficient speed of flow into the mold the extruder output is supplemented by a melt pump 24 placed after the extruder 1 as shown in the figure 4.
Such thin wall trays are ideal for use for modified atmosphere packaging, MAP, of chilled fresh foods such as meat, poultry, fish and pasta which are normally, according to the state of the art, packed in thermoformed PET/PE laminated or in coextruded trays.
The extrusion compression molded PET trays according to the invention will have superior quality, particularly stiffness and clarity, as compared to the thermoformed tray as well as being fully recyclable as they are made from single material PET.
Aside from MAP packaging such trays according to the invention would be attractive for use for high quality fresh and chilled food packs, bringing a much greater range of tray design possibilities and decoration possibilities.
In another preferred embodiment tubs, pots and special trays for frozen and chilled ovenable ready meals have been produced in addition to standard transparent trays.
This market is supplied using so called CPET (crystallized PET) thermoformed trays, which are thermoformed trays made from a PET grade that can be readily crystallized during thermoforming in order to resist the high oven temperatures during meal reheating in microwave and conventional ovens, which standard clear APET (amorphous) trays cannot.
These trays have the drawback that they are opaque and so the meals are not visible except from above and in use they are prone to sag after heating due to the variations in side wall thickness, which is inherent in thermoformed trays .
By use of a specially formulated modified PET resin developed for the extrusion flow molding process according to the invention by Pearl Engineering Polymers Ltd, grade number 101CJ, both opaque and transparent PET trays have been produced with high crystallinity, that are suitable for the ovenable tray market.
This means that food producers can go to the market with transparent ready meals packs that give a totally different aspect in terms of meal quality and freshness.
Both clear and opaque trays were produced in 20 gm with a wall thickness of 400 micron with a level of crystallinity of 25%.
Figure 5 shows a cross sectional view of a preferred embodiment of a 0,5 liter PET preform 25 for carbonated beverages, made following the method according to the invention, and produced in a single cavity tool at the ultra low weight of 15gms.
The preforms 25, given as an example, were produced with a light weight PCO 1881 neck with a body wall thickness 32 of just 1.7 mm, including a step base with a wall thickness 33 of 3 mm, which is not possible to produce using conventional injection molding. Further weight has been optimized in the base of the preform 25 by the use of a step bottom 26 to give the required weight for the petaloid base, whilst keeping a low thickness 35 in the center of the base of 1.5mm.
The inner diameter of the neck 31 was 21,74mm, the length of the neck 37 was 17.00 mm, and the length of the body 36 was 63.00 mm.
In order to produce such part the gate 29 of the fixed, the mold part 13 (se figures 1 and 2) has been modified to give a better performance in terms of crystallinity level and material stretchability.
The gate 29 is wider than normal but the design is such that the wall thickness is lower with resultant reduced crystallization at the gate.
In addition to the design improvements the stretch ratio has been increased from a typical total stretch value of 4.0 (height) x 2.5 (av. width) = 10.0 to a very high ratio of 13.5 = 4.5 x 3.0 giving much improved strength and barrier properties of the oriented PET per given wall thickness.
0.5 liter, 12 gm straight wall petaloid bottles were blown from these preforms 25 and bottles were subject to top load and burst strength testing.
The bottles were able to withstand a pressure of 8 bar. Figure 6 shows a shelf stable food can represented in cross section 27 and in perspective view 28.
Those heat processible transparent food cans were produced in a further embodiment of the invention, in modified PET.
The cans of 0.35 liter capacity, as shown in figure 6, had a wall thickness of 500 micron and were weighing 200 gm.
The outside diameter 38 was 110 mm, the inside diameter 39 was 100 mm, the height 40 was 80 mm, the outside diameter of the bottom 41 was 50 mm, and the radius 42 was 159.
The thickness of the rim 43 in this example was 0,40 mm.
The PET containers in this embodiment were made of a modified PET that has increased thermal stability as a result of a greater level of crystallinity.
The level of crystallinity in these containers was above 35% in order to achieve improved mechanical properties as well as increased thermal resistance for hot fill at 85-90 0 C and pasteurization at 85-90 0 C of high acid foodstuffs (shelf stable fruits, pickled vegetables, jams and marmalades) whilst retaining transparency, by keeping the size of the crystal spherulites as low as possible.
The containers were produced with modified PET, Pearl Engineering Polymers grade 105CHH, using extrusion flow molding conditions according to the invention, as described above for the PET trays.
They were then subject to warm filling at 65 0 C, sealing with a lidding film, before being pasteurized for a 1 hour cycle with a peak temperature of 85 °C for 10 minutes.
In a further preferred embodiment, not represented in the figures, transparent thermal resistant PET containers that could withstand heat sterilization at 121°C were produced following the method of the invention.
These containers were produced from modified PET, Pearl Engineering Polymers grade number 110CHH, using extrusion flow molding conditions as described above for PET trays.
The containers were warm filled at 65 0 C and sealed before being fed to a batch counter pressure autoclave for processing at 121°C for a 1 hour cycle.
In a further embodiment transparent modified PET cans were made for carbonated soft drinks and beer.
These cans were extrusion molded as described above and filled on a metal can line.
After filling, the cans were roll seam closed with standard metal ends.
In a further embodiment transparent modified PET aerosol containers were extrusion molded according to the invention.
In a further embodiment PE tubes were extrusion compression molded.
Following molding of the tube itself the neck thread was separately formed by reheating the neck in a thread mold.
The present invention is in no way limited to the form of embodiment described by way of example and represented in the figures, however, such an improved invention can be realized in various forms without leaving the scope of the invention.