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Our partner - the foundry

We obtain our base material - the iron cast - exclusevely from German production. The small foundry is our trusted supplier and produces this important material for us. This company is - like HOFMANN - long-established and strongly rooted in their regional home. Iron was already cast into shape there since 1896 and a big and valuable treasure trove of experience could be collected since then. These experiences benefit our products and therefore our customers - especially in combination with our own experience values! Enclosed some pictures of the current production.

Why iron cast?

Cast iron containing lamellar graphite (short GGL) has got very good damping characteristics by its material structure. Thereby you get a better running smoothness in the complete machine compared to machines with welded frames or tables. The machine is vibration-poor, which affects the results of milled and planed woods very positively. Besides the basic frame has such an high dead weight, that no additional weight (e.g. concrete or similar things) is needed. These additional weights could only be fixed in the lower area near to the bottom, where they never can cause a vibration-absorbing effect. Iron cast is still the first choice for the usage as a machine body regarding to its proberties.

What's actually "iron cast"?

lamellar graphite cast iron

Iron cast is a group of iron-based alloys with a large amount of carbon (> 2 %) and silicon (> 1,5 %) together with elements like manganese, chromium or nickel. On grey iron cast - short grey cast - the carbon appears in the form of graphite. Iron cast has a density of approx. 7,2 g/cm³, which is clearly lower than the density of steel from pure iron (7,85 g/cm³). In the eutectic range the material has a clearly lower melting point of approx. 1150° C compared to steel, but cannot be forged because of its great amount of carbon. In foundries it's usually melted in cupola furnaces. Grey cast has no recognizable elasticity because of its insufficient elasticity of the inconsistent structure and can be classified as quite rough. Apart from that the carbon provides good thermal conductivity, good damping properties and because of its embrittlement a inherent stability to the material. That's why iron cast is very suitable for the usage as machine body or frame. Added to this there is advantageous self-lubrication, if the lamellars been cut on processing and the carbon itself or instead of it other lubrication materials fill up the hollows. Iron cast parts with intact cast skin also provide a good corrosion resistance.

The production

The forms beeing - depending on pouring temperature, contour and weigth of the workpiece - thermally and mechanically so heavily stressed on casting, that changes of form and measures could happen. This fact has to be considered with foresight on moulding and casting and on modelmaking as well. Furthermore the specific shrinkage of the choosen material has to be considered on manufacturing cast models and metallic moulds, which occurs on cooling down from solidification temperature to the room temperature. The material proberties depend on the chemical composition of the molten metal on one side and on the structure in the solid state on the other side. The developing structure depends on the speed of cooling down and the conditions for nucleation and crystal growth during setting. Different cooling rates in one workpiece depend on the form, espacially the wall thicknesses. A controlled solidification in the cast is needed directing to the downsprue and the feeders (the last place of solidification) to guarantee a homogeneous cast iron structure with best possible mechanical properties. The construction of the casting workpieces and their production as well claim a high amount of technical know-how - that's why a close cooperation of the constructor and the cast iron specialist is necessary!

Lost forms are mostly made of sand with suitable binder materials. A model (a sample of the workpiece) is necessary for contouring and has to be produced with an offset for compensating the shrinkage of the material on cool-down . One ore some more feeders are used for the compensation of the shrinkage in liquid condition during the solidification. You differentiate between permanent and lost models - like on shapes. Furthermore you differentiate between natural and core models. Regarding to their form natural models correspond with the produced piece (larger by the shrinkage rule). Core models have additional core marks, which are something like a bed for the inlaying cores. Cores are needful, if certain contours (mostly hollows) cannot be formed with thepl of a natural model. Permanent models are made of plastics, wood or metal - depending on the functional requirements. They contain not just the copy of the workpiece, which should be poured, but also the gates, which are the channels, where the cast material is filled in and spread and where the air and all the gases go out on pouring. Permanent models are surrounded by sand, which is compressed and stabilized by shaking and pressing. Usually casts are formed double-sided. That's why forms are made of two flasks (the upper and the lower flask) for an easier removal of the model before pouring. Therefore the model must not have undercuts, but has to have slopes to protect the form from damage. Afterwards upper and lower flask are put together precisely and secured with clamps. Then the liquid cast material is filled into the resulting space. The 'separation gap' can often be recognized on the finished workpiece, because there are often marks of postprocessing or leftovers of the grout. The binder material of the sand is selected in a way that the form crumbles and is destroyed by the heat on pouring if possible. Otherwise the form has to be destroyed mechanically to remove the workpiece. Moulding materials with their binding materials are so well developed that they can be used for high-founding metals and also can be re-used. Lost models can be made of one part, because they were not removed before pouring. In this case the forms can have undercuts and don't need to have slopes. The models were destroyed after pouring by vaporizing, out-melting or another decomposition.

The solidification process starts after pouring of the molten material in the cast. This could happen with straight, roughly, spongily (exogenous solidification) or pastily shell-forming (endogenous solidification) walls. On solidification the specific volume of the of the material reduces in depency of the falling temperature of the shrinkage. This shrinkage has to be considered on the construction of the models.