Even though furan is reliable for the high production requirements of cores and molds as a binder system in 3D printing, some small weaknesses are present. So far, the veining on the surface and on the inner contours of castings with furan in combination with sand in complex geometries in high-temperature casting is difficult to avoid. Following consistent development work, ExOne (Gersthofen, Germany) has succeeded in counteracting the veining in furan through the use of additives.
Veining is and remains annoying, not only because the reworking on the surface or on the inner contours of castings always requires a complex post-processing and drives up production costs. For many casters, it is also a personal disappointment after all the preparations in the core manufacturing process and subsequent casting, to come across the annoying casting errors when unpacking the casting.
The ExOne Customer Application Team (CAT) has been intensively involved with the task of combining furan together with a classic silica sand and a number of different additives.
Particularly convincing results have been shown by the use of furan resin in combination with sand and a mineral additive. In all positive test experiments, it could be clearly stated that silica sand in combination with furan binder get a much higher elasticity due to the addition of the additive.
This has completely avoided the typical rupture of the sand core surface in high temperature stressed places, such as the inner contours of the castings, when casting the hot cast material. As a result, the industrial test object, in this case a typical impeller from the pump industry, could be cast without any signs of veining. Thus, furan, now in the first step, rises to a higher level of quality, having once proved this for the shape complexity of an industrial impeller.
Improved quality through additives
In principle, when casting with steel, several factors responsible for the expansion of the sand core surface meet. The expansion of the silica sand results in the so-called quartz inversion. High casting temperatures cause fast quartz conversion and thus lead to increased stresses in the molded part. But the grain size and grain distribution of the silica sand also has an influence on the surface behaviour, because a higher degree of uniformity leads to higher stresses, since all silica grains pass through the transformation temperature at the same time.
Thus, all the basic components of a typical sand-casting mold have a low ductile property, and when the hot metal is poured, there is a high pressure on the surface of the sand mold. The liquid, hot metal surrounding the core after casting causes a temperature gradient between the surface and the center of the core, which is explained as a consequence of structural characteristics of the mold base material. Silica sand, for example, extends linearly up to approx. 400C and then shows a sudden increase in the temperature range of the reversible β-transformation and thus increases the stresses.
Instead of being able to compensate for the pressure due to material flexibility, the rigid property of the sand-binder mixture causes cracks in the surface. The mixture of sand and furan binder, thus organically bonded moldings, gives way and forms surface cracks. The liquid metal now flows into these very fine surface cracks. The fact that silicon dioxide expands in the mold cavities causes the filling of the casting metal in the resulting gap.
Exactly then, with the removal of the cast from the sand mold, the imprint of the previous cracks on the surface will rise, forming a rib-like, vein-like crust, protruding from the smooth surface.
In technical terms, one speaks of thin, irregular metallic protuberances, which occur primarily on the inner contours of a casting mold or in angles, corners and edges of the castings.
Better casting quality with 3D printing
The declared aim of the ExOne test series was to avoid these irregular metallic protuberances. The development department meticulously tested all possible combinations of ExOne certified sands together with the furan binder and various additives. The “benchmark” was to eliminate veining.
Already the refined FS003 sand with a reduced packing density and thus a smaller tension, in contrast to the FS001 sand, was able to achieve a 50% reduction in veining. Pragmatically, it would be the easiest step to use the FS003 sand for improvement in the future.
When blending furan with synthetic sand, no veining could be detected in the test series. With an addition of 2% of the additive, only about 10% of the usual veining effect, as it normally arises, remained. If the addition of the additive is increased from 2% to 4%, i.e. twice the amount, then the end product, which is free from veining, was reliably reproducible, as in the furan-sand mixture. The retrofitting of an additive dosing unit is possible with all existing machines and should be of particular interest to iron and steel foundries.
A piece of the future won with good preparation
In the run-up to the test, however, framework conditions had to be created for this outstanding result, which must be taken into account when evaluating and reproducing other industrial castings. The impeller’s metal castings were produced using 3D printed cores and 3D print production molds from ExOne. For the casting material, the development department used the SS316 stainless steel composition with a casting temperature of 1,622C. ■