CH705222A2 - Method for manufacturing plastic preform in mold for manufacturing e.g. bottle, involves solidifying molten plastic in cavity while applying vertical thrust tending to move mandrel towards upper feeding port for compressing molten plastic - Google Patents

Abstract

The method involves positioning a mandrel (6) into a cavity (2) of a mold so that its end is located at a predetermined distance from an upper feeding port (3) of the cavity. A predetermined amount of molten plastic (11) is introduced by intrusion gravity or low pressure into the cavity by the upper feeding port. The molten plastic is solidified in the cavity while applying a vertical thrust tending to move the mandrel towards the upper feeding port for compressing the molten plastic and compensating shrinkage of the plastic, and a resulting plastic preform is extracted from the mold. An independent claim is also included for a device for manufacturing a plastic preform in a mold.

Description

The present invention relates to a method of manufacturing a plastic object object that can be a finished object or a preform (for example PET) from which a finished object is made (for example a bottle or a technical piece).

Document CH 595 969 discloses a method of manufacturing a plastic preform according to which a plastic material is introduced through a lower orifice of a mold against a retaining action created by an axially displaceable mandrel in the cavity. of the mold and, after having closed the lower material inlet opening, the material located in the mold is compressed by an axial displacement of the mandrel in the cavity of the mold.

This method has the advantage of good control of the introduction and distribution of the low pressure material in the mold but provides an input of the dosed material through lower openings at the bottom of the mold. For some applications it is useful to be able to enter the material from the top by intrusion or gravity. This is for example the case for the production of preforms continuously with a fixed extruder which produces the doses of material which are transferred into molds of preforms housed on a support (round table) in continuous movement.

Also known are methods of forming objects by compression in which is deposited from the top in the cavity of a mold open up (and closed down) a predetermined amount of plastic but without any control of its shape. Then a mandrel is introduced into the cavity of the mold, the mold is closed which compresses the plastic and the force to rise upwards to the neck between the wall of the mold cavity and the mandrel thus forming a preform.

In these systems, the dose of hot plastic material is introduced and placed in the cold cavity of the matrix randomly, and not controlled. In contact with the cold parts of the mold an uncontrollable portion of the surface and the mass of plastic material begins to cool and solidify before the material is compressed by the mandrel which enters the mass of plastic material with pressure raised from the top to force the material to completely fill the cavity. As a result, it is impossible to control and guarantee the quality of the preforms, particularly the state of the voltages in their wall due to uncontrolled cooling.

This method has the disadvantage of not controlling the introduction of plastic material into the mold so that the preform obtained before blowing is not homogeneous, which hinders the obtaining of a quality finished object, the tensions and stresses in the wall of the object are not controlled.

In these known methods, the cooling of the mass of plastic material begins before its intermediate and final shaping.

The object of the present invention is to provide a method of manufacturing a plastic blank or the shaping of the material begins before cooling.

This process is especially interesting for the production of continuous preforms for subsequent operations or can be integrated in a production line of containers by blowing. Another object of the invention is to reduce pressure, temperature and shear forces during shaping of the plastic preform and obviate the aforementioned drawbacks of known devices and methods.

Another object of the present invention is to ensure the quality of the blank by better controlling the introduction and distribution of the plastic in the cavity of the mold to avoid random contact of the dose of plastic with the cold walls of the cavity and thus avoid creating defects, voltages and other random malformations of the blank before the final shaping.

The present invention thus relates to a method of manufacturing a plastic blank in a mold comprising a body, a blank holder and a mandrel movable axially in the mold cavity. The mold is oriented with its vertical axis and its upper filling port at the top. The mandrel is positioned in the mold cavity so that its end is located at a variable but predetermined distance from the upper fill hole of the mold cavity. A metered quantity of meltable plastic is metered at a predetermined temperature by low pressure or gravity intrusion into the mold cavity through its upper feed opening without filling the entire volume of the cavity between the mold and the mandrel. The upper introduction orifice of the mold is then closed with a shutter corresponding to the desired shape of the bottom of the blank. The mandrel is moved towards the upper orifice of the mold to fill the entire cavity of the mold with the plastic. The plastic material is allowed to solidify in the mold cavity while applying to the mandrel a vertical thrust by moving it towards the upper feed opening to fill the entire empty volume and compensate for shrinkage of the plastic material and then extract the blank outside the body of the mold when the blank has reached its demolding temperature and is separated from the mandrel and the blank holder for storage, for subsequent operations or feed directly to a machine for manufacturing a finished part such as a bottle or a technical part.

The accompanying drawings illustrate schematically and by way of example the different successive phases of the method of manufacturing a plastic blank according to the invention.

FIG. 1 illustrates the introduction phase of a plastic dose produced by an extruder (extruder) in a mold through its upper filling port.

Figs. 2, 3 and 4 illustrate how the dose of material is distributed and the intermediate form that it assumes in contact with the mandrel after being placed in the cavity. This shape depends on the weight, temperature and viscosity of the plastic material used, as well as the position of the mandrel which is already located in the mold cavity before the last phase of the final compression.

FIG. 5 illustrates the closure of the mold filling orifice by a shutter having the shape of the desired bottom (which may optionally also add a volume complementary to the mold cavity) and the progressive filling of this mold cavity by a displacement of the mold mandrel towards the upper filling opening of this mold.

FIG. Figure 6 illustrates the process step where the mold cavity is completely filled with plastic by advancing the mandrel toward the top fill port of the mold to compress the material and obtain the final shape before shrinkage compensation.

FIG. 7 illustrates the phase of the process where the shrinkage of the material was compensated for by subsequent displacement of the mandrel towards the upper mold fill hole and the stroke of the mandrel was stopped by reaching the solidified material.

FIG. 8 illustrates the extraction or ejection phase of the finished blank but still hot to be subjected to further operations in a machine for forming a finished object is cooled and stored.

FIG. 9 illustrates another way of introducing a dose of plastic produced by an extruder (extruder), which through a funnel is transferred into a temperature metering chamber and temperature of this plastic. This chamber is also provided with a piston with the anterior shape corresponding to the shape of the orifice for introducing the material into the mold.

The present method is intended for the manufacture of blanks that may be preforms for blow molding bottles or blanks of special shape, for example disc-shaped, or other suitable forms for the production of special and / or technical parts by thermoforming, injection blow molding or other processes. This method seeks to better control the introduction of the dose of plastic material in the mold cavity so that it is reproducible and not random so that all the blanks can be identical and of perfect quality without uncontrolled internal tensions. and unwanted.

To obtain this control of the introduction of the dose into the mold cavity, the mold has a vertical axis and its cavity is provided with an upper orifice for introducing a dose of plastic material. This upper material introduction orifice is the largest possible relative to the part that must be made. The mold cavity may be of cylindrical, conical or other shape depending on the subsequent shape desired. This mold also comprises a shutter of this upper material introduction orifice which forms, when the orifice of the cavity of the mold is open, a funnel facilitating introduction by intrusion or gravity into said cavity of a dose of plastic produced by an extruder, and when the upper material introduction orifice is closed complete the mold cavity by an additional volume having the shape of the desired bottom, (generally hemispherical for bottle preforms), defining the outer shape of the bottom of the sketch.

The mold may be formed of several parts, working in a vertical plane according to the form necessary for the final form to achieve the blank and the requirements imposed by the demolding. The use of open molds allows, according to the profile of the part to be produced, easier demoulding and the exit of the produced objects (for example preforms) with a smaller displacement compared to pieces produced in molds in one piece .

As seen in the drawings, fig. 1 to 8, the mold consists of an upper body 1 having a frustoconical axial passage or another suitable form. The upper end of this frustoconical axial passage or of another suitable shape defines the upper insertion opening 3 of plastic material while the lower end of this frustoconical axial passage, of larger diameter than the upper orifice, defines the diameter of the neck of the blank.

This mold comprises an intermediate body 4 or blank holder laterally deviating defining the shape of the neck, including the thread of the neck of the blank.

The mold further comprises a lower body 5 delimiting the plane of the opening of the blank and serving as an axial guide to a mandrel 6 whose lower part, which is to slide in the lower body 5 is cylindrical, and the part frustoconical upper or another suitable form terminating for example by a spherical cap whose shape corresponds to the inner shape of the bottom of the blank.

The mold also comprises a shutter 7 slidably mounted on the upper face of the upper body 1 of the mold and forming a funnel 8 whose lower portion is cylindrical with a diameter corresponding to the diameter of the upper orifice 3 of introduction of plastic material in the cavity 2.

The sliding shutter 7 and the funnel can form a single piece or be made by two separate parts, integral or not.

The cavity 2 of the mold is the space between the different parts of the mold assembled, that is to say the upper body 1, the intermediate body 4 and the lower body 5 and the shutter 7 and the chuck 6.

In the open position of filling the mold the lower part of the funnel 8 is coaxial with the upper orifice 3 for introducing material into the mold.

In the closed position the shutter 7 is moved laterally so that a hollow 9, (hemispherical in the example shown), is centered on the axis of the mold and therefore come to cap his upper orifice 3 and close the cavity 2 of the mold.

[0031] With reference to the schematic figures 1 to 8, the various phases of the process for manufacturing plastic blanks will be described in detail below.

Figs. 1 to 4 illustrate the phase of filling or introduction of a molten dose of plastic in the cavity 2 of the mold.

The shutter 7 is in the open position and the axis of the mold 1, 4, 5 is disposed under the axis of extrusion of a nozzle 10 of a plodder and a predetermined dose 11 of plastic, to the temperature and the viscosity determined according to the material, is introduced by intrusion or by gravity into the funnel 8 of the shutter 7, passes through the upper orifice 3 for introducing the plastic material and is placed in the cavity 2 the mandrel 6 has been prepositioned axially in the cavity 2 of the mold in a position such that the volume of this cavity 2 of the mold is greater than the volume of the dose of plastic material 11 introduced.

If the dose of plastic material 11 has a high viscosity it falls on the end of the mandrel 6 in a well centered manner and partly matches the shape of the end of the mandrel 6 and the side walls of the cavity 2, as illustrated in fig. 2. Depending on the viscosity of the plastic dose 11 this dose goes down more or less along the mandrel and can take the forms illustrated in FIGS. 2, 3 or 4.

This introduction of the plastic dose in the mold cavity can be done essentially by gravity or by a low-pressure intrusion through the upper opening 3 wide of the mold.

In this phase the dose of plastic material 11 introduced into the cavity 2 does not occupy the entire volume of the cavity 2. As the mandrel 6 is already in position in the cavity 2 during the intrusion of the dose of plastic material 11 this material in the molten state is housed immediately, at least in part, around the mandrel, between the mandrel and the walls of the cavity 2 perfectly centered on the axis of the mold and in an intermediate shape (qu one can call pre-preform) which already corresponds to what the finished draft will have to be. This way of introducing the plastic material 11 into the cavity 2 is perfectly reproducible and symmetrical with respect to the axis of the cavity 2. This intrusion of material is thus perfectly controlled and with minimal forces on the dose of material plastic, the pressures being low practically of the order of the weight of the plastic or a little more. In addition, it is done through a largest possible introduction orifice, corresponding to the diameter of the spherical cap forming the closed end of the blank.

This intrusion of molten plastic into the cavity of the mold being completed, the upper orifice 3 of the cavity 2 is closed by a lateral displacement of the shutter 7. The spherical hollow 9 or another form of the shutter 7 is centered on the axis of the mold thus closing the cavity while adding thereto an additional volume corresponding to that of the recess 9. Then the mandrel 6 is moved axially upwards to distribute, move and compress the plastic material in the cavity 2 to complete the filling. The entire volume between the mandrel 6 and the cavity 2 is filled with plastic before the material is too cold. This deformation of the plastic material is done at low pressure since it is simply a matter of modifying the shape of the material introduced into the cavity by an action exerted through the movement of the mandrel with a large section and of completely filling the cavity with the minimum pressure necessary and controlled. This stage is reached as illustrated in FIG. 6.

Then the plastic cools completely in contact with the mold and the mandrel, retracts and solidifies. During this time the mandrel continues to move up the cavity 2 to compensate for the removal of the material during solidification and maintain the cavity 2, the space between the mandrel and the upper body 1, totally filled by the plastic material by decreasing the volume of the cavity 2 (Figure 7).

The compression of the plastic is maintained by the mandrel until complete solidification of the blank thus compensating for the removal of the plastic material during its passage from the molten state to the solid state. This stage is illustrated in FIG. 7 where we see that the plastic material that has been introduced into the mold cavity reaches its final shape.

Then the mold is opened, the mandrel withdraws from the cavity of the mold and out of the solidified blank but still relatively high temperature. In the case where the blank is immediately used for other operations, this avoids or reduces a subsequent reheating of this blank. The intermediate body 4 of the mold with the shape of the neck deviates to release the blank which can be maintained oriented and output for successive operations such as introduction into a blowing installation for the production of a bottle or in a chain for forming an object or a technical part or stored for later use.

FIG. 9 illustrates another way of introducing the plastic into the cavity 2 of the mold. The melt produced by the extruder 10 is introduced into a supply chamber 12 whose temperature is regulated. Then the plastic material packaged in this feed chamber is pushed by a piston 18 and introduced as previously through the shutter 7 in the cavity 2 where the mandrel has been prepositioned. In this variant also the plastic material is introduced into the cavity 2 of the mold at low pressure since the volume of this cavity 2 is larger than that of the material introduced therein. The quantity of material introduced into the cavity 2 is determined by the dimensions of the feed chamber 12 and the stroke of the piston 18. Once the desired quantity has been introduced into the cavity 2, the shutter 7 is displaced laterally between the upper body 1 of the mold and the feed chamber to place the recess 9 of this shutter opposite the upper feed orifice of the cavity 2. The method of manufacturing the blank then continues as described above.

The molten plastic material delivered to the cavity 2 of the mold may be in different forms. It may be for example a bar or portion of cylindrical tube as illustrated in FIG. 10. It can also be a tubular section as illustrated in FIG. 11. In this case when it falls into the cavity 2 of the mold the dose of plastic material comes to rest on the end of the mandrel 6. Finally, if one wants to manufacture a bi-material blank the dose of plastic material introduced into the cavity 2 of the mold may be as shown in FIG. 12 in the form of a bar whose inner portion is formed of a first plastic material and whose outer portion is formed by an envelope of a second plastic material. It is also possible to provide doses comprising juxtaposed or combined materials, for example materials of different colors. Bimaterial blanks can then be made by blowing or by thermoforming bi-material objects which benefit from a combination of the desired particular characteristics offered by the choice of plastics used.

In a variant of the process of manufacturing plastic blanks can during the introduction of the dose of plastic material in the mold cavity cause a controlled axial recoil of the mandrel, that is to say, to remove his end of the introduction orifice 3 of the material.

In another variant of the process for manufacturing plastic blanks, the mandrel 6 can be rotated during the introduction, distribution and compression phase of the material by advancing the mandrel to completely fill the cavity. of the mold.

This rotation of the mandrel aims to orient the molecules of the plastic material in the walls of the blank which significantly improves the mechanical characteristics of the blank produced.

In this method of manufacturing blanks plastic where the mold comprises an upper body 1 provided with a frustoconical axial passage whose end defines an upper introduction orifice 3 of the mold and where a mandrel 6 is prepositioned in the cavity 2 of the mold before the introduction of plastic material into it, the plastic material introduced into this cavity is placed immediately between the mandrel 6 and the cavity 2 which allows automatic centering thereof and therefore a control precise geometry allowing exact reproducibility. In this way the cooling of the dose of plastic material introduced is accurately controlled and the mechanical and geometrical qualities of the blank are improved. In doing so, the distribution of the internal tensions of the plastics material is also controlled.

There is more like in the existing processes of uncontrollable random cooling of the plastic material when it is introduced into the mold before the start of shaping. The cooling of the plastic and its shaping are completely controlled so that the quality of the blanks is improved and constant.

By implementing this method of manufacturing plastic blanks can reduce the temperatures, pressures and shear forces exerted on the plastic material and thus ensure a better quality of the material and reduce the risk of degradation of it (and acetaldehyde formation in the case of PET).

It also provides a better quality of the blank through the constant control of the flow of plastic material during filling of the mold cavity and the formatting of the plastic dose. The blanks have less internal tensions thus improving the objects that are subsequently produced.

We can control the orientation of the molecules and internal tensions of the blanks using a rotating mandrel during the filling phase of the cavity.

This method of manufacturing plastic blanks where the necessary pressures and forces are reduced also simplifies and lighten the molds and production facilities, the mechanical and hydraulic closing forces of the presses and molds being reduced compared to conventional injection and compression systems.

This method of manufacturing blanks is of particular interest for the production of continuous preform production systems to be integrated online in a bottle blowing system, without intermediate operations or interruption, this as well for linear than rotary, intermittent or continuous installations.

We must also note some important features of the mold for the implementation of the method described.

This mold comprises an upper body 1 having a frustoconical passage therethrough. The orifice delimited by the intersection of the upper face of the upper body 1 of the matrix and its through passage constitutes an introduction orifice 3 or intrusion of large plastic material avoiding shearing and rolling of the material and promoting a transfer of the material by intrusion at low pressure or gravity in the cavity

The mold comprises a shutter 7 moving laterally on the upper face of the upper body 1 of the mold between an open position of the mold cavity for which the introduction orifice 3 of the mold is facing an opening shutter 7 for receiving the material to be introduced into the mold. This opening may be the lower part of a funnel that includes the shutter.

This shutter further comprises a recess 9 with the desired shape of the bottom of the blank opening on its lower face that can be placed on the mold introduction orifice thus allowing its closure and simultaneously the contribution of a complementary volume to the cavity 2 of the mold. This hollow 9 is for example preferably hemispherical in the case of preforms or corresponding shape to the desired bottom of the blank. The shutter 7 comprising the opening and the recess with the shape of the bottom can be made in a single piece or in two separate pieces.

The shape of the mold cavity depends on the shape of the blank that is to be made and of course also the shape of the final object to be made with this blank. This cavity may be provided to form blanks in the form of disks, plates or other special forms, especially for the production of technical parts, packaging or other objects. Of course, one of the most important applications is the manufacture of preforms for the manufacture of bottles, in particular PET bottles.

The mandrel 6 of the described embodiment could, in alternative embodiments of the implementation device of the described method, be formed by any movable part in the cavity 2 of the mold whose shape corresponds to the shape final of the draft to be realized.

Claims (12)

A method of manufacturing a plastic blank in a mold comprising a body (1,5), a blank holder (4) and a mandrel (6) movable axially in the mold cavity, characterized in that positioning the mandrel (6) in the cavity (2) of the mold so that its end is located at a predetermined distance from an upper filling orifice (3) of the cavity (2) of the mold; an intrusion or gravity of a predetermined amount of molten plastic material (11) is introduced at a predetermined temperature into the mold cavity (2) through its upper insertion opening (3) without completely filling the volume of the mold. the cavity (2); closing the upper insertion opening (3) of the mold; moving the mandrel (6) towards the upper mold opening (3) to dispense the plastic material (11) and filling the entire volume of the mold cavity (2) with the plastic material (11) ; the plastic material (11) is allowed to solidify in the mold cavity (2) while applying to the mandrel (6) a thrust tending to move it towards the upper insertion opening (3) to compress the plastic material and compensating for shrinkage of the plastic material and extracting the blank from the mold when the blank has reached its demolding temperature and separating it from the mandrel (6) and the blank holder (4) for store it, to perform subsequent operations or directly feed a machine for manufacturing a finished part such as a bottle, technical part or other finished object. 2. Method according to claim 1, characterized in that during or after closure of the upper orifice of introduction (3) of the mold is increased the volume of the cavity (2) of the mold of a complementary volume. 3. Method according to claim 1 or claim 2, characterized in that while the mandrel (6) is moved axially towards the insertion orifice (3) this mandrel (6) is simultaneously rotated. 4. Method according to one of claims 1 to 3, characterized in that the introduction of the plastic material into the cavity (2) of the mold is directly out of an extruder. 5. Method according to one of claims 1 to 3, characterized in that the introduction of the plastic into the cavity (2) of the mold is via a metering chamber and packaging (12). ) of the plastic material (11) comprising a piston (18) for transferring the plastic material (11). 6. Method according to one of the preceding claims, characterized in that the plastic material (11) in the molten state is introduced into the cavity (2) of the mold in the form of a bar section or tube of plastic material. 7. Method according to one of claims 1 to 5, characterized in that the plastic material introduced into the cavity (2) of the mold is a combination of several plastics. 8. Method according to one of claims 1 to 5, characterized in that the plastic material is introduced into the cavity (2) of the mold in the form of a dose comprising in its interior a first plastic coated partially or completely in an envelope of a second plastic material on the outside. 9. Device for implementing the method of manufacturing plastic blanks according to claim 1, characterized in that it comprises a mold and a mandrel (6) movable axially in a cavity (2) of this mold ; in that an upper body (1) of the mold has an upper insertion opening (3) of the plastic material in the cavity (2) of the mold; and in that it comprises a shutter (7) of the insertion orifice (3). 10. Device according to claim 9, characterized in that the upper body (1) of the mold comprises a through axial passage of frustoconical shape. 11. Device according to claim 9 or 10, characterized in that when the shutter (7) of the mold is in the closed position of the orifice (3) for introducing the plastic into the cavity (2) the upper body (1) of the mold the volume of the mold cavity is increased by a complementary volume contained in the shutter (7). 12. Device according to one of claims 9 to 11, characterized in that the upper body (1) of the mold is in at least two parts that can be opened for the extraction of the blank.