Abstract:
For the manufacture of quickly drying dip coats whose drying time is comparable to the drying time of shells produced on the basis of alcoholic binders, the process uses as a binder for the dip a dispersion of aqueous colloidal silica sol to which synthetic high polymers and/or an emulsion from the group of silicon-organic compounds is formed which after drying is impermeable to water but permeable to water vapor.

Description:
FIELD OF THE INVENTION 
     The invention relates to a process for the manufacture of ceramic shells for use as molds in which 
     a) a fusible or removable pattern of a part to be cast is produced, 
     b) the pattern is dipped in a slurry of refractory material and a binder which contains a colloidal, aqueous silica sol and polymers to form a wet coating the pattern, 
     c) a coarse, refractory powder is sprinkled onto the coat, 
     d) the coat is dried, 
     e) the steps b), c) and d) are repeated until the shell has obtained the desired thickness. 
     The invention also relates to a binder in the above process. 
     BACKGROUND OF THE INVENTION 
     In such a process, pattern clusters of wax or a similar material are provided with a solid ceramic layer with a thickness of a few millimeters by applying several dip coats. The individual layers, after having been applied, are dried or hardened separately. On each individual wet dip coat a coarse, refractory powder or sand is scattered as a binding agent for the following dip coat. Following dewaxing the shells are fired and can then be used for casting whilst still hot or after cooling down. 
     Processes of the type described above which use aqueous binders are limited in their use in that, contrary to alcoholic binders, aqueous binders require a very extended drying process and, ultimately, had to be subjected to suitable vacuum drying to obtain complete dryness. It has been tried to overcome these problems by mixing alcohol, e.g. isopropanol, into the aqueous binder. The alcohol replaced part of the water and had the characteristic feature to evaporate quicker than water. However, the water alcohol mixture was unable to prevent, during the subsequent dipping operations, the water/alcohol mixture from penetrating again into the previously applied and already dried coat, so that with progressive dipping ever increasing drying times were required. Other methods were also tried to solve the pertinent problems. 
     From the U.S. Pat. No. 3,005,244 it is known to add to a 30% aqueous silica sol binder with a quantity of resinous polymers so that the solid content of the resulting binder consists of 60 to 80% by weight of SiO 2  and 8 to 35% by weight of resinous polymer. Ceramic shells which are produced with this binder have, among other things due to the addition of the polymers, an increased green strength of the shell which is said to permit fusion of the wax from the shell without the intermediate use of an autoclave. 
     The U.S. Pat. No. 3,126,597 describes a ceramic shell with an aqueous binder based on silicic acid (30% by weight), to which water is added for dilution and to which sodium fluoride is added for accelerating the sol/gel reaction. The sodium fluoride leads merely to a quicker hardening of the individual ceramic layers applied but does not prevent penetration of the aqueous binder into the previously applied ceramic layers during the subsequent dipping operations. This binder combination requires long drying times and, in the case of shells with a higher number of layers, ultimately vacuum drying. 
     The U.S. Pat. No. 3,165,799 describes a triple dipping process for the manufacture of a ceramic shell for multiple-core investment castings in which the first ceramic dip layer is applied with an aqueous binder with subsequent sanding. Following this, a second ceramic dip layer with aqueous binder, to which 0.5 to 2% by weight of polyvinylalcohol has been added, is applied without sanding or stuccoing. Thereafter, a third ceramic dip layer with aqueous SiO 2  binder with 0.5 to 2% by weight of polyvinyl alcohol is applied under vacuum with finish sanding or stuccoing. The aforementioned dipping sequence has the advantage of permitting the manufacture of investment castings with core parts which can otherwise only be manufactured with ceramic cores. 
     From the U.S. Pat. No. 3,752,689 the manufacture of a ceramic shell by a rapid process with an aqueous binder with 26.4% by weight of SiO 2  and 4.2% by weight of Al 2  0 3  is known, with the aforementioned binder being converted from the sol state into the gel state by respectively organic and inorganic bases. This process permits applying the individual ceramic layers relatively quickly but has the disadvantage that each ceramic layer is infiltrated by the aqueous binder subsequently applied layer, and that lengthy final drying, including vacuum drying, has to be performed. 
     The idea is to discharge the acid SiO 2  /Al 2  O 3  sol which is positively charged, by negatively charged particles of organic and inorganic bases. 
     The U.S. Pat. No. 3,859,153 also describes a rapid process for the manufacture of a ceramic shell in which an aqueous binder with negatively charged colloidal particles is discharged, and thus hardened, by an aqueous sol with positively charged colloidal particles. This process, too, also permits quickly applying the individual ceramic layers but has the disadvantage that the discharge of the negatively charged sol by the positively charged sol applied may already take place in the immersion tank. Another disadvantage of this process is that the shell manufactured by it has to be subjected to a respectively long final drying process. Still another disadvantage is that as a result of the repeated infiltration during dipping with an aqueous binder, the shells become so heavy that there is the risk of breakage. Subsequent vacuum drying is indispensable for economic reasons. 
     The U.S. Pat. No. 3,894,572 describes a manufacturing process for a ceramic shell with a positively charged aqueous binder which is hardened by the sanding material which contains negatively charged colloidal particles. The disadvantage are the same as those described for the aforementioned two rapid processes. 
     The U.S. Pat. No. 3,933,190 describes a process for the manufacture of a pure Al 2  O 3  shell for directional solidification. The ceramic shell is prepared with 15 to 25 parts of an aqueous solution which contains 15% by weight of aluminum polyoxychlorid, 6 to 10 parts of water and 8 to 14 parts of latex. To this fluid Al 2  O 3  powder is added as a filler. For pH control of the binder, 0.5 to 1.5 parts of a 2% aqueous HLC solution are added. In this case the function of the latex added is to increase the green strength of the ceramic layers applied. All disadvantages described for the rapid process are relevant in the case of this shell, too. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     It is the aim of the invention to propose a process and binders of the type described above by which quickly drying shells can be manufactured with the drying time being comparable with the drying time of shells which are manufactured on the basis of alcoholic binders. 
     This aim is solved in that a Back-up-Dip and the binder contain a dispersion of an aqueous colloidal silica sol to which are added synthetic high polymers and/or an emulsion from the group of silicon organic compounds. This causes a coat or film to be formed which after drying is impermeable to water but permeable to water vapour. 
     When high polymers and in particular high polymers with hydrophobizing and/or hydrophobic character are used, a film is formed within the dip layer, also called back-up layer, which although impermeable to water is permeable to water vapour and permits thus finish drying or secondary drying of the ceramic layers which form the shell. When a new dip layer is applied the binder of the subsequently applied wet dip layer cannot penetrate into the already dried or partially dried and sanded dip layer. Drying of the still humid drip layers can be performed in the usual way, if necessary with the assistance of fans. The green strength of the ceramic shells manufactured by this process is just as satisfactory as the strength after firing or burning. Shell defects by fissures of thermal cracks in the castings are not noted. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     By properly proportioning the synthetic high polymers, and/or an emulsion from the group of the silicon organic compounds with respect to the aqueous silica sol, and also by a variation of the refractory material used as a filler, the Back-up Dip can be adapted to the respective conditions or requirements. In this connection, one proposal of the invention suggests for the binder to contain 2 to 20% by volume, in particular 14 to 18% by volume, of synthetic high polymers. Another proposal suggests that the binder contains 2 to 20% volume, in particular 3 to 7% by volume, of a synthetic high polymer to which are added 0.2 to 20% by weight, referred to the weight of the synthetic polymer, of an emulsion from the group of the silicon-organic compounds. 
     It has also been found to be very convenient to use an emulsion of the group of the silicon-organic compounds with droplet sizes &lt;200 nm. In addition, the aqueous colloidal silica sol can contain 0 to 20% by weight of the total water content of the binder, in particular 0.005 to 15% by weight of an emulsion of the group from the silicon-organic compounds. 
     It is also possible to add a surfactant or surface tension-reducing agent to the binder, in particular in a quantity of 0.08% by volume, referred to the volume of the dip-coat mass. The surfactant assists the uniform distribution of colloidal silica sol, high polymer and/or an emulsion from the group of silicon-organic compounds, and improves the wettability of the dip-coat mass without affecting the silica sol, the high polymers and/or the emulsion from the group of the silicon-organic compounds. 
     To prevent segregation of the colloidal silica sol and the high polymers in the case of the long storage times it may be of an advantage to prepare the dispersion of aqueous silica sol and synthetic high polymers and to process it into the dip-coat mass only shortly before it is used. 
     A high polymer which possesses the desired properties has the following characteristic data: 
     
         ______________________________________Solids          50 +/- 1% by weightViscosity at 20° C.           800 to 1500 mPa · s(D = 57.sup.-1)pH-value        7.0 to 8.5Density         approx. 1.02 g/ccmFilm condition  viscoplastic, waterproof,           alkaliproof or alkali-resistant______________________________________ 
    
     An emulsion of the above mentioned silicon-organic compounds which possesses the desired properties has the following characteristic data: 
     
         ______________________________________Quantity of solids 52 to 57% by weightViscosity          25 mPa · sDensity            approx. 1 g/ccmSolvent            waterpH-value           7 to 9______________________________________ 
    
     In the following, the invention is described on the basis of application examples: 
     Characteristic data of the process according to the invention: 
     
         ______________________________________Binder           approx. 30% of colloidal            silica sol or other aqueous            colloidal silica solsDeluting agent:  deionised water, if requiredHigh polymer as film form-            e.g. aqueous high-polymer dis-ing agent:       persion with hydrophobic making            character and/or hydrophobic            character and/or emulsion from            the group of silicon-organic            compoundsFiller:          Filler is the ceramic refractory            material which is mixed into the            colloidal silica-sol binder with            the addition of high polymers            and/or an emulsion from the            group of silicon-organic            compounds, in order prepare            the dip. It consists of            approx. 45% by weight of fused            silica - 12 mesh to &lt;350 mesh,            approx. 45% by weight and            approx. 10% by weight of zircon            silica 350 meshStuccoing material for the            zircon-silica sand1st and 2nd layer:Stuccoing material for the            aluminum silica3rd layer:3rd layer = 1st back-up layerStuccoing material for the            mixture of 50% mullite and4th and all following layers:            50% aluminum silicate            grain size 0.3 to 1.0 mm______________________________________ 
    
    
    
     1st EXAMPLE 
     In a mixing vessel of 600 mm diameter and 650 mm height the following back-up dip was prepared: 
     
         ______________________________________1.  Binder              41.5% by weight = 94 kg2.  Filler              58.5% by weight = 132 kg3.  Total quantity      141 1 = 226 kg    Back-up dip4.  The 94 kg of binder, corre-    sponding to 82 1, consist of:    56 1 of aqueous colloidal    30% silica sol    13 1 of deionished water    13 1 of high polymer5.  The 132 kg of filler consists of:    59.5 kg of fused silica powder                   &lt;350 mesh    59.5 kg of mullite powder                    150 mesh    13 kg of zircon powder                    350 mesh______________________________________ 
    
     The resulting Zahncup No. 2 viscosity of the back-up dip was 22 sec. By increasing the filler quantity to 63% by weight the Zahncup No. 2 viscosity was increased to 37 sec. and was thus within the range of the rated viscosity of 33 to 41 sec. 
     Wax clusters from production and 2 clusters of bending specimens were provided, as usual, with the 1st and 2nd dip coat and then immersed into the back-up of the above described composition as follows: 
     1. Immersion of the 1st back-up layer into the back-up dip and stuccoing by hand with aluminum silicate, grain size 0.3 to 0.5 mm. 
     2. Drying of the back-up dip-coat applied 2.5 to 3 hours. 
     3. Immersion for the 2nd back-up layer into the back-up dip and stuccoing by hand with 50% aluminum silicate and 50% mullite, grain size 0.3 to 1 mm. 
     4. Drying same as under 2. 
     5. 3rd, 4th and 5th back-up layer same as under 3. and 4. This means that the shells were completed in a 2-shift operation. 
     The relative humidity was 50±5% and the ambient temperature 23°±1° C. Drying was performed partially with fan assistance. 
     The resulting multiple-core castings were of very good quality. The following average bending strength values were measured on ceramic flat specimens: 
     
         ______________________________________Condition of the ceramicflat specimens         σ BB:______________________________________Green                  349 N/cm.sup.2Fired                  309 N/cm.sup.2Fired and redipped     400 N/cm.sup.2______________________________________ 
    
     2nd EXAMPLE 
     In a production mixing vessel the following back-up dip was prepared: 
     
         ______________________________________1.  Binder              37% by weight = 423 kg2.  Filler              63% by weight = 720 kg3.  Total quantity of the                   630 1 = 1143 kg    back-up4.  The 423 kg of binder, corre-    sponding to 367 1, consist of    251 1 of aqueous colloidal    30% silica sol    58 1 of deionished water    58 1 of high polymer5.  The 720 kg of filler consist of:    327.5 kg of fused silica powder                   &lt;350 mesh    327.5 kg of mullite powder                    150 mesh    65 kg of fused silica                    350 mesh______________________________________ 
    
     The pH-valve of the finished back-up ranged between 9 and 10 units. The actual viscosity was within the range of the rated viscosity, i.e. between 33 and 21 sc. according to Zahncup 2. The theoretical quantity of solids of colloidal SiO 2  in the binder was calculated to be 21.5%. 
     The theoretical quantity of solids of high polymers in the binder was calculated to be approx. 7%. The total quantity of solids of colloidal SiO 2  and high polymer in the binder was approx. 28.5 %. 
     Relative humidity inside the dipping-room 50±5%. Temperature inside the dipping-room 23°±1° C. 
     Drying time between individual back-up layers 2.5 to 3 hours. 
     After the application of the 1st and 2nd dip the following dipping cycle was adhered to by means of a robot: 
     1. Immersion for the 1st back-up layer into the back up dip and stuccoing by means of a robot with aluminum silicate, grain size 0.3 to 0.5 mm. 
     2. Drying of the back-up layer applied 2.5 to 3 hours. 
     3. Immersion for the 2nd back-up layer into the back-up dip and stuccoing by means of a robot with 50% of aluminum silicate and 50 % of mullite, grain size 0.3 to 1 mm. 
     4. Drying same as under 2. 
     5. 3rd, 4th and 5th back-up layer same as under 3. and 4. 
     The respective shells were produced in a 2-shift operation. A total of 1.334 clusters were produced. 
     The following average bending strength values (σ BB).were measured on ceramic flat specimens: 
     
         ______________________________________Condition of the ceramic flat specimens                  σ BB______________________________________Green                  450 to 660 N/cm.sub.2Fired                  480 to 660 N/cm.sub.2Fired                  590 to 870 N/cm.sub.2______________________________________ 
    
     3rd EXAMPLE 
     In a production mixing vessel the following back-up dip was prepared: 
     
         ______________________________________1.  Binder              36% by weight = 434.8 kg2.  Filler              64% by weight = 737.7 kg3.  Total quantity of the                   636.7 1 = 1172.5 kg    back-up dip4.  The 360.2 1 of binder consist of:    247 1 of aqueous colloidal    30% silica sol    57.2 1 of deionished water    56 1 of high polymer5.  The 737.7 kg of filler consist of:    335.3 kg of fused silica powder                   &lt;120 mesh    335.3 kg of mullite powder                    130 mesh    67.1 kg of zircon silicate                    350 mesh______________________________________ 
    
     The pH-Value of the finished back-up dip ranged between 9 and 10  units. The actual viscosity was within the range of the rated viscosity which, for reasons of production, had been changed in the meantime, i.e. between 28 and 36 sec. according to Zahncup 2. The total quantity of solids of colloidal SiO and high polymer in the binder was approx. 28.25 %. 
     Relative air humidity inside the dipping room 50±5%. Temperature inside the dipping room 23°±1° C. Drying time between individual back-up layers=2.5 to 3 hours. 
     After the application of the 1st and 2nd dip the following dipping cycle was adhered by means of a robot: 
     1. Immersion for the 1st back-up layer into the back-up dip and stuccoing by means of a robot with aluminum silicate, grain size 0.3 to 0.5 mm. 
     2. Drying of the back-up layer applied 2.5 to 3 hours. 
     3. Immersion for the 2nd back-up layer into the back-up dip and stuccoing by means of a robot with 50% of aluminum silicate and 50 % of mullite, grain size 0.3 to 1.0 mm. 
     4. Drying same as under 2. 
     5. 3rd, 4th and 5th back-up layer same as under 3. and 4. 
     The respective shells were produced in a 2-shift operation. A total of 333 clusters were produced. 
     The following average binding strength values (σ BB) were measured on ceramic flat specimens: 
     
         ______________________________________Condition of the ceramic flat specimens                    σ BB:______________________________________Green                    417 N/cm.sup.2Fired                    582 N/cm.sup.2Fired and redipped       696 N/cm.sup.2______________________________________ 
    
     4th EXAMPLE 
     In a production vessel of 600 mm diameter and 630 mm height the following back-up dip was prepared: 
     
         ______________________________________1.  Binder              36% by weight = 73.7 kg2.  Filler              64% by weight = 130.9 kg3.  Total quantity of the                   113 1 = 204.6 kg    back-up dip4.  The 73.7 kg, corre-    sponding to 61.6 1, consist of:    58.7 1 of aqueous colloidal    30% silica sol    2.9 1 of high polymer with    addition agent5.  The 130.9 kg of filler consists of:    59.5 kg of fused silica powder                   -120 mesh    59.5 kg of mullite powder                    150 mesh    11.9 kg of zircon silicate                    350 mesh______________________________________ 
    
     The resulting Zahncup No. 2 viscosity was 43 sec. By reducing the filler quantity to approx. 61 % by weight the Zahncup No. 2 viscosity was reduced to 36 sec. and was thus within the range of the rated viscosity of 28 to 36 sec. 
     Wax clusters from production and 2 clusters of bending specimens were provided, as usual, with the 1st and 2nd dip coat and then immersed into the back-up dip of the above described composition as follows: 
     1. Immersion for the 1st back-up layer into the back-up dip and stuccoing with 50% aluminum silicate grain size 0.3 to 0.5 mm, by hand. 
     2. Drying of the back-up layer applied 2.5 to 3 hours. 
     3. Immersion for the 2nd back-up layer into the back-up dip and stuccoing by hand with 50% aluminum silicate and 50% mullite, grain size 0.3 to 1 mm. 
     4. Drying same as under 2. 
     5. 3rd, 4th and 5th back-up layer same as under 3 and 4. This means that the shells were completed in a 2-shift operation. 
     The relative humidity was 50±5% and the ambient temperature 23°±1° C. 
     Drying was performed partially with fan assistance. 
     The resulting multiple-core castings were of very good quality. The following average bending strength values were measured on ceramic flat specimens: 
     
         ______________________________________Condition of the ceramic flat specimens                    σ BB:______________________________________Green                    366 to 394                    N/cm.sup.2Fired                    558 to 684                    N/cm.sup.2Fired and redipped       725 to 807                    N/cm.sup.2______________________________________ 
    
     5th EXAMPLE 
     In a production mixing vessel the following back-up dip was prepared: 
     
         ______________________________________1.  Binder              37% by weight = 426 kg2.  Filler              63% by weight = 726 kg3.  Total weight of the 637 = 1152 kg    back-up dip4.  The 426 kg of binder, corre-    sponding to 360 1, consist of:    331 1 of aqueous colloidal    30% silica sol    29 1 of an emulsion of a silica-    organic compound5.  The 726 kg of filler consist of:    330 kg of fused silica powder                   &lt;120 mesh    330 kg of mullite powder                    150 mesh    66 kg of zircon powder                    350 mesh______________________________________ 
    
     The pH-value of the finished back-up dip ranged between 9 and 10 units. The actual viscosity was within the range of the rated viscosity, i.e. between 24 and 28 sec. according to Zahncup 2. The relative air humidity inside the dipping room was 50±5%. Temperature inside the dipping room: 23°±1° C. Drying time between individual back-up layers: 2.5 to 3 hours. 
     After the application of the 1st and 2nd dip-coat the following dipping cycle was adhered to by means of a robot. 
     1. Immersion for the 1st back-up layer into the back-up dip and stuccoing by means of a robot with aluminum silicate, grain size 0.3 to 0.5 mm. 
     2. Drying of the back-up layer applied 2.5 to 3 hours. 
     3. Immersion for the 2nd back-up layer into the back-up dip and stuccoing by means of a robot with 50% aluminum silicate and 50% mullite, grain size 0.3 to 1.0 mm. 
     4. Drying same as under 2. 
     5. 3rd, 4th and 5th back-up layer same as under 3. and 4. above. 
     The respective shells were produced in a 2-shift operation. 
     The following average bending strength values (σ BB) were measured on ceramic flat specimens: 
     
         ______________________________________Condition of the ceramic flat specimens:                    σ BB:______________________________________Green                    400 to 600                    N/cm.sup.2Fired                    470 to 690                    N/cm.sup.2Fired and redipped       620 to 750                    N/cm.sup.2______________________________________ 
    
     While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.