Mineral casting is a material that consists of mineral fillers such as quartz gravel, quartz sand and rock powder and a small proportion of epoxy binders. The material is mixed and, depending on the manufacturing process , poured cold as a homogeneous mass into casting molds made of wood , steel or plastic . During the casting process, the mold is shaken to compress and vent the mixture . After a few hours, the part can be removed from the mold and is ready for assembly. The material is also known as polymer concrete or reaction resin concrete ; In English the usual terms are “mineral casting”, “polymer concrete” or “epoxy granite”. The manufacturing process for the production of mineral cast is also called mineral cast, depending on the material.
Mineral casting has been used in mechanical engineering since the late 1970s as an alternative to gray cast iron and welded constructions. Despite its technical and economic advantages, it is only slowly gaining acceptance. Most of the applications can be found in the construction of machine frames and measuring tables, where the good damping properties and temperature stability together with the great freedom of design and cost advantages come into their own. The weight of the parts ranges from a few kilograms to well over 10 tons. In addition to mechanical engineering, there are also applications in plant engineering and the construction industry.
Fillers , also called aggregates, make up 80 to over 90% of the volume , depending on the recipe . Natural and artificial materials of different grain sizes can be used, mainly gravel from hard rock. Since the mineral casting properties are determined by the properties of the fillers and the recipe, i.e. the proportions of fillers and the resin, there are specific requirements for their density , tensile strength and compressive strength , the modulus of elasticity , the coefficient of thermal expansion and thermal conductivity .
In some cases, non-mineral additives (steel, glass ), for example in fiber form, are added to the fillers . Since, depending on the material, shape and volume, they also have a negative impact on the processing and usage properties, there are no clear optimization criteria.
The size and composition of the fillers and the shapes to be produced influence each other. The larger the proportion of binder, the more fluid the mineral casting becomes and the easier it is to cast it. In the opposite direction, the mechanical properties of the mineral cast decrease as the proportion of binder increases. In order to achieve good compaction and homogeneity of the material, the smallest wall thicknesses must be a multiple of the maximum filler grain size.
Binder , also called matrix, consists of resin and hardener and forms from less than 10 to 20% of the volume. The most commonly used resins are epoxies. Compared to methacrylate resins and unsaturated polyester resins , they show less volume shrinkage and longer pot life . The binder also acts as a lubricant during processing. No chemical reaction must take place between the filler and the binder. Cast parts made of epoxy resins are not suitable for longer operating temperatures above 100 ° C.
- Very good damping properties compared to steel or gray cast iron: Mineral cast dampens the vibrations 6–10 times faster than gray cast iron (depending on the design of the components, the mineral casting recipe used and the external influences)
- Low thermal conductivity, high thermal capacity - mineral castings only react very slowly to changes in temperature, which affects the dimensional accuracy of the machines, respectively. the workpieces manufactured on it improved
- Chemically and mechanically resistant to aggressive and abrasive media such as oils , lyes , acids and the usual cooling lubricants ; no corrosion , but high sensitivity to solvents such as acetone .
- Cold casting process: No additional heat input required, low energy consumption
- High molding accuracy, low residual stress , low shrinkage (0.03%)
- Great freedom of design
- Larger components can also be assembled using adhesive bonds.
- Integration of many machine elements such as load anchors, hydraulic , cooling lubricant and electrical lines by casting.
- Disposal and recycling: Mineral cast is chemically inert and can be disposed of.
- The cold casting process without post-processing requires a precise, reusable mold, which causes relatively high initial costs.
- If the material is designed and the quantities are suitable for the process, mineral cast parts in a ready-to-assemble state are up to 30% cheaper than comparable welded constructions or gray cast iron parts.
Optimization of the properties
In contrast to the building industry , the application- related dimensioning is not based on strength , but rather on rigidity : The material is stressed far below the strength limit. Since strength and stiffness correlate to a certain extent, material optimization is still similar. The packing density and the mechanical properties of the fillers have the greatest influence on the material properties. Analogous to cement-bound concrete, gravel mixtures are created according to grading curves from Fuller and Thompson or their further developments. The aim here is a high packing density with the largest possible grains.
The proportion of epoxy depends directly on the packing density. The remaining air content is a few percent by volume and is reduced by vibration in the process. When optimizing the material, the processing properties must also be taken into account. The performance properties can turn out to be considerably worse if the workmanship is poor.
Like welded constructions and gray cast iron parts, mineral castings must be designed according to certain rules. In contrast to other casting processes, due to the dimensional accuracy, the surface quality and the casting at room temperature, post-processing can be avoided in many cases: The mineral casting can correspond directly to the functional part.
The mold design, the shape calculation and the shape construction run parallel to the construction of the part. In order to guarantee the geometrical accuracy and high static and dynamic rigidity, specific construction rules must be observed:
- The main functional area is at the bottom of the mold to ensure accuracy.
- The basic geometry is designed from simple elements; Casting molds are mostly made from plates.
- Integrated machine parts (the cast-in parts such as threaded anchors, transport anchors, lines, containers) must be precisely fixed and must not hinder the flow of material or the ventilation of the mold.
- Metallic cast-in parts such as assembly strips for guide rails must be limited in length to prevent inadmissible stresses.
- The weight can be reduced by means of lost hollow cells made of expanded polystyrene or with the help of plastic pipes.
- The draft angle (taper) is 1–5 ° per side.
- Depending on the recipe, the casting mold can be placed on a vibrating table for compaction.
- Number of planned casts
- Required accuracy and surface quality of the mineral cast component
- Height Weight
- Planned variant production
- Cost and time
Wooden molds are used for prototypes, in the construction optimization phase and for small quantities. As wood absorbs and releases moisture , changes in shape and dimensions result. Wooden forms have a relatively low rigidity and high wear. For small series, wooden molds are therefore also built on steel or aluminum plates to improve rigidity and shape accuracy.
The more expensive steel molds have better properties. For financial reasons, they only come into question from a certain number of pieces (> 20 casts).
Various manufacturing processes are required to manufacture mineral cast . These include u. a. the mold making (also: tool making ), which, as already mentioned, can be preceded by the construction of a few prototypes, the assembly of the tool parts to the finished casting mold, the actual casting process and the subsequent demolding by dismantling the casting mold.
Assembly of molds
The molded parts are cleaned before assembly and treated with a release agent ( wax ) for easy removal from the mold . The seamless, thin and even wax layer is applied with a low-pressure spray system. Either two-component polyurethane varnish is sprayed onto the dried wax layer or a so-called gelcoat is applied . This enables perfect separation and also determines the color of the casting.
The cast-in parts are degreased to ensure adhesion . Most of the cast-in parts such as threaded anchors and steel parts are assembled to the individual mold plates. Plates prepared in this way are assembled and the remaining cast-in parts such as earthing , cables and hoses are attached.
Mineral cast is processed in a mixing plant. First of all, fillers and binders are mixed separately and brought together shortly before casting. The assembled mold is mounted on a vibrating table when external compaction is required. The mineral casting mixture is poured into the mold from a casting vessel with constant compression vibration.
After casting, curing begins automatically as the binder polymerizes . During the hardening process, an exothermic reaction occurs in which the cast heats up to a maximum temperature of 55 ° C. This reaction temperature depends on the proportion of binder and other chemical additives and should ideally be kept as low as possible. Higher reaction temperatures lead to greater shrinkage and more stresses in the finished component. The curing time depends on the shape and weight of the component.
Dismantling of molds
After hardening, the casting mold is dismantled, first the side walls, then the base plate. Demoulding screws or cylinders are often used to dismantle the base plate. To finish, the cast parts are cleaned, disruptive edges are sanded, the pores are filled and the production control is carried out.
Further use, recycling of mineral cast
Mineral cast can be handled like normal building rubble . This is due to the high mineral content and the hardened resin compound, which is harmless to the environment . This means that all types of mineral cast components can be shredded on conventional systems after their use and used for backfilling, for fillers or for soil consolidation .
- Utz-Volker Jackisch: Mineral casting for mechanical engineering - properties, engineering, processing and industrial application of a modern material for high-precision machine frames. Verlag Moderne Industrie, Landsberg / Lech 2015, ISBN 978-3-86236-082-6 .
- Utz-Volker Jackisch, Martin Neumann: Machine frames for highly dynamic production technology. Süddeutscher Verlag onpact, Munich 2014, ISBN 978-3-86236-069-7 .