Plastic Injection Molding

My interest in science focuses on scientific instruments, learning how they work, designing instruments that can be built by an amateur and actually building them. To address the building I have acquired over the years a reasonable collection of metal working tools. I am always on the lookout for an excuse to acquire a new tool. Just recently I needed to cut and bend some sheet metal to build a seismograph. That was the excuse to buy a sheet metal brake/shear.  Sometimes the tools is too expensive to justify or won’t be used all that often. In that case I get tempted to try and build the tool.

A while back I decided to build the Gingery plastic injection molding machine. Years ago I started to build a different machine but it had lots of parts and I got distracted. If you don’t already know Vince and Dave Gingery have written a series of books on building various machine tools, basically from scratch.

I basically followed the plans in the book to build the machine. Of course being an engineer I could not resist the temptation to make a few changes. The plans call for most of the frame to be held together with nuts and bolts. But one connection has to be welded. So I welded all of the frame instead of using bolts. This saved a lot of hole drilling. A chunk of 1″ X 1.5″ cold rolled steel is called out for the heater block. I had a scrap piece of 1″ X 2″ hot rolled steel bar so I used that. Being bigger it allowed me to use two cartridge heaters instead of one. This will allow the block to heat up faster and more evenly. The plans call for a dial thermometer to read the temperature and a mechanical bi-metal thermostat to control the temperature. Since I had already built an electronic temperature controller with digital read out I used that for the temperature control. See http://jimhannon.wordpress.com/2011/02/24/heat-treating/  The sensor for the controller is thermocouple screwed into the heater block. Since the thermocouple I bought was made in China I assumed it would have metric threads and searched all over for a tap to thread the heater block. Turns out the threads were not metric just 1/4-20.

View of completed plastic injection molding machine

As with any machine it takes some learning to use it properly. First I just tried heating some plastic and squirting it out. This helps get the temperature set right and gives you a feel of how to load in the plastic. I cut some strips of plastic from a milk bottle. This plastic is HDPE (High Density PolyEthylene). One thing I found was the plastic sticks to the injector piston and it gets bound up in the guide block when the machine cools. The next time I started it I had to heat the piston with a torch to get it free.

The book recommends making a test mold to try out the machine so that is what I did. The mold makes some 1/2″ diameter pellets of plastic that can be used to feed the machine.

Test Mold

To try it out I cut some strips from the lid of a plastic tote which had no tote to go with it. This plastic is polyethylene. The strips are fed into the top opening of the heater block until the block is filled with molten plastic. The injector nozzle tends to drip a bit when the block is being filled with plastic so I leave the mold off until the plastic is ready to inject. Any drips can be removed just before installing the mold. If you don’t do this the drips will harden in the mold sprue and you won’t be able to inject the plastic.

Above is a picture of the mold with pellets still in in and some pellets still attached to the sprue. I was a bit worried that it would be difficult to get the pellets out of the mold but the plastic shrinks enough that they come out easily.

Coming up in the next post on this subject will be making a mold for something useful.

Making Magnalium

One thing that I like to do is make things from scratch. Particularly things that not everyone makes or can make. Magnalium is a 50 50 alloy of aluminum and magnesium that is used primarily in pyrotechnics. It is quite brittle making it easy to break up into bits or powder.

The only magnesium I have burned is the ribbon so I wanted to see what I was getting into with a whole crucible of burning magnesium. I took about 3 cubic centimeters of magnesium and heated it with a torch until it melted and started to burn.  It was one of the strangest sort of fires I have seen. There was no sparks or flame and little smoke. The metal just gave off a very bright white light and the oxide just expanded and piled up around the metal. The oxide pile was white and looked a bit like cauliflower. From that experiment I could see that the burning was not going to be much of a problem if I could put it out.

Magnesium Burn Test

There are couple of YouTube videos of people making magnalium. In one of them a lid was kept on the melt to keep it from burning. That seemed like a good approach but I thought I would also try something else. Some references mention putting sulfur on the molten metal. The sulfur reacts with the magnesium to make a magnesium sulfide that will cover the melt and prevent it from burning. I am not real sure about that as magnesium sulfide is listed as melting above 2000C. This is way hotter than the melt so it would still be a powder. Commercially when casting magnesium they use sulfur hexafluoride gas  to prevent burning. Using a gas would not work for my setup and I don’t have any sulfur hexafluoride anyway. I got to wondering if one could use charcoal granules to cover the melt. I know that molten magnesium will react with carbon dioxide but I do not think it will react with the carbon. So that is what I set out to do, cover with charcoal and keep a lid on it.

I wanted to use something for a crucible that was expendable and fairly tall so the exposed surface area would be small. In the YouTube videos a tin can was used. This is not a good idea. I have had cans burn through when melting aluminum. It would be a bit messy and waste my metal if there were burning metal running out of my furnace. Something a little heaver would be good. There was an empty torch propane tank laying around the shop so I cut it in half to use the bottom for my crucible. The inside was coated with kiln wash in hopes that the metal would not bond with the steel and make it hard to remove.

The magnesium I have is an old VW transaxle and the aluminum is from my ingot stock made from scrap metal melted down. I weighed out enough of each metal to fit in the crucible and cleaned up the magnesium to try and keep the dross to a minimum.

Metal and Crucible

In the photo above is the piece of transmission case, aluminum ingots and the kiln wash coated crucible.

Complete Melting Setup

The aluminum is melted first and then the magnesium is added. This helps to keep the amount of burning down. At the same time the magnesium was added I added the charcoal. After covering it and waiting for a while for the magnesium to melt I stirred the melt to mix the two metals. The charcoal cover was working as there was little burning magnesium.

Cruclble and aluminum in Furnace

After that the heat was turned off and I went off to do something else while it cooled down.

I came back a while later to see how fast it was cooling off a got a surprise. Something was oozing out of the crucible and had even somehow gotten on top of the lid. It was black and lumpy looking a bit like a fungus growth.

Black Ooze

I had no idea what the stuff was but it looked like using charcoal as a cover was not a good idea. The stuff coming out appeared full of bubbles and had a layer of metal just under the black coating. Under the metal was some white material.

After things cooled down enough to handle I could  look at it a bit closer. Apparently the metal had foamed up and forced its way out of the crucible. There was magnalium in the crucible. It was brittle and I could easily break it up with light hits with a hammer but it was filled with bubbles.

Magnalium Sponge

The charcoal got mixed down into the melt when I stirred it and did not float back to the surface as I expected. Each charcoal particle produced some gas the filled the melt with bubbles. It is really quite beautiful and extremely light with all the bubbles. If you look close at the picture you can see the charcoal bits in some of the bubbles. I have no idea what gas was being given off by the charcoal especially since it had sat there at almost red heat for several minutes before I stirred it up. Next time I will skip the charcoal.

KaBoom II

This year at the Memorial day weekend pyro get together we had a class on making 5 inch spherical shells. In my KaBoom post last year I described making canister shells which are cylindrical rather than spherical. Each of the participants in the class made a plastic shell and a paper shell. Much of the construction is similar except when it comes to sealing them up.

The basic idea is to build the shell in a way that it will burst symmetrically. There is a fuse consisting of several strands of black match that leads from the outside of the shell to the center. This is connected to a time fuse that is calculated to cause the shell to burst at the time it reaches the highest.

Plastic Shell with stars

Above you can see a plastic shell with the fuse extending to the center. The stars, black balls in the picture are what burn to make the color when the shell bursts. They are arranged around the inside of the shell so that they will spread out in a nice pattern when the shell bursts.

Shell halves with powder

A piece of tissue paper is then place in each shell half and filled with rice hulls coated with black powder.

Then comes the tricky part, getting the two halves together without disturbing the stars or powder.

Hit it with a stick

Sometimes tapping the shell with a stick helps settle things in place so it will close properly.

Gluing

After that the plastic shell halves are glued together. Here I am watching my shell getting glued.

It is interesting that there some modern tools in this rather ancient craft. The paper shells rather than being glued are taped with several layers of gummed paper tape. By hand this is a rather laborious task.

Taping machine

Here we have a computer controlled taping machine to do the hard part.

Clamped shells

The paper shells are then set aside to dry overnight and the plastic shells are clamped to let the glue set.

Completed Shells

Here we have the box of completed shells ready to go to the firing line. The lift charge is placed in a plastic cup and taped to the bottom of the shell with a quick match fuse. That fuse is amazing. It burns as a very high rate, something like 300 feet per second. At the firing line the end of the fuse is attached to an electric igniter so that it can be safely fired from a distance with the push of a button.

Above are a couple of the shells build in the class. You can also see a 3 minute video I made of the Saturday evenings shoot at http://youtu.be/7c-aLM5XZos

Credits: All photos by Gary Markley

Colder Than A Well Digger’s Knee

One time during my stay in the middle east I came across an interesting scene. There were two men standing around a large wooden reel with a rope extending from the reel into a small hole in the ground. The reel was supported by posts so that it could be turned by 4 handles attached to each end of the reel. It was about 6 feet long and 4 feet in diameter.  The hole was about 2 feet in diameter and there was a large pile of dirt nearby. They were digging a well. After a while the rope started shaking and the two men started reeling up the rope. They turned and turned and a large amount of rope built up on the reel. Finally a man popped out of the hole. He had one pant leg rolled up above his knee and was covered with mud. Somehow he was able to stand on one leg at the bottom of a hole at least 200 feet deep and dig.

In this part of the world they use a unique system to get water. The geology is such that up in the mountains the water table is higher than it is down lower. So to get water for a village they go up in the mountains a dig a well down to the water. Then starting at the village they dig a tunnel to the bottom of the well. Water can then run out of the well and down to the village without any pumping. These tunnels are called quants. The quants are lined with oval shaped clay tiles that can be turned sideways and slid through each other. At the village the tunnel comes to the surface and the water runs through the village in a ditch, called a jube, along site the road. It is arranged so that each household (compound) can shunt the water into a pool in the compound to be used there. The upper class live at the upper end of the jube and progressively lower class further down the stream.

Incidentally, the expression  “Colder Than A Well Digger’s Knee” only got about 1100 hits in Google. I can sure see how it came about after seeing that man coming out of the ground.

Heat Treating Part II

I Finished assembling the controller for the heat treating oven. Everything is assembled into a small aluminum box. For the AC power connections  I got a short heavy duty extension cord and cut it in half. One half to plug it in and the other half to plug in the oven being controlled. The controller itself is mounted in the front of the box along with a power switch. On the back of the box are the power cords and the terminals to connect the thermocouple.The 20 amp solid state relay is mounted to the box inside. Below is a schematic of the controller.

Here is a picture of the insides of the box.

Here is a picture of the front of the box.

Here is a picture of the back of the box. The end of the thermocouple probe is sticking out at the top. This will be inserted into the oven to sense the temperature.

The next step is to finish assembling the oven and test it to see if it will get hot enough. I have even thought of another use for the controller a plastic injection molding machine based on the Vince Gingery plans.

Shepherds

As I mentioned in a previous post I spent two years living in the middle east. There was a large mountain range just north of the city where we lived.  I really enjoyed hiking in the mountains with some of my friends from school.

Around the time we leaving to go back to the US, I wanted to visit the mountains one last time. None of my friends were available so I took a taxi to the base of the mountains and hired a local guide with a donkey to take me up into the mountains. That was some exciting donkey ride. At times the trail was only 2 feet wide with a sheer drop off of several thousand feet one one side and a wall of rock on the other side.

We rode and hiked until we were at 9000 or 10000 feet altitude. There we came upon a shepherds camp. The shepherds were probably relatives of my guide as we stopped there for lunch.

There was a pen for the sheep which consisted of a stone wall about 3 feet high and 20 feet in diameter. The wall had had a gap filled with brush to serve as a gate. Beside the pen was a rock wall about the same height and about 100 feet long. In the middle of the wall there was a gap about 2 feet wide. There was also a shelter for the shepherds that also consisted of a rock wall about 10 feet in diameter and covered with a sheep skin tent.

As we arrived the shepherds were preparing to milk the sheep. The sheep were herded to one side of the wall. On the other side of the wall there were two shepherds sitting on either side of the gap in the wall with there legs blocking the gap. One of the men would move his leg and let one of the sheep through the gap. If it was an ewe and had milk he would grab it by the hind legs and milk it into a pottery container on the ground between his legs. It was not long before they had all the sheep processed through the gap in the wall and the milk collected.

These sheep are called broad tailed sheep because instead of a small tail they have an enormous dinner plate size lump of fat swinging off their rear end. There was also some goats mixed in with the sheep in the flock.

After the sheep were all milked we were invited in for lunch. We all stooped down and went in the shelter and sat down on the ground. Lunch consisted of flat bread and a form of yogurt made from the sheep’s milk. They had put some chopped onions into the yogurt and added some fresh milk to thin it. You would tear off a piece of the bread and use it to scoop up some of the yogurt and eat it.

After the meal one of the shepherds was getting ready for a several day trip to the other side of the mountain. He packed several pots of the yogurt into a bag along with some bedding. Then he got something wrapped in cloth and opened it, inside was a large piece of mutton. He examined it closely and every so often he would cut off a small piece and feed it to the dogs. At first I thought he was just feeding the dogs but after looking closer I could see that he was cutting off the deposits of fly eggs. He did not want the meat to get maggots while on his trip.

Sitting there I realized that the only direct evidence that I hadn’t stepped back in time to the biblical era was their sandals, which were made of car tire tread held on their feet with leather straps.

PKO Factor

Way back in college one of the required courses for engineering students was economics. I guess they wanted to make sure our education was rounded out a bit. For us as engineering students the math being taught in the course was way behind what we were studying in engineering. As a consequence the class got a bit boring at times. We would often do the math part of homework using things from calculus way advanced from what the economics professor was teaching. If we got exceptionally bored in a class we would get the prof talking about his brother who had been in training to be a kamikaze pilot during the war. As in any science economics tries to measure things and fit the results to theories, often equations, that can be used to predict future behavior. In economics an important thing to try and predict is how much people will spend. By analyzing things one could come up with factors used in the equations. One of these factors is call the Propensity To Consume. For some reason  a group of us thought this was an interesting idea and came up with our own factor, the PKO factor (pronounced pico). This was the Propensity to Kiss Off. Trust me learning engineering is not easy. It seemed like we were either in class or studying. But every once in a while we really had to do something else. So one of us would go around the dorm asking the others in our group “What is your PKO factor?”. If the average factor was over 5 or so we would take off to do something else. Now if we were something other than engineering students this would mean going out drinking. For us it usually meant going to explore a cave. Now the PKO factor often got high late at night and we would get comments like “It is late and dark out your going caving now?” To which we would reply douh.