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On-Line User Help Manual | HotFlo! | |
| This module is used to calculate the main heat flow paths in the die, water cooling line placement, and predicted die face temperatures. |
The first step is to divide the die up into a number of different segments, based on their primary heat flow characteristics. This can be done using general heat flow principles, intuition and experience with other dies. General heat flow principles include:
Typical thermal segments in a Die Casting die include:
DC-CALC permits you to define up to ten distinct thermal segments, but you can have multiples of any segment, giving good flexibility of design.
The other main decision you will have to make is whether to aim for Balanced Cooling or Over-Cooling With Feedback.
With Balanced Cooling you aim for a total balance of Heat Input to Heat Output in the die. This is represented by the Heating and Cooling Power Balance at the end of this worksheet. This is an acceptable strategy for dies which run reliably and predictably. The difficulty here is that if the production of the part experiences significant holdups or changes in production rate (as some dies do) then the balance will alter and the Die Temperature will alter to compensate.
Over-Cooling with Feedback is generally a better strategy because it will still work if the die suffers from occasional brief stoppages. The water cooling lines are designed to give a very low die temperature during normal running. Thermocouples are installed in the die connecting to a Temperature Controller. When the die temperature goes below the setpoint the water flow will cease. It may also bring on die heating (Oil or Electric) if set up to do so.
If you do design the die for Over-Cooling, make sure the thermocouples are actually installed and used in production. If not the die will run very cold and probably produce castings with an unsatisfactory surface.
Enter a description of each segment to assist with interpretation. It is a good idea to draw a sketch of the die showing the boundaries of the different segments.
If you have segments in the die which are regarded as identical from a heat flow perspective, you can define just the one segment, and enter here, the number in the die. This feature is particularly useful for multi-cavity dies.
Here you define the die material for each segment. This gives you the opportunity to design your die with different materials and test the thermal effect it will have.
Here you specify for each die segment, if it is:
F=on the Fixed Die Half (insert or bolster)
M=on the Moving Die Half (insert or bolster)
C=on a moving Core slide.
This is only for clarity of your methodology and future reference. It is not actually used in the calculations, but it may help to identify thermal segments.
The alternatives here are:
H13 - Fully through hardened hot work die steel
P20 - Pre-hardened nitriding steel used in the bolsters
Be-Cu - Beryllium Copper, sometimes used for cores because of its high thermal conductivity.
DC-CALC displays the thermal conductivity of the material chosen for each segment.
DC-CALC displays the mass of the different parts of the casting as entered into the 'Feasibility' and 'Runners' worksheets.
'S&Run' is an abbreviation for 'Sprues, Runners and Biscuits'
'Cavities' means the total mass of all the cavities.
'Overflows' means the total mass of all the overflows (if any).
'Total' means the sum of the above.
Here you have to estimate how much of the total shot mass will be cooled by each segment.
Remember that there are two halves of the die and usually similar amounts of heat will be removed by each half.
Ultimately, all of the heat in the casting must be removed by all the die segments, and this line displays the % of each, based on the mass entered above, and the number of segments.
Adjust these until the grand total is 100%.
The total amount of natural cooling was calculated in the 'Production' worksheet, based on the physical size of the die.
Here you estimate and allocate what % will be removed by each segment.
The main purpose of die spray ought to be to provide lubrication to assist die opening and casting ejection. Die spray also removes heat from the die, and often large amounts. In fact, it is such a rapid way of removing heat that it often becomes a major contributor to die cooling.
However, the extraction of large quantities of heat by die spray cooling has disadvantages with regard to die life. It creates large temperature differences on the die surface, and is thought to be a significant contributor to heat checking.
It is recommended that you design the die to remove most of the heat through properly designed and placed water cooling lines. DC-CALC will help you to do that, at the same time quantifying the amount of heat which will be removed from the unavoidable spraying of the die for the purpose of lubrication.
For each segment, specify what surface area will be covered with die spray (lubricant and/or water) during the die open phase.
Estimate the rate at which the die spray will be applied to each segment. You might have to measure the output rate of a few of the typical die spray nozzles used in your plant to get a more accurate value.
How long do you expect the die spray to remain on while the die is open?
DC-CALC assumes that each segment will be sprayed for the same length of time and that any adjustment of spray between segments will be done by adjusting the output rate of the nozzles.
If the Spray Time exceeds the Die Open Time, a warning message will appear. You must either reduce the Spray Time, or else go back to the 'Production' worksheet and increase the Die Open Time. This will, of course have the effect of decreasing the heat input, and increasing the product cost, both of which will be immediately reflected in the appropriate DC-CALC outputs.
DC-CALC displays the heat transfer coefficient of each segment based on the spray cooling settings selected above.
There are three main fluids used to remove process heat from Die Casting Dies:-
Water has by far, the best heat removel rate due to its heat capacity, heat transfer coeficient and high flow rates. It should be used in thick walled castings, when there is fast production rates and in regions requiring cooling channels with a short length or a small diameter. In other words, water can achieve a high cooling power.
Oil comes next in the cooling power ranking, and can be classified as a medium cooling power fluid. However, the advantage of oil is that it can also be used to heat the die, either during a production halt, or prior to start-up.
Of the three common coolant fluids, air has the lowest cooling power, but there are some instances where it the best solution. If high die temperatures are required, say over 250 degrees C, then air can be considered, especially if the cycle time is long. Another application is the cooling of long thin moving slide cores, where it would be impractical to fit water lines.
Here you indicate which type of coolant fluid will be used for cooling each segment. W = Water, O = Oil and A = Air.
Here you indicate the effective cooling temperature of the fluid. Typical values are :- Water 60 degrees C, Oil 120 degrees C and Oil 180 degrees C.
DC-CALC will display these default values, but they can be overridden according to the application.
Cooling Channel Type
DC-CALC allows for three types of cooling channels, 'Line', 'Fountain' and 'Core', all defined below. For each channel, you must specify which of these three types will be used. The data entered into the cells appropriate for each type of channel is used to calculate the temperature of the die in each segment.
Regardless of the type of cooling channel used, the rate of heat removal is strongly influenced by the distance of the fluid channel to the cavity. Here 'cavity' refers to any region of the die which accepts molten metal, including - the castings, runners, sprues, overflows and biscuits. (Note that the Shot Plunger on a cold chamber machine can also be defined as a heat flow 'Segment' in DC-CALC.)
'Line' Cooling Data'Line' Cooling refers to a cooling channel through a die insert which essentially runs parallel to the cavity as in the illustration to the right. |
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Enter the average width of the cavity which will be cooled by the channel.
Length of 'Line'Here you enter the 'cooled length' of the line. Often it is a good idea to reduce the effective cooling length of a line by drilling out the hole to create a clearance to the threaded connector, as shown here. The 'cooled length' of the line is the dimension L1. For such 'Line' Cooling, the 'Cavity to Channel Distance', as described above, is the dimension D1. |
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Enter here the effective length of the casting running parallel to this cooling line. On the sketch above, it is represented by the dimension L2.
DC-CALC calculates this factor as an intermediate step to establishing the heat flow rate. The channel factor depends on the width of the casting and the distance to the channel, and is turned into an efficiency coefficient for calculation purposes.
'Fountain' Cooling DataFountain cooling, also called 'siphon' or 'point' cooling, consists of a hole drilled into the die insert, essentially at a right angle to the cavity. Coolant travels up a smaller pipe inside the channel, flows past the end of the hole, and returns to drain along the outside. It is a very effective way of cooling the central region of a casting without over-cooling the outside edges. Always put a spherical radius at the end of the hole to prevent creating a crack stress point inside the die insert. |
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Cooled Cavity DiameterSince this type of cooling essentially flows radially towards the fountain, you must estimate the effective cooled diameter of the casting, or part of the casting. This is shown as dimension D2 in the sketch to the right. The 'Cavity to Channel Distance' for this type of cooling is shown as dimension D3. |
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'Core' cooling refers to a fountain cooling line (described above) which is surrounded by the solidifying Die Cast alloy. It is extremely efficient cooling because the shrinkage of the alloy around the core creates a very high heat transfer coefficient. In some instances, this type of cooling is too severe, and can lead to cold flow, cracking of the casting or ejection problems.
Fluid Channel DiameterEnter the internal diameter of the cooling channel in the die. This is marked on the sketch as D4. Cast Surface DiameterEnter the diameter of the outside of the core. If it is tapered, or non-circular, estimate the effective equivalent diameter. This is marked on the sketch as D5. |
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Here you enter the effective cooled length of the internal coolant channel. This is marked on the sketch above as L3. The section of cooling channel below the casting is not very effective in removing heat compared to the section within the core and can basically be ignored.
All the heat going into the die each second, must be matched by heat going out of the die somewhere else. This can be represented by the Power balance equation:
(Casting Heat Input) + (External Heat Input)=
(Natural Cooling) + ( Spray Cooling) + (Water Cooling)
Given the Natural Cooling and Spray Cooling conditions, the Fluid Cooling removes the balance of the heat. We can vary the design of the fluid cooling channels and the fluid flow to control the surface temperature of the die.
This comes from the cycle time and ejection temperature specified in the 'Production' worksheet, the mass of the casting specified in the 'Feasibility' and the mass of the overflows specified in 'Runners' or 'Production'.
You might want to try changing the ejection temperature and the cycle time to see what effect that will have on the die temperature.
Enter the amount of any external heat input, for each (or any) segment. This would include electric heating elements placed in the die to improve surface finish or die filling, as well as any gas heating applied to the outside of the die when it is production.
If external gas heating is applied during production, you will need to enter its heat input to the die. To establish this value, apply the gas nozzle to a container of water and measure:
Then use the following equation to find the heating power of the nozzle, in kilowatts.
Q=4.19 x L x (T2 -T1)/t
For Example: If it takes 2 minutes to heat 10 litres from 25 degrees C to 30 degrees C, then the heat input is:
Q=4.19 x 10 x (30-25)/120
Q=1.74 Kw
This comes from the '% of Natural Cooling in the Segment' which you entered above.
This is the result of the area of die spray, spray rate and spray time which you entered above.
This is the balance of the heat, which must be extracted by the cooling fluid.
These are the die surface temperatures in each segment that will result from all the inputs above. In general, you should aim for the die temperatures to be relatively even, because this generally minimises operational problems such as distortion, cracking, ejection hang up, and surface imperfections.
It is also good practice to get the die surface temperatures less than the target, and control it through the water flow. See Over-Cooling With Feedback above. By placing thermocouples in the die, the process can be quite easily automated. In other words, design for over-cooling, but adjust it with a feedback loop. This is particularly useful in cooling cores, moving slides, and the central regions of a casting.
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In the sample chart above, segment 4 is a good candidate to use automatic cooling with feedback. A thermocouple in the die cavity insert, near this cooling channel, can control the water flow, maintaining the desired temperature. The other cooling channels have insufficient cooling power for automatic control at this production cycle. Slowing the production cycle, or increasing the cooling power are possible improvements.
After you have designed the die and it has been manufactured and trialled, it will be time to run it under production conditions. At this point, you will want to make some measurements to see if the thermal system is performing as per the design criteria. This provides useful feedback for future designs and may identify where the die manufacture has not followed the design. This section of DC-CALC enables you to take some simple measurements, enter the data, and quickly get the answers. There are two types of measurement to take.
This refers to any channels in the die cooled by water.
Method
While the die is running in production, re-route the outlet water from the die into a container of known volume. You may for example use an ordinary plastic bucket, or a larger industrial container. The larger the volume, the more accurate the measurement. Measure the temperature of the water in the container(Outlet Water Temperature) and the length of time it takes to fill the container (Time Taken). Then measure the temperature of the water going into the die (Inlet Water Temperature). Repeat this for each water cooling channel while the die is in production. Now enter the data into DC-CALC.
Below is an example of how the data is entered.
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Cooling Power -actual, is the result of your measurements. This is the rate of heat removal by this cooling channel, as measured. Note that the "Total" value, is the sum of the individual Cooling Powers multiplied by the "Number of Identical Segments" data in the first section of the worksheet.
Cooling Power -target, comes from the data entered in the design section of this worksheet, and Variance is the ratio of these two, expressed as a percentage.
Where you find large differences between the designed Cooling Power and the measured Cooling Power, investigate further. Measure the temperature of the die which relates to this cooling channel. Does a high cooling power correspond to a lower die temperature? Does a low cooling power correspond to a high die temperature? Are these within the design limits?
Where die temperatures are adversely effecting the casting quality or production rate, and these correspond to unfavourable Cooling Power values, you know where to make modifications to the die. There are many things you can do to correct the problem. Changing cooling channels sizes, moving them closer to or further from the cavity, adding or removing channels, adding insulation gaps, adding insulation plates, adding electric heating, adding oil heating. Once you know where the problem lies, and have measured its extent, you are in a position to correct it and start making high quality castings.
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This chart quickly shows the Cooling Power of each channel compared to the original design target. In this example, channel 4 greatly exceeds the target. A good solution would be to introduce automatic cooling control to this channel with a thermocouple. Water would be turned off when the set die temperature is reached. This is the best way to reduce the cooling power. |
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An important factor in evaluating of the thermal performance of the die, is the measurement of the "Temperature of the Casting at Ejection". It turns out that this is rather hard to measure accurately. If you use a hand held thermocouple probe, by the time you get it firmly in place on the casting it will have cooled a few degrees. If you use an infrared detector, the shiny surface of the casting will give an inaccurate reading due to the unknown emissivity value. If you spray the castings with some black paint, that will give a more reliable reading, but the spraying will cool the casting by an unknown amount. If you put some black tape on the casting, at which to point the infra-red detector, that will help, but will also take time, and perhaps remove some surface heat at the same time. It is worthwhile trying all these methods, because some measure is better than none.
However, an alternative is to measure the heat content of the casting directly, and use that to calculate the ejection temperature.
Method
Take a water container large enough to hold about 10 or 20 casting shots so that they are completely immersed in lots of water. It is best to use a container with good thermal insulation. Plastic is a good material, and if it can be placed on a mat to stop heat leaking through to the floor that is excellent.
1. Measure the temperature of the water in the container as it came out of the tap.
2. Drop 10 or 20 castings into the water as they are ejected from the die running in production.
3. Let the temperature of the water stabilise. Stir it a bit to ensure the temperature is even. You want all of the heat to come out of the castings and into the water. You want the water and the castings to be at a new, higher temperature. Don't leave it too long, because you don't want the heat to escape to the atmosphere. Measure the new temperature of the water.
4. Accurately measure the production cycle time (in seconds), the weight of a complete shot (castings, runners and overflows) and count the number of shots added to the water.
4. Enter the data into DC-CALC.
Outputs
The Target Cycle Time comes from the Production worksheet.
The Target Shot Mass comes from the Feasibility worksheet.
The Target Metal temperature comes from the Feasibilty worksheet
The Variances of these are all expressed as a percentage.
The "Heating power from casting" is calculated from the measured values above. The Target value comes from the Production worksheet, based on the target cycle time as well as the designed mass of the casting, runners and overflows.
The "Calculated ejection temperature" is displayed, based on the heat content measured in the castings. Compare it with values obtained using hand held thermocouples or infra-red detectors. Are they generally in agreement? Use this exerience to guide you in entering data for new casting die designs.
The Target value comes from the Production worksheet.
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This chart shows the Heat Input from casting as calculated from these measurements compared to the value used in the Production worksheet. The cycle time has a significant effect on the Heat Input. |
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Conclusion
If any of these "Actual", "Measured" or "Calculated" values varies largely from the original design data, you should go back and review the design. Try entering these measured values back into the appropriated DC-CALC cells. How much does this effect the thermal results? Is this the cause of inadequate casting quality? These are all useful elements of the feedback loop to improve die performance and avoid mistakes in the future.
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