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Handling Large Undercuts In The Brush-On Mold Making Process
Planning is the key to success in any mold making project.  In particular, areas that have deep indentations and or harsh angles, known as undercuts can pose a number of obstacles in the demolding process. Recognizing and handling these areas, and properly constructing the support shell can make even complex shapes easy to mold and cast.  The following demonstrates the brush-on mold making process on a model with severe undercuts underneath the shoulders (see figure 2.), as well as some other challenges. 
 Plaster 3-D model to be molded using the brush-on process.

1 - Plaster 3-D model to be molded using the brush-on process.
 Back view shows 2 large undercut areas under the shoulders.

2 - Back view shows 2 large undercut areas under the shoulders.
 Examine model carefully. Develop a plan of attack before you apply rubber and support shell. Minimizing under cuts now means easier demold later.

3 - Examine model carefully. Develop a plan of attack before you apply rubber and support shell. Minimizing under cuts now means easier demold later.
 Draw  parting lines to help identify individual support shell segments.

4 - Draw parting lines to help identify individual support shell segments.
 Drawing this line also helps identify the cutting line for mold rubber.

5 - Drawing this line also helps identify the cutting line for mold rubber.
 Optional technique: use modeling clay to define outer flange edge.

6 - Optional technique: use modeling clay to define outer flange edge.
 Secure Model to base with hot glue gun.

7 - Secure Model to base with hot glue gun.
 Apply SuperSeal® sealing agent. SuperSeal® does not alter surface detail.

8 - Apply SuperSeal® sealing agent. SuperSeal® does not alter surface detail.
 Apply Universal® mold release.

9 - Apply Universal® mold release.
 Measure out mold rubber. Measure Part B, pack it thoroughly in cup.

10 - Measure out mold rubber. Measure Part B, pack it thoroughly in cup.
 Level off top.

11 - Level off top.
 Measure out an equal volume of Part A.

12 - Measure out an equal volume of Part A.
 Combine material. Adding pigment to every other layer ensures even coverage.

13 - Combine material. Adding pigment to every other layer ensures even coverage.
 1st coat, detail coat 'stipple' first layer on very thin.

14 - 1st coat, detail coat 'stipple' first layer on very thin.
 This shows proper coverage and density for first layer.

15 - This shows proper coverage and density for first layer.
 This shows TOO MUCH mold rubber being applied.

16 - This shows TOO MUCH mold rubber being applied.
 The goal is a thin, even first layer. Create a wide flange at base.

17 - The goal is a thin, even first layer. Create a wide flange at base.
 First coat done? Carefully check model for thorough coverage.

18 - First coat done? Carefully check model for thorough coverage.
 Allow rubber to become 'tacky'

19 - Allow rubber to become 'tacky'
 Sticky to the touch, does not come off model.  About 30 minutes at room temperature. Touch rubber 'Off-Model'

20 - Sticky to the touch, does not come off model. About 30 minutes at room temperature. Touch rubber 'Off-Model'
 Mix and apply second layer.

21 - Mix and apply second layer.
 No color added.

22 - No color added.
 Apply thicker with more fluid strokes. Allow rubber to become 'tacky'.

23 - Apply thicker with more fluid strokes. Allow rubber to become 'tacky'.
 Thicken Rubber With 'Cab-O-Sil'

24 - Thicken Rubber With 'Cab-O-Sil'
 Stir in gradually until desired thickness is achieved. 'Mashed potato' like consistency is perferred by many mold makers.

25 - Stir in gradually until desired thickness is achieved. 'Mashed potato' like consistency is perferred by many mold makers.
 Thickened rubber is good for filling under cuts, andgood For making parting shims that will aid in registering cut rubber. sections during casting.

26 - Thickened rubber is good for filling under cuts, andgood For making parting shims that will aid in registering cut rubber. sections during casting.
 Fill in under cuts. Pack material tightly to eliminate large air voids.

27 - Fill in under cuts. Pack material tightly to eliminate large air voids.
 Make parting shim starting from top.

28 - Make parting shim starting from top.
 Build shim evenly.  Trying to build shim to full thickness in one step often results in 'slumping' or an 'avalanche'

29 - Build shim evenly. Trying to build shim to full thickness in one step often results in 'slumping' or an 'avalanche'
 Sculpt seam evenly from top to bottom.

30 - Sculpt seam evenly from top to bottom.
 Fill in all undercuts.

31 - Fill in all undercuts.
 Mix and apply third layer. Add pigment.

32 - Mix and apply third layer. Add pigment.
 Apply thicker with more fluid strokes. Allow rubber to become 'tacky'

33 - Apply thicker with more fluid strokes. Allow rubber to become 'tacky'
 Thicken rubber with 'Cab-O-Sil' no pigment added.

34 - Thicken rubber with 'Cab-O-Sil' no pigment added.
 Thicken seam.

35 - Thicken seam.
 Build evenly from top to bottom.

36 - Build evenly from top to bottom.
 Complete filling in deep under cuts.

37 - Complete filling in deep under cuts.
 Complete filling all other under cut areas.

38 - Complete filling all other under cut areas.
 Mix and apply fourth layer. No pigment added.

39 - Mix and apply fourth layer. No pigment added.
 Apply thicker with more fluid strokes. Let material cure overnight.

40 - Apply thicker with more fluid strokes. Let material cure overnight.
 Visualize support shell. 'Map out' shell segments can be 2, 3 or more pieces illustrate separation lines with pen or marker.

41 - Visualize support shell. 'Map out' shell segments can be 2, 3 or more pieces illustrate separation lines with pen or marker.
 Create parting shims. Roll into strips with rolling pin: convenient.

42 - Create parting shims. Roll into strips with rolling pin: convenient.
 Purpose: Separate support shell sections from each other. Use soft clay or water clay.

43 - Purpose: Separate support shell sections from each other. Use soft clay or water clay.
 Smooth shim and make an even 'wall'

44 - Smooth shim and make an even 'wall'
 Warning:  Plasti-Paste sticks to everything – especially urethane rubber. Apply Sonite® wax.

45 - Warning: Plasti-Paste sticks to everything – especially urethane rubber. Apply Sonite® wax.
 Apply Universal® mold release to shim and mold.

46 - Apply Universal® mold release to shim and mold.
 Measure and mix Plasti-Paste®  Mix Ratio:  1 part liquid to 3 cups paste by volume.

47 - Measure and mix Plasti-Paste® Mix Ratio: 1 part liquid to 3 cups paste by volume.
 Part B is premixed thoroughly. Blue pigment is added as an optional step to ensure a consistant mix.

48 - Part B is premixed thoroughly. Blue pigment is added as an optional step to ensure a consistant mix.
 Parts B and A measured out correctly by volume.

49 - Parts B and A measured out correctly by volume.
 Plasti-Paste® is applied to create first half of shell.

50 - Plasti-Paste® is applied to create first half of shell.
 Cover evenly making sure edges aren't too thin.

51 - Cover evenly making sure edges aren't too thin.
 Allow 1st half of shell to cure – 90 minutes at room temperature.

52 - Allow 1st half of shell to cure – 90 minutes at room temperature.
 View showing finished  first half of support shell.

53 - View showing finished first half of support shell.
 Remove clay shim.

54 - Remove clay shim.
 Edges of the support shell are 'cleaned up' using a razor knife

55 - Edges of the support shell are 'cleaned up' using a razor knife
 apply Sonite® wax and release agent to cured Plasti-Paste®.

56 - apply Sonite® wax and release agent to cured Plasti-Paste®.
 Second Half of Shell: Mix and apply Plasti-Paste® uniform thickness.

57 - Second Half of Shell: Mix and apply Plasti-Paste® uniform thickness.
 Once again, maintain a uniform thickness.

58 - Once again, maintain a uniform thickness.
 Fill in any holes along the seam. Let cure for 90 minutes.

59 - Fill in any holes along the seam. Let cure for 90 minutes.
 Remove support shell and rubber mold from model.

60 - Remove support shell and rubber mold from model.
  

61 -  
 Extract original model from rubber mold. Get extra pair of helping hands.

62 - Extract original model from rubber mold. Get extra pair of helping hands.
 Use razor knife to cut seam cut slowly - Be Careful!

63 - Use razor knife to cut seam cut slowly - Be Careful!
 Cutting is stopped before the end of the thickened seam.

64 - Cutting is stopped before the end of the thickened seam.
 The mold is 'flexed' off the model.

65 - The mold is 'flexed' off the model.
 The mold can actually be turned inside out, making demold very easy.

66 - The mold can actually be turned inside out, making demold very easy.
 Casting Into Brush-On Mold.  Apply mold release into rubber mold cavity.Apply release agent torubber exterior and shell(in case of leaks) Align cut seams of mold.

67 - Casting Into Brush-On Mold. Apply mold release into rubber mold cavity.Apply release agent torubber exterior and shell(in case of leaks) Align cut seams of mold.
 Assemble shell segments using rubber bands or mold straps.

68 - Assemble shell segments using rubber bands or mold straps.
 Secured mold and support shell ready for casting.

69 - Secured mold and support shell ready for casting.
 Smooth Cast® 300 is poured into the mold and allowed to cure.

70 - Smooth Cast® 300 is poured into the mold and allowed to cure.
 Finished casting is an exact replica of the original.

71 - Finished casting is an exact replica of the original.

                   
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