US3638299A

US3638299A - Process for the production of molds

Info

Publication number: US3638299A
Application number: US885463A
Authority: US
Inventors: Paul Johnson Garner; Thomas Robert Stephen Collins
Original assignee: Imperial Chemical Industries Ltd
Current assignee: Imperial Chemical Industries Ltd
Priority date: 1968-12-30
Filing date: 1969-12-16
Publication date: 1972-02-01
Anticipated expiration: 1989-02-01

Abstract

Producing a mold by forming a metal shell around a model, the model being provided with a lip around its base over which the metal shell is formed to key it to the model.

Description

United States Patent [151 3,638,299

Garner et al. 1 Feb. 1, 1972 [541 PROCESS FOR THE PRODUCTION 0F 58 Field or Search ..29/527.3, 527.2, 527.4; MOLDS 264/338, 312, 54; 249/80; l8/DlG. 30

[72] Inventors: Paul Johnson Garner; Thomas Robert Stephen Collins, both of Welwyn Garden [56] References cued 01y, England UNITED STATES PATENTS [731 Assigne= Imperial Chemical lndusll'ie's 3,000,094 9/1961 Arnoldy ..29/527.2 Eng'and 3,427,185 2/l969 Cheatham ..29/521.2

[22] Filed: Dec. 16, 1969 Primary Examiner-John F. Campbell [211 Appl' 885463 Assistant Examiner-Donald P. Rooney AttorneyCushman, Darby & Cushman [30] Foreign Application Priority Data Dec. 30, 1968 Great Britain ..6l,659/68 [57] ABSTRACT Nov. 3, 1969 Great Britain ..53,693/69 Producing a mold by forming a metal shell around a model, the model being provided with a lip around its base over which U-S. the metal shell is formed to key to the modeL [51] Int. Cl. B23p 17/00, 323p 25/00 17 Claims, 6 Drawing Figures ll fll' llrl PATENTEDFEBI 1072 3,838,299

sum 1 or 3 VIII mamsnrem m2 31638299 SHEET 2 OF 3 Fig.4

wtm x /%AMW PROCESS FOR THE PRODUCTION OF MOLDS The present invention relates to a process for the production of molds and to the molds so produced. In particular the present invention relates to producing molds suitable for the production of articles from thermoplastic materials in particular by injection molding.

' The prevent invention is concerned with the production of molds by forming a metal sheet which will define the molding surface preferably forming by metal spraying and subsequently backing the shell with a reinforcing layer. In particular the present invention relates to an improved method for the production of the metal shell and providing a reinforcement thereto.

According to the present invention we provide a process for making a mold comprising forming a metal shell around a model of the article to be molded, affixing heating or cooling pipes on said metal shell and applying a reinforcing material to provide a backing to said shell and to encase the pipes while leaving their ends exposed wherein the model is formed with a protruding lip around the part of the model which defines the article to be molded over which the metal is deposited to key the metal shell to the model.

We have found that it is particularly convenient to apply the reinforcing material to the shell in two stages. in this way the first stage may be applied to render the shell self-supporting so that it may readily be transported without damaging the shell. The second layer of reinforcing material provides backing to the mold to enable it to be used in molding operations. The extent of backing that is required depends upon the molding operation in which the mold is to be used. For example if the mold is to be used in injection molding where high pressures are used to inject materials into the mold a strong backing is needed. However, if the mold is to be used in blow molding operations the backing material need not be able to withstand such high pressures. We have found that to avoid damaging the metal shell, particularly shells which have been produced by flame spraying, the shell should not be trimmed from the model at least until the first reinforcing layer has been applied to the metal shell. This is particularly important with metal shells produced by flame spraying as stresses tend to be set up in the shell due to the contraction of the metal as it cools on the model. If, however, the shells are produced from low-contraction metals such as bismuth tin alloys the shell can be removed from the model before the reinforcement is applied, although this is not preferred because the shells are less likely to crack when they are reinforced. z

The present invention also provides molds whenever made by this process.

Our invention is particularly useful in the production of molds which are used in injection molding. These molds are generally known as split molds and are made in two halves. Conveniently one-half of the split mold may be made by the process of this invention and when that half has been removed from the model the cavity so produced may be used as a model for the production of the other half.

The model of the article to be molded may be made in any required manner and may be of any suitable material providing it is not adversely affected during the steps of the present invention. The model may be made of wood, metal, plaster, synthetic thermoplastic or thermosetting resins, rubber especially silicone rubber, wax, plasticine, clay or glass reinforced polyester resin. The model must be provided with a lip extending around and beyond the surface of the model which is to define the molding surface so that the metal may be sprayed over this lip and will key to the model, so that it will remain in position in the model until spraying is completed andwill not tend to lift away from the model. We prefer that the model be mounted on a wood or metal base which has angled edges which provide the lip which keys the sprayed metal shell to the model. The model should preferably contain cooling pipes which should be fixed as close to the surface to be sprayed as possible. We further prefer that the model is coated with a release agent such as polyvinyl alcohol before formation of the metal shell; this is particularly preferred when the shell is formed by flame spraying. The release agent allows the metal shell to be parted readily from the model when immersed in water and also assists in the satisfactory laying down of the initial coating of the sprayed metal.

The metal shell may conveniently be formed around the model by electrodeposition or by metal spraying. Any metal from which a shell can be formed around the model may be used but we prefer that if the mold surface is formed by metal spraying it be of zinc. Alternatively, the shell may be formed by compressed air metal spraying using a low-melting, lowcontraction alloy, alloys of bismuth and tin have been found to be particularly suitable. These metals provide a true reproduction of the model surface. The flame sprayed metal shells can be entirely of zinc or a minimum thickness of 0.050 inches of zinc backed by a low-carbon steel, copper or aluminum. lf metal spraying techniques are used aluminum is another particularly suitable material as it is lighter, has less tendency to crack and is readily sprayed at a high rate, i.e., metal deposited per minute. A metal shell thickness of about onefourth inch is preferred for the process of this invention.

After spraying, heating or cooling pipes, preferably of copper, should be positioned and bent to follow the contours of the metal shell. The pipes are preferably fixed to the metal shell with a metal filled epoxy resin as this adhesive gives good heat exchange as is described and claimed in our copending application No. 53692/69. Alternatively, the pipes can be tacked in position with a metal filled epoxy resin and then firmly stuck to the shell by spraying them with a further coating of metal on top of the pipes to adhere them to the originally sprayed shell. If desired thermocouples may also be positioned on the metal shell as this has the distinct advantage that they are in close proximity of the mold face. In our preferred process the metal shell is strengthened by backing it with a reinforcing material while it is firmly keyed to the model by virtue of the shell extending over the lip of the model. Conveniently the shell may be surrounded by shuttering to form a cavity into which mobile reinforcing material such as concrete may be cast. The shuttering may conveniently be provided by screwing wood to the baseboard on which the model is mounted. Steel tie bars may then be bent and fitted to crisscross the formed box and allowed to protrude each side. The tie bars serve two purposes, they provide means for transporting the mold half when it is backed with reinforcement and they also tie it into the second layer of reinforcement if one is used. If the metal shell has been produced by spraying it is advisable to coat it with a water repellent paint before it is backed with the reinforcing material. The reinforcing material such as concrete or aluminum cement may now be cast into the cavity to cover the shell to render it self-supporting. We prefer that the system be well vibrated to ensure there are no voids close to the metal shell. When the backing of the reinforcing material has cured the shell may be trimmed off from over the lip and parted from the model.

If the mold is to be used in high-pressure molding cycles it must be provided with a rigid backing. In our preferred process for making the completed mold the metal shell with cooling pipes and optionally thermocouples attached thereto is provided with a first layer of reinforcement by positioning in a wood or metal bolster, with square section metal blocks at each corner as described in our copending application No. 61659/68 and the bolster subsequently filled with liquid material such as concrete which is then allowed to cure. Providing the reinforcing material in two layers has the added advantage that any cracks which form in the outermost layer of reinforcement will not readily propagate through into the metal shelf.

In the present invention is used for producing split molds the shell of the second mold half may conveniently be prepared by inverting the completed first mold half and using it as a spraying cradle to produce the metal shell for the other mold half. For convenience of operation we prefer that the first mold half is used as the spraying cradle after it has been provided with its first reinforcing layer but before it is provided with the second backing material. An angled beading is preferably fitted around the top of the inverted shell to provide a lip around the first shell to which the second sprayed shell will key. Before any metal is sprayed onto the inverted first shell a material is laid on the shell to the required thickness to define the mold cavity. The pipes already fitted to the first mold half can now be used to cool the system when spraying the second metal shell.

Where the molds are used for injection molding a tapered hole called the sprue through which the polymer may be injected into the mold must be formed in the mold wall. Conveniently a metal block is, therefore, accurately positioned on the cavity defining material before the metal is sprayed so that the block may be later drilled and reamed to provide the sprue. Metal may then be sprayed to the required thickness on to the wax around the metal block and around the key on the angled beading so that the shell is held firmly in position. Alternatively, the metal block may be placed on the back of the sprayed shell but great care must then be taken in drilling a hole to form the sprue so that the metal shell does not crack.

Pipes and thermocouples may then be fitted to the second spray shell in the same way as for the cavity insert and a first layer of reinforcement provided to strengthen the shell. The reinforcing layer may be applied by fixing shuttering together with strengthening rods around the shell and positioning a block of metal to form an extension of the sprue block already positioned on the cavity defining material which will extend to above the level to which the reinforcement is to be cast so that the whole block may be drilled to provide the sprue. Mobile reinforcing material such as concrete or aluminous cement may then be cast around the shell and allowed to cure. This second mold half may then be parted from the first half by trimming the metal away from the angled beading.

If the two mold halves are to be used to produce large moldings in machines which exert a large force during molding they must be provided with a pressure resistant backing and must be backed with a solid reinforcing material which will absorb the molding forces. Our preferred material is concrete. The mold halves must also be provided with means whereby they may be located on a molding machine.

As set out above the mold may conveniently be reinforced by separately mounting each of the mold halves in a bolster which is then filled with reinforcing material, preferably concrete. The bolster should have walls of sufficient height to support the reinforcing material to the required depth. The means whereby the molds may be located on the molding machine conveniently comprise metal blocks which may be located within the bolster, preferably at the corners, where they will be embedded in the reinforcing material as is describedin our copending application No. 61659/68. Alternatively, the blocks may be secured to the outside of the bolster.

It is, however, important with split molds that the two halves are correctly located with respect to each other and this is most conveniently achieved by first locating one half in its reinforcing material and using this to ensure that the second half is correctly located in its reinforcing material. The second mold half together with its dowel blocks may conveniently be located within a bolster which is then filled with concrete so that the mold half is floating" on the concrete and is higher than its final required position. The first mold half located in its holster then lowered onto the second half so that the second half mold insert is depressed to the required depth in the concrete. The two mold halves may then be located in the correct position with respect to each other by means of the metal blocks in the corners of the bolsters; the blocks in one mold half being provided with dowels and the blocks in the other mold half with holes to receive the dowels so that when the dowels lock with the holes the two mold halves are correctly positioned as is described in our copending application No. 61659/68. The second mold half then settles down in the concrete to the required depth and the concrete allowed to set.

The present invention insofar as it relates to the production of split molds is illustrated but in no way limited by reference to the accompanying drawings in which FIG. 1 illustrates the model which is used in the production of the first mold half.

FIG. 2 illustrates the model coated with a metal shell which is provided with a rigid backing material.

FIG. 3 illustrates the mold half shown in FIG. 2 removed from the model and inverted.

FIG. 4 illustrates the method of preparing the mold half illustrated in FIG. 3 as a model for the production of the second mold half.

FIG. 5 illustrates the formation of the second mold half.

FIG. 6 illustrates the second mold half removed from the first mold half and inverted.

With respect to FIG. 1 the model 1 is mounted on a wooden baseboard 2 which has an angled edge 3 over which the metal coating is formed so that the metal will key to the base. Cooling pipes 4 are provided at the back of the model to enhance the setting of the sprayed metal coating. As is illustrated in FIG. 2 the model is first'spray coated with a layer of metal 5 to which are secured pipes 6 and thermocouples 7. The sprayed model is then surrounded by shuttering 8 and keying rods 9 bent to confonn to the shape of the model. Aluminous cement is then cast into the shuttering to provide a reinforcing layer 10 around the shell, when the cement has set the mold half may be removed from the model by trimming the metal away from the angled edge 3 of the baseboard to release the shell from the model and inverted as is shown in FIG. 3 where one may see the mold cavity 11.

The mold half illustrated in FIG. 3 is then used as the model for the production of the other mold half as is shown in FIG. 4. The surface 11 of the cavity insert is first coated with a layer of wax 12 to the thickness required for the mold cavity. A metal block 13 is then positioned on top of the wax in the position where the sprue is to be formed in the mold. The wax coated mold half is then sprayed with a layer of metal 14 which keys over the lip formed by the cement layer 10 to form the other mold half as is illustrated in FIG. 5; thermocouples and pipes are provided as is illustrated by 15 and 16. A metal block 17 is also provided behind the block 13, shuttering 19 placed around the edges of the sprayed metal shell and aluminous cement 18 cast around to back the shell while leaving the end of the metal block 17 exposed. The second mold half may then be removed from the first half as is illustrated in FIG. 6.

We claim:

1. A process for making a mold comprising forming a model of an article to be molded and providing said model with a protruding lip surrounding the part of the model which defines the article to be molded, forming a metal shell around said model and spraying metal over the protruding lip thereby keying the metal shell to the model, afiixing heating or cooling pipes on said metal shell and applying a reinforcing materials to provide a backing to said shell and to encase the pipes while leaving their ends exposed.

2. A process according to claim 1 in which the metal shell is at least one-sixteenth inch thick.

3. A process according to claim 1 in which the shell is provided with a reinforcing backing before it is removed from the model.

4. A process according to claim 1 in which the reinforcing material is applied in two stages, the first stage being a protective layer to render the metal shell self-supporting and the second stage to enable the mold to withstand force during molding wherein the shell is removed from said model after the first stage of the reinforcing material has been applied and before application of the second stage.

5. A process according to claim 1 in which the model is mounted on a base which has angled edges which provide the lip over which the metal shell keys.

6. A process according to claim 1 in which the model is coated with a release agent before formation of the metal shell.

7. A process according to claim 1 in which the metal shell is formed by electrode position.

8. A process according to claim 1 in which the metal shell is formed by metal spraying.

9. A process according to claim 8 in which the metal shell is produced by flame spraying and is entirely of zinc.

10. A process according to claim l-l in which the metal shell consists of a layer of zinc at least 0.050 inch backed by lowcarbon steel, copper or aluminum.

11. A process according to claim 1 in which the shell is formed by compressed air spraying of molten metal.

12. A process according to claim 11 in which the metal is an alloy of bismuth and tin.

13. A process according to claim 1 in which the metal shell is applied to a thickness of about one-fourth inch.

14. A process according to claim 1 in which the model is provided with heating or cooling pipes.

15. A process according to claim 1 in which at least the layer of reinforcing material adjacent to the metal shell is of aluminous cement.

16. A process according to claim 1 in which the reinforcing material is concrete.

17. A process according to claim 6 in which the metal shell is coated with impervious paint before it is backed with the reinforcing material.

* i i t

Claims

1. A process for making a mold comprising forming a model of an article to be molded and providing said model with a protruding lip surrounding the part of the model which defines the article to be molded, forming a metal shell around said model and spraying metal over the protruding lip thereby keying the metal shell to the model, affixing heating or cooling pipes on said metal shell and applying a reinforcing materials to provide a backing to said shell and to encase the pipes while leaving their ends exposed.

10. A process according to claim 1 in which the metal shell consists of a layer of zinc at least 0.050 inch backed by low-carbon steel, copper or aluminum.