Investment Casting with Ice Patterns Made by Rapid Freeze Prototyping
Wei Zhang, Ming C. Leu
Department of Mechanical and Aerospace Engineering and Engineering Mechanics
UniversityofMissouri–Rolla,Rolla,MO65409-0050,E-mail:************
Chao Feng, Rong Ren, Renjie Zhang, Qingping Lu, Jubu Jiang, Yongnian Yan Department of Mechanical Engineering, Tsinghua University, Beijing 100084, P. R. China ABSTRACT
One of the most promising applications of rapid freeze prototyping (RFP) is making metal parts by investment casting with the built ice parts. The integration of RFP and investment casting allows fast creation of complex net-shape metal parts directly from their CAD models. The advantages of doing so include no part geometric complexity limitation, no experience of parting line design and asmbling needed, clean and low-cost of process operation, and good performance. In this paper, we will prent our recent study on binder lection, slurry making, ceramic shell making, and the casting results of the metal parts.
KEYWORDS
Solid Freeform Fabrication, Rapid Prototyping, Rapid Tooling, Rapid Freeze Prototyping
Ice Patterns, Investment Casting
1.INTRODUCTION
Rapid Freeze Prototyping (RFP) is a solid freeform fabrication process that builds three-dimensional ice parts directly from the CAD models. To make a functional part (metal or plastic), some traditional shape duplication process are needed, which is similar to some other SFF process. Previous studies have demonstrated the feasibility of making plastic parts from ice patterns by Ultra-Violet (UV) light curable silicone molding, sand casting, and investment casting[1-4]. Investment casting appears to be the most promising application and there are some studies related to this application. In the late 1940s, the Mercast process ud frozen mercury to make net-shape parts. The mercury pattern was first formed in a special aluminum mold and was then invested in alcohol/silica slurry, which eventually formed the mold cavity for casting. Mercast was a successful process that yielded excellent quality castings. But the molds were expensive and mercury is hazardous to health, eventually the process was forced into disu. Dry ice was also ud as frozen pattern in the cast process developed later[4]. The industrial u of the process provided valuable information o
n the low-temperature investment casting slurry for the potential application of rapid freeze prototyping. In 1991, an investment casting technology, which was patented in 1991 (US Patent # 5,072,770), has demonstrated the possibility and advantages of investment casting with ice patterns. The technology, called Freeze Cast Process (FCP), was developed by DURAMAX Co. It starts with the building of solid master and silicone mold. Then ice patterns are made with the mold and dipped into refrigerated ethyl silicate slurry and stuccoed. After repeating the dipping and drying process, a ceramic shell is made and then it is put in room temperature and allow the ice pattern to melt, drain, and dry. The shell is obtained and needs to be heated before ready to pour molten metal. Figure 1 illustrates the operation steps of the process. FCP has veral advantages over the competing
casting process, including low cost, high quality, fine surface finish, no shell cracking problem,easy process operation, and faster run cycle. One major concern in the FCP process is ice pattern making [4-6]. The traditional method of making ice patterns is by injecting water in a mold and making it frozen. Some main issues in this method include compensation of water expansion during freezing, air bubble removal, ice pattern de-molding, and part complexity limitations.With RFP it is possible to make accurate ice patterns directly from CAD models in a short time,without the cost
and other issues of mold making. This is especially valuable in ca that a small amount of complex metal parts and thus ice patterns are needed.
Figure 1. Freeze Cast Process Operation Steps
Though FCP has demonstrated the success of using ice patterns to make metal parts by investment casting, there is no detailed information reported yet. The following ctions are bad on our recent study, conducted mainly at Tsinghua University with clo collaboration with University of Missouri – Rolla. The study was aimed to find the best slurry composition (including binder, catalyst, ceramic powder, and parating agent), operating process and condition, and required processing of ceramic shells.
2. MATERIAL FOR LOW TEMPERTURE INVESTMENT CASTING
The slurry ud for investment casting with ice patterns (operating at a low temperature)should have different features from slurry ud for regular investment casting. For example, the slurry for low-temperature investment casting should contain no water, not freeze at sub-zero temperatures, and has medium drying speed.
2.1 Binder
Binder is one of the major slurry materials. Water glass, ethyl silicate, and silica gel are the most often ud binder materials for investment casting. However, in order to be suitable for low-temperature investment casting, the binder material should not freeze, not lo effectiveness,and have good fluidity at a sub-zero temperature. The ceramic shell made with water glass has low strength and low accuracy, thus it is not considered for our study. Silica gel is made with water glass and contains 40~50% water. It will freeze once it is put in low-temperature environment. So it is not possible to u silica gel as the binder for our study either. Only ethyl silicate satisfies the requirements and is ud as the binder in our study [7].
Pure ethyl silicate can be ud to produce a binder but it is more usual to employ a condend or co
ncentrated form containing about 30% to 40% silica by weight. In our experiment, the 40# ethyl silicate, which contains 40% silica by weight, is ud. The ethyl silicate directly received from the vendor has no binding property. To be ud as binder, the ethyl Make a Master Part Make a Soft Mold Make Ice Patterns
Dipping Pattern in
Slurry to Make
Ceramic Shell
Firing the Shell Cast a Metal Part
silicate must be chemically decompod by reaction with water. The reaction produces alcohol and silica in an active state-the colloidal state which contains silica particles larger than a solution but smaller than a slurry[8]. By strictly controlling the amount of each composition, we can obtain a stable colloid which contains no free-state water. Since ethyl silicate is not soluble with water, the reaction only occurs on the contacting surfaces and is thus slow. Alcohol is soluble with both ethyl silicate and water. By adding alcohol, the reaction speed can be substantially incread. The compo
sition is listed in Table 1. The adding of hydrochloric acid is to decrea the pH value and keep the colloid stable.
Table 1. Composition of the binder
Distilled water40# ethyl silicate Alcohol Hydrochloric acid
12 ml200 g232 g 3.2 ml
2.2 Ceramic Powder
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Fire-proof ceramic powder is another important factor for the success of shell making. Good powder can result in smooth surface finish, high accuracy, and good property of the castings. Factors that needs to be considered when choosing the powder material include the density, melting point, linear expansion coefficient, chemical composition, cost, etc. In practice, the fire proof powder material usually has veral particle sizes with a certain mixing ratio. The ratio of fine/medium/coar powder is critical for the shell quality. In order to obtain smooth ceramic shell surface, a certain amount of fine grade powder is needed. Table 2 gives the mixing ratio of our ceramic powder.
Table 2. Mixing ratio of the clay bad alumino-silicates powder
270 mesh200 mesh70/100 mesh30/60 mesh
30%30%35%5%
During the heating and firing treatment of the ceramic shell afterwards, alcohol and water will evaporate, resulting in volumetric shrinkage, tiny holes, and tiny cracking. The more percentage of the alcohol and water, the more rious the shrinkage and defects. So increasing the powder percentage in the slurry (i.e. reducing water and alcohol percentage) will help reduce the shrinkage and defects caud during heating and firing treatment. In our study, we try to increa the powder percentage as long as the fluidity can satisfy the casting requirement. The percentage we u is (Powder : Binder) 300g : 100 ml.
2.3Catalyst
玻璃猫鱼Catalyst is required to make the binder for low-temperature investment casting. Without catalyst, the gelling of the slurry directly mixed with powder can take days or longer. The gelling time depends on temperature, binder composition, and pH value. Among the three factors, pH value is the most important one. Existing studies indicate that the slurry is the most stable when pH = 2.0, thus the gelling time is the longest. When pH = 5.0~6.0, the slurry is the most unstable and the gelling is the f
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astest. When pH < 1.0, the slurry is also unstable.
Adding catalyst can change the pH value of the slurry from its stable value to its unstable value, thus improving the gelling speed and productivity. The adding amount of catalyst determines the gelling speed. Practical shell making operation needs moderate gelling speed. A
too fast gelling speed usually results in wor slurry fluidity, causing it difficulty to fill the whole cavity.
The catalyst can be either acid (to change pH value towards 1.0) or alkaline (to change pH value towards 6.0). The popular acid catalyst includes H 2SO 4, HCl, H 3PO 4. The alkaline catalyst includes NaOH, Ca(OH)2, MgO, CaO, Mg(OH)2, Na 2CO 3 and various organic materials. In our study, we u alkaline catalyst.
2.4 Separating Agent
Separating agent is needed for investment casting with ice patterns. The reason is that the slurry contains alcohol and water. The alcohol and water will interact with the ice pattern surface. The decomposition process releas heat which caus more decomposition. As a result,the ice pattern surface will be riously damaged. The parating agent material must satisfy the following requirements:
§ Not soluble with water or alcohol.
§ Not reacts with water or alcohol.
§ Not freeze at sub-zero temperatures.
§ Have good coating property on ice surface at low temperatures.落梅
§ Non-toxic and no pollution.
In our experiment, we u the mixture of silicate oil and kerone (1:1 ratio). Before coating the ice patterns, the parating agent is cooled to the ice pattern temperature.
3. INVESTMENT CASTING
EXPERIMENTS
Since the following study is
focud on the process of investment
casting with ice patterns, the ice
patterns can be built either by rapid
freeze prototyping or by traditional
molding method. In order to evaluate
the casting accuracy, the ice pattern
should be simple and easy to measure.
In this study, we u a cylindrical ice
pattern made by traditional molding
method. The ice pattern is made with a
steel mold with a diameter of 30 mm,
as shown in Figure 2. Separating agent
is coated on the outer and interior
cylinders before injecting water to help
remove the ice pattern after freezing.3.1 Ceramic Shell Making
The steps to make a ceramic shell with an ice pattern are as follows:
§ Prepare binder with composition listed in Table 1 and mix ceramic powder material with mixing ratio given in Table 2.
01-ba 02-outer cylinder 03-interior cylinder 04-aling ring 05-fixing axle Figure 2. Steel mold for making cylindrical ice patterns
美国名字§Measure 20 ml binder, 60 g powder mixture, and 3 ml catalyst. Put them parately in a low-temperature environment of –5°C and allow them to cool down.
§Make cylindrical ice pattern with steel mold shown in Figure 2 and measure the diameter of the obtained ice pattern.
§Cover the ice pattern with the outer cylinder (part 02 in Figure 2) and coat the interior surface of the outer cylinder with parating agent.
§Mix binder with powder and then add the catalyst.
§When the slurry is about to gel, inject the slurry into the outer cylinder and keep vibrating to let the air in the slurry out.
§After the slurry has completely t, take the mold out and put in room-temperature environment to melt the ice pattern and drain the water.
§Take the ceramic shell out of the outer cylinder and measure the interior diameter of the shell cavity and obrve the surface quality.
The shell making experiments have shown that the shell dimensions keep changing with time at room temperature, as given in Table 3.
Table 3. Ceramic shell diameter (mm) vs. time at room temperature
Samples Original size
(after demolding)
24 hours48 hours72 hours
129.8229.6229.5629.54
229.7829.6629.6229.60
329.7629.6029.5629.52
429.7629.6229.5829.54
It can be en from Table 3 that the shells all have the largest diameters originally and then the sizes become smaller and smaller at room temperature. The sizes change most significantly during the first 24 hours. The sizes vary 0.12 ~ 0.20 mm during the first 24 hours, 0.04 ~ 0.06 mm during the cond 24 hours, and 0.02 ~ 0.04 mm during the third 24 hours. The shrinkage is caud by the evaporation of the residual water and alcohol in the shell. To make the shell dimension get stable quickly, we lect to fire the shell after removing from the mold. Table 4 gives the shell diameter variation with time at room temperature. Comparing Table 4 with Table 3, we can e that after firing treatment, the shell dimension becomes more stable. The variation in 48 hours is negligible.
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Table 4. Ceramic shell diameter (mm) vs. time at room temperature with firing treatment
Samples Original size
(after demolding)
Firing 30 min. and
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五个月宝宝吃什么辅食最好48 hours
129.5029.5229.52
229.7029.7629.76
329.7429.7829.80
3.2 Investment Casting Experimental Results
The ceramic shell obtained above still needs to be fired at a higher temperature for a longer time becau the firing treatment mentioned above is only for the purpo of keeping shell dimensions stable and the firing treatment is short (only 30 min.) and mostly occurs on the
