Read about the Hot_box process phases and the types of binders used, as well as the specifications of the necessary sand. Discover the advantages of this process, but also its disadvantages such as high energy consumption and the generation of harmful substances.
The Hot Box process is a method in which the cores are hardened directly inside the core box, resulting in immediate strength. Hardening is achieved by the application of heat, which is generated electrically or by gas. The process was developed by Renault in the 1960s and involves the use of fluid synthetic resins as a binder and a hot-acting catalyst.
The process involves five stages:
- Preparation of the mixture (sand + organic binder and catalyst)
- Transport of the material to the core shooting machine
- Injection of the material into the preheated (180-250°C) core box using electric or gas heaters; to ensure complete mould filling, the shooting pressure must be 6atm, but this causes rapid wear
- hardening of the material in contact with the hot mould
- Removal of the core and post-hardening of the central part. After formation, the cores must rest for at least 2-3 hours to avoid the subsequent formation of bubbles during casting
The Hot-Box moulding process involves the use of three different types of binders: phenolic, urea, and furan resins.
Phenolic resins require a higher temperature for polymerization and a more powerful catalyst, and are suitable for cores that require high hardness or very thin cores, but have difficulties with shakeout. Conversely, urea resins have a faster cure time and can be used at lower temperatures, allowing the use of milder catalysts. However, the cold mechanical strength is weaker, while hot strength allows for easier shakeout. This resin has the advantage of having a moderate cost which helps to lower the binder cost. Furan resins have intermediate characteristics and provide good cold mechanical strength and polymerization speed.
For the Hot-Box process, it is necessary to use alkali-free quartz sand, which must be washed and dried, and the mixture must be composed of 1.4 to 2.3 parts of resin and 0.25 to 0.45 parts of hardener for 100 parts of sand.
The addition of additives allows the fluidity and preservation of the material to be controlled. This process offers advantages such as high productivity and forming precision, but requires a high amount of energy for heating and can generate harmful substances.
The process involves the use of sand combined with phenolic or furan resins and ammonium salts (phenolic-furanic).
The two options differ in the fact that phenolic resin is subject to gas development but good for avoiding crack formation, while furan resin develops less gas and is more rigid, facilitating crack formation (if the core is rigid during the metal casting phase, cracks with relative metal penetration can be created, forming “rat tails” or small crests on the casting).
The system varies depending on the thickness and configuration of the casting, and the sand must be prepared just before use. This method is more suitable for the production of cores with reduced thickness and dimensions. If the core were too large, the curing time would be excessive.
To prepare the mixture, it will be necessary to use a mixer together with a Cold-Box moulding plant. The most suitable mixer for solutions containing solvents is the intensive or scraping blade mixer. After mixing, the mixture is conveyed into a removable cartridge to allow cleaning of the mixture that can accumulate inside. The cartridge helps to convey air onto the mixture and to fill the core box. Finally, the Cold-Box type moulding machine, with suitable modifications, can be equipped with a core box heating system, which can work from 180 to 240°C.
In the Hot-Box method, simply mixing the resin with sand and then curing it in the heated core box is not sufficient, as the binder polymerization would be too slow. It is necessary to add an appropriate catalyst to the system, which is usually an ammonium salt of a strong acid to accelerate the process.
Special additives are also added to the catalyst to accelerate catalysis, improve the workability of the mixture, reduce annoying emissions during core extraction, and improve shakeout after casting.
- A mixture of quartz sand and fluid synthetic resins with a hot-acting catalyst is used
- The mixture is sprayed at 180°C-250°C onto the pattern or in the heated core box where it hardens by polymerization
- Final hardening occurs subsequently
- The castings have a weight ranging from 1 to 100 kg and a good surface finish.
- Use of toxic and harmful catalysts
- High costs, especially for heating the mould
- Variable regeneration possibilities depending on the type of resin
- Various environmental and safety issues for the operators
To carry out Hot-Box moulding, some fundamental components are necessary:
- The sand, for example, must have specific mineralogical characteristics, such as a monocrystalline quartz structure of 99.7%, no more than 0.2% of clay, no more than 0.07% of calcium carbonate, and a grain size between 40 and 60 A.F.S; the grain size must be uniform and not exceed a mean value, while the amount of dust must not exceed 0.02%
- The resins used are thermosetting furan type, composed of condensation products between furfuryl alcohol, urea, and formaldehyde
- The catalyst consists of a mixture in an aqueous solution of ammonium and technical urea salts, which combine to form the acid necessary for the resin hardening reactions
For Hot-Box and Warm-Box moulding processes, three different categories of binders can be used: phenolic, furanic, and ureic.
Phenolic binders are obtained from the condensation of phenol with formaldehyde;
furanic binders are obtained from the condensation of furfuryl alcohol with formaldehyde, and
ureic binders are respectively obtained from the condensation of urea with formaldehyde.
By combining these resins, it is possible to create different sub-categories of binders, which are the most used in foundry, such as:
- Phenol urea
- Phenol furanic
- Furan urea
- Phenol furan urea
To fluidize these binders, a mixture of vegetable pitch (cellulose processing waste) and non-hazardous organic solvents is used.
Adding the fluidizer to the sand, resin and catalyst mixture in the Hot-Box process allows for several functions:
- Increases the flowability of the mixture during shooting
- Facilitates the detachment of the core from the core box
- Has a hydrophobic function to improve the preservation of the cores over time
- Improves the core sand after casting
- Prevents the rapid drying of the mixture in the presence of high temperatures
The fluidizer is introduced into the mixture or pre-diluted in the catalyst.
Other additives are added to improve the life of the mixture and reduce the drying effect of the sand in summer.
In winter, sometimes they are added at the end of the working day to keep the mixture “shootable” until the next shift starts, thus avoiding cleaning the core shooting head, but this leads to a slight increase in cooking times the following day.
The use of Hot-Box technology offers advantages such as:
- Reduced gas formation during casting,
good mechanical resistance
- A satisfactory emptying of the casting
- A good productivity when working with thin cores
However, there are also disadvantages, including:
- The high cost of metal core boxes
- The short lifespan of the mixture in hot environments
Types of mixers
Types of mixers for the preparation of mixtures
Mixers are used for the preparation of sand and resin mixtures, which can be classified into three groups. In general, these mixers can be used for all processes that involve the use of liquid resins, both hot and cold. However, it is important to carefully evaluate the characteristics of the mixers and choose the one that is most suitable for one’s needs, avoiding purchasing an oversized device.
The models of the mixers are:
- Intensive: they are able to mix a pre-set amount of sand (maximum 15/18 kg) with liquid resins in a few seconds, which are dosed through specific volumetric dosers. This type of mixer is suitable for feeding a single machine and the blend has a limited bench life
- “Scraping blades” or “muller”: it mixes sand, resins, and any catalyst through blades that scrape the bottom and walls of the container. In this case, the dosing of the resin can be done manually or automatically.
The product is suitable for blending high-density resins and silicates. When using this type of mixer, care must be taken with the mixing time, especially at high ambient temperatures, because the mixture could overheat and cause the loss of product properties. In colder periods, instead, the mixing time can be extended to bring the mixture to the right temperature and improve sand coverage, facilitating mixing for core filling
- Continuous: to obtain the desired quantity of sand and resin, thanks to a calibrated gate and dosing pumps. It is recommended to empty the auger every time the mixer is stopped, in order to always have a fresh blend available