Cold Box Process

Fundamentals
As the name already indicates, the PU cold box process is based on the reaction of two components with polyurethane. A polyaddition of the part 1 component, the phenol-formaldehyde resin, and the part 2 component, the isocyanate, is initiated through basic catalysis, usually by means of gassing with a tertiary amine.

Performance parameters
The hardening reaction is very fast, which makes the PU cold box process attractive for the highly productive production of series components in particular. The high strength level enables fast and automated core production with process reliability. The cores can be cast just a short time after production and feature high thermal stability, which also allows the dimensionally accurate production of water jackets or oil duct cores. Due to their almost pH-neutral properties, high proportions of mechanically or thermally treated used sands from cold box production can be reused.

Properties and advantages

  • Rapid model change possible (cold core boxes)
  • Excellent thermal stability
  • Short cycle times and high productivity thanks to rapid hardening
  • Secure core extraction, low core fracture thanks to high initial strength
  • High dimensional accuracy
  • Smooth core surfaces
  • Low tooling and energy costs

The main reason why the cold box process is so successful is that it makes it possible to achieve complicated core geometries with high dimensional accuracy and high productivity. When looking at the overall process of core production, the cold box process is distinguished by the fact that the cores that were shot can be mounted to core packages and coated directly after production, i.e. short cycle times are possible from the shot to the core that is ready to use. Excellent disintegration after casting and various possibilities of regeneration with very high reuse rates round off the picture.

Fundamentals of the process
The components, phenol-formaldehyde resin and isocyanate, are mixed with the mold material, compressed in a core box and hardened with a catalyst. The addition rates can vary depending on the application and mold material; in relation to the mold material, they are usually between 0.4% and 1.2% per part. The binder bridges that develop during the reaction (see SEM photo) ensure that the molding material compound is stable. After casting, the casting heat has weakened the binder bridges to the extent that the sand can be removed from the cast part by means of mechanical input.

Advantages
Economic and automatic core production and excellent possibilities for “online production.” The cores are inserted into the ingot or green sand mold as soon as possible after production and then cast. The productivity is about twice as high as that of the Croning method.

Requirements for cold box
The most important prerequisite for standing one’s ground in the international competition is to produce high quality cast parts with intricate geometry at a reasonable price. The most important market requirements for the PUR cold box resin are:

  • High reactivity
  • Reduced emission and odor pollution or a low concentration of monomers (free phenol and free formaldehyde)
  • Reduction of amine consumption
  • Long processing time (bench life) of the sand mixture
  • High core box cleanliness
  • High strength level
  • High thermal resistance (thermal stability)
  • High stability with respect to water-based coatings (hydro-stability)