Introduction: Casting Coasters
In this project you will create a master mold of a coaster then use a silicone mold to be able to produce many copies of that design.
This project will require the use of a CNC milling machine.
Step 1: Start Laying Out a Vector Design
Vector designs use points and lines rather than pixels. These file types are much easier to use on CNC machines.
For the design I suggest using Inkscape (free open source software). It works well for designing vector graphics. Make sure your design looks good as a black and white image. Also consider limiting detail to make the CNC milling process easier.
Here I choose to do two example designs. The family crest and the logo of my friend's company.
Step 2: Trace a Bitmap to Add Elements to Design
If you have trouble finding a vector (svg) graphic and are not an artist yourself, there is another option. Find a black and white silhouetted image in PNG or JPG formats (known as bitmap or raster formats). You can import this to Inkscape (software mentioned earlier) and use the "Path" > "Trace Bitmap" tool to convert you pixel based image to one that is vector based and easily imported to the CNC software.
Step 3: Design and Generate Gcode
Take the SVG formatted design and upload to a program that can generate Gcode. Gcode is simply a description of the past that the tool must follow to cut out the desired shape. The software you use will depend on the CNC mill you will be using. You can see the family crest generated in Easel.
Assuming you will be using Easel you open a new project and select "File" > "Import SVG". From there resize the design to about 4 inches in width and height. You can use the "simulate" button to get a visual preview of what the cutting path will look like.
You can experiment with seeing how different bits will impact the design and cutting time. It is very likely it will take a long time to cut, this is why creating a mold to make copies is more efficient.
Step 4: Create the Mold Pocket
We dont just need to cut out one coaster but create a mold from which many can be produced. We do this by creating a pocket where the silicone (mold making material) is made in the master mold.
The overall process is:
Master Mold (wood) --> Production Mold (Silicone) --> Final Coaster (Plastic Urethane).
The master mold must be created so that the silicone mold has a rim around the edge to hold in the plastic in the final step.
To do this make a big circle in the design software and set to a depth of 3/8th of an inch. Then a second smaller circle inside the first at a depth of 1/4th of an inch.
You may wish to CNC this out with a larger bit than when you cut the design as a time saver.
Step 5: Finalize the Design in the Pocket
Before cutting on the CNC you will need to locate the design inside the pocket. You can do the entire things in one step, but since you will need a small cutting tool to get the detail you want that might take more time than you want. Another technique is to rough out the general pocket quickly with a bigger bit, then in a separate operation, with a smaller bit go back in for the detail.
Step 6: Cut the Desgin
Let you CNC run, and cut out the designs you have created.
Step 7: Prep the Master Mold
Right out of the CNC the mold was a little rough. With sanding paper and a knife I was able to clean it up. Tip: by using a wire brush on the grain I was able to create a deeper texture which will show up in the coasters you cast.
Further there were some places where important details of the design were chipped away. With woodfiller I was able to fix some of these trouble areas.
Step 8: Create the Silicone Mold
The silicone comes in two parts that must be combined. Smooth On has a number of products which work well for this, OOMOO is a smart choice. This (or similar) can be purchased at Online at Reynolds Advanced Materials or your local SmoothOn distributor.
Basically you mix two liquids together and they harden in the mold to make silicone rubber. Make sure it gets into all the fine details of your design.
There are a lot of other details about making good molds so I suggest you watch this video to get an overview of the capabilities of the material and process.
Step 9: Create Resin Coasters
This will be another two part mix. Again Smooth On products are a good choice here for example Smooth Cast. There are different options with different cure rates.
They will produce heat as they cure and you can likely see the change as they harden first in the thickest areas.
Be careful to avoid bubbles.
Step 10: Demold and Repeat
Once cured take the plastic coaster out of the mold then repeat the process to create as many coasters as you wish.
Step 11: Stain and Finish
Stain the resin coasters like you would any wood product. Expect to do some experimentation here to figure out what look is best.
Add finishing touches like gluing green felt or a thin layer of cork to the underside.
Let dry and enjoy.
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