Introduction to Molding & Casting

A brief introduction to Molding & Casting. What is a mold, how to cast an object using various techniques, understanding material behavior and interaction between materials. I used this slides in a lecture I gave to the students of the environmental design class at the College of Design and Innovation of Tongji University, Shanghai, China, to help them on an assignment about concrete casting.

1. MOLDING & CASTING
INTRODUCTION TO

2. KEYWORDS
mold
model/master
cast/copy

3. MOLDING & CASTING
KEYWORDS
mold
model/master
cast/copy
The mold is the negative shape. It is the starting
point of the process and it can be designed and
fabricated from scratch or it can derive from an
existing object (model or master) that you want to
copy.
For a successful job, the mold must be carefully
engineered and fabricated, taking into account the
complexity of the shape, the behavior of the
materials (flexibility, stiffness), the number of copies
to reproduce.
The cast (or copy) is the positive shape,
complementary to the mold, and it is the final result
of the process.

4. Molding and casting in industrial manufacturing:
Ceramic factories: plaster molds and slip (liquid clay).
Plastic factories: metal molds and injection molding, rotational
molding, blow molding.
Glass industry: metal or graphite molds and blown molten silica.
MOLDING & CASTING

5. 3+1 basic types of molds:
One part (open face)
Two part (close box)
Multi-part (for complex shapes)
+ special “One time” molds
MOLDING & CASTING

6. ONE PART MOLD
Wooden mold for mooncakes. The mold is directly carved into wood.

7. ONE PART MOLD
Rigid plastic mold for ice cubes

8. ONE PART MOLD
Silicone mold for complex ice shapes

9. ONE PART MOLD
Rigid mold can’t be used if model has undercuts

10. ONE PART MOLD
Flexible molds allows extraction of models with undercuts

11. TWO PARTS MOLD
Silicone rubber mold. An original is needed to fabricate this
mold. It can be used for multiple copies.

12. TWO PARTS MOLD
Silicone is flexible and reproduces fine details. “Registration
keys” are needed for the alignment of two parts molds.
REGISTRATION KEYS

13. TWO PARTS MOLD
3D printed mold: plastic filament is rigid, not suitable for highly
detailed objects.

14. TWO PARTS MOLD
No original is needed to fabricate this mold, the 3D model
must be designed as the negative part.

15. TWO PARTS MOLD
High rigidity of the mold allows only simple shapes to be casted

16. MULTI-PART MOLD
Complex shapes, with extreme undercuts or intersecting components,
require a multi part mold that must be precisely engineered.

17. Complex shapes, with extreme undercuts or intersecting components,
require a multi part mold that must be precisely engineered.
MULTI-PART MOLD

18. Complex shapes, with extreme undercuts or intersecting components,
require a multi part mold that must be precisely engineered.
MULTI-PART MOLD

19. Complex shapes, with extreme undercuts or intersecting components,
require a multi part mold that must be precisely engineered.
MULTI-PART MOLD

20. “ONE TIME” MOLD
Fabricated with any material it must be destroyed to be
able to extract the cast

21. “ONE TIME” MOLD
Fabricated with any material it must be destroyed to be
able to extract the cast

22. Cardboard soaks too much water from concrete and bends loosing
any structural strenght. Plastic film can be used as isolation.
“ONE TIME” MOLD

23. A 3D model is “unfolded” onto a plane using a dedicated software
(Pepakura), printed on paper and assembled.
“ONE TIME” MOLD

24. Mold: paper (to avoid soaking, the paper can be laminated).
Cast: colored plaster or concrete.
“ONE TIME” MOLD

25. Laser cut plastic sheet, folded, assembled. Tape makes it watertight
“ONE TIME” MOLD

26. This mold can be reassembled and reused. The choice of material is
optimal: concrete doesn’t stick to plastic and the surface is smooth.
“ONE TIME” MOLD

27. Laser cut sliced mold. First a box is created around a 3D model.
“ONE TIME” MOLD

28. Using a dedicated software (123D MAKE) the model is sliced into
4mm thick sections. Laser cut plans can be exported as a .dxf file.
“ONE TIME” MOLD

29. Each slice is laser cutted on 4mm cardboard and stacked following
the given order to make the mold.
“ONE TIME” MOLD

30. Plaster is poured into the mold. The materials used are not optimal:
the fluid plaster fill the corrugated cardboard. The resulting texture is
very interesting, but needs a lot of clean up.
“ONE TIME” MOLD

31. The same technique shown before can use styrofoam sheets instead
of carbboard, thus creating a smoother surface texture.
“ONE TIME” MOLD

32. Cast material is usually a substance in its liquid or fluid state, able to
solidify after being poured into the mold. Nevertheless, anything that
is able to fill an hollow space and keep the shape is a suitable material.
Take a look at this model of a room.
WHAT KIND OF MATERIALS CAN BE USED?

33. This is a multi-part mold. It appear to be the final model, but in this
case we need the cast to be around the mold and not into it. As seen in
the previous image, the final model will be a room. It is composed of
small parts for easy disassembling.
WHAT KIND OF MATERIALS CAN BE USED?

34. The mold is wrapped with a substrate rich in nutrients and sealed into
a plastic bag for a week
WHAT KIND OF MATERIALS CAN BE USED?

35. After a week the substrate has turned into mushrooms that grow
around the mold filling any hollow space.
WHAT KIND OF MATERIALS CAN BE USED?

36. The mold is then disassembled and removed and the resulting cast is a
model of the interior of a room.
WHAT KIND OF MATERIALS CAN BE USED?

37. Answer: mushrooms.
WHAT KIND OF MATERIALS CAN BE USED?

38. Vacuum-forming gives great, fast results when used in combination
with 3D printing. Open-source plans for a vacuum table are available.
The plastic material used comes from a recycled milk/water bottle.
ADVANCED MOLD MAKING TECHNIQUE
http://www.thingiverse.com/thing:1951311

39. To measure the amount of cast material you need to fill the mold, you
can fill the mold with rice or plastic balls, and measure the volume.
PRO TIP #1

40. You should always use a release agent to prevent your cast to stick to
the mold (especially when using styrofoam).
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