FR2462989A1 - Blow moulding bottles from saturated polyester - including maintaining the blown article against the mould for a short time to stabilise it - Google Patents

Abstract

A bottle is blow moulded from a tubular parison of saturated polyester closed at one end by heating the parison to orientation temperature (in the range 70-130 deg. C), blowing it in a mould, and then maintaining it in the mould for at least 5 seconds with the mould kept at a temp. between the glass transition point and the crystallisation temp. The portion of the mould contacting the base of the blown parison is cooled during this period. The bottle has superior strength and very small age and temperature deformation.

Description

 The invention relates to a method for manufacturing a bottle-shaped container made of saturated polyester, and more particularly to a method for manufacturing a bottle-shaped container having a very high mechanical resistance, a very low degree of deformation with the aging and at high temperatures, and advantageous dimensional stability.

A transparent thin-walled, blow-molded container made of biaxially oriented plastic is produced by implementing a process consisting in heating a cylindrical part or blank whose bottom is made of saturated polyester, for example terephthalate polyethylene, at temperatures between the glass transition point and the crystallization temperature, to load the heated part in a fully automatic rotary blow molding machine, of the multi-station type, by means of a loading device, and
give a biaxial orientation, in the lateral and longitudinal directions, to the part placed in the blow mold. Since the saturated biaxial orientation polyester container has a high transparency, a low permeability to water and gas and a absence of toxicity, it is suitable for a wide variety of applications in the fields of liquid condiments, soft drinks, alcoholic drinks, etc.

 However, the biaxially oriented saturated polyester container is designed in such a way that its molecular chain and its microcrystals improve the mechanical resistance and the impermeability to gases, while simultaneously allowing the maintenance of a significant stress inside the wall of this container. When the aging stress is subjected to ambient temperature or to a temperature higher than ambient temperature, for example at a temperature of approximately 70 ° C., it tends to disappear, which causes withdrawal of the container. This removal process also takes place in the container during its cooling, after it leaves the mold, which raises a great difficulty in giving this container good dimensional stability.

 The invention relates to a method of manufacturing a bottle-shaped container made of saturated polyester, which consists in charging a cylindrical part, the bottom of which is heated, at a temperature which makes it possible to obtain an orientation during blow molding, in a mold whose size is adjusted according to the shrinkage affecting the container during its cooling, after the part has been blow molded and while it is discharged from the mold, to impart a biaxial orientation, in lateral and longitudinal directions , to the part placed in the blowing mold, then to bring the outer surface of the wall of the container body closely against the inner wall of the mold maintained at a temperature of the order of the glass transition point, for at least 5 seconds, to thermally fix the blow molded container so that it acquires good dimensional stability.

 Given that the container thus manufactured according to the process of the invention has, as described in more detail below and after it has been discharged from the blow mold, a very low degree of deformation with aging at room temperature and at higher temperatures, for example at temperatures of about 70 ° C., and since it also has good dimensional stability, there is no risk of its commercial value decreasing by deformation after molding by blowing, and this container can be supplied to consumers safely.

 It is necessary to use a blow molding mold, the dimensions of which are adjusted according to the shrinkage occurring during natural cooling, after the container has been unloaded from the mold, since the container, which is thermally fixed and to which a biaxial orientation has been imparted, shrinks considerably at the curing temperature and during the curing time, after unloading it from the blow mold, so that the capacity may vary from that calculated according to the shrinkage occurring at the outlet of the mold.

 The curing temperature is generally higher and the duration of curing is shorter the greater the withdrawal of the container thus oriented, thermally fixed and cooled, at the time of unloading. More particularly, whereas it has been confirmed that the shrinkage of the container, thermally fixed for approximately 8 seconds, is approximately 4.3% at a temperature of approximately 700C, of 0.6% at a temperature of approximately 800C and about 5.3% at a temperature of about 900C, it is necessary to use a blow mold orientation having a dimension allowing to cancel withdrawals which vary with the duration and the temperature of hardening.

 The method of the invention therefore makes it possible to manufacture a bottle-shaped container having greater # possibilities of dimensional stability, without significant shrinkage, with aging, at room temperature, by simple determination of the size of the blow mold in taking into account the shrinkage of the mold occurring during hardening, during blow molding of the container. When a 1 liter bottle-shaped container weighing 40 grams, made of polyethylene terephthalate, for example, is blow molded so as to receive a biaxial orientation, the dimensions of the blow mold are so that the capacity of this mold is changed by about 6%, this rate being obtained by adding about 2% shrinkage of thermal curing to about 4% shrinkage of the mold. Then, when the piece of polyethylene terephthalate has received its biaxial orientation, it must be maintained at a temperature between the glass transition point, which is about 70 C, and the crystallization temperature which is about 1300C. More particularly, the blow mold is maintained at a temperature between 70 and 1300C, advantageously between 80 and 1000C and preferably between 85 and 95 C, and the blow molded container is subjected to thermal hardening, inside the mold for a time between S and 10 seconds, before being discharged from the mold. In the case where the thermal curing time is less than 5 seconds, the container cannot acquire good dimensional stability, and if the curing time is greater than 10 seconds, it is disadvantageous for the container molding cycle.

 The invention therefore relates to a method of manufacturing a bottle-shaped container of saturated polyester. According to the method of the invention, it is easy to give this container a thin and transparent wall and good mechanical properties by a biaxial orientation. The container produced by the process of the invention has a very low deformation on aging and a very low degree of deformation at high temperatures, as well as an advantageous dimensional stability. The method of the invention uses a biaxially oriented blow mold, the dimensions of which are determined taking into account the shrinkage occurring during thermal curing, the shrinkage considered being the shrinkage of the mold occurring during blow molding. of the container.

The invention will be described in more detail with reference to the accompanying drawings by way of non-limiting example and in which
- - Figure 1 is an elevation, with axial half-section, of a part intended for the production of a container in the form of a bottle in the implementation of the method of the invention
- Figure 2 is a partial axial section of the part shown in Figure 1, fixed to a template used in the method of the invention
- Figure 3 is a partial axial section of the orientation mold by blowing, this mold being shown in the position it occupies during it confers a biaxial orientation to the workpiece
- Figure 4 is a graph showing the total height and capacity (overflow capacity) as a function of the thermal hardening time of the 1 liter container, 30 minutes after a biaxial orientation has been imparted to this container by blow molding
- Figure 5 is a graph showing the total height and capacity as a function of thermal hardening time of the 1 liter container filled with water at high temperature; and
- Figure 6 is a graph showing the total height and capacity as a function of the thermal curing time of the container left in the upright position in the ambient air at 400C and 75% relative humidity.

 The figures, and more particularly Figure 1, show a part from which a bottle-shaped container must be produced by implementing the method of the invention. The same reference numerals designate the same elements in the different figures. The part 10 is made of saturated polyester, for example polyethylene terephthalate having an intrinsic viscosity greater than 0.55. To this end, a method of molding an extruded blank or a method of molding an injected blank is implemented. The part 10 is shaped so as to be cylindrical, to have a bottom and to have a thickness. relatively large as a whole, and it comprises an end section 12 forming a neck, having an opening and whose thickness is greater than that of the body II of this part, a previously finished bead 13 being formed at a short distance from the section ex truce 12 of so as to be adjacent to the body 11.

 Figure 2 shows the part 10 fixed in the inverted position on a template or mounting 15. This template 15 is intended to detachably hold the part or the container in the form of a bottle during the operations consisting in heating this part, in loading it in the blow mold of a fully automatic orientation and blow molding machine (not shown) with multiple rotating stations, to blow mold the part in order to give it a certain orientation, to unload the container in the form of a bottle thus formed and evacuated.

The template 15 comprises a cylindrical mandrel 16, a circular disc 17 connected to the upper end of the mandrel 16 with which it is made in one piece, a support 18 intended to hold the end section of the piece 10 forming the neck, a core guide element 19 made of heat-resistant and wear-resistant synthetic resin, for example fluorine resin, disposed inside the support 18, and a core axis 20 capable of rising inside of the mandrel 16. The support 18 supports the end section 12 forming the neck of the part 10 by its outer edge and it rests on the upper surface of the disc 17 by means of a plate 21 of thermal insulation. 20 of the core has an upper end piece 22 in the shape of an inverted cone or of a spherical shape, made of thermal insulation material, for example made of fluorine resin, intended to oppose the conduction of heat coming from part 10 and to the adhesion of the bottom of this piece 10 to this tip lo rsque said part 10 is oriented longitudinally while the axis 20 is in the extended position. The axis 20 has, at its lower end, a T-shaped opening 23 and vents 25 in the form of grooves formed in the peripheral surface of the axis, these vents extending upwards from the mouths 24 of the hole 21. A element 26 for holding a nozzle is also fixed to the lower end of the core axis. As shown in FIG. 3, the mandrel 46 has a hole 27 intended for the passage of a cylindrical rod 28 during the operation. consisting in conferring a longitudinal orientation on the part placed R inside the blow molding machine.

 FIG. 3 shows the part 10 oriented longitudinally, then oriented laterally in the mold 29, so that this part 10 is blow molded to form a container 30 in the form of a bottle.

 Since the crystalline structure of the part 10 increases according to a phenomenon called "bleaching" during the progressive cooling after the molding operation to thereby reduce the transparency, the part must be cooled rapidly from the molding temperature to room temperature.

 In the method of manufacturing the container in the form of a saturated polyester bottle, the part 10, injection molded in saturated polyester, is placed in the mold 29 brought to the orientation temperature allowing the part 10 to be molded by blowing and an orientation biaxial is imparted to this part which is then thermally fixed during the final phase of orientation by contacting, for 5 seconds or more, the outer surface of the wall of the container 30 with the inner surface 31 of the wall of the mold 29. It is necessary to bring the part 10 to a temperature between the glass transition point of the saturated polyester, preferably equal to around 70 ° C., and the orientation temperature making it possible to orient the molecules, this temperature being lower than the melting point which is preferably about 1300C, and it is also necessary to heat the mold 29 to a temperature within the same range, between the transi glassy saturated polyester and the orientation temperature, in order to thermally fix the container 30 formed from the part 10 in the case where the latter is loaded into the blow mold 29 and where it receives a biaxial orientation in this mold 29.

 The container 30 in the form of a bottle is produced from the part 10 during the operation consisting in loading this part 10, brought to the orientation temperature and supported by the template 15, in the blow mold 29, to be raised the cylindrical rod 28 for advancing the core axis 20 in order to orient the piece 10 longitudinally according to a coefficient of between approximately 1.4 and 2.5, and to introduce air inside the piece 10 compressed from inside the cylindrical rod 28 and passing through the hole T-shaped and in the vents 25, in order to give this piece 10 a lateral orientation with a coefficient equal to about 2, for molding the container 30 in the form of a bottle as shown in FIG. 3. The body, the bottom and the shoulder of the container 30 are thus expanded so as to be able to come into contact with the internal surface 31 of the wall of the mold 29 for 5 seconds or more , which allows the thermal fixing of these parts, and l he container 30 is then discharged from the mold 29 by opening the latter so that the container 30 can be removed therefrom.

 It should be noted that, in the case where the mold 29 is brought to a temperature above the upper limit of the orientation temperature at the time of the thermal fixing of the container 30, a phenomenon known as bleaching appears at the bottom or at any other analogous part of the container 30, while the orientation is almost zero. On the other hand, if the mold 29 is not carried to the glass transition point of the saturated polyester, the container 30 cannot be thermally fixed in order to allow the elimination of its stresses. It should also be noted that in the case where the container 30 is not placed in sufficient contact with the internal surface of the wall of the mold 29, that is to say for a period of less than 5 seconds, it cannot to be sufficiently freed from its constraints.

 The mold 29 also has channels 32 allowing the passage of a heating medium (liquid) which make it possible to obtain a predetermined temperature for thermal fixing by circulation of this liquid in the mold.

Consequently, the container 30, sufficiently oriented and thermally fixed for more than 5 seconds, is then discharged from the mold 29 while retaining its high temperature, without deforming, and it is then cooled as it is.
However, the part which has not been sufficiently oriented, for example the bottom of the container 30, is deformed. To prevent this partial defect reaching this part of the container 30 during the thermal fixing operation, a channel 34 is formed in the bottom ring 33, as shown in FIG. 3, in order to allow the circulation of a cooling independently of the fluid flowing in the channels 32. More particularly, only the bottom ring 33 is cooled when the container 30 is unloaded from the mold 29, which prevents partial deformation of the container 30 by causing it to harden during cooling.

EXAMPLE
A cylindrical piece of polyethylene terephthalate, having a bottom, is molded by the implementation of an injection molding process making it possible to obtain a blank. After this part has been brought to an orientation temperature of 95 ° C., it is loaded into a blow mold of 225 mm in height and of 1 liter capacity, this mold being heated to the temperature of thermal fixing which is of 86 C. The part is given a biaxial orientation by advancing the core axis and blowing compressed air inside the mold, thus obtaining a container in the form of a bottle, with a thin and transparent wall. .Three types of container are molded using thermal fixing times equal to 6, 8 and 10 seconds respectively,
The duration of the thermal fixing, during which contact is made between the external surface of the wall of the part and the internal surface of the wall of the mold, is considered to be substantially equal to the duration of the blowing in this manufacturing process. from the bottle-shaped container.

 The total heights and capacities of the containers thus obtained, 30 minutes after the completion of their manufacture, are indicated respectively in millimeters and in cubic centimeters on the graph of FIG. 4.

 When the duration of the thermal fixing is increased from 6 seconds to 10 seconds, the height and the capacity of the container increase as clearly shown in the graph.

 The total heights and capacities of the containers of the three types indicated above are shown in FIG. 5 in the form of the difference between the values measured after the containers have been left at room temperature for 2 hours, after having been filled with water. at 70 ° C, 30 minutes after the end of the blow molding process, and those measured before filling the containers with hot water.

 The heights and total capacities of the containers of the three types indicated above are shown in Figure 6 as the difference between the containers kept in an oven at 400C and 75% relative humidity for an additional period of 1 to 5 days. , 30 minutes after the end of the blow molding process, the measurements being then made at this time, and the values measured before the containers are placed in the oven.

 It is obvious, in FIG. 5, that the bottle-shaped container made of saturated polyester thus obtained by implementing the method of the invention has superior properties as regards its dimensional stability at high temperatures, as well that a sufficient service life when it has been filled with liquid at a temperature of 70 C. FIG. 6 also shows in an obvious manner that the container obtained by the process of the invention exhibits a very small deformation with aging, as well as superior dimensional stability.

 It goes without saying that many modifications can be made to the method described and shown without departing from the scope of the invention.

Claims (4)

 1. - Method for manufacturing a container in the form of a saturated polyester bottle from a cylindrical piece of saturated polyester comprising a bottom which is brought to a temperature between the glass transition temperature of the polyester and the temperature of orientation of this polyester, the realization of the container consisting in giving a biaxial orientation to the part in a blow mold, the method being characterized in that it consists in bringing the part to a temperature between 700 and 1300C approximately, to load the part in the blowing mold brought to a temperature comprised substantially between the glass transition temperature of the saturated polyester and the orientation temperature making it possible to thermally fix the container, to mold the part by blowing giving it a longitudinal orientation, then a lateral orientation, inside the mold, to make the bottle-shaped container, to thermally fix the dila container tee during molding, this fixing being obtained by bringing the outer wall of the container into contact with the inner wall of the mold maintained at the orientation temperature, for at least 5 seconds, and cooling the bottom ring of the mold for the duration of the thermal fixing, and then open the mold to discharge the container thus molded.  2. - Method according to claim 1, characterized in that the mold is made so that its dimensions are adjusted to oppose the withdrawal of the container occurring during cooling after unloading of said blow molded container.  3. - Method according to claim 1, characterized in that the mold is made so that its capacity is adjusted with a tolerance of about 6% obtained by the addition of about 2% shrinkage occurring during thermal fixing about 4% of mold shrinkage.  4. - Method according to claim 1, characterized in that the mold comprises a template comprising a cylindrical mandrel, a circular disc produced in one piece with the mandrel, a support intended to maintain the end section, forming a neck, of the part, a core guide element disposed inside the support and a core axis which can rise inside the mandrel.