The die lubrication phase has a determinant role on both production rate and production quality.
Water based release agents are sprayed to remove most of the heat deriving from aluminium cooling and solidification, which is about 250 Kcal per kg. of alloy and create a film on the die cavity which facilitates aluminium flow and reduces physic-chemical adhesion effect.
The “sticking” effect of aluminium on steel is well known and becomes more evident when the metal temperature, speed and final pressure grow higher, also known as “metallization”.
This paper shows the results achieved “on site” with the “Smart Lubrication System” technology: die lubrication control system according to the surface temperature.
Keywords: die-casting cell, die-casting, temperature, die surface, lubrication, release agents, warm-up, temperature control, monitoring.
During die filling, excess amounts of release agents develop gas causing casting porosity and can cool excessively the die during heating transient increasing the number of scraps.
At production start up, when the die is in Warm-Up phase the optimum solution is to use release agents with specific low heat and high lubrication capacity and to be able to change the spray time during production in function with die temperature variations when the machine is in full operation.
During production the die temperature should remain constant on all the surface area and at the same cycle time, but in reality it is conditioned to the down times due to various reasons and to variations due to outside factors such as: metal temperature, cooling water temperature and in some cases also environmental conditions.
The use of die thermo regulators has increased but the excess quantity of 250 Kcal to be extracted from the die per Kg. of cast alloy is removed by the release agents sprayed on the die surface.
The cooling channels in the die have geometrical restrictions due to the shape of the cavity, presence of ejectors and need of not passing near the cavity surface to avoid premature braking of the inserts.
To obtain fast production cycles there must be high cooling capacities therefore high fluid flows at low temperature but this can cause breaking due to the temperature gradient.
Hot steels with 5% Cr used for inserts are bad heat conductors therefore the temperature difference between the surface in contact with the cast metal and where the cooling channels slide generates stretching due to thermal expansion.
The most critical phase of the die life is during cooling when the water base release agents are atomized on the hot surface of the cavities because of strong stress tractions due to contraction of superficial layers respect to the underlying part which cools much slower because of the low thermal conductivity.
It is known that the traction breaking load is less compared to compression and fatigue strength.
This shows that die pre-heating is important to speed up production but has a limited effect on the surface breaking due to thermal fatigue.
Another important factor is the effect of the die temperature on the casting quality and it is evident how die thermoregulation units are not very efficient because the fluid temperature is adjustable but not the die surface temperature. Thermocouples can be fitted directly into the die but unless they are very close to the surface, causing mechanical resistance, they have a great hysteresis and the correction effect given by the fluid temperature variation is restrained and with a very high time variable.
Using water limits the fluid temperature to 120°C (pressurized circuits) whereas thermal fluids that can reach a temperature of 350°C have a specific heat which is half the value of water therefore are not suitable to remove great heat quantities in short periods.
The simulation models used, allow to make precise simulations of the die thermal dynamics with different geometries of the cooling channels, therefore we can say:
§ Constant die temperature is necessary for product quality and for die life
§ The cavity surface temperature is the most important because it is subject to important thermal cycles
§ The most important factor for removing heat from the die is to spray release agents and to keep a thermal balance.
SLSTM “Smart Lubrication System” Technology
To allow die casters to deal with the above topics and provide an efficient device that can be used in difficult foundry environments, Idra, Baraldi and Venezia Tecnologie have developed a system that monitors the die surface temperature with infrared rays and necessary protection to allow the use near the die, the acquisition system and software for processing data and its use on the lubrication cycle.
The monitored temperature is used to control the lubrication circuit that atomizes oil at low temperature during production start-up and after cycle stops that exceed a certain period time, as well as to modify lubrication time during production when die temperature variations occur that exceed the set tolerances.
In traditional running, oil spraying at start up and injections with low speed and low pressure are manually carried out by the operator. The number of castings for warm-up and above all the ones after cycle interruptions, represent a significant percentage of the total machine scrap when producing difficult castings. To be able to manage the transition phases generates important economical advantages.
Here below is a comparison between traditional warm-up values and the ones using the SLSTM system, carried out in an important Italian foundry that has a well structured operational situation with processes and training given to operators.
Testing protocols have been determined which foresee the transition surface temperature from low speed/pressure to 290-300°C, production of 5 castings with simulated stop and die cooling using the lubricator and re-start cycle.
In Protocol 1, operations are according to the traditional method, therefore after 5-6 castings the passage to the production conditions is manual. In the protocols the points on the curves between the two blue lines represent the low pressure injections (warm-up cycles).
It was experimented that the transition temperature evidenced is at limit between good castings and scrap castings. The average number of parts scrapped at every re-start is 4,6.
Protocol 3 shows the temperature monitored to pass automatically from the anhydrous lubricant to the water base type. Considering the same transition temperature of 290-300°C. 3 reduced cycle were averaged to reach the working conditions.
The transition is automatically managed and not conditioned to interpretations or errors that can be made by the operator.
Management of restart is very delicate and involves quality staff. The risks are: lack of selecting scrap castings or on the other hand scrapping an excessive number of parts.
In both cases with repercussion on the total cost of the lot.
The average number of castings produced in “reduced” conditions was 3 with a reduction of 35% in comparison to the conditions without anhydrous lubricant and automatic management of Protocol 1.
Graph 1 and 3 above show the temperature trend for each cycle of the 2 tests. It clearly indicates how the number of cycles under the transition temperature in Protocol 3 are less and how the die surface temperature increased more rapidly.
The Warm Up Lube 04TM in effect reduces notably the capacity of extracting heat from the die during the warm-up phase. The low thermal capacity (less than half in comparison to the water type – Tab.1) brings a rapid increase of the release film temperature, reducing drastically the cooling capacity. The result is also sustained by the low thermal conductivity.
Capacity at 20°C
103 J/m3 K
Thermal Conductivity at 20°C
Density at 20°C
WARM UP LUBE 04TM
The main components of the Warm Up Lube 04TM, have been selected and carefully examined in laboratory tests and specific tests conducted directly on field for different types of castings.
The product creates a film on the die surface that has a very high lubricating capacity and guarantees smooth sliding of the moving parts for dies with movements and ejectors, avoiding cracking and braking during the delicate production start up phase. Furthermore the Warm Up Lube 04TM has an excellent separating power and doesn’t deposit oily residuals on castings, therefore reducing pollution in remelting baths.
The following table (Tab.2) was determined by thermo gravimetric analysis on a sample of 5.0 mg of Warm Up Lube 04TM: The product is heated very slowly by means of an automatic cycle (2 °C/min) and the weight variation (loss) is registered.
% by weight
As noted in the table, there is no change in the product up to 200°C whereas up to 300°C 74% of the initial weight remains stable. A temperature of 300°C is reached in an average to large die only at the end of the warm-up cycle, when the system passes automatically to the traditional release agent. Whereas at the beginning of the warm-up cycle it is normally much lower with values varying from <100°C to max 180°C.
To be noted that the Warm Up Lube 04TM boiling temperature is >300°C so decomposition starts before reaching the boiling temperature. Therefore the product is not capable of removing a significant amount of heat with the evaporation mechanism as occurs with the traditional release agent.
We have highlighted and verified that the special characteristics of the Warm Up Lube 04TM allow to pass from low to high injection speeds without defects due to die lubrication, from the first casting produced in standard conditions with high injection speed. The automatic application is very rapid and eliminates manual brush lubrication, which is a normal foundry procedure.
The Warm Up Lube 04TM has also many advantages on environment and health conditions of the working area. The product is 100 % synthetic and 90% biodegradable with no harmful or irritating effects. By replacing during warm up the use of lubrication pastes or greases or anti-sticking products eliminates the dangerous steams which the operator is exposed to.
Economical evaluation and conclusion
Considering an average size machine producing castings with a technical cadence of 65 parts/hour, excluding efficiency, with 10 stops in the range of 24 hours (approx. 1 stop every 1 ½ hours), which represent an average value for completely automatic diecasting cells producing critical castings, but without considering particularly delicate dies for which you can easily consider also twice the number of stops, you have a time saving of 15’ per day, representing 1% of the available time.
Hard to evaluate but real is the decrease of “outside” scraps not intercepted by “Quality Control” but found after mechanical machining with relative costs.
The SLSTM is a simple and manageable system to measure the die surface temperature without complicated solutions which make die change difficult. Foundry activities are already difficult therefore solutions need to be simple and robust. The proposal to use sensors in the die, like other devices, has been eliminated and put on foundry shelves.
With the feedback module, the monitored temperature is also used to control “Idra” lubricators to change spraying time and keep constant surface temperature during the process, but this will be the next topic in discussion.
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