GB2246736A

GB2246736A - Removing a meltable core from a moulded plastics article by immersion in a heated salt solution

Info

Publication number: GB2246736A
Application number: GB9016389A
Authority: GB
Inventors: James Frank Yardley
Original assignee: Dunlop Ltd
Current assignee: Dunlop Ltd
Priority date: 1990-07-26
Filing date: 1990-07-26
Publication date: 1992-02-12
Also published as: FR2665103A1; ES2047413B1; ITMI912081A1; GB9016389D0; MX9100398A; CA2047812A1; IT1251123B; ES2047413A1; DE4124315A1; ITMI912081A0

Abstract

The moulded article of plastics material which may contain an embedded metallic component, is formed about a core of a material which melts at a temperature below that of the plastics material. The core is removed by immersing the assembly of plastics material and core in a bath containing a heated salt solution, and this avoids undesirable heating of the metallic component that would otherwise also occur with induction heating of the core, or difficulties that arise when using heating oils. <IMAGE>

Description

C (1 1 -3 MANUFACTURE OF PLASTICS ARTICLES This invention relates to the

manufacture of articles of plastics material and in particular to the manufacture of articles by a "lost core" type of process in which plastics material is moulded with the aid of a fusible melt out mould core member which, subsequent to moulding, is removed by heating the mould core member to a molten state in which it can flow out from a cavity thereby defined by the moulded plastics material. In particular, though not exclusively, the invention relates to the manufacture of composite type plastics articles comprising a combination of plastics and metallic materials.

In the manufacture of precision type engineering components it is conventional to use a low melting temperature metal alloy such as leadtin-antimony or tin-bismuth for the mould core, these materials being suitable for pre-forming accurately to a required core shape.

Subsequent to moulding of pla.gtics materal around the metal alloy core, the core is melted either by an electrical induction heating technique or by thermal conduction such as from the heat of a hot oil bath.

The induction heating technique is not always suitable, especially if the moulded plastics material has been pre-moulded integrally with metal components. Thus it may be required to provide the moulded plastics article with a large number of screw-threaded brass inserts to facilitate subsequent attachment of other components. The induction heating results in undesirable local heating of the inserts and the immediately adjacent plastics materia.l. In some cases it is possible to shield the metallic components from the induction field, but this is not always possible and in the case of an article having e g a large number of distributed inserts it is not practical.

The hot oil bath technique therefore has generally been employed for composite type articles 1 i 1 i i 2.

of plastics and metallic material. This avoids undesirable local heating but other problems are encountered. often the mould core is formed from lead tin antimony which melts at 1870C; particularly in this case it is therefore necessary to use oil at approximately 200-2100C but the conventional oils used for this purpose, e g water soluble polyethylene glycol type oils, have a flash point of 215"C and there is therefore a potential safety problem.

The use of a hot oil bath technique also leads to other potential problems. The oils degrade typically over a period of 3 months during which time the oil viscosity becomes progressively greater and thereby reduces the ability for good thermal conduction; thus the heating time required to melt a core becomes increased. Also the oil darkens with use and where the inserts are of an anodised type it is found that they become adversely marked by the oil.

The present invention seeks to provide an improved method for the manufacture of plastics articles by the lost core technique and a method particularly but not necessarily suitable for the manufacture of composite plastics articles comprising in combination metallic and plastics materials.

In accordance with one of its aspects the present invention provides a method for the manufacture of articles of moulded plastics material comprising forming a core member of a material which melts at a temperature below that of the moulded plastics material, moulding the plastics material around the core member then immersing the assembly of plastics material and core member in a bath containing a heated salt solution and maintaining the article in the salt solution for sufficient time to remove the core material.

It is envisaged that the salt solution will be maintained at a temperature greater than the melting point of the core material.

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However, the invention does not exclude the possibility of a multi-form heating technique in which induction heating is used to augment salt bath heating. In that case the salt bath need not be heated to above the melting point of the core material but the salt bath temperature should be selected such that additional heating by an induction field, in the case of a moulded article having integral metal components, does not result in potentially damaging heating of those metal components and thus of the surrounding moulded plastics material.

A suitable salt solution is a eutectic mixture of inorganic salts. An example of a suitable solution is a eutectic mixture of potassium and sodium nitrates.

The method of the invention further envisages that the article may be retained in the salt bath solution for a time greater than that necessary to remove the core material whereby the salt bath may be employed to effect any necessary post-cure heat treatment of the plastics material.

It is envisaged that the invention will be of particular use in the manufacture of plastics articles of thermoplastics materials but it may be employed also for articles formed from thermosetting materials such as phenolics.

The moulded plastics article may be comprised substantially only of plastics material or it may be of a composite nature comprising plastics material and metallic material. Typically it will be of a composite construction comprising a moulded plastics body having embedded therein one or more metallic components.

It is envisaged that the method may comprise use of a conveyor type installation to sequentially introduce one or a group of moulded assemblies of plastics material and moulded core into a salt bath and then sequentially remove said assembly or groups of assemblies from the bath. Subsequent to immersion j i i i i 4. in the salt bath the articles or groups thereof may be sequentially immersed in a first water tank for preliminary washing and then rinsed in a second water tank.

The invention further envisages that contaminated water from the rinse or pre-wash tank may be fed to a zone of the salt bath, e g by spraying thereon, whereby the water is evaporated due to the heat of the salt bath and thereby to result in the salt remaining in the salt bath. The need for special waste treatment of salt contaminated water in the pre-wash bath (and also the rinse bath) is thereby avoided. It is envisaged that the rinse bath may be supplied with fresh water and an equal volume of water transferred to the pre-wash bath from which, in turn, a similar volume of the salt contaminated water is fed to the salt bath for heating to vaporise the water and collect the salt. The salt bath may be of a zoned construction in which a first zone is used for treating the assemblies of plastics material and moulded cores and a second zone is used f or treatment of the salt contaminated water.

one embodiment of the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawing which shows schematically apparatus for use in performing the method of the present invention.

An overhead conveyor system 10 is used to slowly move a series of moulded assemblies 11 into a salt bath 12. Each moulded assembly comprises a thermoplastics body portion moulded around a metal alloy core of lead-tinantimony. The conveyor system is operated to move slowly and result in each moulded assembly being submerged in the salt bath f or a period of typically 2 hours. Subsequently the conveyor system 10 lifts the respective moulded articles from the salt bath, the cores then having been melted out from the moulded articles. The articles are then sequentially i f 1 i i 1 i i i i 1 i i 1 1 i i 5. moved by the conveyor through the wash and rinse baths 13 and 14 respectively.

The salt bath in this embodiment of the invention contains a eutectic mixture of potassium and sodium nitrates at a temperature of 2100C where the moulded assemblies comprise moulded cores of lead tin antimony which melts at 1870C.

It will be seen from the accompanying drawing that the salt bath 12 has an auxiliary zone 15 separated f rom the main zone 16 by a vertical baf f le 17. The purpose of the zone 15 is to provide an area of heated salt solution 12' into which salt contaminated water from the wash bath 13 may be sprayed. A collection hood 18 facilitates extraction of steam from above the salt bath zone 15. Pipes 19 and 20 respectively interconnect the wash and salt baths and the rinse and wash baths whereby when fresh water is added to the rinse bath an equal volume of water may pass to the wash bath and in turn an equal volume of the salt contaminated water pass as a spray into the zone 15 of the salt bath.

Means, not shown, is provided for continuously circulating the salt solution through the bath 12 and heating the salt solution. Means, also not shown, additionally is provided for collecting the mould core material which has been melted from the moulded articles and has sunk to the bottom of the bath 12.

The use of a salt bath f or melting the mould cores avoids the aforedescribed problem of undesirable localised heating in the case of articles having metal inserts and also avoids the problems associated with the conventional use of an oil bath. Further advantages arising from the use of e g a eutectic mixture of inorganic salts are that they have a higher specific heat than oil, thereby either enabling the melt time to be reduced or a lower temperature bath to be used for the same period of immersion as compared with oil. Also they are non-inflammable and do not degrade OP- a 6.

significantly.

1 1 i 1 i 1 1 1 1 i 1 1 1 i 1 1 7.

Claims

CLAIMS:

1. Method for the manufacture of articles of moulded plastics material comprising forming a core member of a material which welts at a temperature below that of the moulded plastics material, moulding the plastics material around the core member then immersing the assembly of plastics material and core member in a bath containing a heated salt solution and maintaining the article in the salt solution for a sufficient time to remove the core material.

2. Method according to claim 1 wherein the salt solution is maintained at a temperature greater than the melting point of the core material.

Method according to claim 1 or claim 2 wherein induction heating is used to augment salt bath heating.

4. Method according to claim 3 when dependent on claim 1 wherein the salt bath is not heated to above the melting point of the core material.

5. Method according to any one of the preceding claims wherein the salt solution is a eutectic mixture of inorganic salts.

6. Method according to claim 5 wherein the salt solution is a eutectic mixture of potassium and sodium nitrates.

7. Method according to any one of the preceding claims wherein the article is retained in the salt bath solution for a time greater than that necessary to remove the core material.

8. Method according to any one of the preceding claims wherein the moulded plastics article is of a composite nature comprising plastics material and metallic material.

9. Method according to claim 8 wherein the article comprises a moulded plastics body having embedded therein one or more metallic components.

10. Method according to any one of the preceding claims wherein the plastics material is a thermoplastics material.

4 t 8.

11. Method according to any one of claims 1 to 9 wherein the plastics material is a thermosetting material.

12. Method according to any one of the preceding claims wherein a conveyor type installation is used sequentially to introduce one or a group of moulded assemblies of plastics material and moulded core into said salt bath and then sequentially remove said assembly or groups of assemblies from the bath.

13. Method according to any one of the preceding claims wherein subsequent to immersion of the moulded assembly of plastics material and moulded core the article is immersed in a first water tank for preliminary washing and then rinsed in a second water tank.

14. Method according to claim 13 wherein salt contaminated water from the rinse or pre-wash tank is fed to a zone of the salt bath thereby to return zhe salt to the salt bath.

15. Method according to claim 14 wherein the contaminated water is sprayed into a zone of the salt bath.

16. Method for the manufacture of an article of moulded plastics material according to claim 1 and substantially as hereinbefore described.

Published 1992 atIbe Patent Office. Concept House, Cardiff Road, Newport. Gwent NP9 1 RH. Further copies rokV be obtained from Sales Branch. Unit 6. Nine Mile Point, Cwnifelinfach, Cross Kqys. New-port. NPl 7HZ. Printed by Multiplex techniques lid. St Marv Cray. Kent.

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