Release Agents for Die Casting: General Aspects and State of the Art
Some basic features are common for all kind of release agents (R.A.). They are:
Incompatibilty with the product to be demoulded both chemical and physical. If moulded material absorbs the release agent, release power is extremely poor and surfacial defects are highly probable. Lubricating effects to help moulded material flow and moulded item extraction.
R.A. should not be washed off mechanically by moulded material flow.
In the case of die casting a basic property is thermal resistance at high temperature.
Materials technically available in die casting release agents formulation are mineral oils, special silicones, waxes and esters.
We need also surfactants because our products are water based.
The job in formulating release agent is mostly empirical and requires a lot of experience. A set of analysis the obtain a coherent picture of thermal resistance of materials commonly used was not exaustive because of the instruments limitations.
Die casting R.A. are water based because the water evaporation heat is very useful to cool down moulds. For this reason are used at high dilution.
Photo of "Best in Class" Magnesium Front Sub-Frame, Meridan Lightweight Technologies Inc., Ford Motor Company, Strathroy, Ontario Canada on display at the 121st AFS Metalcasting Congress, April 27, 2017, Milwaukee, WI. Material: AE44 Magnesium Process: Cold chamber high pressure diecasting Weight: 23.1 lbs. Application: Automotive mounting structure for steering, gear, sway, bar and engine roll restrictor. Originally designed as a weldment of steel stampings, the magnesium diecast structure reduced mass by nearly 9 lbs. and consolidated 15 parts into one large casting with corrosion mitigation and load distribution parts. The subframe was a CAE-driven design with stringent stiffness targets. The final design met all stiffness, durability, dimensional, and strength performance requirements, as well as requirements for elevated temperature proof load at attachment points.
Review of article by Dr. Giovanni Rampi, Jodovit s.r.l. - Lonate Pozzolo - Italy, 2001
When we make something by moulding normally we need a release agent or a lubricant to extract it from the mould. It is a condition common to almost all type of materials: rubbers, plastics, concrete, metals, baked foods. Die casting release agents are called in foundy world also lubricants because they must have also a very good lubricating power. Are commonly water based and applied at high dilution rates to use the great water evaporation heat to cool the moulds. We do here a short but general overview on these materials.
The necessity of using the so called Release Agents when we produce moulded items is a need common to all types of materials like, for instance, rubbers, plastics, building concrete and metals, each one with its peculiar release requirements, easy or difficult to satisfy. If we consider in general the great world of release agents from the point of view of the intrinsic properties we notice that there are some of them that are common to all kind of products that they must fulfill to reach the target of giving a good release effect.
In other words a good release agent must have the following properties-
Be inert toward the material to demould. In fact if it is absorbed, it cannot be a good barrier between the mould and the material to demould.
This condition has another aspect. Being inert means also to avoid any interference, chemical or physical, with the surface of the piece moulded to avoid defects of any kind like porosity, stains, flow marks and others.
Must adhere to the mould and be resistant to the flow of cast material. If the release agent film is removed, even in a few points, the release is compromised because the moulded piece sticks to the mould and production is slowed or even stopped. Also when it is possible to demould the piece the sticking damages its surface to a certain extent and leads to rejects.
Must have some lubricating effect and help the cast material flow in the mould.
Must have lubricating properties to help the piece be extracted easily.
Must leave no residuals in the mould which are difficult to remove.
In the die casting field we work at very high temperature. A specific condition is the thermal resistance, good enough to allow the release agent to work well in difficult conditions. I means it should resist at the mould temperature first, to form a film stable enough to survive the time between the application and the metal injection in open air that is an oxidizing situation; then at the aluminium temperature to a certain extent to provide a barrier and lubricating effect.
Lubricating properties are of fundamental importance in die casting field because aluminium is a completely stiff material and does not allow even the small deformations that help demoulding rubbers and plastics. In addition to that, in die casting moulds there are many mobile parts and so called pins that require a good lubrication to work normally. When some of these parts are damaged because of lack of lubrication, the work to repair is very difficult, takes long time and the cost in term of production loss is dramatic. For this reason in the die casting world release agents are more often called lubricants.
Everyone today knows that die casting release agents are water based. The active ingredients are emulsified (emulsified itself means emulsified in water). The practice of using water based products comes from a long time ago. Using solvent based products is very dangerous considering the temperature levels common in our field. Solvent based products are dangerous for the health, for the environment, and because of the risk of explosions. But another consideration led to the use of water based release agents: the very high evaporation heat of water is greatly helpful in cooling down the mould. The cooling effect was in the past the only way to control and reduce the mould temperature and is still important today when moulds temperature is commonly thermostatically controlled.
To put water in the ingredients which we need to use in our formulations is a difficulty of primary importance for release agents suppliers because to produce emulsions of good quality, stable at the storage under different conditions, and stable also at high dilution degree, requires skilled technicians in laboratory, skilled workers in production and very efficient plants.
Every emulsion becomes unstable when is diluted. The higher the dilution, less is stability. Another instability factor is the shear effect produced by fast recyclings and strong pumpings, like stroke pumps and these negative conditions have a dramatical influence at the dilution rates encountered in die casting field.
Last condition that makes our work more difficult today is the ecological conscience that reduces the range of products and surfactants readily available to be used without restrictions due to workers health or environment pollution.
PRODUCTS COMMONLY USED IN R.A. MANUFACTURING.
Many years ago, when die casting technology began, the products more often used were heavy mineral oils containing low molecular weight paraffines and polycyclic aromatic compounds, as they came directly out from petroleum first distillation. During the following years due to the technology evolution, heavy oils were abandoned and other materials came into use, such as natural products like refined mineral oils or synthetic. The most important synthetic products was silicones family. Resinous synthetic polymers have no chances in our field because they do not melt well but remain like semisolid or show a further polymerisation; so they make build-up on the mould and are practically an obstacle to a good lubrication. May be the release itself can be achieved but because of lack of lubricity working with them is impossible.
We can divide the products normally used by Release agents Formulators in five big families.
Mineral oils. Now we use mineral oils more refined than in the past but the content of paraffines and aromatics is still high. Lower is their viscosity and more are refined. Oil classification is based on ISO rules that set a certain number of viscosity levels (at 40°C) as oils official grades.
In our field the most widey used are ranging from 100 Cts grade to 460; 680 is less used today.
Ester oils. Are both natural and synthetic. Natural ones are vegetable oils like soybean, sunflower or castor oils. Synthetic ones are monofunctional esters like fatty acid esters with monooxydrilic alcohols or trifunctional esters of trimethylolpropane. Oleic acid is the most used fatty acid. Ester oils are widely employed in lubricants formulations but we find them only in few release agents for die casting. The reason may be found in the fact that it is sometimes difficult to emulsify them in the normal way like mineral oils. Other reasons are not disclosed.
Silicone oils. Die casting silicones are called Alkyl aril polysiloxanes. Commonly known as Silicones oils are polydimethyl siloxanes. The general family is the same but the specific properties are very different. Our silicones have better lubricant properties and are easily paintable. Thermal resistance is very high as every type of silicone.
Waxes. Today the modern technology offers us an interesting family of synthetic products. It is the family of polyethylene waxes. They are widely used because they can substitute mineral oils. Are used grades with a melting point of 100°C and when are molten their viscosity is very low. At high temperatures they behave like oils but do not contain harmful substances. Negative point is that such as materials solidify on the cold parts of moulds and can create a hard build-up.
Surfactants. Are necessary to emulsify the active ingredients. They are the bond between water and oils or waxes. The quality and the stability of emulsions depend as first instance on surfactants choice. It is very difficult to schematize them because the surfactants world is extremely wide and complex. Roughly we divide them into three wide families.
Non ionic surfactants. Are the most commonly used because their versatility and efficacity is very high. Negative point is their low biodegradability and their degradation is achieved only with very efficient plants of water treatment. Anionic surfactants. Are less but still used, because they are more biodegradable. Are less versatile than nonionic ones and are greatly sensitive toward pH variations and water hardness.
Cationic surfactants. Are not used because they have poor efficiency and because cationic emulsions are acidic. Corrosion could easily happen in acid media; cationc emulsions are not used at all in our field.
In which way we choose the ingredients for our formulations and how do we evaluate their influence on the overall performance of a release agent?. It is an embarassing question and responding to it seriously is quite impossible because we do not know what happens in the mould but we can only suppose it. We do not know which are the situations that a release agent film is exposed at, point by point, because we do not have analytical devices that can tell us what we need in the real situation. In die casting injection times are milliseconds; No one analytical device can measure temperature rises, thermal exchange, viscosity drop happening in few millisecond.
We do not know what happens to aluminum also in the mould during this short time; we make computer simulations but not measures. A release agent technician has only four tools. A generic knowledge of materials properties, his fantasy, intuition and experience. He can only extrapolate a behaviour of a certain material at supposed working conditions.
Our job is based on practical tests in production where we compare situations and performances and try to understand which are the focal points that make difference. We suppose a certain behavior and if it happens we believe to have been right, if not we exclude it, with the complication that results are often apparently contradictory. In conclusion making release agents, of any kind and for any application, is only empirism with all the negative inconveniences arising from that. We must admit we are not still at a sufficient level of scientifical objectivity.
We said above that the job of release agent technician is empirical. The hope for the future is to know with sufficient precision the liquid metal flow so that we can see point by point what is the local temperature and heat exchange ratio to see the thermal stability required. It is also necessary to know the local pressure on mould walls to establish the resistance to the washing necessary for a specified geometry. Of course thermal resistance and flow resistance are not independent but must be considered as two faces of the same problem. We must know what happens in the very short time during the injection.
Maybe the level of precision and reliability of computer simulations of moulding parameters could be already sufficient to give the information we need, or will be sufficient in a short time. But if that is also true, another point remains set in stone. This point is the knowledge of thermal behaviour of materials available for our purposes with the sufficient precision and reliability. Without it we always remain in the same empirism as before and today. Today we can obtain data on thermal stability of all materials with the existing equipments but these data are not referred to the real injection conditions; if we do not know exactly but continue to imagine and suppose and compare, we remain in the same empirism. Efforts must be done to bring light on first instants. Release agents become important only when for some instance there is a problem and they do not work well and all the production stops. When everything is running normally, these materials are put in a corner. But they represent the base, the starting point of production possibility: to extract from the mould what we produce. Without them it is not possible to make moulded items. It is very strange that an entire world, the die casting world, is based on the sole empirism and up to today no researches are made by scientifical groups to clarify the focal parameters that govern the matter. It is not easy but is necessary; the progress comes from knowledge. Click here to read the entire "Release Agents for Die Casting" article.