Casting Aluminium With Lost PLA Investment Mold

I made a home made CNC with a spindle with external Ø=65mm (Ebay link), and at that time I had available a plastic bracket that I used to hold the spindle on the machine. Right now I need more rigidity, so I decided to make two brackets and mill them to the spec.

So just to clarify, I could make the part directly on the CNC (But, where is the fun of it?), so I decided to make it by casting.

Precedents on aluminium casting:

First plaster mold aluminium casting video

First Lost PLA in plaster mold aluminium casting video

This videos show the learning curve and the improvements that I made on my process, and the actual project will add even more improvements, I will try to show you all the information that is relevant to you don´t make the same mistakes that I had already made.

Step 1: Fusion 360 3D CAD Modeling

Since we have to begin somewhere, I decided to start with CAD modeling!

I started to use Fusion 360 as a hobbyist and I enjoy it, they made lot of improvements last years.

So I made some sketch on paper to have an idea what I will model, make sure you get your dimensions, restrictions and ideas clear before start!

For the CAD model I made the sketch, extrude and add the features that I need to connect to the machine and to hold the spindle. After that I modeled the sprue, runner and gates for the casting process.

Exported the 3D models to STL files to be able to use them on Sli3r(or other Gcode generator). Just be careful about export STL file quality, if your definitions on are very rough the file will have a very low definition.

Step 2: 3D Printing

My printer was made in 2013, i used Slic3r since then, you may use your preferred G code generator.

So at this point I used the follow setup to print PLA (Ebay PLA link):

Layer thickness: 0.25mm

Bottom/Top/Vertical shells layers: 2

Infill: 8% rectilinear

Support material: Yes

Extruder Temperature: 205ºC (use your best temperature setting for PLA)

Bed temperature: 60ºC (use your best temperature setting for PLA)

I printed two spindle brackets and one sprue&gating, after that had join them together with some glue.(you can other methods for joining, like hot glue, heating both parts, ect.)

***Don´t forget to increase part size between 3-5% due casting shrinkage.

Here you can see the Step 1 & 2 video:

Step 3: Plaster Mold Preparation

The preparation of plaster mold is the most critical part of the process, mixing the components, drying the plaster, burning the PLA, you can easily put your work on the garbage. You should prepare you work before start with plaster mixing.

For the plaster (Ebay example) I used more or less 1:1 water/plaster ratio and I added 2/3 of plaster weight in sand to improve mechanical resistance after drying the mold. If you use cold water the plaster setting time will increase, I just cooled the 5L bottle on the freezer until 5-8ºC.

You should calculate the total volume of plaster that you need, in my case it was a box of 20*12*20cm=4800cm^3, you could use USG plaster calculator to calculate how much plaster you need.

I used:

Water: 5L (cold water 8ºC)

Plaster: 4.5kg

Sand: 3kg

Always put the water in first place, never the other way, or you will get lot's of plaster agglomerates. You could add the sand alone or pre-mixed with plaster, I prefer to do it separate, so I add the sand to the water and after that I add the plaster slowly to avoid formation of agglomerates. After add the plaster wait 1-2min to wet it with water.

You should mix the slurry during 2-3min slowly to avoid air entrance, this is very important for your casting details.

After that is time to pour the plaster slurry, just do it slowly to avoid air bubbles. ***Tip: Fix your PLA part somehow, the part will have lots of buoyancy (see the video)

Let the plaster set for 20-30min, and here you have the plaster mold!

Step 4: PLA Burnout

This is the most important part of the process, fails that you should expect that will ruin your casting:

-Cracks on the mold

-Broken mold

-PLA not fully burned

-Particles inside of the mold

How to avoid cracks and keep good mechanical resistance on the mold:

Every plaster/water ratio will change this a little bit, but you can see on the image above how I normally do the mold drying and PLA burn out.

You should dry the water from the mold during 2-3h at a temperature between 100-120ºC, this should be done slowly, if you increase the temperature too much the water will be retained inside of the mold due wall calcination.

After that the mold still contain residual water, this water SHOULD BE REMOVED before casting or it may lead in mold EXPLOSION during casting!!! So to avoid that you should dry the mold during 2-3h at 200-240ºC

To burn the PLA from the mold you should heat the PLA more than 400ºC, remember that plaster has a very low thermal conductivity, so this process will take a long time and is always better to let the mold more hours in this step to assure that all the PLA was burned out. I maintained the temperature at 500ºC during 6h with forced air in to the mold, this worked out form me.

Step 5: Aluminium Casting

This step I think that don´t need too many details, there are lot's of information about how you can melt the aluminium.

Just tree tips:

-Always use protection!

-Always melt more aluminium that what you need, you don't want to ruin all the work on the last process!

-When you start the pour, don't stop until the mold is filled.

Step 6: The Casting Result

I would say that the result was very good, the detail on the parts exceeded my expectation!

Anyway, there are a lack of feeding on the top of the parts, next time is good idea to implement some feeder.

You could see on the last image that the initial slug has been retained on the runner, this was done on purpose to avoid the initial slug to get on the parts.