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.

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.

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

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.

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.

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.

One of the things needed for the instrument is a special leaf spring. Whenever I do a project that needs a special tool I use the project as an excuse for getting or making the tool. The spring is to be made of a special stainless steel that has to be hardened after cutting it to size. The material is 17-7 PH stainless steel. The PH stands for precipitation hardened. The process for hardening this steel is considerably different than for high carbon or tool steel. Basically for tool steel there are three heat treatments annealing, hardening and tempering. Annealing consists of heating the metal to above it’s curie temperature, the point where it becomes nonmagnetic, and then allowing it to cool very slowly. Hardening consists of heating the metal to above it’s curie temperature and cooling it very quickly. Tempering consists of heating hardened steel to a specific temperature depending on the degree of hardness desired. For the 17-7 steel there several different processes one even includes cooling to -100F and holding for 8 hours.

Luckily I need the hardest you can get called condition  CH 900 and the material I have to start with is in condition C. To get from condition C to condition CH 900 all you have to do is heat to 900 +/- 10 F and hold for one hour followed by air cooling to room temperature.

To keep the temperature within plus or minus 10F will require some sort of active temperature control.  The other day I spotted a nice all stainless steel pizza oven on sale for about half price. This will be my basic oven. It has nice Calrod heating elements rather that just some Nichrome coils. I have also ordered some ceramic blanket insulation to help it get to and survive operating at 900F.

On Ebay I spotted some nice digital temperature controllers for not too much money. Putting this all together should give me a nice heat treating oven for the springs. I also see it being useful for tempering tool steel items, annealing glass, re-flow soldering circuit boards and even still able to bake a pizza if desired.

The article in the March 2010 TCS http://sas.org/tcs/weeklyIssues_2010/2010-03-05/feature2/index.html on chemical kinetics inspired me to learn more about the subject. Chemical kinetics is not something that I had encountered in my meager chemistry education.

Background

Basically chemical kinetics is the study of the rate at which chemical reactions occur
and what factors affect that rate. Temperature and for reactions in solutions the concentration of the chemicals involved are big factors. To keep things simple this experiment is investigating the concentration effects at a fixed temperature. Chemical reactions can occur over a wide range of rates from almost instantaneous to many
months or years. Reaction between oxalic acid and potassium permanganate is convenient to use as an educational experiment because it occurs slowly enough to easily observe but not too slow that it would take months to complete the experiment. A dramatic example of a very fast reaction would be burning hydrogen. Concrete setting, which can take years to complete, would be an example of a slow reaction. Knowledge of chemical kinetics has a lot of practical applications. For example designing epoxies with specific setting times.

The Math

For many chemical reactions a general equation has been developed that can be used to describe the rate at which the reaction occurs based on the concentration of the reactants. Other equations take into account other factors like temperature. For this experiment only the concentration will be studied. The equation is: r = kA^mBⁿ In this equation r is the rate of change of the concentration of the reactant. The constant k is used to make the numbers come out right. The concentrations of the two reactants are A and B. The exponents m
and n describe the shape of the curve or how sensitive the rate is to the concentration. In the case of the exponent being zero the concentration of the reactant has no effect on the rate of the reaction. This is called a zero-order rate. This happens because A⁰ = 1. The concentration is no longer in the equation. If the exponent is 1 the reaction rate is called a first-order. In this case there is linear relation between the concentration and the reaction rate. For example if you double the concentration of a reactant the rate will also double. A third case is where the exponent is 2. This is called a second order rate equation. Here the rate of the reaction is more sensitive to the concentration than in the first order. The rate will vary with the square of the concentration. The equation is general and would allow for any exponent but only zero, first and second order are likely to occur. When the reaction occurs the reactants are consumed and over time the rate of the reaction changes for first and second order reactions.

Integrated rate equation

We have an equation that shows the relationship between rate and concentration but what
we really need for the experiment is an equation that shows the relationship between concentration and time. Rate is the change in concentration per time, often expressed in units of moles per liter second. This is similar to speed expressed in miles per hour, for
example. If we were traveling at a constant speed, the distance traveled can be computed by multiplying the speed by the time spent traveling at that speed. Things get a bit more complicated if the speed is not constant. This is where calculus comes in handy. To further simplify things we will assume one of the concentrations in the rate equation is constant. Then by applying some calculus and integrating the rate equation with respect to time we come up with equations describing the concentration vs. time for the three orders of interest.

For zero-order

For first-order

For second-order

The concentration of the reactant is A. A₀ is the initial concentration at time = 0. k is the same constant as in the rate equation. T is time and e is the number e.

In the graph below I have made plots of what the concentration verses time would look like
for zero, first and second order reactions with some arbitrary numbers for A₀ and k. The green trace is the zero order plot and as you can see it is a straight line. The blue trace is the first order plot. The red trace is the second order plot. As the order gets higher the traces become steeper. The zero order plot shows the reaction continuing beyond zero concentration. This actually cannot happen in real life as there is no more reactant to
consume at this point. The first and second order plots never actually reach zero concentration. But in reality when the graph says the concentration is less than one molecule in the solution there is really zero concentration because you cannot have less than one molecule of a substance.

Design of the Experiment

The goal of the experiment I want to perform is to determine the order of the rate
equation with respect to potassium permanganate and the value of k
for the reaction of acidified potassium permanganate and oxalic acid.

• To keep the temperature from affecting the results, the solutions will be
  allowed to settle to room temperature before proceeding with the
  experiment.

• To eliminate the effect of the concentration of oxalic acid on the rate, the oxalic
  acid concentration will be made large with respect to the potassium
  permanganate concentrations.

• To get an idea of the color change expected, perform a pre-experiment just to observe the color change.

• To ensure enough data to plot, take at least four data points.

• To keep possible contaminants from affecting the experiment, use distilled water to make up the solutions.

In a future post I will describe the results of my experiment.