A'dam FabLab Academy

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Molding and Casting

Assignment: Design a 3D mold, machine and cast design.

Final cast with Smooth-on 325
click image to enlarge

3D image

Character design

Used Rhino3d to draw character within mold box.

  1. Character should have strong corners perhaps deeper then looks good in a render. This character's antlers weren't thick enough making them very weak with some of the casting materials.
  2. Draw a complete character, place in box, add long cylinders for mold matching pegs, then finally add plane through center and do a number of Boolean2d to make the front and back of mold.


Material used for mold was Machinable wax

PartWorks 3D

Preparing design

  1. Save drawing as a STL
  2. Open PartWorks 3D on ShopBot
  3. Load the STL file
  4. Enter units and tool size because STL file does not contain units
  5. Enter scale, for example, 70%
  6. Enter material size
  7. Set zero on surface that isnt moving
  8. Rough cut
    Tooling: 4 (four) fluted .125 inch endmill
    Cutting Parameters: Passdepth: 4.0, Stepover: .635, SpSpeed: 12000, Feed: 60, Plunge: 10
    Toolpath: Rapid: 2.0 mm, M-Allow .5, ZLevel: Raster X 4.0
    Finished cut
    4 (four) fluted .125 inch ball endmill
    Cutting Parameters: Passdepth: x.x, Stepover: .xxx, SpSpeed: 12000, Feed: 60, Plunge: xx


How much mold rubber do I need?

Example calculation:

  1. Using PMC® -121-30 Dry - Easy To Use Urethane Rubber
  2. h * w * d = VOLUME (rubber needed to make the mold). Then divide the VOLUME by the specific volume yield of the mold rubber (constant 963 cm3)
  3. (h * w * d)/ 963 cm3 = Weight in kg of PartA and PartB need to make mold
  4. (8.8 height x 9.0 width x 2.0 depth) / 963 cm3 = Approximate Weight of Product Needed 0.16 Kg of Part A and 0.16 Kg of Part B

Other calculation options:

Smooth-on has a free iPhone applicationto calculate how much PartA and PartB you need.

Smooth-on has the same calculatoron their site

Smooth-on has an explanation of this calculation on their FAQ


Experimented with different casting materials

Plus: dried quickly
Minus: too thick to pour in molds small holes
Smooth-Cast- worst results.
Plus: Quick curing time, minutes!
Minus: Ruined my mold.
Crystal Clear - best results.
Plus: Easy to pour. Colors can be added.
Minus: Long time to cure 16+ hours. Shrinks.
Final cast with Smooth-on 325
Plus: Easy to pour. Colors can be added.
Plus: Cure in 20 to 30 minutes.

Lessons Learned

Mold design: While designing mold think how the cast material will flow into the finished mold. Place pouring holes to allow the best flow of casting material.

Wax is brittle a small piece of the design broke off, for example the characters nose. However the finish pass had enough to cut a small nose.

Know what end-mills are available before creating file in PartWorks. The 1/16 inch ball nose end-mills had a 1/4 inch shank that would have skimmed the interior of mold therefore making it unusable for my design.

End-mill stick out was to short rough end-mill kissed the top of the material when doing the peg holes.

Amount of mold release should be light spritze which is then brushed with soft brush, a few minutes later a second light spritze.