IE921493A1

IE921493A1 - Process for manufacturing a blow moulding

Info

Publication number: IE921493A1
Application number: IE149392A
Authority: IE
Inventors: Klaus Landrock
Original assignee: Rehau Ag & Co
Priority date: 1991-05-10
Filing date: 1992-07-01
Publication date: 1992-11-18
Also published as: FI922113A7; DE4115236A1; ES2051138T3; EP0512444B1; FI922113A0; DK0512444T3; EP0512444A1; NO921634D0; ATE102867T1; NO921634L

Abstract

The invention relates to a process for producing a blow moulded article by the process of extrusion blow moulding. A split blow mould (4) is used for this. The parison is introduced as a tube (31) in the thermoplastic material state into the opened blow mould (4). After the closing of the blow mould halves (41, 42), the parison is brought into contact with the inner contours of the blow mould (4) by a defined blowing pressure and, after the stabilisation, is removed from the reopened blow mould (4) as a finished moulded article (3). The invention consists in the hollow inner space of the blow moulded article being filled with foam (71) between the end of the blowing process and the opening of the blow mould with the utilisation of the process heat remaining in the wall of the moulded article.

Description

METHOD OF MANUFACTURING A BLOW MOULDING This invention concerns a method of manufacturing a blow moulding by way of the extrusion blow moulding process using a split blow mould. In this method, the parison is introduced in bubble form while still in a thermoplastic state into the opened blow mould where it is pressed by a defined blowing pressure against the inner contours of the blow mould after the mould is closed. After having stabilised, it is removed from the open blow mould as the finished moulding.

Blow moulding methods of this type are readily familiar, e.g. DE-OS 25 56 186. This specification describes an injection blow moulding process for the manufacture of thermoplastic blow mouldings in which a hollow thermoplastic extrudate is produced in consecutive process steps, the end of which is closed off to form a pressure-tight seal. The cavity in the closed hollow extrudate section is inflated by admitting a pressurised gas into the mould until the hollow extrudate hugs the mould wall. After cooling, the mould is opened and the finished moulding removed.

State-of-the-art methods of this type can be used to manufacture blow mouldings of any type. In order to improve the mechanical or thermal properties of such blow mouldings, it is known that they can be filled entirely or partially with foam. Here, reference is made to DE-OS 21 06 633. This specification describes a method of foam-filling elongated mouldings in which a solid foam intermediate in the form of a rod of appropriate cross section is inserted longitudinally into the cavity where it expands as a function of time or by implementing suitable measures, e.g. application of heat.

Page 2 In the descriptive part of chis preliminary publication, it is pointed out that a further familiar method of foam-filling closed hollow sections involves introducing a viscous foam mixture into the closed hollow sections where it is then expanded. However, the brief period of time available until the foam starts to expand proved to be of a disadvantage. According to this preliminary publication, the situation is alleviated by the use of probes which either serve part of the closed hollow sections or which feature a series of holes serving the entire cavity through which the viscous foam mixture is admitted in individual portions.

All familiar methods of foam-filling closed hollow sections are associated with one and the same drawback, i.e. for process-related reasons they must be performed in two separate stages. The first stage of the process involves manufacturing the blow-moulded parts as far as the point where they are removed from the opened blow mould.

The second stage of the process involves filling these blow moulded parts with foam. For reasons of stability, the finished blow moulding must generally be placed in a heated support mould while the foam expands. After the foam has been admitted and allowed to stabilise, the foam-filled blow moulding can be removed from the support mould.

DE-OS 34 45 542 describes a method of blow moulding a hermetically sealed container with a sterile internal discharge outlet. This container is manufactured by extruding a relatively hot plastic parison. This parison is held between two moveable mould halves. The inside surfaces of these moveable mould halves define the shape of the container and at least part of the container neck.

Page 3 Once the mould halves are closed, an injection mandrel is lowered into the tubular parison whereupon the container is blow-moulded. The sterile pharmaceutical fluid is admitted into the blow-moulded container through the injection mandrel before the mould halves are opened. The injection mandrel is withdrawn after the container has been filled with pharmaceutical fluid and the container closed by means of a stopper pressed into the container neck and forming a liquid-tight seal.

This is a complicated and costly technique in which a blow-moulded container is filled with pharmaceutical fluids before it is removed from the mould and then immediately sealed in a sterile state. For medical reasons, the costs of such methods must be accepted; however such expense need not be tolerated in the case of industrial containers.

The invention is aimed at avoiding the above described drawbacks associated with state-of-the-art techniques and represents a simple and cost-effective method for enhancing the mechanical and thermal properties of blow moulded parts. The invention proposes filling the cavity of the blow moulding in the period between completion of the inflation process and the point at which the blow mould is opened, utilising the process heat present in the blow moulding wall.

The invention takes advantage of the fact that the outer contours of the blow-moulded part, formed during the extrusion blow moulding process, fully assume the contours of the split blow mould before it is demoulded. Depending on the type of material being processed, the wall of the blow-moulded part in this phase has a process-related temperature of between approx. 100 and 180° C.

Page 4 Whereas in conventional extrusion blow moulding techniques the above described process steps are followed by the phases of stabilisation, cooling and demoulding, this phase provides the innovatory method with two fundamental conditions for filling the blow moulding cavity with foam: 1. While closed, the blow mould serves as a support counteracting foam pressure 2. The blow moulding is still heated in this state.

Consequently, there is no necessity for the support mould otherwise necessary in state-of-the-art techniques during the subsequent foam-filling process and the blow-moulded part need not be reheated.

In one version of the innovatory method, the foam used for filling the cavity of the blow moulded part consists of preformed foam particles which firmly coalesce with the inside wall of the blow moulding upon contact.

On filling the moulding cavity with foam particles, the residual process heat of the blow-moulding wall should not exceed 120°. Higher temperatures might adversely affect the foam particles. Experiments have shown that ESP or PP particulate foam produces optimum results up to the specified maximum temperature of 120° C.

In another version of the newly invented method, the foam can be generated directly inside the blow moulding as a result of mixed foam components reacting with each other.

Page 5 In this case, the residual process heat should not exceed 60° C since higher temperatures would have an adverse effect on foam expansion.

According to the invention, the blow moulding is filled at an internal pressure of 2-8 bar required to stabilise the blow moulded part. The filling pressure at which the particulate foam or foam components is/are injected into the blow moulding must therefore exceed this stabilisation pressure.

The particular advantages of this innovatory technique are illustrated particularly clearly by the fact that using particulate foam to fill the blow moulding, the filled blow moulded part can be demoulded after a brief stabilisation phase of between 0.5 and 3 minutes, depending on moulding wall thickness. If foam components are admitted to the blow moulding cavity (where they expand to form foam of desired density), demoulding times will vary between approx. 1 and 6 minutes, depending on moulding type.

In applications involving the use of particulate foam, any foam particles not in direct contact with the moulding wall can be sintered by injecting a fluid whereby this process also utilises the residual heat present in the container wall.

The drawing shows in schematic form the stages involved in the innovatory process for manufacturing a blow moulded part. It depicts an extruder 1 with feed hopper 11 for the polymer material 12 from which the blow moulding wall 3 is made. The polymer material 12 is drawn into the flight of an extruder screw 13 where it is plastified and discharged via the die 14 in the form of a tubular semi-finished product 31.

Page 6 The tubular semi-finished product 31 is advanced into the split blow mould 4 which comprises blow-mould halves 41, 42. As soon as the semi-finished product 31 has enveloped the tip of the blow mandrel 5, the blow mould halves 41, 42 are closed and the blow mould 4 is removed from the extruder 1. Blow air is then immediately admitted into the semi-finished product 31 causing the parison to hug the inner contours of the blow mould 4.

The next stage of the process involves detaching the blow mandrel from the compressed-air source and connecting it to a feed injector 6 which is fed by a silo 7 charged with pre-expanded particulate foam. The feed injector 6 is connected to an air source 61 which supplies the feed air. The feed injector 6 delivers the particulate foam 71 from the silo 7 into the cavity 43 of the blow-moulded part.

The blow mould 4 is then detached from the feed injector 6 and cooled in a stabilisation phase in preparation for demoulding the filled blow moulded part. Before the cooling process is completed, moist air may, if necessary, be injected to sinter any unbound foam particles .

In the last schematic drawing, the blow mould halves 41, 42 are reopened and the finished blow moulding 3, now filled with particulate foam, can be disconnected from the blow mandrel 5 and demoulded.

Proceeding from the schematic drawing and accompanying description, the hollow cavity of the blow moulding is filled via the blow mandrel 5. This filling method is particularly suitable for large-volume blow-moulded parts. However, blow mouldings of smaller volume may also be filled with familiar injection devices.

Page 7 In cases where the blow moulding is filled with foam components, the blow moulding 4 is connected to the foam component supply lines (not illustrated in the drawing) instead of to the feed injector 6.

- Patent claims -

Claims

PATENT CLAIMS

1. Method of manufacturing a biow moulding by way of the extrusion blow moulding process employing a split blow mould whereby the tubular parison, while still in a thermoplastic state, is introduced into the opened blow mould, pressed against the inner contours of the blow mould at a defined blow pressure after the mould is closed and following a period of stabilisation then removed from the reopened blow mould as a finished blow moulding, characterised by the fact that in the period between completion of the inflation process and the point at which the blow mould is opened, the residual process heat in the moulding wall is utilised for the process of filling the hollow cavity of the blow moulding with foam.

2. Method as described in claim 1, characterised by the fact that the foam comprises preformed foam particles which, upon contact with the inner blow moulding wall, firmly coalesce with it.

3. Method as described in claim 1, characterised by the fact that the foam material is generated directly inside the blow moulding as a result of the mixed foam components reacting. 4. Method as described in claims 1 to 3, characterised by the fact that the residual process heat must not exceed 120° C. 5. Method as described in claims 1 to 3 , characterised by the fact that the residual process heat must not exceed 60° C. 6 . Method as described in claim 1, characterised by the fact that while the blow moulding cavity is being filled, a blow pressure of up to 3 bar must be maintained. Page 9 Rxgaxk

4. 7. Method as described in claims 1 and 2, characterised by the fact that the foam particles not in direct contact with the blow-moulding wall are sintered by additionally injecting a fluid.

5. 8. A method of manufacturing a blow moulding according to any preceding claim substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.