Mineral Casting: Vibration-Free Precision with Environmental Responsibility

Structure of the basic geometry

The mould is made from wood, steel, aluminum, PVC, silicone, polyamide or a combination of these materials. The main criteria for the selection of the suitable form material are:

•  the estimated number of castings to be produced by a mould,
• the required accuracy and surface quality of the mineral casting components,
• size, weight,
• planned variants in the casting,
• costs and time

The design of the mould, the calculation of the mould and the construction of the mould all run parallel to the construction of the part. The design criteria of the mould must be considered in order to ensure the geometrical accuracy and high static and dynamic rigidity:

• The design of the main functional surfaces the mold are critical , in order to ensure the accuracy.
• Moulds are mostly made from plates. Basic geometry is design from simple elements.

Integration of mechanical components

Threaded inserts, steel plates, transportation anchors, cables and trunking, s and hollow cells can be cast directly in the mineral casting component as it is a cold casting process.

In order to ensure that it is in optimal working order later on, all mechanical components must be fastened securely as their position can no longer be corrected later on and substantial lift forces develop during the casting procedure, which could change the position if the fastening is not secure enough. It must be ensured when the component is being sesigned and more importantly later with the construction of the mould, that this insert does not obstruct the flow of material or the venting of the mold.

If the threaded holes are too close to the edge or if the mineral casting component has hole pattern with mating holes, special cast parts are moulded in. These special cast parts (mouldings, plates) are anchored to the mineral casting component with hexagonal screws.

Operational surfaces

As described in the chapter on accuracy, a maximum accuracy of approximately +/- 0.1 mm/m can be attained with good moulds. Many components however, have a mating surface for other parts, which clearly require a greater accuracy, for example for the attachment of linear guide systems, sliding or mounting surfaces. It is therefore imperative to integrate these functional surfaces in order to attain the specified tolerances. Today SCHNEEBERGER essentially uses four procedures as below, in order to create the necessary surfaces:

• Machining of mineral casting - with this procedure the mineral casting is poured with an excess and afterwards machined to the exact size by milling or grinding.
• Machining of cast parts - with this procedure, metal parts, such as steel or cast iron plates, are cast into the mineral casting and machined after the casting has hardened.
• Replication - with this procedure the mineral casting is poured approximate 2 mm undersize. After removing the mould there is a second casting stage where very fine material is cast with a precise setting jig.
• Subsequent casting in of metal parts - With this procedure high-precision cast parts are precisely cast in after removal of the mould using positioning gauges.

FEM analysis

By using the FEM (finite element method) one can carry out calculations of different characteristics of component geometry and machine designs. A typical use is the calculation of deformations on machine bases or components and the derivative of optimized geometry from these results. In addition, temperature dynamics and vibration response can be determined very accurately nowadays.

Some practical examples, such as the decrease in the deflection of components of a textile machine (Cetex) or the reduction of the regenerative effect on turning lathes (Boehringer Werkzeugmaschinen GmbH) thanks to the better absorption behavior of mineral casting in direct comparison to welded or cast designs, or the optimization of the thermal behavior (SCHNEEBERGER AG).

Transport and set-up

Transport

The mineral casting components must be securely fixed during transport. A crane, fork-lift truck and truck are used for transport. In addition suitable anchors and rails are to be provided.

Set-up

In principle similar rules for bases made from steel or gray cast iron also apply to the set-up of mineral casting racks. Special foundations are very important particularly with large machines because, in their typical suitable construction as an engine foundation with 70-90% of the total mass of a machine, they guarantee a high overall torsional stiffness with the high inertial mass and they can attain the necessary accuracy thanks to the smooth running of the machine.

A correspondingly substantial dimensioning of the machine foundation is of great importance particularly with extremely heavy component parts and/or a correspondingly large workforce or with sudden impact loads, because otherwise it results inevitably in the partial sinking of the machine and so the geometrical accuracies typical of today are no longer attained over the entire working space. Machines with a three-point set-up are an exception to this rule, because their accuracy does not depend on the foundation.

Materials tailored to specific projects

Our experts at the SCHNEEBERGER mineral casting facility develop various compositions which are tailored to specific projects/individual customer applications. In doing so, our customers receive the best possible combination of technical benefits and affordable prices.

Mineral casting and its areas of application

SCHNEEBERGER Mineralgusstechnik covers a very wide range of sectors and products, as you can see from these examples.

Presence: We have our own local production facilities in both the European and Asian markets, and our own sales offices in all major industrial countries. 

Sectors: The number of areas where mineral casting offers unbeatable benefits for our customers is growing fast. Here are some of the main sectors that we are active in:

• Machine tool industry
• Pick & place
• Packing industry
• Laser industry
• Electronics industry
• Optical industry
• Medical technology
• Food
• Flat panel (inspection equipment)

Products: Our applications cover a full range of sizes. Here are a few examples from different sectors.

A single cast.

Mineral casting from SCHNEEBERGER offers unbeatable advantages, which come to the fore in the optical industry.

Anyone who works with optical components and surfaces, such as industrial lenses or glass for spectacles, cannot afford to cut corners when it comes to surface quality, precision and productivity. After all, their customers’ sight depends on it, as does the ability of machine manufacturers to compete in the optical industry.
Powerful machines can manufacture well over 100 spectacle lenses per hour, with consistently reproducible levels of quality. In order to realise the high engineering dynamics and required product geometry while maintaining the utmost precision, machine constructors require a machine bed that has exemplary vibration dampening and thermal stability, such as can only be achieved by mineral casting.
This means we are an indispensible partner for manufacturers of production equipment, not only for high quality industrial optics but also for fine spectacle lenses.
SCHNEEBERGER produces its mineral casting in a cold casting process using minerals and epoxy resin. Thanks to the especially perfected production methods, we can achieve exceptionally flat and parallel reference surfaces, whether for guideways or other precision components such as measuring scales or linear motors.
As SCHNEEBERGER manufactures both mineral cast constructions and linear bearings, we are able to adjust the bearing support surfaces so that they are an optimum match for each bearing type and for the specific application. This ensures the most linear, parallel and flat operation possible for linear movements, resulting in optimum technical and economic solutions.

Sample application

History

Mineral casting and the geometry of the products

A significant advantage of mineral casting is that the most geometrically complicated structures/ bases can be manufactured without the need for further processes. This necessitates an accurate tool (mould), which can usually be amortized over a short time period. Threaded inserts, mounting plates, spindle attachments etc. are pre-cast as moulded-in parts and provided for in the design phase of the mould, thus removing the need for any complex, cost-intensive finishing or machining. This of course, means shorter turn-around times, which plays an increasing role in modern times.

Bonding agent

The bonding agent, also called a matrix, consists of resin and after the recipe is agreed – the application- specific fillers are added. Epoxy resin is used, as opposed to Methacrylate resin, and polyester resins exhibit less contraction (volume shrinking) and an improved processing time. When poured the bonding agent also acts as a lubricant between the elements of the filler, thus improving the flow characteristics. There can be no chemical reaction between the filler and the bonding agent. Cast parts containing epoxy resins as bonding agents are not suitable for operating temperatures of over 80°C.

Advantages of epoxy resins

• Low viscosity gives good flow characteristics
• Guarantee that the filler used is well lubricated
• Long gel time
• Slow hardening by precipitation, combined with low exotherms
• Good mechanical characteristics (tension, compression, bending, elastic modulus, thermal expansion)
• Good chemical characteristics, i.e. resistant to aggressive liquids
• Good long-term behavior
• Epoxy resins are certified for the food industry

   

Quartz rock as a filler

Fillers, also known as aggregates, form the basis of the mineral casting. Various recipes or mixes are available from SCHNEEBERGER mineral casting, this ensures and determines the optimum properties of each mineral casting product meet the demands of the customer’s specific application.
A natural quartz rock is used in different grain sizes. Since the mineral casting characteristics are determined predominantly by the characteristics of these fillers there are specific requirements for their density, tensile and compressive strength, elastic modulus, thermal expansion coefficients and heat conductivity.

Non-mineral additives can also be added in part to the fillers.

Quartz pebbles that have high degree of purity and very good mechanical characteristics are used.