Process for core removal from molded products

Abstract

A method of removing a core from a molded product in which the core is formed of a particulate inert material, such as sand-bound sand, bound together by a cured binder of a water soluble carbohydrate alone or mixed with a silicate is disclosed. The silicate is preferably an alkali earth metal silicate, preferably sodium silicate, and the carbohydrate is preferably a saccharide or starch. The binder is cured by heat. The core and molded product are exposed to water, preferably heated water in a bath or steam, to rapidly disintegrate the core and remove it from the molded product.

Description

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention is directed to a method for the removal of a molding core from a molded product after the product has been molded, and a cured core and product made thereby.

Over the years considerable attention has been directed to the development and expansion of lost core technology for the molding of products having complex shapes, undercuts or negative drafts or complex cavity configurations. In lost core technology such complex shapes and configurations which cannot typically be formed utilizing permanent, reusable molding forms are formed by using a shaped core or other mold form on a one time basis to form the portion of the product which is of complex configuration and is then removed from the complex portion of the molded product by disintegrating the core away from the molded product.

Various materials and procedures have been employed in the forming and removal of such cores, all of which have their disadvantages.

One such prior procedure involves the use of low melt metals, such as tin, bismuth or other low melt alloys. In this procedure the low melt metal is first shaped into the negative of the complex shape which is to be present in the finished molded product. This metal core is then positioned in the mold and the material from which the finished product is to be molded is pored or injected into the mold about the core. Once the material from which the product is to be made has solidified, the molded product together with the core are removed from the mold and are heated to melt the core away from the finished product.

This low melt metal procedure suffers a number of disadvantages. In the first instance the procedure can only typically be employed in the molding of materials which are of a higher melt temperature than the low melt metal core material. Thus, the procedure is not generally usable in the molding of plastic polymers which have a lower melt or decomposition temperature than the metal of the core. Another disadvantage is that the heat and pressures during molding tend to deform the core. Moreover, the core material is heavy and expensive, and it may be toxic. Therefore, the low melt metal is difficult to handle and process. The low melt metal procedures are also energy intensive requiring large amounts of heat in the melting process, and they frequently require high temperature oil baths which are both expensive and hazardous. The low melt metal procedures are also difficult to control during the core removal to prevent damage to the molded end product, and the low melt metals are hard to reclaim. Still another disadvantage is that the low melt metal procedures typically require relatively long periods of time for the removal of the core which may be upwards of 45 minutes or more.

Water soluble polymers, such as amorphous acrylic base copolymers, have also been employed as core materialsand be hardly soluble in water. Also they should not react with or be soluble in the material from which the desired molded product is formed, and should not adhere to any great extent to the product material.

A wide variety of sands may be used as the particulate inert material for the core, including most conventional foundry sands, such as 45-130 GFN silica, lake and bank sands. Chromite, zircon and olivine sands can also be used, as well as reclaimed sands. Examples of other particulate inert materials which may be employed as core materials include small steel shot or glass beads or bubbles, small polypropylene pellets, aluminas and clays. A combination of two or more of these materials may also be used.

The water soluble carbohydrate binder may include saccharides, such as dextrose and molasses, and starches such as corn starch. These carbohydrates may be utilized alone as the binder or they may be supplemented with a silicate. The silicate when used is preferably an inorganic alkali earth metal silicate, such as sodium silicate. A suitable silicate-carbohydrate binder for use in the practice of the method of the present invention may be for example Adcosil NF available from Ashland Chemical Inc. which contains approximately 10% dextrose of the total solids, has a SiO₂ /Na₂ O ratio of approximately 1.95, and a viscosity of approximately 2.1-2.8 Stokes at 25°C

The molding procedure of the present invention may include injection molding, but is not limited thereto.

The core may include reinforcements such as metal rods, wires or the like in order to strengthen the core. The core may also be coated, if desired, with known non-water based coatings to improve its surface smoothness.

The significant feature of the method of the present invention is that it has been discovered that cores formed of the aforementioned materials may be quickly and easily removed from the molded product simply by exposing the molded product and core to water or steam, preferably heated water. The water may be in the form of pressurized water or steam jets or a bath. If a water bath open to the atmosphere is employed, the temperature of the water is preferably less than 100°C to avoid the added expense of boiling and the attendant loss of water due to evaporation. It has been found that in the method of the present invention, simple immersion of the molded product in a water bath results in a very rapid, and in some cases almost instantaneous disintegration and removal of the core from the molded product without regard to whether the core has been exposed during the molding process to extreme high metal melt temperatures or low plastic melt temperatures, such as the melt temperature of polyethylene. Although it is believed that the foregoing description of the present invention together with the knowledge of those skilled in the art are sufficient to enable one in the art to form the core, complete the molding operation and remove the core, a brief description of those steps by way of example follows.

The particulate inert core material and the binder are first mixed in a container. The particulate material, preferably dry, is added to the container and the binder material in the amount of about 5 wt. % in a suitable inert carrier liquid such as water, is added to the particulate material. Where the particulate inert material is sand, after mixing the material in the container has the appearance of wet sand.

The next typical step is to form the molded core by placing a portion of the mixed core material in a core box having a configuration of the desired final complex configuration of the molded product. The core material is then cured in the core box in that configuration by using heat, preferably in the form of microwave radiation. Microwave radiation levels on the order of those of a regular kitchen microwave will cure the core in about 11/2 minutes, depending upon the size of the core.

Binders containing silicates have typically been cured in the past by passing carbon dioxide through the core material in the core box. It has been found in the present invention that the use of carbon dioxide to cure such silicate containing binders results in greatly reduced water solubility and greatly increased core removal times. Thus, carbon dioxide curing is not preferred in the present invention. Heat curing is the preferred method of curing. It not only results in excellent water solubility of the core binder, but permits reduction in the amount of the binder material to as little as about 2 wt. % of the particulate material without adverse result on the core strength.

The cured core C which is bound by the cured binder into the configuration in which it is to be inserted into the exterior mold 18 is positioned in the mold. The core C preferably has slightly larger portions than the overall size of the final molded product 10 to form "core prints" 22 to insure accurate positioning of the core C in the mold 18 as shown in FIG. 2.

Once the core C is positioned in the mold 18, the metal or plastic material of which it is desired to form the final molded product 10 is then injected into the remaining spaces in the mold 18 between the core C and the interior faces of the exterior mold 18 parts 19 and 20.

These injected product materials may include a wide range of plastics, including polyesters, nylons, polysulfones, polycarbonates, PTFE or phenolics. They may also include a wide range of metals including aluminum, bronze, brass, steel or iron. It will be understood that the aforementioned materials are not exhausitive of all of the materials which may be molded employing the method of the present invention.

The mold 18 is then cooled in order to solidify the molten molded product material into its final shape.

After the product material has solidified, the mold 18 parts 19 and 20 are separated and the molded product 10 and core C are removed from the mold 18. In accordance with the present invention, they are together exposed to pressurized jets of water or steam or plunged into a water bath. The water is preferably heated in order to accelerate the disintegration and removal of the core material from the molded product 10. It has been found that even cores C of relatively large size completely disintegrate and are removed from the molded product 10 within a matter of 15-30 seconds when immersed in a hot water bath.

The sand from the disintegrated core will settle to the bottom of the water bath and may be readily reclaimed. The silicate, if present, and the dissolved water soluble carbohydrate, if it is still present and has not been decomposed by the heat during molding, are removed from the bath by maintaining a flow of fresh water through the bath.

The following are examples of core materials and binders which may be employed in practicing the method of the present invention. They are given as exemplary only and are not to be considered as limiting the invention.

______________________________________
Particulate
Inert
Binder      Material  Comments
______________________________________
Sodium silicate/
Sand      Good strength. Good water
Dextrose1        solubility with core disinte-
grating in less than 1 minute.
Pure Corn Starch2,
Sand      Good strength. Even better
Sodium silicate/      water solubility than just
Dextrose1        sodium silicate/dextrose.
Molasses3
Sand      Fair but acceptable strength.
About same water solubility as
corn-starch - sodium silicate/
dextrose binder.
Sodium silicate/
Glass     Good strength. Good water
Dextrose1
Beads     solubility with core disinte-
grating in less than 1 minute.
Sodium silicate/
Steel     Good strength. Good water
Dextrose1
Shot      solubility.
______________________________________
1 Adcosil NF, Ashland Chemical Inc.
2 Argo, CPC International Inc.
3 Grandma's (unsulfured), Motts USA.

In each of the above examples, the amount of binder used (together with its water carrier) was about 5 wt. % of the particulate inert material. Where both sodium silicate/dextrose and an additional carbohydrate binder were employed, they were a 50--50 mixture of each of a total of 5 wt. %.

In each of the above examples, the core disintegrated extremely rapidly--on the order of less than one minute. It has been found that where the cores are exposed to high molten metal temperatures, for example in the molding of aluminum, the core remains water soluble, but the time needed for disintegration is somewhat longer.

Although the core C as shown in the drawings is shown for the formation of an interior cavity in the molded product, it will be appreciated that the term "core" as employed herein is not limited to only the formation of interior surfaces or cavities. The method of the present invention also contemplates the use of molded cores and removal from the molded product for the formation of complex undercut exterior surfaces with equal facility.

It will also be understood that the preferred embodiment of the present invention which has been described is merely illustrative of the principles of the present invention. Numerous modifications may be made by those skilled in the art without departing from the true spirit and scope of the invention.

Claims

1. A method of removing a molding core from a molded product wherein said core comprises a particulate inert material which is formed into a discrete configuration conforming to the configuration of at least a portion of the molded product and the particulate inert material is bound in said configuration by a heat cured binder comprising a silicate salt and a water soluble carbohydrate, said method comprising exposing said bound core with said water soluble carbohydrate binder therein and said molded product to water after the product has been molded to disintegrate and remove the core from the molded product.

2. The method of claim 1, wherein said

binder also includes a silicate.3. The method of claim 2 1, wherein said silicate salt is an alkali

earth metal silicate. 4. The method of claim 3, wherein said silicate

salt is sodium silicate. 5. The method of claim 1, wherein said water soluble carbohydrate is selected from the group consisting of

saccharides and starches. 6. The method of claim 5, wherein said saccharide is selected from the group consisting of dextrose and molasses.

. The method of claim 5, wherein said starch is corn starch.

8. The method of claim 2, wherein said water soluble carbohydrate is selected

from the group consisting of saccharides and starches.9. The method of claim 1, wherein the particulate inert material is selected from the group consisting of sand, metal shot, plastic polymers, glass, alumina, clays

and mixtures thereof.

10. The method of claim 2, wherein the particulate inert material is selected from the group consisting of sand, metal shot, plastic polymers, glass, alumina, clays and mixtures

thereof.11. The method of claim 1, wherein said water is heated. 12. The method of claim 11, wherein the heated water has a temperature of

less than about 100°C 13. The method of claim 1, wherein said

water is steam. 14. The method of claim 1, wherein the core and molded product are immersed in a bath of said water to disintegrate and remove

the core. 15. The method of claim 1, wherein said molded product is formed of a material selected from the group consisting of plastics and metals. PAR

16. The method of claim 2, wherein said molded product is formed of a

material selected from the group consisting of plastics and metals.17. The method of claim 16 15, wherein said silicate salt is sodium silicate, said water soluble carbohydrate is selected from the group consisting of saccharides and starches, and wherein the core and molded product are immersed in a bath of heated water

to disintegrate and remove the core. 18. The method of claim 17, wherein the particulate inert material is selected from the group consisting of sand, metal shot, plastic polymers, glass, alumina, clays and mixtures thereof.

19. The method of claim 1, wherein said binder is cured by

heating.20. The method of claim 19 1, wherein said heating binder is heat cured by microwave energy.

21. The method of claim 2, wherein said binder is

cured by heating.

22. The method of claim 21, wherein said heating

is by microwave energy.

23. The method of claim 5, wherein said

binder is cure by heating.24. The method of claim 23 5, wherein said heating binder is heat cured by microwave energy.

25. The method of claim

9, wherein said binder is cured by heating.26. The method of claim 25 9, wherein said heating binder is heat cured by microwave energy.

27. The

method of claim 11, wherein said binder is cured by heating.28. The method of claim 27 11, wherein said heating binder is heat cured by microwave energy.

29. The method of claim 18, wherein said binder is cured by

heating. 0. The method of claim 29 18, wherein said heating binder is heat cured by microwave energy.