INJECTION MOLDING & TYPES

PRINCIPLE :
o melting a material, usually plastic, and injecting it under pressure into a
mold cavity where it cools and solidifies, taking the shape of the mold

• Injection molding is a widely used manufacturing process for producing

plastic parts by injecting molten material into a mold. The basic principles
behind this process include.

• PROCESS:
1. Melting the Material:

• Plastic pellets (usually thermoplastics) are fed into a heated barrel.

• A rotating screw pushes the material forward while heating it until it melts
and becomes a viscous liquid.

2. Injection of Molten Plastic:

• The molten plastic is then injected into a closed mold cavity at high
pressure using the screw or a plunger.
• The pressure ensures the mold is completely filled and the part takes the
desired shape.

3. Cooling and Solidifying :

• The plastic inside the mold cools and solidifies as it takes the shape of the
mold.
 • Cooling time depends on the material, part thickness, and mold
temperature.

4. Mold Opening and Ejection:

• Once the part solidifies, the mold opens.
• Ejector pins push the part out of the mold cavity.
The mold then closes again for the next cycle.

❖ INJECTION MOLDING TYPES

1. Gas-Assisted Injection Molding

• In gas-assisted injection molding, a pressurized inert gas (typically

nitrogen gas) is injected into the mold right after the molten plastic,
forcing the material into the mold walls and leaving a cavity where the
gas once was. This gas allows for built-in hollow sections in a mold, but it
also promotes cooling and prevents distortions by evening out wall
thicknesses, especially in thick walls in an injection mold. Uses for gas-
assisted injection molding include material reduction in large parts,
hollow products, and other porous designs.

• The benefits of this type of plastic injection molding revolve around its
ability to reduce material usage and cooling time. The gas pocket will fill
 thicker sections, resulting in less warping and faster cooling times as the
molten plastic flows towards the colder mold walls. The pressure also
causes less shrinkage to occur when cooling and minimizes the chance of
sink marks from appearing. Gas-assisted injection molding also requires
lower clamping forces and
overall residual stresses,
leading to a more stable
injection molding process.

• Gas-assisted injection
molding does not come without its limitations; it can only really be applied
to single-cavity molds, otherwise, the gas channeling becomes too
complicated (this is exacerbated if the multiple cavities are unique from
each other). Also, some materials may react with the gas or become foggy
due to the nature of the gas interfacing with the material's surface. Clear
plastics are especially prone to this effect, and so are not usually capable
of being gas-assisted injection molded
 2. Thin-Wall Molding :

In thin wall molding, parts maximize material and cost savings by utilizing thin
wall thicknesses relative to the part’s overall size. Thin wall molding allows for 1-
2mm thick walls that both decrease cycle time but increase needed injection
pressure. Specialized thin wall injection molding machines have the highest
precision specifications among injection molding machines, as thin-wall molding
is often used for small parts. Uses for thin-wall injection molding are small, tight-
tolerance applications like electronic parts, enclosures, medical device
components, tubing, etc.

The advantages of thin-wall injection molding, as previously mentioned, are cost

savings and speed. Thin wall plastic parts require less material, resulting in lower
material costs and overall resource consumption when compared to traditional
injection molding methods. Cycle time is also drastically reduced, and parts
result in fewer emissions from shipping due to their decreased weight. A thin-
wall injection mold can also be made of recyclable plastics, and thin-wall parts
reduce weight (and therefore
emissions) in fuel-based applications
like heavy equipment and automotive
vehicles.
 3. Liquid Silicone Injection Molding

• Liquid silicone injection molding allows for the mass production of

silicone rubber products. This type of injection molding is distinct from
others in that silicone rubber is technically a thermoset rubber, therefore
it will require vulcanization (the process that provides rubber its
beneficial material properties like durability and flexibility). Opposite to
typical injection molding where molten plastic is injected into a colder
mold, cold silicone rubber is injected into a heated mold cavity and
vulcanized. This injection molding technology requires specialized
equipment such as mixers, metering units, perfectly sealed molds, and
other components. Uses include products for sealing applications,
connectors, over-molding for other plastic products, infant products,
biocompatible medical products, baking equipment, insulating products,
and more.
• Liquid silicone injection molding has several unique advantages. The
material does not require any melting and can be kept in its liquid form,
ready to use. Silicone is generally quick to solidify and produces little to
no burrs/waste if the machine is engineered correctly. This injection
molding technology typically automates the injection process to reduce
operator errors and offers a lower risk process as at no point is the
material hot except inside the mold. Silicone products are one of the
only biocompatible materials and are impressively resistant to chemicals,
temperature, and electricity, making them a unique offering from an
otherwise thermoplastic-laden injection molding selection.

• Liquid silicone injection molding

is not perfect; the vulcanization
of silicone material is an
irreversible process, meaning
once a part is set, there is no
going back. There is no
 recycling of silicone products, and if a mistake is made, the material
cannot be turned back into stock as with other types of injection
molding. Also, liquid silicone requires its own unique equipment that
may be more difficult to procure and regularly maintain.

• Materials commonly used in liquid silicone injection molding are:

➢ Standard silicone
➢ Medical grade silicone
➢ Resistant silicones
➢ Optical silicone
 4. Structural Foam Molding :

Structural foam molding allows for the mass-production of very large parts,
thanks to the introduction of a composite material formed from a polymer mixed
with an inert gas (such as nitrogen) or a chemical blowing agent. Material is kept
separate in liquid state and then mixed inside the mold. The gas/ chemical
blower is then added to the mixture, causing a change in the chemical reaction
that results in the formation of a low-density, rapid expansion of foam. As the
foam expands and cures, the interior retains its high porosity core while the
surface interface between the foam and the mold collapses, forming a high-
density protective skin on the outside of the part. The resulting injection mold is
a lightweight, flexible, and strong foam product. Uses for structural foam
molding include car roofs, housings for medical equipment, skis, interior and
exterior automotive parts, and other large parts.
The advantages of structural foam injection molding lie in its ability to make
large, lightweight, strong, and cost-effective parts. Structural foam parts can
expand to the size of car roofs and other large products and use less material
thanks to their inherent porosity. Also, this porous nature contributes to weight
reduction with no tradeoff for strength; structural foam parts are strong,
durable, and up to 8 times stiffer than solid polymers. Structural foam injection
molding is easy to mold, as it easily conquers variable thickness walls,
experiences less stress during the process, is resistant to warping, and has a
lower incidence of damaging molds and machines. Structural foam parts are also
quick to produce, easily painted, and are largely unaffected by temperature
differentials.
The disadvantages of structural foam molding are mainly a result of the foam
material, as surface finishes are rough, wall thicknesses cannot be under ¼ inch
thick, and generally require more post-processing than other types of injection
molding. Also, structural foam molding has lower production speeds than other
injection molding techniques.
Materials commonly used in structural foam injection molding are:
• Polyurethane
• Polycarbonate
• Polyphenylene oxide (Noryl)

• Polybutylene terephthalate
• Acrylonitrile butadiene styrene
 5. Metal Injection Molding :
➢ Probably the newest and most unique type of injection molding, metal
injection molding allows for the production of injection-molded metal
products. In the process, metal powder and binder are mixed and
granulated. This so-called “feedstock” is then shot into an injection unit,
where the raw product is molded. After molding, the part is cleaned in a
solvent and thermally debonded of its binder material, leaving behind a
fully metal (yet still porous) product. The debonded product is further
strengthened in a sintering oven where metal particles coalesce into a
solid matrix (typically under vacuum to reach high solid density), and the
part is then ready for additional post-processing procedures like finishing,
heat treatment, etc. This process is used to mass-produce small (<100g)
metal components in a single step such as hand tool heads, mechanical
components, linkages, automotive and aerospace parts, and more.
➢ Metal injection molding has the advantage of being able to create metal
parts that were previously impossible to produce using more traditional
methods. Also, a high volume of parts can be molded in one step, which
can rival the more common casting process for small metal parts. It also
offers high fidelity features for the final product such as knurling, holes,
and other fine details. Wall thicknesses can be made with narrow
dimensional tolerances (hundreds of micrometers) and there is virtually
no waste (a significant benefit when compared to all other metal
manufacturing techniques).
➢ The major downside to this injection molding technology is that it is
somewhat expensive and limited to lower volumes and sizes of parts. The
equipment needed for metal injection molding is quite expensive, and so
far, the uses of metal injection molding are in high-end applications
requiring incredibly complex and detailed metal parts.

• Materials commonly used in metal injection molding include:

• Stainless steel

• Tungsten carbides
• Various alloys
• Cobalt-chrome
• Titanium
• Nickel super alloys

• Molybdenum-based metals
• Composites