How to make a wood casting flask

I suppose it goes without saying that you need a flask if you plan on casting anything other than an ingot.  A casting flask should be durable enough for repeated use and should be able to withstand the stresses of pounding the sand in it.  A flask should also have some means of allowing the cope to be drawn straight from the drag and allow it to be put back together assuring good alignment between the two.

I have a project that I am working on that requires me to cast a plaque that will be mounted to a cast block to use as a marker for our dog.  After deciding on the the layout and size of the plaque I discovered I did not have a flask large enough to hold it.  So I decided I needed to make one.  This article is how I done it and perhaps there is something you can take from it and use for yourself.

The Flask Size

The plaque that I am going to cast measures 8" x 12" and is 1/4" thick.  I want a couple of inches of sand around the pattern when I cast it, so the finished inside size should be 12" x 16".  I will make each half of the flask 3-1/2" tall for a total flask height of 7".

I have made flasks in the past from 3/4" pine boards with butt joints fastened together with screws.  While they worked just fine, I felt like they were a little thin and would flex when I rammed the sand in.  This time I plan to make the flask from two layers of 1/2" cabinet grade plywood.  This will make the flask 1" think and should give me more rigidity when I go to ram the sand into it.

Preparing the stock

Pieces cut out for the new flask. Gluing up the pieces for the sides of the flask. The sides of the flask all clamped up. I cut 4" wide pieces of plywood long enough to get a long and a short side of the flask from.  Cutting them wider allows me to trim them down in the event that pieces slide around and end up uneven.  I find it helpful to have a heavy flat surface to clamp my glued up pieces together on.  I have a piece of old fire door that I use for that purpose because it is thick and pretty straight.  Also, using a roller to spread the glue reduces the mess and time that it takes to glue the pieces together.  The images to the left tell the story.  You can click on the images for a larger version as well.

A Note on Assembly

Finger joints cut on the end of a test piece. Finger joints fitted together. There are a number of ways you can assemble a flask, ranging from butt joints and glue, to screws, finger Joints (box joins) to bolting.  I guess it all depends on how rigid you want it and how much time and effort you want to put into it.  My first flasks were made with butt joints put together with glue and screws.  These worked just fine for the first several molds that I made, but they did work loose over time.  For this flask, I am looking for more rigidity. This time I will make the flasks with Finger Joints, sometimes called a box joint.  This type of joint is commonly used to make drawers.  By making interlocking fingers you dramatically increase the surface area that you can glue, making the joint very strong.

Now, I don't pretend to be a wood worker with any skill, but if I can do this, i am sure you can too.  I found a couple of YouTube videos that were of a great help to me.  The first one was how to build a cross cut sled for the table saw using the 5 cut method.  This method allows you to make final adjustments to your fence to bring the blade within a thousandths of an inch to being perfectly 90° to the fence.  It is a great video and worth sharing.  William Ng does a fantastic job of putting the cookies on the bottom shelf!  I made one of these to make my flask with.  The time was well spent and didn't cost that much money.  I have embedded the video below for your viewing pleasure.

The next thing I done was to make a Finger Joint Jig to help cut the finger joints.  William has another video describing how to make a finger joint jig.  He does a great job in describing how it is made and why he takes the time to get things set up proper.  With the sled above and the finger joint jig, cutting the joints for the flask was easy as can be.  When I made the finger joint jig, I made mine to accommodate the one inch thickness material I was using for my flask sides.  I have embedded the finger joint video below for your viewing pleasure!

Flasks glued up and clamped. When I had the two fixtures completed, I set the blade height to just a little over the thickness of the material I was using for my flask sides.  Next I cut the finger joints using Williams method he shows in the above video.  Now I have to be honest.  My joints turned out to be pretty tight.  The problem was I set yo the jig and then made a couple of blade changes.  I should have made some test cuts and adjusted the jig again before I started cutting the finger joints for the flask that I made.  Next time, I will heed that little piece of advice.  Nonetheless, I got them cut out and glued together.  The image to the left shows the two flask halves glued and clamped up.  Another thing I noticed was the plywood didn't like being persuaded into position with the mallet.  A lamination or two on a couple of the fingers spread apart.  Now if I would have adjusted the fence properly, I think this would not have happened.  A better material for the flask would have been some hardwood milled down to the thickness I wanted.  Oh well, live and learn.  It will be a serviceable flask nonetheless.