WO1998023425A2

WO1998023425A2 - Process and apparatus for neck crystallisation of a preform

Info

Publication number: WO1998023425A2
Application number: PCT/EP1997/005819
Authority: WO
Inventors: Antonius Wilhelmus Franciscus Danen; Hendrik Jan Willem Elbertsen; Maria Gerarda Van Buuren-Teuwsen
Original assignee: Constar International Holland B.V.
Priority date: 1996-11-28
Filing date: 1997-10-22
Publication date: 1998-06-04
Also published as: WO1998023425A3; AU7409498A

Abstract

A process for manufacturing a preform for a container made of a plastics material which becomes crystalline when heated, for example PET, PEN or copolymers of PET/PEN. A container blank is heated at least in a mouth section (4) to crystallisation temperature, so that it is crystallised by 20 %. During crystallisation, the outside diameter of the mouth section (4) shrinks from an initial value (A) to a smaller intermediate value (Z). In a further step, the mouth section (4) is expanded by an expanding mandrel (15) against an external mould in such a way that its outside diameter has a final value (E), which is between the initial value (A) and the intermediate value (Z). The wall thickness of the mouth section (4) is thereby reduced from an initial value (d1) to a final value (d3).

Description

Preform Manufacture

This invention relates to preform manufacture. In particular, it relates to the manufacture of a preform for a blow moulded container in which the mouth of the preform is treated by crystallising and shaping procedures .

Preforms for containers such as bottles are typically produced by an injection moulding process. Such preforms comprise essentially cylindrical hollow bodies open at one end. The preforms are then shaped at the open or "neck " end and finally, in a further operation, a container is produced by blow moulding. This blow moulding operation considerably reduces the wall thickness of the preform.

When shaping the neck, an external thread is usually moulded in the neck to enable a screw closure to be used on the container.

Refillable bottles are typically washed in a caustic solution at high temperatures in order to enable them to be reused. Such bottles must maintain stability in the shape of the neck, so that the bottle may be closed and maintain a good seal even after repeated caustic hot washing. Hot washing leads to particular problems with material shrinkage due to thermal processing and may lead to the production of stress cracks. It is therefore essential that the original shape of the neck is precisely defined so that good sealing is obtained and may be maintained even after repeated use of the container .

US 4,499,044 describes a process for the manufacture of preforms, in which the upper edge of the neck is formed exclusively by a continuous shoulder on a moulding mandrel. The end area of the preform is enclosed by a mould and the material of the preform is stretched by a mandrel and pressed into the mould. The wall thickness of the neck is thereby reduced considerably, which may lead to stress cracks in subsequent cleaning and refilling operations.

US 4,476,084 describes a process for increasing the robustness of a container neck, in which the neck is heated to crystallise the material and is then shrunk onto a mandrel lying inside the neck. The disadvantage of this method is that the outer neck is not defined exactly by a tool, but is only formed by shrinking. With this process it is therefore difficult to obtain exactly defined sealing regions with tight tolerances.

From US 4,933,135, a process is known for producing a preform or a container with a dimensionally stable neck. This process also has the disadvantage of a big reduction of the wall thickness of the neck, which may lead to stress cracks.

The present invention seeks to provide a solution to these problems by crystallising and shaping the neck of a preform or of a container, in which the neck has a seal which has tight tolerances and which remains dimensionally stable even after several refilling and washing cycles. This invention also seeks to provide an apparatus for manufacturing a preform in which the mouth is formed with small movements of material. Large movement of material involves the risk of stress cracks and also of air bubbles, which can form in the plastics material or on the surface between a tool and the plastics material. According to the present invention, these problems are solved by a process according to the characterising part of Claim 1 and 8 and by an apparatus having the features of the characterising part of Claim 9.

A preform for a blow moulded container is initially produced from a blank of a plastics material which can be converted to a crystalline state by heating. For this purpose, plastics such as PET and PEN and also copolymers or blends of PET and PEN may be used. The preform is produced by injection moulding and, prior to any shaping of the neck, has an essentially cylindrical exterior. In the injection moulding process, outwardly projecting threads for a closure and projections, for example a transfer ring or a bead for supporting a tamper evident band, may be produced. The neck, in particular, may include threads for screwing on a closure, above which there may be a sealing region. This invention relates in particular to the shaping of this neck.

The blank is first heated at least in the sealing region or mouth section, and preferably in the whole neck, to at least crystallisation temperature and is crystallised preferably by at least 70 % . During this crystallisation, the outside diameter of the sealing region decreases by shrinking from an initial value to a smaller intermediate value.

In general, the whole neck is crystallised, which leads to shrinkage. The shrinkage of the topmost mouth section is particularly critical since this may form the sealing surface for a closure. After shrinking, the circumference of this outermost mouth section no longer has the required tolerances, and different shrinkage values may be observed even due to variations of the wall thickness . In a further step, the mouth section is expanded against an external mould, in such a way that the outside diameter of the mouth section has a value, which is bigger than the outside diameter after the shrinkage, but smaller than the outside diameter of the mouth section in the original, injected form.

By expansion of the mouth section using an expanding tool, for example, by a mandrel inserted into the open end of the blank, the mouth section is pressed into the mould, whereby an exactly defined outer surface with low tolerances is formed. If a thread is already formed in the neck during production of the blank, the blank must be aligned in a mould which is provided with recesses for the threads. In this case, not only is the topmost mouth section expanded, but also the rest of the neck. For alignment, the blank may be provided with a projection, which can be detected by a sensor. Instead of a projection, air bubbles, notches (for example in a transfer ring) or discoloration¹ s may be used. When forming of an axisymmetric retaining bead for a snap-fit closure, such alignment is of course superfluous.

The mouth section is enclosed, before its expansion by the mould, so that this is in contact or almost in contact with the surface of the mouth section. The external mould is thus completely closed, before the forming process of the mouth section. Consequently, the danger of material being squeezed out between individual parts of a multipart mould is avoided. It has been found that the use of a three-part mould, in which each part encloses approximately a circular sector of 120°, is particularly advantageous. Of course, the mould may have recesses in the neck, underneath the mouth section, into which eventual threads on the blank may be pressed during expansion. In this way, threads can be formed in the neck. It is also possible to compress the whole neck in the axial direction. The advantages of this are that not only is the upper edge of the mouth section precisely defined, but also material is available to compensate for wall thickness reduction due to the expansion. In a particularly preferred embodiment, the reduction of the outside diameter of the mouth section between the initial value (the diameter of the unworked blank) and the final value (outside diameter of the completely formed blank) is between 1 % and 3 %, preferably 2 %.

It is particularly advantageous, if the unconstrained inside diameter of the neck is expanded by at least 1 %.

As a result of the reduction of the outside diameter and the expansion of the inside diameter, the wall thickness of the mouth section is reduced by at most 20 % and advantageously not more than 10 °> .

In a further embodiment, at the end of the crystallisation process, at least the mouth section is heated to at least glass transition, or softening, temperature. During crystallisation, the temperature of the mouth section increases continuously because of a constant heat supply. The process is especially economical, if, after reaching the desired degree of crystallisation of at least 20 ■- , the temperature of the mouth section reaches glass transition temperature. The shaping of the mouth section can be carried out immediately following the crystallisation without further heating.

The process according to the invention is carried out particularly advantageously by a mandrel which tapers conically in the insertion direction. The mandrel is inserted into the opening of the blank and expands the neck. Owing to the conical shape, a gentle expansion of the mouth section results.

Because the material of the blank has reached glass transition temperature during the expansion process, there is a danger of the inner face of the blank and the mandrel sticking together. This risk is reduced by the use of an expanding mandrel with a lubricating coating. A coating made of polytetrafluoroethylene is particularly advantageous.

The apparatus according to the invention comprises an expanding mandrel and an external mould. The external mould is used for enclosing the outer face of the mouth section and has a free inside diameter, which is smaller than the outside diameter of the mouth section of the blank and bigger than the outside diameter of the shaped mouth section. The expanding mandrel is insertable in an insertion direction into the opening of the blank and is substantially cylindrical with a conically tapered end section. The mouth section of the blank is enclosed by the mould, while the neck is pressed against the inner face of the mould with the aid of the expanding mandrel. The mould may be provided with recesses for forming injected threads on the blank. In the case of asymmetrical threads, there may also be an alignment device, which aligns the blank relative to the mould. According to a particularly advantageous embodiment, the mould of the apparatus according to the invention may be provided with a lubricating coating. The use of polytetrafluoroethylene is particularly preferred but other coatings may be used which improve the lubrication and prevent adhesion of the surface of the expanding tool to the thermoplastic plastics material heated to glass transition temperature.

Preferred embodiments of the invention will now be described, by way of example only, with reference to the drawings, in which:

Figure 1 is a side view of a blank, part-sectioned, before shaping of a mouth section;

Figures 2a to 2d are schematic side sections showing individual steps for the shaping of the mouth section;

Figure 3 is a side section through the neck of a preform;

Figure 4 is a schematic side view of an expanding mandrel; Figure 5 is a schematic side view of an apparatus according to the invention; and

Figure 6 is a side view of the neck of an embodiment with a retaining bead.

Figure 1 shows a blank 19 for the production of a preform. The blank 19 is produced in an injection moulding process and has a transfer ring 20 and a retaining bead 21 as well as thread 25, which are also formed in the injection moulding process. The blank 19 is essentially hollow and cylindrical and is closed at one end. Starting from the blank 19, a preform may be produced for blow moulding a container, such as a bottle. Projection 31 is used for aligning the blank 19 in a mould.

The blank 19 has a neck 2 at its open end 22, which is shaped to receive a closure, in particular by being provided with sealing zones and threads.

Mouth section 4, which is situated adjacent to the open end 22 of the blank 19 is particularly essential for the present invention. The mouth section 4 is shaped so as to form a sealing surface. Figure 2a shows an enlarged view of the neck 2 of the injected blank 19. The mouth section 4 shown in Figure 2a is unshaped. The outside diameter of the mouth section has an initial length A and the free inside diameter has an initial length Ii. The neck 2, and in particular the mouth section 4, is crystallised by heating. The heat may be applied by an infrared heating device (not shown) .

Figure 2b shows the crystallised neck 2 of the blank 1. During crystallisation, the neck shrinks, particularly the mouth section 4, so that its outside diameter reduces to an intermediate value Z and the free inside diameter obtains a value I-. The heat supply is maintained until the degree of crystallisation in the neck has reached at least 20 %. In a further step (Figure 2c) , the blank 19, in particular its mouth section 4, is enclosed by a mould 10. The mould 10 comprises three parts each extending over an angle of 120°, which can be joined, so that they lie with their inner faces close to or touching the mouth section 4. In this way, an annular gap 11 is formed between the mouth section 4 and the inner face of the mould 10, the gap having a form which is defined exactly by the inner face of the mould 10. In addition, a second mould 12 is shown in figure 2c, which is used for shaping threads 25 in the neck 2 of the blank 19. The mould 12 is provided with recesses 26 for accommodating threads 25. According to the step shown in Figure 2d, the blank 19, particularly its mouth section 4, is expanded by an expandable mandrel 15. A preform 1 for the production of a container is thus formed at the end of this step. The mandrel 15 is insertable into the open end 22 of the blank in insertion direction R. The expanding mandrel 15 is essentially cylindrically over its length with an end section 17 tapering conically in the insertion direction R at one of its ends. The cylindrical part of the mandrel 15 has an outside diameter which is larger than the free inside diameter I₂ of the blank shown in Figures 2b or

2c. By inserting the expanding mandrel, the blank, and in particular in its mouth section 4, is expanded and pressed against the inner face of the mould 10. The inner face of the mould 10 has an inside diameter, which corresponds to the desired final value E of the outside diameter of the mouth section 4. The mouth section 4 is pressed by the expanding mandrel 15 against the mould 10 and so receives an exactly defined, regular surface with tight tolerances. The threads 25 are pressed into the recesses 26 and receive an exactly defined structure.

It is also possible to form threads, not during the production of the blank, but by shaping them through thermal deformation during expansion. In this process, which is not described in detail, the neck 2 is pressed into the recesses 26 during the expansion. The advantage of this process is that the blank does not have to be aligned. The disadvantage is in the relatively large movement of material required in the area of the thread 25.

The outside diameter of the mouth section 4 changes during shaping from an initial value (see Figure 2a) via an intermediate value (see Figure 2b, 2c) to a final value (see Figure 2d) . The size of the final value of the outside diameter is important particularly with respect to sealing properties of the mouth section 4.

In Figure 2d it is also clear how the thread of the neck 2 is pressed into the second mould 1, whereby the external thread in the neck 2 is shaped.

It is also possible to shape the end face 23 of the mouth section 4 precisely by compression.

During the shaping process, which is shown in Figures 2a to 2d, the free, unconstrained inside diameter I of the mouth section 4 will of course also change. By shrinkage during crystallisation, the unconstrained inside diameter is reduced from an initial value Ii to an intermediate value I-. Following expansion of mandrel 15, the inside diameter is increased to a final value I₃.

This also has an effect on the wall thickness, which is reduced from an initial value d, to a final value d₃ by the shaping process. By using the shaping process of the present invention, it is possible to reduce the wall thickness d of the mouth section 4 during the shaping process by less than 20 %, advantageously by less than 10 % . The free inside diameter is expanded during the process by at least 1 ^:λ (ratio of I₃ to I. ), whereas the outside diameter is reduced from the initial value A to the final value E by 1 - to 3 "-- during the shaping process . Example

A blank having a maximum outside diameter in the mouth section 4 of 25.33 mm and an inside diameter in the mouth section of 20.5 mm was used to produce a preform. The preform has an inside diameter of 20.75 mm and an outside diameter in the mouth section 4 of 24.95 mm. The maximum outside diameter of the mouth section 2 is defined by threads, and in this example is 27.45 mm.

The end face 23 may be compressed in the insertion direction R by a tool in order to form its surface with tight tolerances. As a result, the whole neck 2 is compressed, which may make additional material available for the shaping of threads.

The intermediate value Z of the outside diameter of the mouth section 4 (see Figures 2b and 2c) depends on the degree of crystallisation and in the example described above is 24.4 mm.

Figure 3 shows a cross section through a side wall of a preform 1 with a ready formed neck 2. In the mouth section 4, the preform 1 has an outside diameter E and a wall thickness d . The outside diameter is determined by the shape of the mould 10 and the wall thickness di is determined both by the mould 10 and also by the outside diameter of the expanding mandrel 15. As Figure 3 shows, threads in the neck 2 define a maximum and a minimum outside diameter. In this embodiment the minimum outside diameter between the threads is selected to be smaller than the final value E of the outside diameter of the mouth section 4. Figure 4 shows schematically an expanding mandrel 15 with an end section 17 tapering conically in insertion direction R. To reduce the adhesion of the thermoplastic plastics material heated to glass transition temperature to the surface of the expanding mandrel 15, this is coated with a lubricating coating 16.

Figure 5 shows schematically an apparatus according to the invention, which is provided with an alignment device 30 for aligning the blank 19 in the mould 10. The device 30 turns the blank in such a way that the threads 25 of the blank fit into the recesses 26 of the mould 12. The device 30 has a sensor for determining the correct position of the blank on the basis of a specific shaping feature, for example a notch 31.

Figure 6 shows the neck 2 of a blank, which has been formed for the production of a bottle closeable with snap-fit closures. The neck is provided with an annular bead 26, which may be gripped underneath by a snap-fit closure. The bead 26 may likewise be pressed during the expanding process into correspondingly shaped recesses of a mould. With axisymmetric projections as in this case, the provision of an aligning device 30 is of course superfluous.

By providing threads 25 or beads 26 in the neck 2 of the blank 19 which are already preformed and pressed into corresponding recesses in the mould, movement of material during expansion may also be reduced, which results in smaller incidence of stress cracks and air inclusions.

It is clear that with the present process, in which parts of the blank are expanded against an external mould, measures must be taken, which facilitate the escape of air contained in the annular gap 11. Such measures are known to the expert and are not the subject of this invention.

Claims

CLAIMS :

1. Process for manufacturing a preform (1) for a container from a blank (19) made of plastics material which is capable of crystallisation when heated, such as PET, PEN or copolymers of PET/PEN, characterised in that: at least a mouth section (4) of a blank (19) is heated to crystallisation temperature and crystallised by at least 20 , in which the outside diameter of the mouth section (4) is reduced by shrinkage during crystallisation from an initial value (A) to a smaller intermediate value (Z); and in that the mouth section (4) is then expanded against an external mould (10) in such a way that its outside diameter has a final value (E) , which is no larger than and preferably smaller than the initial value (A) , and the same as or larger than the intermediate value (Z) .

2. Process according to Claim 1, characterised in that the reduction in outside diameter from initial value (A) to final value (E) is from 1 % to 3 Ψ, , preferably 2% .

3. Process according to Claim 1 or Claim 2, characterised in that the free inside diameter (I) of the mouth section (4) is expanded during shaping from an initial value (Ii) by at least 1 % to an final value

(10 -

4. Process according to any one of Claims 1 to 3, characterised in that the wall thickness of the mouth section (4) is reduced during the shaping process from an initial value (dO by at most 20 -,, preferably less than 10 % , to an final value (dO •

5. Process according to any one of Claims 1 to 4, characterised in that the mouth section (4), when it reaches a degree of crystallisation of 20 % , is at least at the glass transition temperature of the thermoplastic plastics material.

6. Process according to any one of Claims 1 to 5, characterised in that the mouth section (4) is expanded by a mandrel (15) tapering conically in an insertion direction (R) .

7. Process according to Claim 6, characterised in that the mandrel is an expanding mandrel (15) which is coated with a lubricating coating (16), for example polytetrafluoroethylene .

8. Process for manufacturing a preform (1) for a container from a blank (19) made of plastics material which is capable of crystallisation when heated, for example PET, PEN or copolymers of PET/PEN, characterised in that: the preform includes a neck (2) which is expanded against an external mould (10) in such a way that the inside diameter (I) of the neck (2) is increased by 1 % to 3 %, whereby the wall thickness of a mouth section (4) of the neck is reduced by not more than 20 ^' , and preferably by 6 % to 10 .

9. Apparatus for expanding at least a section (4) of a neck (8) of a blank (19) for shaping a preform (1) for a container, characterised in that the apparatus comprises: an substantially cylindrical expandable mandrel (15) which is insertable into the neck 8) and which has an end section (17) which tapers conically in an insertion direction (R) ; and an external mould (10), against which a section (4) of the mouth (8) of the blank (9) is expandable.

10. Apparatus according to Claim 9, characterised in that the expandable mandrel is provided with a lubricating coating (16).

11. Apparatus according to Claim 10, characterised in that the lubricating coating (16) comprises polytetrafluoroethylene .

12. Apparatus according to any one of the Claims 9 to

11, characterised in that the mould (10) comprises at least two pieces.

13. Apparatus according to any one of the Claims 9 to

12, characterised in that the apparatus includes an alignment device (30) for aligning the blank (19) in the mould (10), whereby the mould (10) has recesses for accommodating threads (25) of the blank (19) .