Further work in similar lines by Bassoli and Atzeni20 analysed the compressive strength of cylindrical samples printed using ZCast 501 and baked at 160-250C (320- 482F) for 4-8 hours led to the observation that the com- pressive strength decreases with increasing baking time and temperature. Compressive strengths of baked sam- ples ranged from 2.6-6.2 MPa, much higher than those observed by the current authors. Thermogravimetric anal- ysis showed temperature as the primary influencing factor during baking with respect to compressive strength. Sam- ples lost 6-7% weight after baking for 4 hours at 150C (302F), but the current authors observed that baking time is also significant.
Kaplas and Singh21 investigated the effectiveness of shell
moulds printed using ZCAST 501 and cured at 110C (230F) for one hour, and reported that a shell thickness of 2mm was sufficient while casting zinc, and radiographic testing of castings showed decreased shrinkage and gas levels at lower shell wall thickness. Effectively, lower wall thickness means lower cost moulds, leading to a time and cost savings of almost 40%. Aluminium was cast into printed shell moulds and moulds produced from sacrifi- cial patterns printed using starch on a Z Corp 310 Plus 3D printer and the tolerance ratings in both cases were found to be within normal limits.22
Surface roughness values
of castings produced using the investment casting tech- nique were a slightly lower, around 4µm, when compared to direct printed moulds, showing an average roughness of around 7µm. The shell thickness, when reduced from 12 mm to 6 mm was found to offer better dimensional stability and mechanical properties, possibly due to an enhanced heat transfer rate. Similar results were also ob- tained while casting lead.23
While 3D printing of moulds appears to be promising for quickly producing functional prototypes and end-use parts in small quantities with specific metals though rapid cast- ing, there is very little information available on the actual process, the mould metal interactions and the significance of process parameters. While most existing research is cen- tred on aluminium, there was no significant attention paid to other light alloy systems, other than a few trials with relatively less significant metals like zinc and lead. The on- going research into mould materials suitable for both 3D printing and ferrous casting is yet to offer some solutions, it is clear that rapid casting is an immediate possibility with light metals, and it is time a scientific investigation into the effects of various parameters on the effective utilisation of the method was initiated. This paper reports the results of statistical experimental investigations carried out to es- tablish the combined and individual influences of various factors such as mould materials, coatings and pouring tem- peratures, in combination with a few light alloys, on the ef- fectiveness of the rapid casting procedure employing rapid prototyped moulds.
International Journal of Metalcasting/Summer 2011
Mould Materials
proportions. The normal grade for 3D printing for other purposes is ZP131, which is essentially plaster with a small quantity of crystalline silica and varying quantities of vinyl binder, carbohydrate and surface salt. The bind- ing materials are essentially water-based solutions mixed with small quantities of glycerol, sorbic acid salt, surfac- tant, pigment as in the case of ZCast 501 or humectants, and a polymer as in the case of ZP 131. The basic chemis- try and the bonding mechanisms in these material systems are not widely researched and reported as in the case of traditional foundry materials, but this is not the main ob- jective of this research. For casting purposes, it is impor- tant to know the overall characteristics of the moulding material such as permeability and compressive strength, under varying baking conditions.
Different grades of materials were supplied by Z Cor- poration for 3D printing, and all are proprietary mate- rials, with limited information on the basic ingredients and overall properties. The ZCast 501 is the material ap- proved specifically for 3D printing of moulds and mainly consists of CaSO4
, 0.67H2 O and MgSiO4
Earlier research by the current authors using response surface methodology19
established optimum baking con-
ditions for ZCast 501 as stated earlier. Sieve analysis re- sults indicated a wide range of grain sizes, and the fine particles acting as interstitial material, would have ad- verse effects on the permeability. During the initial cast- ing trials, it was a surprise to find that the plaster mate- rial ZP131 was also effective as a mould material and the aluminium castings produced were of much better sur- face quality. Based on this, ZP131 was also identified as a potential candidate for the 3D printing of moulds, and subsequent experimental evaluation in similar lines as in the case of ZCast 501, using central composite design, revealed that the best baking time and temperatures were 150C (302F) for 3.2 hours for the maximum permeability and 150C (302F) for 7 hours for the maximum compres- sive strength.
While the two material systems exhibited similar perme- ability and strength, ZCast 501 was slightly more porous than the ZP131 with mean permeabilities at 2116.7 mD (millidarcy) for ZCast 501 and 2099 mD in the case of ZP131. A T-Test on the permeability revealed high values of probability, at 0.9914, meaning that the two popula- tion variances are not statistically different and that the difference in the values are due to natural variation. The maximum compressive strength was higher for the plaster material, probably due to the finer grain size, consuming more binder due to larger grain boundary area and the T- Test resulted in high values of probability as in the case of permeability. Overall, the statistical testing on the re- peated central points of the permeability and compressive
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