RAPID PATTERN BASED POWDER SINTERING TECHNIQUE AND RELATED SHRINKAGE CONTROL

Transcription

1 RAPID PATTERN BASED POWDER SINTERING TECHNIQUE AND RELATED SHRINKAGE CONTROL Jack G. Zhou and Zongyan He ABSTRACT Department of Mechanical Engineering and Mechanics Drexel University 3141 Chestnut Street Philadelphia, PA19104, U.S.A. Phone: , Fax: Rapid Pattern Based Powder Sintering (RPBPS) is a new rapid tooling technique proposed by the authors. It is an integration of three techniques: rapid prototyping, lost wax casting and powder sintering. The main advantages of the RPBPS technique compared with other tooling techniques are low investment and production cost, short production cycle and a variety of materials of products. One of the key technical problems is how to calculate and control the shrinkage during three processes in the RPBPS to improve the accuracy of products. The total linear shrinkage of the products made with the RPBPS can be controlled within 3.5 % by using the selected binder type and binder ratio, suitable compacting pressure and specially designed ramp temperature program in the sintering and infiltration processes. RPBPS Technique and its Shrinkage Control Key words: Rapid pattern; Powder sintering; Green compact; Shrinkage control; Binder 1

2 INTRODUCTION Rapid prototyping/tooling and manufacturing have experienced tremendous growing and drawn great attention in national and international manufacturing industry. Although rapid prototyping has brought in a new revolution in manufacturing processes of materials by using additive and layer by layer material processing technique, its crown has gradually shifted to rapid tooling/manufacturing, i.e. not only prototype but also functional products. The research presented in this paper is an effort toward the rapid tooling/manufacturing direction. Rapid Pattern Based Powder Sintering (RPBPS) technique proposed by the authors is a new rapid tooling technique. It is an integration of three techniques: rapid prototyping [1], lost wax casting [1] and powder sintering [2]. The new method includes the following main steps. A master pattern made of polymer material is first fabricated by a rapid prototyping machine based on a 3-D solid model designed in a CAD system. The pattern is positioned on a substrate in a metal box or frame, and then a mixture of metal (ceramic or polymer) powder and binder is cast around the pattern under certain pressure. After removing the pattern and separating the substrate, a green compact having desired cavity/geometry can be obtained. Then the green compact will be sintered and/or infiltrated with selected metals in protective atmosphere to finally form a tool, mold or part. Figure 1 shows the detailed procedures to make an injection mold with the RPBPS technique. Figure 2 is an example of making an injection mold of a clutch component for a company using the RPBPS technique. The reason that the RPBPS is better than other rapid tooling methods can be concluded as that it overcomes the defects of other RT methods and adopts the merit from various tooling techniques. Table 1 is a detailed 2

3 comparison of conventional tooling, current rapid tooling [1, 3, 4, 5] and the new RPBPS tooling technique. From Table 1 it can be seen that the RPBPS technique is suitable for a variety of materials and any complex geometry, and also has the advantages of rapid process and low cost. To design mold and cores in a 3-D CAD To make the mold and cores by RP machine To mix metal powder and binder in certain ratio To cast powder /binder mixture in a container under a pressure if not qualified To examine and test the product To sinter and infiltrate the green compact To remove RP patterns using lost wax casting method if qualified Final product Figure 1. Main technical steps for making an injection mold with the RPBPS technique CAD Solid Model Rapid Prototype Green Compact Mold Final Plastic Part made from the Mold Finished Mold Sintered and Infiltrated Mold Figure 2. Making an injection mold for a clutch component with the RPBPS technique 3

4 Table 1 Comparison of various tooling techniques (A: Conventional B: Rapid Tooling C: RPBPS) A B C Process Time Cost Qty Advantages Disadvantages CNC 2~6 $2,000 to 1000 Close Tolerances Design Limitations Machining Weeks $20,000 to Short Lead Time High Cost Material 10,000 Aluminum Tooling Silicon Rubber Epoxy Composite 10~12 Weeks 1~2 Weeks 6~8 Weeks Keltool 4~6 Weeks Spray Metal New RPBPS 3~6 Weeks Less than a week $10,000 to $20, to 20,000 $1,000 to $5,000 1~50 $2,000 to $10,000 $2,000 to $7,000 $2,000 to $10,000 less than $2, to 2, to 20, to 2,000 more than 20,000 High Volume, Production Material, Close Tolerance Very Low Cost Short Lead Time Production Material Volume Production Material High Volume Production Material Volume Various Materials High Volume & quality Low cost, Short time High Cost Long Lead Times Low Quantities Non-Production Materials Design Limitations Surface Finish Cost and Lead Time Design Limitations Size Limitations Cost, Lead Time Design Limitations Surface Finish Cost and Lead Time Needs potential users evaluation and supports to refine the process SHRINKAGE STUDIE IN THE THREE STAGES OF THE RPBPS A series of key technical problems have been studied in the RPBPS. For example, one of them is to find suitable binder materials that can be solidified in a short time (< 24 hours), and the solidified binder should have enough strength at higher temperatures (>110 0 C) in order to keep original shape of the green compact during the step of removing the master pattern (see Figure 1). The second problem is to find some efficient and rapid way to remove the master pattern. For a pattern with small volume it can be removed by heating the polymer pattern in a temperature range from C to C, which is similar to lost wax casting. For a pattern with simple shape and draft angle, it can be pulled out directly. However, for general patterns with large volume or complex shape 4

5 we have to use some special solvents to dissolve them. The third problem is to infiltrate ceramic matrix green compact. We have found a method of infiltrating α-al 2 O 3 matrix green compact with aluminum alloys under special protective atmosphere. In this paper we will mainly discuss one critical problem on calculation and controlling of shrinkage in the RPBPS process. Shrinkage Analysis During the Fabrication Process of Master Pattern Considering the requirements of high accuracy and low cost, we chose the 3-D Printing RP machine (a product of Sanders Prototype, Inc.) to make the master pattern. The building material provided by this company is a kind of crystal polymer that has a melting point of C. The building material is heated first by using electrothermics. An attemperator is installed to control the heating temperature. Depending on surrounding temperature the suitable heating temperature range is C C. Then the molten building material is pressured to flow into a nozzle that is heated to the same temperature as that of the building material. Once the liquid is sprayed from the nozzle to a special substrate, it will solidify in second, and the temperature of the solidified material will drop to the surrounding temperature very quickly. The total shrinkage in this processing stage includes three parts. (1) The shrinkage of the build material in liquid state from the heating temperature (117 0 C C) to the melting point (110 0 C). Due to the small temperature change, the shrinkage is very small and can be neglected. (2) The shrinkage caused by phase transformation and (3) The shrinkage of the solid pattern from the solidification temperature to the surrounding temperature. The shrinkage of (2) and (3) is relatively larger and needs further consideration. If a master pattern is formed by 5

6 casting or other one-step forming method, the phase transformation may cause obvious shrinkage, and the distortion and thermal stress of the pattern may be inevitable due to different shrinkage occurring in different regions of the pattern during the cooling stage of the solid pattern. The Sanders 3-D Printing RP machine, we are using, has been designed to have some special features to solve these problems. First, as we all know, the pattern is fabricated layer by layer, and the thickness of each layer can be controlled from to mm. Each layer is formed by using a jet nozzle to print on the surface of the last layer. The bore diameter of the nozzle is only mm, so the width of the scanning line will not be over mm. Using this forming method the distortion and residual stress caused by the difference of shrinkage in the pattern can be reduced greatly. Second, before the printing of each layer the nozzle will first draw a outline of the pattern in order to keep an accurate outline dimension. Thirdly, the height of each layer can be controlled by using a milling process following each layer printing. When the pattern is milled its temperature will drop to the surrounding temperature. It has been found that the linear shrinkage of the pattern fabricated by the Sanders 3-D Printing machine can be controlled not over 0.01%, and this shrinkage can be neglected without effect to the accuracy of final products basically. Experimental Study on the Shrinkage During the Hardening Process of Green Compact Experiments show that the shrinkage in this process is affected by several facts: a) the type of binder, b) the ratio of binder in the mixture, c) the compacting pressure, d) the solidification time, and e) the dry temperature. As mentioned before, when choosing a 6

7 binder we mainly care its strength and solidification time, but not its shrinkage. The green strength mainly depends on the binder s type and the compact density. Figure 3 shows some experimental results of the effects of different binders and compact density on green strength of 316L stainless steel material. The ratio of binder is related to the binder s viscosity and the grain size of the metal powder. Since there are very small difference of the viscosity and the specific gravity among the three binders used in the experiments, the ratios of the three binders are chosen same when the used powder is same. In general, if the green strength is sufficient, in order to reduce the shrinkage in the sintering process the binder ratio should be as small as possible [2]. The specific gravity range of polymer binders is about (g/cm 3 ), and the viscosity range is (Pa s) which depends on the surrounding temperature. To obtain a better surface finish the grain size of the powder is chosen as 325 mesh. Under these conditions, according to our experiments, the weight ratio of the binder should not be more than 2%. On the other hand, if the solidification time is long enough ( for most binders 48 hours is enough) and dry temperature is not too low ( for most binder room temperature is enough), the effects of solidification time and dry temperature on the shrinkage are very small and can be neglected. Green density mainly depends on the compacting pressure, therefore after the type and ratio of the binder are determined the major factor affecting the green strength, green density and the shrinkage in this stage will be the compacting pressure. Figure 4 shows some experimental results on the relationships between compacting pressure and green density. Figure 5 shows another experimental result on the effects of compacting pressure to the linear shrinkage, from which one can see that the higher the compacting pressure is, the smaller the linear shrinkage will be. This is because that the porosity in the 7

8 green compacts will be reduced with the increase of the compacting pressure. However, this kind of shrinkage reduction must have a limit because of the inside cast plastic pattern. It has been found that the total shrinkage in this process is less than 0.37%. 18 Powder: type 316L stainless steel grain size 325 mesh Binder 1: specific gravity 1.2 (g/cm 3 ) 16 viscosity 3 X 10 2 (Pa s) weight ratio 2 % Green compressioin strength, 100 psi (0.69 MPa) g / cm g / cm g / cm g / cm g / cm g / cm g / g cm / cm g / cm 3 Binder 2: spcific gravity 1.16 (g/cm 3 ) viscosity 2.7 X 10 2 (Pa s) weight ratio 2 % Epoxy resin: specific gravity 1.21 (g/cm 3 ) vicosity 3.6 X 10 2 (Pa s) weight ratio 2 % Zinc stearate: weight ratio 2 % Solidification time: 24 hours Dry temperature: 25 Sample sizes: 0 C 1.0 in 1.0 in Zinc stearate Binder 1 Binder 2 Epoxy resin Figure 3 Effects of binders and green density on green strength of 316L stain steel compacts 8

9 7.0 : Binder : Epoxy resin Green density, g / cm Powder: type 316L stainless steel grain size 325 mesh Binder ratio by wt: 2 % Solidification time: 24 hours Dry temperature: 25 0 C Sample sizes: 2.08 in 0.5 in in Compacting pressure, tons/ in 2 ( 1.38 X 10 3 MPa) Figure 4. The relation between compacting pressure and green density for two binders X Powder: type 316L stainless steel grain size 325 mesh Binder ratio by wt: 2 % Solidification time: 24 hours Dry temperature: 25 0 C X : Binder 2 : Binder 1 Linear shrinkage (%) X X : Epoxy resin Sample sizes: 2.08 in 2.15 in 0.5 in X X Compacting pressure, tons/ in 2 ( 1.38 X 10 3 MPa) Figure 5. The linear shrinkage of the green compacts under different compacting pressures 9

10 Shrinkage in the sintering and infiltration processes Experiments have shown that the linear shrinkage of the green compact during sintering and infiltration processes is much higher than that occurred in the former two stages. The green compact may be out of shape due to large shrinkage especially when the plate used to support the compact reacts with the powder at high temperature or the distribution of the heating elements is not uniform. It has been found that the density of compact, the sintering time and temperature, the type and grain size of the powder, and the type and ratio of the binder all affect the shrinkage and distortion of the green compact in this stage. Usually we don t have much choices on the powders and binders, but we can control the ratio of the binder and raise the density of the compact as high as possible. We also can carefully design the sintering program to control the shrinkage and reduce the distortion of products. There have been many researches on the theoretical calculation of the shrinkage in the powder sintering process [6, 7]. We have used a simple method to design the program of sintering and infiltration and estimate the linear shrinkage in this stage. The basic equation for this kind calculation can be written as: 1 - L s / L p = 1 - (V s / V p ) 1/3 (1) where L s and L p denote the linear dimensions of a porous body (green compact) after and before sintering, respectively; V s and V p denote the volumes of the porous body after and before sintering, respectively. The ratio V s / V p can be further calculated as V s / V p = v s / v p (1 - d p / d c ) + d p / d c (2) 10

11 where v s and v p denote the volumes per unit weight (cm 3 / g) of the pores after and before sintering, respectively, d p denotes the density of the compact before sintering and d c denotes the density of the solid substance (without pores). Experiments show that for any kind of metal powders the ratio v s / v p will decrease with the increases of sintering time and temperature. In the one hand, we want a smaller shrinkage after sintering. On the other hand, a smaller shrinkage means that the sintered compact still has a lot of porosity, and after infiltrated by copper the formed mold or part will have a lower surface hardness due to containing more copper. As shown in Figure 1, in the RPBPS technique the sintering and infiltration are carried out simultaneously in order to save time and energy. Figure 6 is a conceptual diagram of this method. Figure 6. Sintering and infiltration of a green mold in the furnace Although there are a lot of published experimental data in the effects of sintering temperature and sintering time on the ratio of v s / v p, these data can not be used as the ground to calculate the shrinkage in our RPBPS directly. This is because that in the 11

12 RPBPS sintering and infiltration are conducted simultaneously, and the melted copper will penetrate the porous compact very quickly due to the capillary action, which will prevent the further shrinkage of the compact. Experiment shows that a porous compact. With size of 50.8mm 50.8mm 25.4mm, can be filled by melted copper completely in only one minute, and from then on its volume will not shrink even being kept in a high temperature for a long time. A suggested shrinkage calculation procedure and the sintering/infiltration temperature program can be introduced in the following steps. 1) According to the function of the product to determine its hardness range; 2) According to the hardness to choose the type of the powder and determine the proportion f of copper used in infiltration; 3) According to the composition of the powder to calculate the value of d c ; 4) According to the shape of the master pattern and its stress status to determine the compacting pressure p; 5) According to the designed compacting pressure p to estimate the value of d p based on the experimental relationship between p and d p (see Figure 4). In general, the shape of real compact is always more complex than that of the samples used in experiments (see Figures 3, 4 and 5), the estimating value of d p may be little higher than the real value, which needs to be amended according to specific circumstances. 6) If the amount of the binder (not over 2% by wt) in the compact can be neglected for simplification, based on the following formula the value of v p can be calculated, v p = (1 / d p ) - (1 / d c ) (3) 12

13 7) According to the proportion f of the copper in the product to calculate the value of v s based on the following formula v s = (1 + f ) / d c - 1 / d c (4) 8) According to the value of v s / v p to choose suitable heating temperature T and time t based on the existing experimental data of the effects of T and t to v s / v p which have been published in various literature for past several decades. 9) Calculating the shrinkage of this stage based on the above procedures and equations. We use the part shown in Figure 2 as an example to further describe how to calculate and control the shrinkage. We planed to make an injection mold for the clutch component by using RPBPS technique. Due to a large demand of the parts, more than a million, the mold needs to have a longer lifetime, for which its surface hardness should not be lower than HRC 50. According to our experiments, when using the composite of tool steel powder and pure copper as mold and infiltration materials, the proportion f of the copper should be controlled not over 18 % (volume ratio) in order to get enough hardness. According to the composition of the tool steel powder (C , Si 0.35, Mn , S< 0.03, P< 0.035), the parameter d c is calculated as 7.78 (g / cm 3 ). Since the shape of the master pattern is not complex and all faces of the pattern will only bear compression stress, the compacting pressure can be designed as large as 15 tons / in 2. If selecting epoxy resin as binder (weight ratio 2 %) for casting the green mold in the pressure of 15 tons / in 2, the green density of the compact can reach (g / cm 3 ) (see Figure 4). A mean value of 6.1 (g / cm 3 ) is determined as the value of d p. Based on 13

14 the Equation (3) we have v p = (1/6.1) - (1/7.78) (cm 3 / g). If selecting f = 18 %, based on Equation (4) we have v s = ( ) / / (cm 3 / g). Thus v s / v p = / According to the experiments on the sintering of various metals in Ref. [7], for the above value of v s / v p, the sintering temperature should be controlled between C to C, and the sintering time should be controlled within a half hour. Based on Equations (1) and (2), the linear shrinkage of the green mold in this stage is about 3.1 %. If the compacting pressure can be raised to over 23.3kg / mm 2, d p will reach (g / cm 3 ), according to our calculation the linear shrinkage in this process can be controlled not over 1% and the mold still has a hardness not lower than HRC 50. However, as mentioned above we can not increase the compacting pressure infinitely because the master pattern is made of a crystal polymer which can not bear very high pressure. Thus the linear shrinkage during the process can be controlled in the range of 2.95 % %. The total linear shrinkage during the three processing stages can be controlled not over 3.50 %. Figure 7 shows a designed program of sintering and infiltration for this clutch component mold. It includes five stages. In Stage 1 (from room temperature to C) a medium heating rate (300 0 C / h) is helpful to the binder s melting and volatilization, a high rate may cause distortion of the green compacts. In Stage 2 (from C to C) the heating rate is small ( C / h) in order to maintain a uniform shrinkage of the compacts during the volatilization of the binder and the simultaneous sintering of the powder particles. In Stage 3 (from C to C) a very high rate (700 0 C / h) is designed to reduce the sintering time and the shrinkage of the compacts before infiltrated by copper. In Stage 4 the highest temperature ( C) is kept for only one and a half 14

15 hours for the melting of the copper and the infiltration process. Stage 5 is the cooling stage; it will take 8-10 hours. During the entire sintering and infiltration processes, Nitrogen needs to be provided continuously to protect the products from oxidization. temperature, T, ( 0 C) Sintering material: Tool Steel powder compact Binder material: epoxy resin (2 % by wt) Infiltration material: copper Protective gas: nitrogen Cooling way: air cooling Stage 1 Stage 2 Stage Time, t, (hour) Stage 4 Stage 5 Figure 7. Program for sintering and infiltration of tool steel mold CONCLUSIONS In our new developed RPBPS technique, shrinkage control is one of the major problems. After extensive study we can control the shrinkage in an acceptable range. Due to special manufacturing techniques and smaller temperature change, the shrinkage during the fabrication process of the master pattern can be neglected. Experiments show that the green strength of the green compacts mainly depends on the binder s type and green density, while the green density has a nonlinear positive relationship with the compacting pressure. After the type and the amount of the binder are determined the major factor 15

16 affecting the shrinkage during the hardening process of green compact is also the compacting pressure. Based on the needed hardness of the products, the program of the sintering and infiltration temperature can be designed following certain steps. The shrinkage during this process can also be calculated according to the proposed formulas and some data coming from the experiments on the green density and available literature. According to the measurements and calculations, the total linear shrinkage of the products manufactured with the RPBPS can be controlled not over 3.5 % by using suitable binder and binder ratio, suitable compacting pressure and carefully designed temperature program of sintering and infiltration. REFERENCES [1] P.F. Jacobs, Sterelithography and other RP&M Technologies, from Rapid Prototyping to Rapid Tooling, ASME Press, New York, 1994, pp [2] D.L. Dyke and H.D.Ambs, Powder Metallurgy: Applications, Advantages and Limitations, ASM, Metals Park, Ohio, 1983, pp [3] D.A. Vanputte, Rapid Tooling is a Key Factor in Future Achieving Rapid Product Development, Eastman Kodak Company, 27th ISATA Conference, Auchen, Germany, Nov. 1994, Paper 94RA024. [4] Wet Pouring and Rapid, Prototyping Proceedings of 1st European Conference on Rapid Prototyping, Editor P.M. Dickens, Nottingham, England, 1992, pp [5] R. Flint and D. Ellis, Second Tooling Using Rapid Prototyping, Proceeding of SME RP&M 94 Conference, Dearborn, MI, April 1994, pp

17 [6] C.M. Sierra and D. Lee, Modeling of Shrinkage During Sintering of Injection Molded Powder Metal Compacts, Powder Metall. Int., vol. 20, No. 5, 1988, pp [7] G. C. Kuczynski, D. P. Uskokovic, H. Palmour and M. M. Ristic, Sintering 85, Plenum Press, New York, 1985, pp