GB2050232A - Thermoforming sheet material - Google Patents

Abstract

A method of thermoforming sheet polypropylene comprises the steps of causing the polypropylene sheet 1 to intermittently pass through a continuous heating zone A in which it is brought to a temperature close to, but lower than, its melting temperature, conveying the sheet to a forming zone B, carrying out a sheet preforming operation by means of a punch 5 and then a final forming operation by blowing a jet of pressurized fluid through a port 11 onto one surface of the sheet. The article so formed is then cooled by the mould 2. <IMAGE>

Description

SPECIFICATION Method of thermoforming high crystalline content polymers, and particularly sheet polypropylene This invention relates to an improved method of thermoforming high crystalline content polymers, and particularly sheet polypropylene.

High crystalline content polymers, and polypropylene especially, have currently achieved preminence among thermoformed or hot moulded plastics, by virtue of some advantageous properties, such as high resistance to chemical agents, high mechanical and thermal resistance, impermeability to liquids and gases, etc., which make them specially suitable for manufacturing food packages.

Also widespread is nowadays the processing of high crystalline polymers, and in particular polypropylene, by means of a "solid state" thermoforming process, which is characterized in that the material subjected to thermoforming or hot moulding is heated, rather than above its melting point, a few degrees below this limit, thereby the material can be processed in the solid state. Thus, for example, US Patent No. 3,642,415 teaches that, for a polypropylene having a 168 C melting point, a preferred temperature range for solid state thermoforming is from 149 C to 166 C.

Prior art methods of thermoforming sheetlike plastics materials have proposed a wide range of processing conditions, among which those of cooling the mould or die at its forming cavities (refer to US Patents Nos.

2,967,328 and 2,282,423), or by contrast, of heating the mould to about 66-110 C (refer to US Patent No. 3,121,920).

Also known is to subject to a process of thermoforming sheet or strip-like plastics materials, those materials which have a fluidity rate which varies within a wide range, e.g.

from .2 to approximately 5 as measured with the ASTM D-1238-52T method, in accordance with the teachings of the cited US Patent No. 3,121,920.

The relevant literature further teaches (refer to US Patent No. 2,967,328) the use of heated punches in thermoforming plastics material sheets. The processing conditions mentioned above have also been applied to conventional methods of thermoforming high crystalline polymers, in particular polypropylene. However, in spite of the extreme variety of the conditions employed in prior art polypropylene thermoforming processes, the latter have failed to prove entirely satisfactory, especially as relates the production output and the quality of the resulting products.

It has now been found that by adopting a peculiar combinaton of steps, some of which are known per se, an improved method of thermoforming polypropylene is achieved which obviates the cited disadvantages, while affording a high production rate and articles moulded with high precision, i.e. of excellent quality.

These and other advantages, such as will become apparent herein below, are achieved by an improved method, according to the instant invention, of thermoforming sheet-like polypropylene by processing it in the solid state, characterized in that it comprises the steps of causing the polypropylene sheet to pass with intermittent motion through a continuous heating zone the longitudinal extension of which is at least four times as long as the useful length of a successive forming zone such as to heat the sheet gradually to a temperature close to, but lower than, its melting temperature, conveying the sheet to the forming zone, and applying a controlled pressure to the peripheral areas of the sheet located at the forming zone for the purpose of anchoring said sheet, carrying out a sheet preforming operation between a punch contacting the sheet at least partially and a hollow mould which are in motion relative to each other, said punch being heated at least at its sheet contacting area to a temperature by approximately 16 to 34 C below the sheet temperature, and carrying out a final forming operation by blowing a substantially instantaneous jet of pressurized fluid onto the sheet surface facing the punch, such as to cause said sheet to adhere to the mould walls surrounding said cavity, said walls being maintained at a temperature below 10 C.

Further features and advantages of the invention will become more clearly apparent from the following description of a preferred, though not limitative, embodiment of the instant polypropylene thermoforming method, with reference to the accompanying drawing, where the one Figure shows a flow diagram of the method according to this invention.

In the thermoforming method according to the invention the start material is a sheet of polypropylene, preferably having a low fluidity value, i.e. below .7 as measured with the ASTM D-1238-65T method at 230 C and 2.16 kgs.

In actual practice, it has been found that by utilizing a low fluidity rate material, i.e. one of high viscosity, easier distribution and greater thickness uniformity are achieved, as well as an improved molecular orientation of the formed products, which, for a given weight, contribute towards a better consistency of the articles, or in other terms, a better material economy.

The polypropylene sheet, indicated in the drawing at 1, is first passed to a heating zone A, of much elongated configuration, such as to allow for a sufficient residence of the sheet in the heater to achieve a progressive and gradual increase of temperature up to about 161-168"C, which is the required tempera ture for the solid state thermoforming. Advantageously, the heating zone A has a length which is at least four times as great as the useful length of the successive forming zone, such as to ensure a uniform heating throughout the thickness of the polypropylene sheet, with due regard for its low diffusivity (namely, low thermal conductivity to thermal capacity ratio). The sheet residence time in the heating zone is at least 5 seconds, for sheets about .2-.3 mm thick, and increases with the sheet thickness dimension, albeit at a lesser rate.

In order to ensure graduality and a uniform as possible application of heat, the heating zone A comprises several series of ceramic thermoelectric elements of conventional design. which are controlled by electronic regulators, such elements being preferably arranged above and below the sheet path up to a point close to the forming mould or die.

Uniformity of heat application is also provided by an accurate arrangement of said ceramic thermoelectric elements in succession and closely adjacent one another.

The sheet advance movement is ensured by suitable toothed chains (not shown) entraining it from the beginning of the heating zone A, or alternatively, if a preliminary free stress relieving step is desired, after the sheet has already advanced by a distance through the heater. The sheet is caused to advance intermittently, and in order to minimize the dead time for feeding in the mould or die, as well as to prevent thermal disturbance of the sheet, the movement is imparted by cams which are subjected to high accelerations and decelerations, in the order of 10 m/sec2, quite compatibly with the sheet strength, within a shorter time than .4 seconds. The sheet, after it has reached a temperature of 161-168"C, is delivered to the forming zone B.For the forming step, a hollow mould or die 2 is utilized, the cavity 3 of which is a negative image of the configuration of the article to be formed, as well as a countermould 4, in which a shaped punch 5 is movable which goes through a bell-like movable member 6. The latter is elastically suspended in the counter-mould 4, e.g. by means of springs 6a, and may be actuated from the outsides by admitting a pressurized fluid through ports 7 formed in the countermould 4.

The mould is also mechanically controlled by cams actuated in synchronism with those cams which control the intermittent movement of the sheet 1 through the heating zone A.

Thanks to the mechanical control system, the overall movement time of the mould is less than about .8 seconds. This time economy, when projected over the sequence of operations carried out, allows for the carrying into effect of the thermoforming step, which as explained herein below, involves three stages, namely punch preforming, pressurized fluid forming, and residence of the article in the mould for cooling, with durations in the order of about one second each.

The mould 2 is cooled, at least at its walls defining the cavity 3, to a temperature below 10 C, preferably to a temperature between 4 and 6 C, such as to produce a rapid cooling of the formed article as the latter contacts the walls of the cavity 3. For this purpose, there are provided in the mould 2 channels 8 for circulating a coolant such as water and an anti-freezing agent. In the mould 2, there are also provided one or more ports 9 for venting the air or possibly apply a vacuum during the forming step, or even for injecting backpressure air during any of the preforming, forming or ejecting stages. To facilitate the ejection of the formed articles from the mould, an axially movable member 10 is further provided which has a stem protruding outwards from the mould 2 to permit the same to be actuated from outside.The movable member 10, which in the embodiment described defines the bottom of the cavity 3, is also cooled, as are the sidewalls of the mould defining the cavity 3, down to 4-6 C.

At the thermoforming zone B, the sheet 1 is first anchored by application, at its peripheral areas, which surround the mould mouth or opening, of a controlled pressure through the bottom edge of the movable member 6. The movable member 6 is enabled to apply on the peripheral sections of the sheet a sufficient pressure to block or clamp the sheet, or alternatively, to allow a controlled sliding movement of it into the mould cavity, such as to prevent crimping or abnormal build-ups.

After the sheet has been thus anchored, it is engaged by the punch 5 in its movement towards the cavity 3 of the mould 2. Advantageously, the punch 5 is heated, at least at its outer surface contacting the sheet being processed, to a temperature which is by about 16-34 C lower than the sheet temperature, i.e. to a preferred temperature of about 134-145"C. Preferably, the smallest values in the temperature range mentioned above are selected when it is desired to impart to the sheet a particular configuration, as conferred by the punch shape; in this instance, the higher temperature differential between the punch surface and the sheet being processed "freezes" the surface of the contacted sheet into the shape conferred thereto by the punch. By contrast, the highest values in the above temperature range are selected when it is desired to achieve, at the bottom of the formed article, a reduced thickness dimension, or to impart to that bottom a configuration more in agreement with the shape of the mould than of the punch.

The punch 5 is heated, e.g. electrically, and is controlled independently of the movements of the mould or counter-mould.

To provide a more uniform thickness distri bution in the formed article, the punch has a low friction coefficient, at least at its surface contacting the sheet, which is achieved by either mirror-like chrome plating said surface or coating it with a low friction coefficient material such as polytetrafluoroethylene (Teflon) Registered Trade Mark. Moreover, the punch is preferably made of a low thermal conductivity material.

Upon completion of the preforming phase by means of the punch 5, the forming is effected by injecting into the cavity defined between the mould 2 and counter-mould 4, when these are closed, pressurized fluid, preferably air, directed onto the sheet surface facing the punch 5. This can be obtained through specially provided ports fitted with valves, e.g. similar to the one schematically indicated at 11 in the drawing and formed in the counter-mould 4. Generally, the pressure of the fluid admitted through the ports 11 exceeds about 3.5 kg/cm2.

Advantageously, the pressurized fluid is injected as an instantaneous jet which is cooled down to a temperature below about 10 C.

Thus, a simultaneous and rapid cooling of both the sheet surfaces is achieved, namely of the surface caused to adhere to the walls of the cavity 3 by the jet, and of the surface whereon the pressurized fluid cooled jet impinges, to favour a good dimensional stability in the product as well as a high degree of accuracy of the details thereof.

Advantageously, the method according to the invention contemplates a further operation, to be carried out in the mould, which comprises trimming of the formed article. This is achieved by providing ridges and mating grooves, having sharp edges, respectively on the mould 3 and counter-mould 4. The trimming operation being carried out while the article is still clamped in the cooled mould to 4-6'C, in addition to eliminating subsequent cutting steps, gives rise to an important advantage in thermoforming polypropylene, i.e.

that of eliminating any distortion and return heat from the residual portion of the sheet, namely the flush, to the formed article. This enhances the accuracy of the edge finish, an essential condition for any subsequent edge rolling operation to be carried out on the formed article.

The article, thus formed and cooled, is ejected from the mould by actuation of the member 10 or, if required, by application of a backpressure through the channel 9, and can be discharged, for example, through a movable conveying channel 12 into an underlying stacker.

From the description given herein, it will appear that the method according to the invention fully achieves its objects. Thus, by virtue of the peculiar combination of temperature levels in the various members and elements which contribute to the thermoforming of polypropylene, the cycle dead times are minimized and heat dissipation from the sheet is effectively prevented, to achieve useful thermoforming steps having durations in the order of one second. The inventive method, therefore, affords a production rate which, depending on the thickness dimensions employed, may exceed 30 to 35 cycles per minute.

Moreover, articles of high precision can be produced which exhibit remarkable accuracy and dimensional stability characteristics.

The method is useful both for processing homopolymer and copolymer resins of polypropylene, and for processing polypropylene, as charged up to 40% by weight with inert filler materials usually employed for charging polypropylene, such as calcium carbonate, chalk, talcum, wood, etc.

The invention as described in the foregoing is susceptible to many modifications and variations, all of which are intended to fall within the invention scope. Thus, for example, the mould could be provided with several cavities and a corresponding number of punches for the purpose of simultaneously forming a desired number of articles. Furthermore, during the preforming stage with the punch, a slight vacuum or moderate backpressure may be applied to the opposite side of the sheet, as required. Further modifications and variations will present themselves to the expert in the art.

Claims (14)

1. An improved method of thermoforming sheet-like polypropylene by processing it in the solid state, characterized in that it comprises the steps of causing the polypropylene sheet to pass with intermittent motion through a continuous heating zone the longitudinal extension of which is at least four times as long as the useful length of a successive forming zone such as to heat the sheet gradually to a temperature close to, but lower than, its melting temperature, conveying the sheet to the forming zone, and applying a controlled pressure to the peripheral areas of the sheet located at the forming zone for the purpose of anchoring said sheet, carrying out a sheet preforming operation between a punch contacting the sheet at least partially and a hollow mould which are in motion relative to each other, said punch being heated at least at its sheet contacting area to a temperature by approximately 16 to 34 C below the sheet temperature, and carrying out a final forming operation by blowing a substantially instantaneous jet of pressurized fluid onto the sheet surface facing the punch, such as to cause said sheet to adhere to the mould walls surrounding said cavity, said walls being maintained at a temperature below 10 C.

2. A method according to Claim 1, characterized in that the polypropylene comprises either a homopolymer or copolymer of poly propylene having a fluidity value in the molten state (as measured with the ASTM D-1238-65T method at 230 C and 2.16 kgs.) below 0.7.

3. A method according to Claim 1, characterized in that the polypropylene comprises at least 60% by weight of a polypropylene homopolymer or copolymer and up to 40% by weight of an inert filler.

4. A method according to one or more of the preceding claims, characterized in that the heating step is effected by means of a plurality of ceramic thermoelectric elements which are controllable and adjustable, being arranged along said heating zone closely adjacent to one another.

5. A method according to Claim 4, characterized in that the ceramic thermoelectric elements are arranged above and below the path of said sheet through said heating zone.

6. A method according to one or more of the preceding claims, characterized in that the step of applying a controlled pressure to the peripheral areas of said sheet is carried out by means of an elastically acting member effective to block the sheet in the mould.

7. A method according to one or more of Claims 1 to 5, characterized in that the step of applying a controlled pressure to the peripheral areas of said sheet is carried out by means of an elastically acting member such as to allow for a controlled sliding movement of the sheet in the mould.

8. A method according to Claim 7, characterized in that, after said controlled sliding movement, a sheet blocking step is provided through compressed air acting on said elastic member.

9. A method according to one or more of the preceding claims, characterized in that said punch has a low frictional coefficient, at least at its surface contacting said sheet.

10. A method according to one or more of the preceding claims, characterized in that g said sheet as it enters said forming step has a temperature of about 161-168 C and said punch is heated to a temperature in the 134 to 145 C range.

11. A method according to one or more of the preceding claims, characterized in that the pressurized fluid for effecting the final forming step is cooled to a temperature below 1 0"C.

12. A method according to Claim 1, characterized in comprising an additional step of trimming said sheet in the mould, said step being carried out by means of sharp edge ridges and mating grooves provided respectively on said mould and said counter-mould.

13. A method according to one or more of the preceding claims, characterized in that said mould is provided with a plurality of said cavities and corresponding plurality of said punches for simultaneously thermoforming several articles.

14. An improved method of thermoforming sheet-like polypropylene by processing it in the solid state, substantially as herein de scribecrlvith reference to the accompanying drawings.