Week 12 - Molding & casting


Molding & casting

Molding and casting are very versatile processes, especially, when it comes to producing larger qualities of goods. Injection molding is probably the most popular process for making high volumes of components from plastics, but also for metals casting, it is one of the dominant processes. Metal casting however mostly works with lost forms from sand molds held together by tailored binder materials.

In this week, we will design a part - in my case the rim of a wheel - and CNC mill the actual part from wax. In a second step, the mold is created from 2K-silicone. The mold will consist of two parts, an upper and a lower half which. The third step will involve making the rim from two components forming polyurethane resin.

Like every manufacturing process, also molding comes with certain constraints. In our case, we will have to consider the constraints of CNC-based manufacturing during the mold making process, i. e. to respect the size of available milling tools, their maximum cutting depth, the tool diameter and the avoidance of undercuts as well as the ones for the casting process like the taper angle to ensure smoooth removal of the rim and the mold. Autodesk Fusion comes with a lot analysis tool to simulate the mold making and account for casting constraints using draft analysis.

Modelling & CAM process


Fusion's draft analysis tool revealed severe violations of the draft angle constraints of moulding of my first design. Even though this is not really a problem when casting in silicone, as the material very flexible and allows for undercuts, I modified the design to comply with these constraints. I iterated the design, using the taper angle function in order to pass the draft angle analysis. Three degrees of draft angle should be enough be ensure easy removal of the finished mold, respectively the finished part.

image DraftAngle1 Initial rim design not respecting draft angles
image DraftAngle4_DraftFeature Improved rim design

CAM process

In general, settings CAM process in Fusion involves setting an origin point. The orientation of the axis should equal the orientation of the CNC machine coordinate system.

Generic CAM workflow

image 01_CAMSetup_CoordinateSystem Creating a new setup
image 02_CAMSetup_CoordinateSystem Setting origin point

Select the correct orientation by click the end or middle point. Switch to stock material to enter the dimension of the stock.

image 03_CAMSetup_CoordinateSystem_AccordingToBigCNC Correct orientation
image 04_StockSize xxx

The tool's parameter have to entered in the tool library. The tool length has to estimated. Before the CNC machine is started, the tool size needs to be check. The actual tool size should not be shorter than your estimation. In order to be not too repetitive, adding a tool will be discussed in the specific workflow for the rim.

image 05_SelectTool Tool library
image 06_NewTool New tool

Another important parameter are the clearance height and the retract height. The clearance height is from which the tool path is started. The retract height is the height the tool goes to before going to the next cutting point.

image 07_Clearance Setting clearance height & retract height
image 08_BottomHeight Setting an offset for the clearance height

In general, the milling process comes with a roughb> and a finishing cut which will involve different tools. The majority of material will be removed by the rough cut, whereas finishing will result in a smooth surface. Choose adaptive from 3D to set the parameters for the cut.

image 09_StepUp Rough cut paramters, e. g. stepdown
image 10_Calculate Calculation of the tool path

With post processing the tool path will be exported into machine readable Gcode.

image 11_PostProcessing Choose post processing to output Gcode
image 12_PostProcessing_Done Set your machine and save file

For the finishing cut, there are several strategies available. The best results for each strategy depend on the geometry of your part.

image 13_Smoothing CAM finishing strategies

Once the Gcode is generated, Brackets opens for you to confirm that the code was generated correctly.

image 13_Finished_Gcode Post processing output: Gcode

Workflow for the model

The part, I would like to cast is this rim. Initially, I thought about having a second mold and a cavity that form the tire. Due to time contraints this was not possible, so I decided to just make the rim.

I used a flat end mill roughing tool of 6 mm diameter and a ball end mill as a smoothing tool of 3 mm diameter. All parameters can be retrieved from the pictures below.

image ActualRoughTool Roughing tool
image ActualSmoothingTool Smoothing tool

As I wanted to make sure, that the milling tool would not cut the outer surfaces of the stock material, I made the stock size a bit smaller than the part size. This will generate a warning.

image 17_Stock Setting the origin point
image 18_Stock_FixedSize Setting the dimension of the stock material

I select adaptive from the 3D menu calculating the rough cut tool path, choosing the tool for roughing and smoothing from the library. I used the contour strategy for smoothing.

image 19_AdaptiveClearingRough Setting up the rough cut
image 20_RoughCutSimulation Calculating rough cut
image 24_SettingsToolspeed_rough Roughing: the tool's spindle speed and cutting feed rate need to choosen carefully (tool still needs to be selected correctly!).
image 24_SettingsToolPasses_rough The stepdown should be around half of the tool diameter.
image 25_SettingsToolspeed_smooth Smoothing parameters
image 25_SettingsToolPasses_smooth Smoothing stepdown

If everything was done accordingly, the simulation will show the expected outcome.

image 23_SimulateToolPath To check the simulation, choose simulate by right clicking setup
image 27_NoCollisions Check for collisions

Now, we can generate the Gcode for the Roland MX-40 as described above and set up the CNC machine. We will have to files, one for roughing, one for smoothing. You can also have one, as the machine will stock to expect the tool change.

CNC Milling

Now, we are almost ready to mill. What is left to do, is to...

For setting the origin in z you can use the sensor to measure the tool length.

image Origin Move to the origin of the CAD model
image Origin_2 After applying the origin point (using tool sensor)

Click cute to start the job.

image 28_StockMaterialInPlace Stock material attached to the machine bed
image 31_CuttingStarted Rough cut

The wall of the mold got pretty thin. I broke it away later on and modified the smoothing path to account for it.

image 32_RoughingProcess Rough cut creates thin wall
image 32_RoughingProcessB Finished roughing step
image ToGetRidOfWax Smoothing: Wax that sticks to the milling tool may damage the mold.
image 32_RoughingProcessC Reducing the cutting & spindle speed helped

After the finishing was completed, I realized that the cut was not completed as the tool did not go down to the bottom of the mold. I check my simulation for collision but did not realize the some surface weren't cut accordingly.

image Problem Failure was in fact already visible in the simulation.
image Problem_2 Modification of the design, removal of the walls.

I tried to fix these errors by modifying the design and a new finishing cut. I also change the pivot elements to ensure the completion of the cut.

image Solution_1 Initial pivot design
image Solution_2 Modified pivot design
CNC milling: rough cut
CNC milling: finishing

Mold making

I used a product called NEUKADUR ProtoAmid 60 as the resin and NEUKADUR ProtoAmid Komp. B 4 as the hardener. Both components are mixed in a 1:10 ratio. Information on the product can be retrieved from safety data sheet. The material is designed for vaccum processes. The wax molds rim's height was increased by MDF plates clamped to it.

image 01_PrepareMolding Clamped MDF rim
image 02_PrepareMolding Clamped MDF rim

The silicone components were mixed by hand, respecting the stoichiometry (scale). As shear introduces to lot air, the resin needs to be put the vacuum chamber. A pot life of 80 minutes gives enough time for venting and casting.

image 03_WeightingComponents Silicone resin and hardener
image 04_StockSize Mixing both components
image 05_SiliconComponents Wearing gloves is highly recommended.
image 06_VacuumProcess Venting in the vacuum chamber
image 07_DefoamingStarts Air is venting
image 08_DefoamingSpills Resin spills over due to venting.

As the casting process also introduces some air bubbles, the mold goes back into the vaccum chamber after casting.

image 09_RemainingResin Resin becomes clear after venting.
image 10_PouredResinDefoamed After casting is completed.

The resin will take about 12 hours to cure. As I making the second half from the same wax mold, it took me another 12 hours to complete the mold making process.

image C00_CastingPrepation Finished upper and lower part of the mold


As I did not account for a sprue in the design, I had to make one afterwards. I took a power drill and a copper pipe that was sharpened on a lathe to drill a hole into the silicone top mold.

image C01_CastingPrepation Short feed pipe
image C02_CastingPrepation Drilling of the sprue
image C03_CastingPrepation Ready for casting

I will use a polyurethane resin which takes polyols and an isocyanate. Especially, isocyanates should be dealt carefully due to toxicity.

image C04_PolyolIsocyanate NEUKADUR resin (polyurethane)
image C05_Weighting_2 Dosing the components: Comp B
image C05_Weighting Dosing the components: Comp A

Both components, the polylol and the isocyanate are placed in the chamber individually and will be mixed within the vacuum chamber. They will be vented and stirred.

image C06_Inlets Both components - inlay (one time use) + cups
image C05_WC07_ResinInChambereighting Stirring unit
image C08_MouldAdded Stirring unit, components and the mold

The resin has a pot life of 8 minutes. As the mold is filled, more air will emerge from the resin and the mold.

Vacuum chamber: mixing the two components
Vacuum chamber: Casting

The reaction of both components is exothermal which makes the resin heat up significantly. After about 20 minutes the resin is cured.

Casting results:

image CastRimResult Finished rim
image CastRimResult_2 All steps

The quality of the cast was good except for a few air bubbles in the finished rim. However, there was slight misalignment of the two parts of the mold. What I also did not take into consideration was the direction of the spokes of the scimitar design. The intension was to have these oriented in one direction, so I would have been in need for a second CNC milling mold.

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