EP0838405A1

EP0838405A1 - Container and blowing mould for production of the same

Info

Publication number: EP0838405A1
Application number: EP97850099A
Authority: EP
Inventors: Mikael Quasters
Original assignee: PLM AB
Current assignee: Rexam AB
Priority date: 1996-06-27
Filing date: 1997-06-19
Publication date: 1998-04-29
Also published as: NO972965L; NO972965D0; SE509009C2; SE9602551D0; CZ200697A3; SE9602551L

Abstract

A container for drinks comprises a transparent bottom portion (2) and a body (3), which is extended from the bottom portion (2) and at the top passes into an opening. The bottom portion (2) has a first wall portion (7) which curves to the interior of the container and which is formed coaxially with the longitudinal centre axis (8) of the container and whose outer circumference substantially coincides with the outer circumference of the bottom portion (2). Moreover, the bottom portion (2) has a second wall portion (9), which curves to the interior of the container and which is formed within and coaxially with said first wall portion (7). The wall of the bottom portion (2) is formed such that a ray (S1, S2, S3), which at an angle of incidence (α) falls on the outside of the bottom portion (2) at an arbitrary point (a) and which leaves the inside of the bottom portion (2) at a point (b), satisfies the following condition:

wherein γ is the increase in inclination between the wall surface of the bottom portion (2) at the point (a) and the wall surface of the bottom portion (2) at the point (b), _c is the critical angle for total reflection at the point (b) and n_D is the refractive index of the bottom portion (2) at the point (b).

A blowing mould for producing such a container has surfaces for forming the wall portions (7, 9) which are curved to the interior of the container (1).

Description

Technical Field

The present invention relates to a container, especially for drinks, of the type defined in the preamble to claim 1.

The invention also concerns a blowing mould for producing such a container.

Background Art

The invention is particularly, but not exclusively, directed to blow-moulded bottles of a plastic material, which are refillable and intended for aerated soft drinks. Examples of such bottles are disclosed in e.g. EP-A-0 521 841 and EP-A-0 247 566. This kind of bottle is thus produced from a preform which is heated and blow-moulded to a finished bottle.

Refillable containers of the above-mentioned type must be inspected after each filling operation for detecting impurities or cracking in the container bottom. Equipment for such inspection is disclosed in e.g. EP-A-0 647 846, which describes a method for inspecting a transparent container having a bottom which curves to the interior of the container. In the method, the container is arranged between a direct or indirect light source and a detector, which is a digital camera, a so-called CCD camera. The light source illuminates the bottom of the container, which thus is penetrated by light, and part of the light passing through the container is caught by the detector, which registers this light as an image. Defects in the bottom, such as cracks, or impurities on the bottom inside the container, such as liquid or particles, deflect the incident light and thus result in small amounts of light reaching the detector from these areas. The resulting image therefore presents darker portions in the area of a defect or an impurity. In the detector, this image is digitised at a number of intensity levels, whereupon the image is processed in a computer assigned to the camera. With a view to demonstrating a defect or an impurity, use is suitably made of a threshold value in respect of the intensity levels of the image, the threshold value being selected so that the container is considered unacceptable if the intensity of light within an area of the image is less than this threshold value. If the intensity of light over the entire image or a selected part thereof is greater than the threshold value, the container is determined to be acceptable.

After each filling operation, containers of the above-mentioned type must be washed, which usually takes place in an alkaline solution and at a temperature of about 58-60°C. The higher temperature used, the better washing result, but the plastic material implies restrictions. However, the market demands containers that withstand washing temperatures of up to about 75°C.

The modern technique (see for instance EP-A-o 442 836, EP-A-0 571 262 and WO-A-90/18005) makes it possible to produce containers that withstand the above-mentioned high washing temperatures. This technique requires high crystallinity in the container wall, which in turn places demands on the preform that is used. For the purpose of preventing the bottom portion of the preform from being opaque during cooling, the preform is made thinner than before. Strength problems and a risk of cracking in the container may therefore arise, particularly in the bottom portion thereof.

As further examples of prior-art technique, mention can be made of the containers disclosed in WO-A-95/25041, WO-A-95/06593 and DE-A-28 07 185.

Summary of the Invention

In consideration of the above, the object of the invention is to provide a container that withstands temperatures of up to about 75°C and has improved strength in its bottom portion and which can be inspected by radiation of the bottom of the container.

According to the invention, this object and also other objects that will appear from the following specification have now been achieved by means of a container having the features as defined in appended claim 1 and by means of a blowing mould having the features as defined in appended claim 7. Preferred embodiments are defined in the subclaims.

Thanks to the changed shape of the container bottom, preferably by means of two inwardly directed, coaxial curved portions, the container bottom will obtain improved strength compared with corresponding prior-art bottom constructions. The internal pressure that may prevail inside the container and that is established, for instance, when filling the container with an aerated soft drink, is efficiently counteracted in the inventive container by the modified bottom with an annular reinforcement in the transition between the curved portions.

It has been found that the above-mentioned design of the container bottom with a second wall portion curving to the interior of the container may sometimes cause problems when radiating the thus designed bottom. More specifically, dark rings, which arise in the area of the second curved wall portion, cannot be distinguished from impurities on the container bottom, e.g. an annular accumulation of liquid. In the area of these dark portions, it is besides impossible to demonstrate defects, such as cracking, in the material of the bottom.

Also this problem has been obviated by means of the container as defined in claim 1.

Brief Description of the Drawings

The invention will now be described for the purpose of exemplification and with reference to the accompanying schematic drawings, which illustrate a currently preferred embodiment.

Fig. 1 is a schematic side view of a prior-art arrangement for inspection of a container,

Fig. 2 is a part-sectional view of the lower part of an inventive container,

Figs 3a-b are part-sectional views corresponding to the one in Fig. 2, of containers which are not designed according to the invention,

Fig. 4 is diagram of the area, as permitted according to the invention, of the increase in inclination γ, versus the angle of incidence α.

Description of Preferred Embodiments

Fig. 1 illustrates an example of an arrangement, as described by way of introduction, for inspecting an arbitrary container 1, the container being shown in cross-section for reasons of clarity. The container 1 has a transparent bottom 2, a body 3 which is extended from the bottom 2 and passes into an opening 4. As mentioned above, the invention is particularly directed to containers for drinks, and the container shown in Fig. 1 may be, for instance, a refillable bottle of thermoplastic material, intended for aerated drinks. The container 1 is arranged between a light source 5, which illuminates the bottom 2 of the container 1, and a detector 6, e.g. an electronic camera, which registers light passing through the bottom 2. The bottom 2 of the container 1 is preferably illuminated for a short space of time, for example by making the light source 5 generate a flash of light. The light rays passing through the bottom 2 of the container 1 and falling on the detector 6 are registered by the detector in the form of an image. For further details regarding the method of inspection, reference is made to the above-mentioned EP-A-0 647 846 and the references incorporated therein.

Fig. 2 is a sectional view of the lower part of a container 1 which is designed according to the invention. The bottom 2 of the container 1 has a first wall portion 7, a so-called dome, which curves to the interior of the container 1. The dome 7 is formed coaxially with the longitudinal centre axis 8 of the container 1, and the outer circumference of the dome 7 substantially coincides with the outer circumference of the bottom 2. Further the bottom 2 has a second dome 9, which curves to the interior of the container 1 and which is formed coaxially with and within the first dome 7.

Fig. 2 shows how three essentially parallel rays of light S1, S2, S3, which fall on the outside of the bottom 2, pass through the material of the bottom and leave the inside of the bottom 2. The designations used henceforth will be explained below in connection with the ray S1 in Fig. 2. The ray S1 falls on the outside of the bottom 2 at the point a at an angle of incidence α relative to the normal to the wall surface at this point. This ray is refracted according to the refraction law: n1 sin α = nD sin β, where n₁ is the refractive index (≈1) of the air on the outside of the container 1, n_D is the refractive index of the material of the container at the point a, and β is the refractive angle in relation to the normal to the wall surface at the point a.

This ray S1 passes on in the material and falls on the boundary layer towards the interior of the container 1 at the point b at the angle of incidence , whereupon it is refracted according to the refraction law. If the wall surfaces of the bottom 2 are parallel at the points a and b, the angles β and  are the same, and therefore the ray leaving the inside of the bottom 2 is parallel with the incident light ray. If this is not the case, and if the light ray experiences an increased inclination at the point b compared with the one at the point a,  will be greater than β. By the expression "increased inclination" for a surface is meant a surface which is more parallel with the centre axis 8.

Practical experiments have demonstrated that when designing the bottom 2 of the container 1, regard must be taken to this increased inclination γ. If the inclination becomes too great, the angle of incidence  will in fact, at the point b, exceed the critical angle for total reflection _c: c = arcsin n1 nD , whereupon the incident light will undergo a total reflection and therefore not reach the detector. This case is illustrated in Fig. 3a, which shows a container 1 which is not designed according to the invention. The ray S1' arrives at too steep a portion of the bottom 2 of the container 1 and therefore experiences such a great increase γ of the inclination between the wall surface of the bottom 2 at the point a and the wall surface of the bottom 2 at the point b that the condition for total reflection is satisfied at the point b, and therefore the ray S1' does not reach the detector 6, which registers a dark annular portion in the area of the second dome 9.

The design according to Fig. 3a besides suffers from the drawback that the outside of the bottom 2 in the area of the second dome 10 is essentially parallel with the centre axis 8 of the container 1. This results in only a fraction of the incident ray S1' being refracted into the material in this steep area, whereas the main part of the radiation will be reflected towards the outside of the bottom 2 since the reflectance of a surface is essentially equal to 1 for angles of incidence close to 90°. This contributes to the detector 6 registering a dark annular portion in the area of the second dome 9. According to a preferred embodiment, the outside of the bottom portion 2 therefore is formed with an inclination which all over exceeds about 10° relative to the centre axis 8.

For preventing total reflection when radiating the bottom 2 of the container 1, the following condition must thus be satisfied:

In Fig. 4, this connection is illustrated for a container of polyethylene terephthalate (PET) having a refractive index of 1.576. The above condition is satisfied in the area indicated by dashed lines.

Fig. 3b shows a further examples of a container which is not designed according to the invention. In this case, the wall thickness of the bottom 2 increases in the direction of the centre axis 8, and therefore the incident ray S1" experiences such an increased inclination γ between the points a and b that total reflection arises at the point b. In this area, the detector 6 thus registers a dark portion which cannot be distinguished from impurities on the bottom surface and which prevents the demonstration of defects in the actual material of the bottom.

The expert realises that there are a number of conceivable modifications of the invention within the scope of the appended claims. For instance, it may be mentioned that the bottom of the container may have a plurality of coaxial, inwardly curved portions. Possible thermoplastic materials for the production of a container according to the invention are e.g. polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polypropylene (PP) or polyamide (PA). Of course, also mixtures of different plastic materials can be used.

The invention is also not connected to the use of a particular method for radiation of the bottom of the container. The most important thing is that defects and/or impurities can be demonstrated in such a radiation, preferably as dark portions. The method of inspection may utilise either directed or diffuse radiation.

The invention also concerns a blowing mould (not shown) for producing a container of the above-mentioned type. The mould comprises a first curved portion for moulding the first dome 7 (cf. Fig. 2), and a second curved portion, which is formed within and coaxially with the first portion and which is adapted to mould the second dome 9. According to a preferred embodiment, the outside of the blowing mould is inclined in relation to a centre axis of the blowing mould.

Claims

A container, especially for drinks, comprising a transparent bottom portion (2) and a body (3) which is extended from the bottom portion (2) and at the top passes into an opening (4) opposite the bottom portion (2), the bottom portion (2) having a first wall portion (7) which curves to the interior of the container and which is formed coaxially with the longitudinal centre axis (8) of the container and whose outer circumference substantially coincides with the outer circumference of the bottom portion (2), characterised in that

the bottom portion (2) has a second wall portion (9), which curves to the interior of the container and which is formed within and coaxially with said first wall portion (7), and that

the wall of the bottom portion (2) is formed such that the following condition is satisfied for a ray (S1, S2, S3), which at an angle of incidence (α) falls on the outside of the bottom portion (2) at an arbitrary point (a) and which leaves the inside of the bottom portion (2) at a point (b):
wherein γ is the increase in inclination between the wall surface of the bottom portion (2) at the point (a) and the wall surface of the bottom portion (2) at the point (b), _c is the critical angle for total reflection at the point (b) and n_D is the refractive index of the bottom portion (2) at the point (b).
A container as claimed in claim 1, wherein the outside of the bottom portion (2) is inclined relative to the centre axis (8) of the container.
A container as claimed in claim 2, wherein the inclination of the outside of the bottom portion (2) in relation to the centre axis (8) is greater than about 10°.
A container as claimed in claim 1, 2 or 3, which is refillable.
A container as claimed in any one of the preceding claims, which is a blow-moulded bottle of thermoplastic material.
A container as claimed in claim 5, wherein the thermoplastic material is polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polypropylene (PP) or polyamide (PA), or mixtures thereof.
A blowing mould for producing a container according to any one of claims 1-6, said blowing mould comprising a first projecting portion for moulding a first bottom wall portion (7) of the container, characterised by a second projecting portion, which is formed within and coaxially with said first portion and which is adapted to mould a second bottom wall portion (9) of the container.
A blowing mould as claimed in claim 7, wherein the outside of the blowing mould is inclined in relation to a centre axis of the blowing mould.