Synthetic moulding sands or manufactured green sands is commonly made up of silica sand(coarse) and to this is added bentonite and coal dust or pulverised coal.

Water is required to be added as well. The moulding sand is prepared by blending the ingredients and mixing in a muller. This is then used to form the mould into which the molten metal is poured.

After cooling the shake-out occurs where the mould is broken and the product released. Some users of repeat products tend to want to re-use a mould several times and so require a different performance of their synthetic moulding sands.  

Recycled moulding sand (hot-sand) in some foundries is re-used indefinitely but there are foundries that use moulding sand selectively. It has been found that hot sand recycling does increase the rate of scrap produced (Scott, undated).

Bentonite is a common ingredient in the manufacture of green sand as a binder.

Bentonite is introduced in two places:

  • As a biding agent (40-65kg/t);
  • As part of the coal dust where bentonite (±30%) is premixed with coal dust to eliminate explosiveness.

The bonding ability of bentonite can deteriorate with successive uses so it is important to keep adding new bentonite to the mixture.

There is calcium and sodium bentonite that can be used.

For foundry engineers and supervisors we will ask for:

  • Amount of bentonite in recycled material
  • Size distribution of silica sand used (90% in #40 – #100 sieve range)
  • Current amount of water added (in % or litres H2O/ton greensand)- usually 2 – 7%

①         Deterioration Temperature

  • Calcium bentonite (aka southern bentonite) deteriorates at 600o This makes it suitable in the aluminium and aluminium alloy casting sector
  • Sodium bentonite (aka western bentonite) deteriorates at 1,180o This makes it suitable in the iron and chrome alloy mould sector
  • Activated bentonite can have a deterioration temperature between 800oF-1,100oF

②         Benefits of Calcium and Calcium-based Bentonites

  • Ideal for aluminium and aluminium alloy foundry sands
  • Flowability of sands, defined as mould densification, is better with calcium bentonite than sodium bentonite (Vingas and Zrimsek, 1964)
  • When used as a binder in re-cycled foundry sands, and no additives added, provides a more flowable sand

③         Benefits of Yellowstar Bentonite

Our FoundryStar™ bentonite has the following benefits

  • Cost effective (up to 30% cheaper) when compared with competitors products
  • Non-ferrous casting goes best with FoundrySTAR™
  • Product consistency
  • Ideal when you want greater Green Compressive Strength and easier shakeout.
  • Excellent in providing a higher mould density
  • Delivered by pneumatic truck directly to silos or in 1t bulk bags and flows freely
  • Can be used in high-production lines or bespoke lines

④ Application of FoundryStar™

  • Dosage: Works well at 45kg/t to 70kg/t dosage
  • Water: In plant use 31l/t – 33.5l/t water required to get maximum bonding effect at 55kg/t
  • Swelling: Non-swelling or low swelling;
  • Moisture: <11%. We have supplied a bentonite with 0.12% moisture (105oC)
  • Particle Size: Milled to the correct specification (<5% retained as grit)
  • Flowability: High across a range of water levels making it easier to work with and removing the need to include additives
  • Compression: Increased by increasing water or ramming energy can be increased to make a stronger mould
  • Compressive Strength: Rammed energy increases of moulds dramatically

⑤ Switching to Yellowstar’s FoundrySTAR™

Switching from your current bentonite to ours is easy and has been done as follows;

  1. Develop a trial protocol and commence with 30-60t
  2. Gather performance data and see what changes are required to the bentonite mixture
  3. Recommence trial of next 30-60t and make final adjustments at mixture and plant level.
  4. Commence new product

Duration: 10 shifts