Release agents are of central significance in die casting technology.
Excerpt from the January 2020 Issue of Metalworking World by Fabio Boiocchi
Release agents affect the quality of the cast parts, the lifetime of molds and length of casting cycles. Release agents must be matched with the respective casting material and should meet high requirements in terms of economic efficiency and environmental compatibility.
Release agents are indispensable for the trouble-free production process in a die casting foundry. The most widely used release agents for die casting are water-based chemicals . They form a separating film between the casting mold and the casting material during the casting process and cool the casting mold, which has a strongly elevated surface temperature after a casting process. In the long run, such a temperature load – in the case of aluminum, for example, up to 500°C  – would cause severe wear of the mold. Therefore, release agents also protect the casting molds.
A release agent, in many cases a wax and water emulsion, is sprayed onto the shaping surfaces of the mold by a spraying device that moves into the opened mold between two casting cycles. While the water evaporates, cooling the mold, the paraffin remains on the mold surfaces and generates the separating film. It prevents the melt from sticking to the mold and facilitates the removal of the finished casting from the mold.
In addition to their releasing and cooling ability, release agents must possess several other properties, such as a good wetting capacity, chemical stability at high mold temperatures, environmental compatibility and the ability to not clog the channels and nozzles of the spraying system and the gaps between the mold's moving parts. Water-dilutable release agents contain additives that cause the organic raw materials to form a stable mixture in the aqueous phase and protect die casting machines and molds against corrosion. A biocide protects the release agent concentrate from microbiological attack and the decomposition associated with it. Release agents should also meet the demand for resource efficiency, so that for each casting cycle only small amounts are needed and the expenditure of recycling or disposal is low. When using micro-spraying technology, further savings can be achieved in reduced wastewater disposal. With this technology, the least amount of releasing and cooling liquid is sprayed onto the casting mold.
Water-based and water-free release agents
Regarding die casting, a distinction is made between the use of water-based and water-free release agents. They can be divided into three groups according to their basic raw materials .
1. The first group consists of mineral oils, semi and fully synthetic oils and oils obtained from renewable raw materials (ester oils). Oils have the property of spreading well on the casting mold, and ester oils also have a favorable effect on the CO2 balance. The use of such oils, however, is limited by the maximum permissible mold temperatures.
2. The second group comprises synthetic waxes produced from crude oil. Their particular strengths lie in promoting a good flow of the molten metal and a good adhesion (adhesiveness) on the mold. This is important in order to avoid metal adhesions. Further advantages are a higher temperature resistance compared with oils and a good mold removal capability at higher mold temperatures. Wax-based release agents, however, tend to form deposits.
3. A third group, which is relatively young, are release agents based on R-polysiloxanes. They adhere well to the mold surface, promote the flow of the molten metal and the formation of high-quality casting surfaces and ensure an easy removal of the die cast components even at very high mold temperatures. Unlike wax residues on cast part surfaces, residues of release agents based on an R-polysiloxane, which are due to its decomposition products, are relatively easy to remove.