Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
2000-05-23
2002-05-14
Cain, Edward J. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C523S142000
Reexamination Certificate
active
06387983
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to foundry mixes and their use in preparing foundry shapes (molds and cores) and in casting metals. The foundry mixes comprise a foundry aggregate and an organic foundry binder that contains an organic polyisocyanate and an aliphatic mercaptan. The binder systems react to form thiocarbamate polymers when mixed with sand. Preferably, a tertiary amine curing catalyst is used to promote the reaction of the organic polyisocyanate and aliphatic mercaptan.
BACKGROUND OF THE INVENTION
In the foundry industry, one of the procedures used for making metal parts is “sand casting”. In sand casting, disposable molds and cores are fabricated with a mixture of sand and an organic or inorganic binder. The foundry shapes are arranged in casting assembly, which results in a cavity through which molten metal will be poured. After the molten metal is poured into the assembly of molds and cores and cools, the metal part formed by the process is removed from the assembly. The binder prevents the molds and cores from disintegrating too rapidly when the hot molten metal is poured over them during the casting process.
Two of the prominent fabrication processes used in sand casting are the no-bake and the cold-box processes. In the no-bake process, a liquid curing catalyst is mixed with an aggregate and binder to form a foundry mix before shaping the mixture in a pattern. The foundry mix is shaped into a mold or core by placing it in a pattern, where it is cured until it is self-supporting and can be handled. In the cold-box process, a gaseous curing catalyst is passed through a shaped mixture (usually in a corebox) of the aggregate and binder to cure the mixture.
A binder commonly used in the cold-box fabrication process is a phenolic-urethane binder. The phenolic-urethane binder is mixed with an aggregate to form a foundry mix. The foundry mix is blown into pattern, typically a corebox, where it is cured by passing a gaseous tertiary amine catalyst through it. The phenolic-urethane binder consists of a phenolic resin component and polyisocyanate component. Phenolic-urethane binders are widely used in the foundry industry to bond the sand cores used in casting iron and aluminum. An example of a commonly used phenolic-urethane binder used in the cold-box process is disclosed in U.S. Pat. No. 3,409,575, which is hereby incorporated by reference. More recently amine curable cold-box binders based on acrylic-epoxy-isocyanate were developed, such as those shown in U.S. Pat. No. 5,880,175, which is hereby incorporated by reference. These cold-box processes are high production processes with short cycle times, i.e. typically less than one second.
After the metal has been cast, it is necessary to separate the sand mold and cores from the casting. One of the problems with using organic binders to form foundry shapes is that they can be too effective in binding the aggregate together. The result is that the foundry shapes are not readily separated from the metal part formed during the casting process. Consequently, time consuming and labor intensive means must be utilized to break down the binder so the metal part can be removed from the casting assembly. This is particularly a problem with internal cores, which are imbedded in the casting assembly and not easily removed.
Iron castings are made around 1500° C. Since a phenolic resin core binder undergoes thermal decomposition at this temperature, an internal core will decompose and can be readily removed from the casting. On the other hand, aluminum is cast at around 700° C. At this lower temperature, a phenolic resin core binder does not readily decompose, making complete removal of the internal core laborious. Since aluminum castings, as well as other light alloy castings, are used increasingly in place of iron as a means of reducing the weight of vehicle components, such as engine blocks and manifolds, there is a need to develop new methods of which facilitate the removal of internal cores.
One method of facilitating the removal of sand, used to make an internal core, from a large aluminum casting (e.g. an engine block) or a complex aluminum casting (e.g. a water pump housing), is by baking the casting in a forced air oven at a high temperature, typically for four to ten hours. Exposing the casting to the high temperature causes the binder to slowly decompose. This process is called “thermal core removal”. This procedure reduces productivity and requires forced air ovens and large amounts of energy. Alternatively, some aluminum castings can be violently shaken until the internal core is released (mechanical core removal or “shakeout”). This procedure is inefficient and reduces productivity.
Another approach is shown in U.S. Pat. No. 4,293,480. This patent discloses a binder containing a modified isocyanate component that promotes better shakeout. On the other hand, U.S. Pat. No. 4,352,914 discloses a polyurethane binder with improved shakeout where the resin is modified.
It is also known to add simple compounds to phenolic urethane binders that will improve shakeout without affecting the stability of the binder. For instance, it is known to add polyester polyols ((U.S. Pat. No. 4,982,781) and polyether polyols (U.S. Pat. No. 5,132,339) to improve shakeout. The English translation of the abstract of Japanese Patent 57 050585 discloses the use of organosulfonic acids, as well as carboxylic acids, in amine-cured phenolic-urethane binders to improved shakeout, while the English translated abstract of Russian application SU 79-2844061 indicates that sodium salts of certain organosulfonic acids can improve the “knock-out” of foundry shapes made from certain inorganic binders.
Because of problems associated with the shakeout of foundry shapes made with phenolic urethane, other cold-box binder systems are often used for casting aluminum that provide good core removal. For example, furan cold-box resins display excellent core removal characteristics in aluminum casting. However, furan resin binders build a tar like residue on tooling. This requires frequent cleaning, higher tooling costs, and lowers foundry productivity. Thus, there is no “perfect” binder that provides the high productivity of a cold box binder system (e.g. instantaneous cure with a gas catalyst at room temperature) and facile thermal decomposition for rapid internal core removal when casting aluminum.
Binders for foundry aggregates based on organic polyisocyanates and thiophenols, cured with a volatile amine curing catalyst, are disclosed in U.S. Pat. No. 3,925,296 (Stone et al.). The advantage of the binder described in Stone is that the binder cures at low temperatures, or even room temperature. See column 1, lines 10-13 of the Stone patent. Stone does not disclose anything about improvements in casting and core removal. Aliphatic mercaptans are not disclosed in Stone and this patent indicates that an inert solvent is preferred or required.
SUMMARY OF THE INVENTION
The invention relates to a foundry mix comprising:
(1) a major amount of a foundry aggregate; and
(2) a minor amount of a foundry binder comprising:
(a) an organic polyisocyanate; and
(b) an effective amount of an aliphatic mercaptan,
where (a) and (b) are applied to a foundry aggregate as separate components or as a mixture.
Particularly preferred as (b) are polymercaptans. When the aggregate and binder are mixed, a thiocarbamate polymer is formed that holds the sand together in its shape. Preferably, a catalyst is used to promote the reaction of the organic polyisocyanate and aliphatic mercaptan. The preferred catalyst is a tertiary amine catalyst.
The invention also relates to a method of preparing a foundry shape, the shapes prepared, a method of making a metal casting, and metal castings prepared by this process. The foundry binders are particularly useful for preparing foundry shapes by the cold-box process using a volatile amine curing catalyst.
There are several advantages of using component (b) in the binder compositions, including the following:
1. When used in making foundry shap
Ashland Inc.
Cain Edward J.
Hedden David L.
Wyrozebski-Lee Katarzyna
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