Binder composition and method of forming foundry sand cores and molds

Abstract

A cost-effective starch/water-based binder composition and related method for forming silica sand cores and molds for foundries, wherein the sand grains are pre-coated with starch having additives making the coated sand effective for blowing said cores and molds. One or more additives are included with the starch; preferably sodium tripolyphosphate and silicon or Silres BS16. The preferred starch is a tapioca starch. The binder of the invention is highly competitive due to its low cost and effectiveness for forming silica-sand cores and molds, being particularly effective for use in aluminum foundries for the automotive industry.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a process block diagram illustrating the main steps of a method illustrating one preferred embodiment of the invention for the manufacture of sand molds and cores utilizing an artificially-modified tapioca type starch.

FIG. 2 shows a second preferred embodiment; being a process, similar to that in FIG. 1, modified to utilize native tapioca type starch.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention will be better understood with reference to the following detailed description of certain specific embodiments thereof.

Silica sand cores are made from sand grains bound by a suitable binder which provides a strong bond between such sand grains in such a manner and with a sufficiency to withstand the handling of the cores and the assembled molds without losing the precise dimensions and form needed for impressing the desired geometry and surface quality to the casting under the temperature and pressure of the molten metal used to form said casting.

Currently, sand cores are shaped by mixing sand and a binder, blowing the binder-sand mixture into a mold having the desired shape, and curing or hardening the binder in the mold so that the mold geometry is fixed in the cores after their removal from said mold.

There are a variety of synthetic resins used as binders as is well known in the art. According to the present invention, a water-based binder and a method for forming the sand cores provide a number of advantages over the current state-of-the-art binders regarding manufacturing costs and core qualities.

The binder of the invention is mainly composed of starch. Though starch has been utilized as an additive to prior-art binders, mostly in proportions of less than 20% of the binder composition, to the best of applicants' knowledge it has not previously been effectively used as the main constituent of a sand core-making binder in mass production foundries. According to a preferred embodiment of the present invention, two additives are added to the starch, in the proportions and in the form explained below, which in combination with the starch, provide the desired qualities to the formed cores. These additives are sodium tripolyphosphate and silicon or Silres BS 16. More broadly, the separate functions of the two additives with certain starches may be found in only one single additive.

There are several types of starch suitable to be utilized as the basis of the binder composition of the invention. One difference among them is the amount of starch needed to achieve the same mechanical strength. For example potato requires more than 2% weight (based on sand) and for maize close to 2% weight. However, when using Tapioca starch, the core needs significantly less starch content to achieve the same strength.

Tapioca starch is thus preferably used but other varieties of starch can also be utilized according to broader aspects of the invention. There are two types of tapioca starch: (a) native (i.e. unmodified) starch and (b) artificially-modified starch. Both types have proven effective as binders for the particular application of core making. The method of the invention may have some differences depending on which type of tapioca starch is utilized.

The core-making method of this embodiment of the invention will be described first as applied to the utilization of artificially-modified tapioca starch. With reference to FIG. 1 showing a diagram of the method steps for forming such cores, silica sand 10 is mixed with the artificially-modified starch 12 in a proportion of from about 0.5% to 3.0% by weight on the basis of the sand weight, in a paddle mixer 14 for a period of time from about 30 seconds to about 120 seconds. This period of time must be sufficient for achieving a good dispersion of the starch over the surface of the sand particles. After this first mixing step, water 16 is slowly added to the sand-starch mixture preferably in a proportion from about 5% to 10% by weight on the basis of the sand weight and the sand-starch-water mixture is mixed in said mixer 14 for at least 1 minute, producing starch-coated sand grains 18 which are then allowed to dry naturally (indicated by dotted line 19) or optionally in a suitable drying furnace 20 for accelerating the drying process and thereby increasing the process productivity. The furnace 20 can be of the type having forced air 17. The dry starch-coated sand 18 is then treated in a suitable mill 22 (for example, a ball mill or a vibrating shaker) for destroying the sand lumps which may have been formed during the previous step. Thereafter, the milled coated sand 23 is screened on a screen 24, for example utilizing a sieve # 30 for assuring that all the sand grains have a homogeneous particle size. Sand lumps 25, separated from the coated loose sand grains, are recycled to mill 22. After screening, the starch-coated sand 23 undergoes a second mixing step in a mixer 28 where water 35 is added in a proportion preferably of at least 2% by weight on the basis of the sand weight, also sodium thipolyphosphate 32 is added in a proportion of from 0.1% to about 0.2% by weight on the basis of the sand weight, and silicon 34 from about 0.1% to 0.2% or Silres BS 16 from about 0.02% to 0.08%, resulting in sand ready for core blowing in blower 36 where the final core 38 is formed.

Referring now to FIG. 2, wherein same numerals designate the same elements, the method therein described is the method followed when native-type tapioca starch is used for binding the sand grains in cores and molds. Silica sand 10 is pre-heated to a temperature in the range from about 110° C. to about 130° C. in a furnace 13. The preheated sand is then transferred to the mixer 14 to follow the rest of the method in the same manner as above-described for the artificially modified starch.

Water 16 is also preferably heated to a temperature of about 70° C. in order to preserve as much as possible the temperature of the sand above about 70° C. The amount of water 16 added at this mixing step should be sufficient to reach a humidity level in the range from about 2% to about 4% of the humid sand weight.

The rest of the method illustrated in FIG. 2 is the same as in FIG. 1.

It is of course to be understood that in the above specification, only certain specific embodiments have been included for purposes of illustrating the principles of the invention and that the invention is not intended to be limited thereto. It will also be evident that numerous changes may be made to the embodiments herein described without departing from the spirit and scope of the invention which is limited only to the extent set forth in the appended claims.

Claims

1. A method of forming silica sand cores and molds for foundries utilizing a starch-water-based binder comprising mixing silica sand grains with starch in a suitable mixer, adding water to said sand and starch mixture and continue mixing said sand, starch and water so that the sand grains are coated with starch; drying the starch-coated sand grains and treating them in a mill for destroying lumps formed during the starch-coating step; adding water to said coated sand grains and screening said sand grains for homogenizing and loosening said grains; adding at least one additive selected from the group consisting of sodium tripolyphosphate, silicon and a concentrated water solution of 1-5% potassium hydroxide plus 40-70% potassium methyl siliconate; and blowing said cores and molds utilizing said starch-coated sand grains.

2. The method according to claim 1, wherein said silica sand is mixed with starch in a proportion of from about 0.5% to 2% by weight on the basis of the sand weight.

3. The method according to claim 1, wherein said sand grains and starch are mixed in a paddle mixer for a period of time from about 30 seconds to about 120 seconds.

4. The method according to claim 1, wherein water is added to the sand-starch mixture in a proportion from about 5% to 10% by weight on the basis of the sand weight.

5. The method of forming silica sand cores and molds according to claim 1, wherein said starch-coated sand grains are allowed to dry naturally.

6. The method of forming silica sand cores and molds according to claim 5, wherein said starch-coated sand grains are dried in a drying furnace for increasing productivity.

7. The method of forming silica sand cores and molds according to claim 1, wherein the dry starch-coated sand is treated in a ball mill or a vibratory shaker.

8. The method of forming silica sand cores and molds according to claim 1, further comprising screening the starch-coated sand and thereafter mixing said starch-coated sand with water in a proportion of at least 2% by weight on the basis of the sand weight.

9. The method of forming silica sand cores and molds according to claim 1, wherein said additive is sodium tripolyphosphate, which is added in a proportion of from about 0.1% to about 0.2% by weight on the basis of the sand weight; and a further additive is silicon being also added in the same proportion.

10. The method of forming silica sand cores and molds according to claim 1, wherein said additive is sodium tripolyphosphate, which is added in a proportion of from about 0.1% to about 0.2% by weight on the basis of the sand weight; and a further additive is iron oxide is also added in the same proportion.

11. The method of forming silica sand cores and molds according to claim 1, further comprising heating said sand to a temperature in the range from about 110° C. to about 130° C. prior to mixing said sand with water and starch.

12. The method of forming silica sand cores and molds according to claim 1, wherein said starch is artificially modified tapioca starch.

13. The method of forming silica sand cores and molds according to claim 1, wherein said starch is native tapioca starch.

14. The method of forming silica sand cores and molds according to claim 11, wherein said starch is native tapioca starch.

15. A starch-water-based binder composition suitable for forming silica sand cores and molds for foundries, comprising more than about 80% starch; sodium tripolyphosphate and silicon.

16. A starch-water-based binder composition suitable for forming silica sand cores and molds for foundries, comprising from about 1.0% to about 1.5% by weight of starch; from about 0.1% to about 0.2% by weight of sodium tripolyphosphate; and from about 0.1% to about 0.2% by weight of silicon; all on the basis of the sand weight.

17. A starch-water-based binder composition suitable for forming silica sand cores and molds for foundries, comprising more than about 70% of starch; from about 0.1% to about 0.2% by weight of sodium tripolyphosphate; and from about 0.1% to about 0.2% by weight of Silres-BS16; all on the basis of the sand weight.

18. A starch-water-based binder composition according to claim 15, suitable for forming silica sand cores and molds for foundries, wherein said starch is tapioca starch.

19. A starch-water-based binder composition according to claim 15, suitable for forming silica sand cores and molds for foundries, wherein said starch is artificially modified tapioca starch.

20. A starch-water-based binder composition according to claim 15, suitable for forming silica sand cores and molds for foundries, wherein said starch is native-tapioca starch.