Molding Processes
A mold or mould is a hollowed-out block that is filled with a liquid like plastic, glass, metal, or
ceramic raw materials. The liquid hardens or sets inside the mold, adopting its shape. A mold is
the counterpart to a cast. The manufacturer who makes the molds is called the mold maker. A
release agent is typically used to make removal of the hardened/set substance from the mold
easier. Typical uses for molded plastics include molded furniture, molded household goods,
molded cases, and structural materials.
Powdered Metals- Sintering and Cermets
Sintering- The fabrication of metal shapes from small particles of metal by sintering is a
technology over 1000 years. The sintering of metal and ceramics on a large scale is however a
much more recent development. Sintering is mainly used for cermets which are components
made for a combination of metal and ceramics
Sintered normally involves three processes : Production of the powder
Shaping of the component
Heat treating for strength
Advantages of Sintering
1. The parts produced have an excellent surface finish, and good dimensional accuracy.
2. The porosity inherent in sintered components is useful for specialized application such as
filters and bearings.
3. Refractory materials which are impossible to shape using other methods can be fabricated
by sintering with metals of lower melting points
4. A wide range of parts with special electrical and magnetic properties can be produced.
Plastic molding- The most common type of plastic molding process used for large scale
manufacturing is the injection molding process. There are eight major types of plastic
molding processes:
1. Injection molding- A manufacturing process for forming objects, as of plastic or metal, by
heating the molding material to a fluid state and injecting it into a mold. Feed screw heats
material and meters how much goes into the mold and it has three zones.
1. Feed Zone-A constant depth feed that pushes the material together and rids it of air.
2. Melt Zone- An angled that forces the beads together to generate heat
3. Metering Zone- Constant depth and controls how material goes into the mold
http://www.youtube.com/watch?v=eUthHS3MTdA
2. Rotational molding- Or rotomolding is a relatively slow process that is capable of
making large one-piece hollow plastic objects at a cost lower than most other plastic molding
processes. The rotational molding process starts with a closed, split mold. The molding
compound, usually a thermoplastic in the form of pellets or granules, is pre-measured and
loaded into the molding chamber. The chamber is then sealed and mounted on a multi-axial
rotation station. The chamber is heated and the compound inside is melted. Preferably, the
molten material has low viscosity and can be distributed easily. The rotation station begins to
rotate slowly on multiple axes simultaneously such that the molten material is evenly coated
on the internal surfaces of the mold while the temperature continues to drop in a controlled
manner. The rotation and cooling continue until the molding compound solidifies. The mold
can then be opened and the finished part is released. The rotational speeds, heating and
cooling rates need to be carefully controlled throughout the process. Unlike other common
plastic molding methods, such as injection, extrusion, and compression moldings, there is no
external pressure applied during the entire molding process. Most thermoplastics are suitable
for rotational molding.
http://www.youtube.com/watch?v=Fr_s_xG78AA:
Pros








One-piece product design reduces or eliminates assembly cost.
Allows complex mold split lines.
Allows molded threads and mold-in inserts.
Allows a wide range of surface finishes (textured, smooth, or polished).
Very little waste in material.
Low residual stresses. There are thermal stresses during the cooling
process but no forming stresses because no external pressure is applied.
Tooling is less expensive. Wall thickness of the molding chamber can be
thinner than those for other molding methods.
Suitable for both low-volume prototypes and high-volume production.
Cons


Slow production speed. It usually takes about one hour to finish a cycle,
as opposed to just a few seconds for a typical injection molding process.
Lower precision.
3. Thermoforming- A plastic thermoforming process begins with a sheet of
thermoplastic material formed by the extrusion process using a slotted die. The
sheet of plastic material is first heated to become a flexible membrane. This soft,
rubber-like membrane is placed on the mold and stretched to cover the entire
surface. Vacuum, external air pressure, and mechanical forces are used to rid the
air bubbles and improve the surface quality. The plastic part remains in the mold
until it solidifies. Excess material is trimmed after the part is removed from the
mold. Hollow or double-walled parts can be created by twin-sheet thermoforming
that fuses two separate sheets simultaneously formed in their respective molds.
http://www.youtube.com/watch?v=_z6RgDJxZSs
Pros





Low initial setup costs
Fast setup time
Low production costs
Less thermal stresses than injection molding and compression molding
More details and better cosmetics than rotational-molded products
Cons


Geometries limited to thin shells or shallow shapes
One side of the product can be precisely controlled by the mold
dimensions while the other side cannot.
4. Structural foam molding-The key element in structural foam molding is
low pressure. Unlike conventional injection molding, which utilizes
extremely high pressures to force materials into a mold's cavity, structural
foam molding takes advantage of a part's configuration, its generally thick
wall sections (that act as runners), and the foaming action (supplied by either
chemical reaction within the resin blend, or the introduction of a compressed,
inert gas into the mold), to allow the molten resin to flow much further, and
with far lower pressure, than the typical injection molding process would
allow. http://www.youtube.com/watch?v=RQbfGSttIFU
http://www.youtube.com/watch?v=WYj6HrzyGAc
5. Gas assist injection molding-The first patents for gas assist injection
molding came in the early 1970s. But the technology languished many years
as patent holders and equipment manufacturers battled in the courtroom. Gasassist injects pressurized nitrogen into the interior of the polymer melt. The
gas follows the path of least resistance, penetrating the part via carefully
designed networks of thicker cross-sectioned gas channels. Nitrogen seeks the
area in the mold cavity with the lowest pressure and travels most efficiently
in the hotter core regions of the polymer melt. It has a viscosity much lower
than its polymer host and remains isolated within the channel as it flows in
the direction of the melt front. The displaced molten polymer is pushed
towards the melt front by the advancing gas, filling and later packing the
mold cavity.
6. Blow molding- A molding process in which air pressure is used to inflate soft
plastic into a mold cavity. It is an important industrial process for making onepiece hollow plastic parts with thin walls, such as bottles and similar containers.
Since many of these items are used for consumer beverages for mass markets,
production is typically organized for very high quantities. The technology is
borrowed from the glass industry with which plastics compete in the disposable
or recyclable bottle market. Blow molding is accomplished in two steps: (1)
fabrication of a starting tube of molten plastic, called a parison (same as in glassblowing); and (2) inflation of the tube to the desired final shape. Forming the
parison is accomplished by either of two processes: extrusion or injection
molding. http://vimeo.com/6328444
7. Compression molding- Compression molding is one of the original
processing methods for manufacturing plastic parts developed at the very
beginning of the plastics industry. In fact, it was widely used in the bakery
industry for cookie or cake molding before plastic materials existed. Although it
is also applicable to thermoplastics, compression molding is commonly used in
manufacturing thermoset plastic parts. The raw materials for compression
molding are usually in the form of granules, putty-like masses, or preforms. They
are first placed in an open, heated mold cavity. The mold is then closed and
pressure is applied to force the material to fill up the cavity. A hydraulic ram is
often utilized to produce sufficient force during the molding process. The heat
and pressure are maintained until the plastic material is cured.
http://www.youtube.com/watch?v=c7lvK-Gw8cc
8. Film insert molding-Prior to insertion of the film into the injection mold, dust
and impurities are removed from the film surface by blowing with ionized air.
The film is inserted by means of a handling unit so as to achieve precisely
reproduced and efficient production. The film can be held in place in the injection
mold through its geometry. In many cases, however, further measures (e.g.
electrostatic charge, mechanical holding) are necessary to improve the retention
of the film in the mold. In the event of long flow paths, the use of needle valve
nozzles, which are opened according to the filling procedure through cascade
control, has proven effective for avoiding wrinkling. In the case of large part
dimensions, special consideration must be given to wrinkling, washing out,
distortion, delaminating and overflow of the melt in the exposed area