Publisher: FAT GmbH, Germany
Casting plants establish demanding requirements of flexibility and productivity of their cold resin moulding shops in order to be competitive. Mechanised moulding plants in the classic moulding shops (figure 1) allow for a higher efficiency, an optimised use of resources, and for sophisticated production methods and process cycles. In this process, the selection of a suitable binding agent system is an essential factor with respect to casting quality.
Figure 1: Graded molding plant using the alpha-set method
Casting pieces are used in more and more areas of machine construction. This is, however, also linked with higher production numbers, with more and more complicated geometries, as well as with increasing quality requirements. Apart from manufacturing ever increasing numbers of casting pieces with increasingly demanding geometries, casting plants frequently have to significantly improve the casting quality at the same time.
This is also done in order to decrease efforts for finishing in the cleaning shops. Thanks to optimised production methods and process cycles, to the integration of existing resources and capacities, as well as to using first class auxiliary materials, productivity and casting quality can significantly be improved. The task in most cases includes an entire moulding plant including a sand reclamation and dedusting plant (figure 2).
Figure 2: Alpha-set sand reclamation plant
The Right Concept
Different concepts including flask systems or flaskless systems, as well as different binding agents have to be discussed in order to solve this task. Selecting the most suitable binding agent is also essential. Criteria for selecting the most suitable binding agent system are determined by the specific casting process and by the necessity of producing good, and most of all, continuously good reclaimed material features.
For the most part, furan resins are used as binding agents in moulding shops of iron and aluminium foundries. For steel casting and high-alloy special steels, the A-stage resin (resol) method, or better know as the alpha-set moulding method, has proven its worth.
FAT GmbH from Niederfischbach has had great success with delivery of alpha-set moulding plants to steel foundries e.g. in the Ruhr district (figure 3) in the past. Application of the alpha-set method is, however, less popular in Germany. For this reason, we’d like to describe the implementation of an alpha-set moulding plant in a steel foundry below in more detail.
In this article, the steel foundry (Figure 3) established by the FAT company in Germany is taken as an example.
Figure 3: Alpha-set resin and grade casting plant
Alpha-set moulding plants have the following benefits over other cold resin moulding methods (e.g. the furan resin method) in steel and special steel foundries:
- Very good casting surfaces,
- Low predisposition to defects like thermal cracks, formation of flashes, finnings and erosion effects,
- It is a nitrogen, sulphur and phosphor free system,
- Easy pattern stripping,
- Good moulding-material collapse features,
- Reusability of the reclaimed material of up to 90 %,
- Low odour development during moulding and casting,
- Low pollutant category of the discharged moulding material for landfill disposal.
The decision for the most suitable binding agent system was preceded by an industrial trial, in which a casting piece was cast in a mould bound with furan resin on one hand, and in a mould bound with alpha-set resin, on the other hand.
The differences in quality were significant. The widespread opinion that reclaiming the moulding materials if bound with alpha-set resins is difficult was also disproven. Since there was only little experience with moulding plants of this type in the steel foundry, primarily technical characteristics for the material reclamation, like annealing loss and dust concentration of the reclaimed materials, were fixed in contracts, apart from the strength values determined during the trials.
The second essential issue for selecting the most suitable concept was if the moulding plant was to be designed as a flask or flaskless system. The FAT concept was based on a flask moulding plant. Although the flask moulding plant has the disadvantage that the flasks have to be returned by transport on one hand, it also has many advantages on the other hand. Flaskless moulds, for instance, in general require more sand and the addition of a higher quantity of binding agent in order to achieve the necessary sand mould strength.
When using flasks, it is possible to use a leaner mix of binding agents. Furthermore, and in particular in steel casting, flask clamping can be designed much simpler and much more efficient than with barrel clamping. It is obvious that handling a barrel with dimensions of 2000 mm x 1400 mm x 500/500 mm during pattern stripping, rolling over and closing is more problematic due to the strength problems, than it is with a flask system. Due to the above mentioned benefits, the steel foundry opted for the flask moulding plant of FAT, which is based on an alpha-set binding agent system and includes a sand reclamation and a chromium-ore sand separation system (figure 4).
Figure 4: Chromite sand separation system
Mastering the Challenges
During the plant design several challenges had to be overcome. For instance, an entire mechanised moulding plant for medium casting and a mould circulation system for heavy casting had to be integrated into an existing building. Additionally, apart from the existing space, the manufacturing facility’s infrastructure had to be considered, such as the furnace location, the location of the core shop and of the cleaning shop for finishing the casting pieces. The locations and routes of existing cranes were also to be considered. Collisions between cranes would have caused undesirable waiting periods and would have put the performance of the moulding plant agreed in the contract at risk due to increased cycle times.
Another important issue when designing the moulding plant is a realistic consideration of auxiliary and handling times. Since these times strongly depend on which specific casting process is used, a close cooperation with the client prior to making any design decisions is an essential prerequisite. Until the detail design was started, many discussions with the clients had to be held in order to optimise the concept and to adapt it perfectly to the existing conditions.
The moulding plant as it was installed had a central mixer station including two cold stream mixers. These mixers can serve two filling positions, one for medium casting, and one for heavy casting. The cycle time of the moulding plant for medium casting amounts to 5 minutes per mould half. In the heavy casting process, the production cycle is significantly longer, and auxiliary times for preparation strongly depend on the type of casting piece. Another challenge was to adjust the binding agent mixture to the specific casting procedure. The alpha-set method varies from other cold resin methods with respect to the chemical reaction during application. The A-stage resin (resol) is hardened by adding esters. These esters are delivered as a solution, in which the water required for the chemical reaction is already mixed in by many binding agent manufacturers. But this doesn’t allow a sufficient flexibility during mould production. This is why FAT proposed using an alpha-set binding agent, in which the water is only added during mixing in the cold stream mixers. The cold stream mixers are equipped with a suitable water dosing unit (an additional pump including a metering valve). While moistening new sand to be used as facing sand is not desired, this equipment allows, for example, moistening with reclaimed materials if so desired by the customer. As a result, the addition of water can be adjusted to the specific mould production process according to the customer requirements.
Sand moulds are evenly compacted by vibrations during the filling process, if possible, and subsequently left to harden on the downstream roller conveyor stations until they are separated from the pattern. Where exactly the filling stations are positioned and how many down-stream hardening stations can be used, are essential factors for achieving the production performance as previously determined by theory.
The mould is separated from the pattern by using a knock-out table, in which the mould is lifted by means of pins inserted in a lifting plate. Hydraulic clampings hold the pattern plate in place. This way, lifting is a parallel process and no edges are broken off.
A chain turnover device turns the flask over and puts it on the roller conveyor for further transportation. Drying the mould prior to smoothing is not required with furan resins, since the hardening of alpha-set resins is a polyaddition reaction, and not a polycondensation reaction. As a result, water is not released but chemically bound. The subsequent smoothing process is carried out above a dip tank, where a chain turnover device positions the flask accordingly. The smoothened mould is put on top of a steel plate conveyor and is skin-dried. With this process, the flask is moved through a tunnel in order to safely extract all exhaust gas.
Afterwards, the cores are inserted by using the overhead crane, and the flasks are added subsequently. The moulds are cast on the foundry floor in a separate casting section. After cooling, the flasks are emptied on a vibrating grate (figure 5), and the sand is fed into the sand reclamation unit including chromium-ore sand separation.
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