Author Archives: george

Its Snowing

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“Snow” doesn’t beautify a interior

Snow for Christmas day like we had this year is wonderful. It makes the world seem clean and bright. “Snow” inside a die cast plant just makes a mess. Yes we all know that die cast dies require venting to work properly. The trick is to vent out the gas without venting out the aluminum. It is the vented aluminum that we in the die cast community call “snow”

For those practitioners who have not invented a strategy for controlling snow, it is discovered after the die is put into service. At that point in time the production demand for parts prevents implementing proper fixes. Been there. Just because it is not possible to completely fix the die at that time, doesn’t mean that it is not possible to do something to get better results. In all cases vent features on the die increase the frontal area of the molten metal. The applied pressure multiplied by the frontal area determines the tonnage required. It really starts to “snow” when the applied pressure exceeded the tonnage of the machine because the die blows open. Die vents need to be attached at the last place to fill. Do a short shot to determine where that is. Vents attached to earlier places in the fill only bleed off metal that should have been available to fill porosity at the far end. In many cases a line of weld can block these off or undercuts can stick the overflow into the die filling the unneeded vent.

Every cavity replacement for dies prone to “snowing” includes a redesign of the vent system. Waffle style vents achieve vent path shut off using less frontal area. Zig zag vent shut off works for other dies that are not as close to consuming the available machine tonnage. The knowledge gained from this Kaizen activity makes it possible to launch “snow” free new jobs out of the box.

More is Better?

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When you are choosing materials to create a chiller block- more heat conductivity is better. Beryllium copper has 5 times the heat conductivity of H13. Fast freezing of the aluminum creates the crumbly shut off that makes chill vents work. Somewhat messy as the leading crumbs fill the casting machine pit. When choosing the main insert material, more heat conductivity may not be better. Most customers do not wish to buy crumbly parts. Otherwise we would make all of the inserts out of beryllium copper. Fortunately H13 has poor conductivity. It makes it possible to get the molten metal to the far side of the mold before it freezes. Interestingly a thermocouple drilled into a H13 insert to 6mm from molten metal does not rise in temperature until almost one full cycle time has elapsed.

A few proponents of conformal cooling have chosen to over simplify. A die does not run faster simply because more is better. Quickly making good quality parts occurs when proper choices are made. In a 390 alloy (740C cast temp) valve body heat is a problem. An Anviloy ( 3 times the conductivity of H13) roto pocket sub insert proved to be more effective than a conformal cooling sub insert. ( Having tried both) The scale in the water passages of the Anviloy sub insert could be drilled out each run such that the casting quality did not deteriorate with die age.

OLYMPUS DIGITAL CAMERA

However I believe more is better when it comes to the Nanaimo bars that my Canadian wife makes for me. I can even put up with the fact that they crumble.

FORMer CONTACT

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Contact forms eliminated incoming email spam

We all quickly discovered that posting e-mail addresses on our websites made them the target for spam. This ushered in the contact form. Contact forms historically were filled out by sales people who built a relationship to the point where the potential customer would give them the information. Web designers hoped that they could force users to fill out the forms by blocking access to the website until that was accomplished. This worked the first time. Users quit filling out forms when they discovered that giving out their e-mail only made them a target for spam. The impersonal nature of the contact form and the lack of actual response by the company was to blame. This brings us to where we are today. Clicking on “CONTACT US” brings up the “HTTP/1.1 New Session Failed” error. Companies do not bother to even fix the website functionality. What went wrong with the contact form? The answer is embedded in the name. Potential customers make inquiries. Salesmen fill out contact forms. An inquiry is actually a question. Questions are one of the best indications of buying interest. In this COVID 19 era is is better to replace “CONTACT US” button with an “MAKE AN INQUIRY” button. Educate your sales people to build a relationship by trading timely response to questions asked for contact information. This is the best time to train your sale staff in remote selling. (they cannot visit in person) Yes web developers, cookies can be used to send answers to the questioner the next time they log in without pressuring them to give out their e-mail upfront. Trust takes some time and effort to rebuild. A “BUY NOW” button to replace the “CONTACT US” button only applies if you want to to do the massive investment needed to take on web players like Mc Master.

100C

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100 degress centigrade is the boiling point of water. Usually die casters wish to keep the water in the die thermal passages below the boiling temperature. This is because the cooling tower water we use contains minerals. The evaporation process that the cooling tower uses concentrates these minerals to the saturation point. If boiling occurs within the die thermal passages the minerals deposit as scale. Scale blocks the thermal heat transfer we require to make good castings. Usually the set temperature of the hot oil units that we use is greater than 100C. During operation the water within the die does not boil because it is flowing. When we shut down for the holidays, setting the hot oil units back to 100C avoids boil out of the water in the die. In many cases the die remains warm enough to avoid start up scrap even whan shut down for a few days. As the picture shows only a small fraction of the power is required to keep the die hot.

Thermal Gradient

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die details also react badly to hot and cold exposure.

If your feet are in the fire, placing a block of ice on your head will not make you feel better. Yes, on average the temperature you are exposed to is correct. The thermal gradient is the problem. Die casting dies are the same. Molten aluminum at 1200F or more one one side. Cooling water at 80F beneath the surface. H13 splits if the water is closer than 3/4 inch to molten aluminum. The thermal gradient is too high for the material. Water cooling was added to the H13 core pin forming the central shaft hole in the pictured front air conditioning compressor casting. The added water passage was closer than 3/4 inch away from molten metal. The core pin split in three shots. Maraging die steel tolerates a higher thermal gradient. This same water cooled core pin out of maraging die steel did not split. This is probably why conformal cooling details 3D printed out of maraging material achieve higher cooling performance.

Alloy

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New alloys underpin innovative automobiles

In the heyday of Detroit, popular cars like a 57 Chevy included a die cast aluminum alloy automatic transmissions. Success leads to wide spead adoption. The aluminum alloy used to make the automatic transmission became the world standard. Die cast aluminum alloy became a commodity listed on the London Metal exchange. The purchasers of 57 Chevy .autos enjoyed the advancement. This is similar to the improvement in human lives that became possible with the creation of metal alloys for making a surgical scalpel. Time marches on. Today, we would not consider using anything but a scalpel made of the best surgical steel alloy. The buying public feel the same way about thier automobiles. The enthusiasic public acceptance of electric automobiles are causing new aluminum die cast alloys to displace the old standards. The brittle corrosion prone old standards are being replaced with ductile aluminum die cast alloys that are strong without heat treat.

Thermal Barrier

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Induced convection causes the holder sides to be cold
Infrared thermal picture over die picture

Die Casting is a thermal process. The casting die removes heat from the molten aluminum and creates a casting. However the first step is to get molten aluminum to the far side of the die before it solidifies. Usually this is accomplished by rapid injection into the die. For dies creating parts with long fill paths like the 3500 ton structural casting die the picture it is also necessary to have a warm die surface. A thermal barrier to transfering heat to the cover die naturally occurs due to shinkage due to solidifaction. My guess is that the casting is no longer in contact with the cover die surface beyond half way into the dwell time. Accordingly the majority of the heat of fusion is transferred into the ejector die and slides. The net result is many cover dies require oil heating. This is especially true for thin wall structural castings with long fill paths. Since oil heaters only delivery 1/10 of the heat input of the molten metal we also need to engineer die insulation. The 100 degree F outer holder temperature due to induced convective cooling is wonderful on the ejector side where we are trying to get rid of heat. The same cooling on the cover side adds heat input power requirements to the cover side oil heating units. More practical insulation used in the example picture is implemented as a cover platen face plate. This element reduces heat tranfer to the cover platen for every die run the machine. Otherwise the cover platen would be 50 degree hotter assuming that larger oil heating units were used.

“X” spurt

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dry seal pipe threads in aluminum castings like to leak

“X” is the unknown quantity “Spurt” is the drip under pressure. Die cast engineers understand the feeling. Especially when the customer is complaining that many of the dry seal pipe thread ports are leaking. High volume mudane alumininum castings like air brake valves will be with us for a while even as sexier electric vehicle castings capture our attention. In all cases it comes down to doing the basics well.

Avoiding leaks in dry seal pipe threads is all about forming a thick skin in the cored passages. In most cases it will be necessary to tap the pipe thread into the casting without premachining. There is no advantage in going to the trouble of engineering the die to form a thick skin layer if you machine it all away. Usually it will be necessary to make the core pin out of maraging material. Normal H13 cores almost instantly split when the internal water passage is close enough to achieve the desired skin thickness. I only manifold cooling water inlet lines because it is important to avoid boiling within cores like this. An outlet manifold hides the evidence that the water flow is insufficient. Cores with this much internal cooling are easily overcooled between shots. Certain brands of die casting machines (like Buhlers) can be bought with timed water cooling valves which can stop the water flow each cycle before the core pin making a dry seal pipe port becomes too cold. Even though it is possible to use hot oil within a pin like this, the 50% reduction in heat removal related to switching from water to oil spoils the thick skin creation that is needed.

Draft

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Tall fin parts like to stick in the mold

True to my college days, I would rather write about beer when the topic of draft comes up. In the emerging electric vehicle marketplace, die casters will be charged with the task of creating parts with tall cooling fins. As you might suspect the taller fins are more likely to stick in the mold. This is due to a lack of space to add draft. Certain historic die cast parts like fan clutches also required tall cooling fins.

Only a small subset of die casters have produced fan clutches due to their difficulty. They are supported by premier die shop that have the skill to make the required details. Draw polishing the cavity making the pictured parts costs US$5000. As a part of my task to return a casting shop to profitablity, I found it necessary to teach my selected die shop how to create a mold without using draw polish. They took some convincing, so we agreed to draw polish two diagonal cavites on a 4 cavity die. As I expected the draw polished cavities stuck in the mold on the first shot. I was not surprised. Normal EDM sinking creates ribs with negative draft. It is called underburn. It takes a lot of draw polishing to restore posiitve draft

Heat Check

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Toothed machining locators help mitigate heat check

I learned die casting from a fanatic. He honed his skill to the point where he was able to achieve unbelievabe die life. In the five years that I worked with him, he never welded a casting die surface (introducing heat check) that contacted molten aluminum. I have to take responsibility for the one time that it happened. The OEM engineering group forced us to implement a panic shape change. Welding the die was the only option to meet timing. Because of his fanaticism we regulary achieved die lives up to 500,000 shots on heat check sensitive parts like transmission stators.

Torque converter performance require heats check free blades

The die casting industry is learning how to economically make the emerging structural castings. These castings are made from alloys that have a hotter casting temperature. Higher casting metal temperatures result in more heat check. Heat check begins in as little as 5000 shots. In most cases it is a good practice to use toothed machining locators The teeth ignore some heat check and improve located consistancy. When the volume dictates multiple machining nests, removing a tooth makes it possible to mark the processed casting to indentify which nest was used.