Discover the advantages of the No-Bake process in the production of cores and molds for foundries, such as improved mechanical strength and reduced excess metal. Also, read about process compatibility and the different types of resins used.

Introduction

 

Advantages

 

The No-Bake production system for creating cores and moulds using a self-hardening process is becoming increasingly popular due to its mechanical strength and ease of use. This system enables the creation of precise dimensions for shapes, minimizing the amount of excess metal in the castings. However, it is important to pay close attention to the models and release agents used to facilitate the removal. Additionally, in response to the productivity demands of foundries, technological developments in the machinery have greatly simplified the work of production staff.

The low cost of the No-Bake process depends on several factors, such as:

  • The use of a continuous mixing system, which is less complex than the grinding mill
  • It enables the production of forms of various sizes without wasting time
  • It eliminates the need for mounting brackets
  • It makes the machinery more compact and easier to manage
  • It allows a considerable energy savings.

In many cases, it is possible to avoid painting by using metals that do not require high casting temperatures, good moulding materials, and paying particular attention to the compactness of the mixture.

 

Compatibility of processes

 

For moulding moulds, it is possible to use recovery sand that has already been used with different resins, such as those from cores, but it is important to choose compatible products among them. Otherwise, small amounts of sand mixed with incompatible resins can be used, but more new sand will need to be added to ensure production.
It is important to monitor the sand based on the acid demand, granulometry, loss on ignition, and moisture, with a frequency dependent on the number of cycles that the sand will have to endure in a given period of time.
It is worth noting that a pH of 0 indicates the highest acidity, a pH of 7 is neutral, and a pH of 14 indicates the highest alkalinity.

 

The moulding processes of self-hardening systems for cores and moulds No-Bake

 

The No-Bake self-hardening system for moulding cores and moulds uses resins and catalysts that, when mixed with sand, react with each other to achieve the hardening of the form at room temperature.
No-Bake self-hardening systems can be classified into three main groups based on the type of catalyst used:

  • Acid catalysis for phenolic, furan, and phenolfuran resins
  • Basic catalysis for urethane resins
  • Ester catalysis for alkaline phenolic resins

Phenolic resin

Phenolic resin is a widely used material in the production of cores and moulds for foundries that cast iron. It contains phenol and formaldehyde, which create a distinctive odor during mixing. However, due to the condensation of water vapor, the mechanical strength of the mould can be reduced on the surface.
Moreover, erosion during casting can be caused by incorrect handling, improperly compacted sand, unsuitable release agents or facing.

Due to its high viscosity, during cold periods, it is necessary to control the mixing to avoid the formation of lumps and possible variations in the flow rate of the pumps.
During the winter, it may be necessary to request modified resins from the supplier with the addition of solvents.

The duration of the sand-resin mixture to form the moulds depends on the ambient temperature and that of the pattern, which can cause uneven hardening. The catalysts used can be sulfuric acid or sulphuric acid, and the mixture duration ranges from 5 to 15 minutes, while the forming time varies from 20 to 60 minutes.
Once the mixture starts to bind, the sand should not be moved as it will lose cohesion and the desired strength.

Among the advantages of this process are:

  • A good casting strength
  • A good decoring
  • An ideal storage temperature of 20 degrees
  • Low cost

However, there are also disadvantages such as the need for strong acids and odors during forming and casting.

 

Phenolic – alkaline resin

 

Phenolic-alkaline resin is a type of binder used in the production of steel castings in foundries.
Due to its composition of liquid esters, this resin is particularly suitable for this type of application. During the casting process, the binder hardens completely and offers good resistance to erosion, as well as to the formation of fins.
Additionally, the resin has a low odor, which makes it easy to handle.
However, the recovery of sand used during the foundry process is still problematic.

Furan resin

Furan resin is often used in steel and nodular iron foundries for castings of various sizes. It is mainly composed of furfuryl alcohol, and the amount of this component determines its properties.
For example, if furfuryl alcohol constitutes 70% of the resin, the nitrogen content will be 5%, while with 80% it will be 2.5% and with 90% it will be 0.5%.
Thanks to its low viscosity, the resin adheres easily to sand, simplifying the moulding process. Additionally, it allows for precise adjustment of bench life, from 2 to 30 minutes depending on the amount of catalyst used, and the removal of the mould can occur from 10 to 60 minutes.
Since it is difficult to remove during the sand recovery phase, it is important to minimize the amount of catalyst used.

Advantages:

  • High heat resistance
  • Good shakeout properties
  • Excellent mechanical sand recovery
  • Easy to manage lifespan
  • Good storage time
  • Low consumption of catalyst

Disadvantages:

  • High cost of resins

Urethane resin

Urethane resin is a binder system that can be divided into three types: phenolic-alkaline, polyether.
Each resin manufacturer makes appropriate modifications to the three basic systems and markets them under their own name.
Therefore, the urethane system can be used in various types of foundries, including those that produce light alloys and those that work with steel, depending on the specific modifications made to the system.

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