Molding is a manufacturing
process that involves shaping a liquid or malleable raw material by using a
fixed frame; known as either a mold or a matrix. The mold is generally a hollow
cavity receptacle, commonly made of metal, where liquid plastic, metal,
ceramic, or glass material is poured. In most cases, the mold is derived from
the initial pattern or template of the final object; its main objective is to
reproduce multiple uniform copies of the final product. As the liquid cools and
hardens inside the mold, the final configuration is achieved. Its removal is
facilitated by the use of a release agent or ejection pins.
We are surrounded by both ordinary and complex objects that were manufactured
as a result of the molding manufacturing process. Molding has occurred
throughout the millennia. Evidence of its usage has been discovered dating as
far back as the Bronze Age, where stones were used as molds to produce spear
tips.
Modern molding processes include plastic injection molding, Liquid Silicone
Rubber (LSR) molding, overmolding, and insert molding. Customized prototypes
and end-use parts are produced with the plastic injection molding process. The
standard process eliminates the use of embedded heating or cooling lines within
the molds so that molders, also known as molding technicians, can carefully
monitor fill pressure, aesthetics, and overall part quality.
· Liquid Silicone Rubber (LSR) molding is a highly flexible material that is considered a thermosetting polymer, meaning its molded state is permanent and it can’t be remelted like a thermoplastic could. A specific LSR molding tool is designed with CNC machining, thus providing different surface finish options for the end-use LSR part.
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· Overmolding allows a single part to contain multiple materials. Once a substrate part’s total run is molded, overmold tooling is setup on the press. It is then hand-placed into the mold and overmolded with either a thermoplastic or liquid silicone rubber material. Insert molding is similar to overmolding, but most commonly uses a preformed metal substrate part that overmolds it with plastic to create the final part.
As its name implies, injection
molding is the manufacturing process of injecting material into a mold to
produce a part. While the most common materials used for injection molding are
metals, thermoplastic polymers and thermosetting polymers, other possible
materials include glass, elastomers, and confections. Die-casting specifically
refers to the injection molding of metals.
The first injection molding machine was patented in 1872 by the American
inventor brothers John Wesley and Isaiah Hyatt, who eventually used it to
produce collar stays, buttons, and hair combs. A German inventor patented the
injection molding of plasticized cellulose acetate, a much less flammable
material than cellulose nitrate, in 1939. World War II was responsible for the
industry’s rapid expansion as demand exploded for affordable, mass-produced
products. The industry went on to witness the invention of the first screw
injection machine in 1946, which, today, accounts for the vast majority of all
machines.
Later in the 1970’s, the first gas-assisted injection molding process was
developed, making it possible to produce complex, hollow objects that cooled
quickly. This greatly improved design flexibility as well as the strength and
finish of manufactured parts. It also reduced production time, cost, weight,
and waste. Today, the plastic injection molding industry produces a broad range
of products across numerous sectors, which include the aerospace, automotive,
construction, consumer goods, packaging, plumbing, and toy industries.
Although injection molding is a versatile process, it is critical that careful
attention is given to a mold’s design and material, the material used, the
part’s desired shape and features, and the specifications of the molding
machine. Molds are generally made from steel or aluminum and are
precision-machined to form their specific features. A liquid material is fed
into a heated barrel, mixed, and fed into the mold’s cavity, eventually cooling
and hardening to the mold’s configuration.
Optimal for high-volume production, a diverse variety of parts from small
components like bottle caps, packaging, musical instruments to toy cars, all
the way up to entire body panels of cars, mechanical parts and gears, and most
plastic parts on the shelves today, are produced thanks to injection molding.
Thanks to advances in 3D printing technology, photopolymers can be used for
manufacturing some simple injection molds considering that they don’t melt
during the injection molding of low-temperature thermoplastics.
The equipment used in injection molding includes injection molding machines,
molds or dies, and injection and ejector molds. Due to their high cost, custom
molds are handled and stored very carefully with special attention being given
to environmental temperature and humidity levels in an effort to prevent
warping.
The two main methods for constructing molds are: standard machining (CNC) and
Electrical Discharge Machining (EDM). Standard machining has historically been
the more conventional method and developments in CNC (Computer Numerical
Control) have allowed the fabrication of more complex molds with greater speed.
EDM, also known as spark erosion, has also been widely adopted in mold making.
Tool steel is the most common material used in mold making. Well-designed molds
built of modern hard aluminum (7075 and 2024 alloys) are easily capable of
100,000, or greater, part life with proper mold maintenance. Mild steel,
aluminum, nickel or epoxy are only suitable for prototype or very short
production runs.
Rotomolding, also called
rotational molding, entails filling a charge, or shot weight, of material into
a heated hollow mold, which is followed by slowly rotating the mold, causing
the softened material to disperse and adhere to the mold’s walls. The mold
continues to rotate at all times during the heating phase to achieve and
maintain an even thickness throughout the part. This rotation also prevents
sagging, or deformation, during the cooling phase.
The distinct advantage of rotomolding is that it is an easier process than any
other when it comes to producing large, hollow parts, such as oil tanks or
chairs. In addition, the molds used in rotomolding are significantly less
expensive than other types of molds. Very little material is wasted with
rotomolding; excess material can often be reused, making it both economic and
ecological.
Another advantage lies in the molds themselves; they necessitate less tooling,
which means they can be manufactured and put into production much faster than
other molding processes. This is especially valuable for complex parts. Rotational
molding is also the process of choice for short runs and rush deliveries. The
molds can be exchanged quickly, or different colors can be used without purging
the mold. With other processes, purging may be required to exchange colors.
The main drawbacks are the hard-to-reach areas in the mold and a long cooling
duration that leads to significant mold downtime.
The first application of bi-axial rotation and heat was documented in 1855,
mainly to produce metal artillery shells and other hollow vessels. The initial
objective of using rotation was to create consistency in wall thickness and
density. Eventually, rotational molding was used for hollowing wax objects;
shortly thereafter rotomolding was used to fabricate chocolate eggs. It was
subsequently applied with the use of plaster-of-Paris in the 1920’s. In the
1950’s, it was originally applied to plastics and was slow to receive industry
adoption because of its sluggish productivity rate and the limitation caused by
a small number of suitable plastics. The first rotomolded products were doll
heads, which led to the creation of other plastic toys, eventually creating
road cones, marine buoys, and car armrests. The resulting popularity
accelerated the development of larger machinery and eventually led to the
creation of a worldwide trade association called The Association of Rotational
Molders (ARM).
New plastics like polycarbonate, polyester, and nylon, were introduced to
rotational molding in the 1980’s, leading to new applications for the process,
such as fuel tanks and industrial moldings. Most recently, the development of
plastic powders and process control improvements has led to a considerable
increase in its application.
A broad spectrum of equipment sizes can be found among the various rotational
molding machines. Generally, a rotational molding machine is comprised of
molds, an oven, a cooling chamber, and mold spindles. Uniform coating of the
plastic inside each mold is achieved with the spindles being mounted on a
rotating axis. The quality of the molds, usually aluminum-based, is directly
linked to the quality and finish of the final product.
The different rotomolding machines are as follows:
· Rock and roll machine
· Clamshell machine
· Vertical or up-and-over rotational machine
· Material jetting
· Vertical or up-and-over rotational machine
· Shuttle machine
· Swing-arm machine
· Carousel machine
The rock-and-roll machine is specialized to mainly produce long narrow parts.
The clamshell machine is a single-arm rotational molding machine that heats and
cools in the same chambers and takes up less space than the shuttle and swing
arm machines. Vertical rotational machines are considered small-to-medium in
size (in comparison with other rotomolding machines), and are energy-efficient
thanks to their compact heating and cooling chambers.
Although a single-arm shuttle machine exists, most shuttle machines have two
arms that alternate the molds between the heating chamber and cooling station.
The arms are independent of each other and they turn the molds bi-axially. The
swing-arm machine is beneficial for companies with prolonged cooling cycles or
lengthy demolding time. It can have up to four arms with a bi-axial movement.
Each arm is independent of the other since it’s not necessary to operate all
the arms simultaneously. The carousel machine is one of the most common
bi-axial machines in the industry. It can have up to four arms and six
stations, and is available in a wide range of sizes.
The original principle of blow
molding is derived from glassblowing. Essentially, blow molding is a
manufacturing process that forms hollow plastic parts.
Blow molding is first launched by melting down plastic and forming it into a
parison, which is a tube-like segment of plastic with a hole in one end that
allows compressed air to pass. A “preform,” rather than a parison, is used with
injection and injection stretch blow molding (ISB). The parison is then clamped
into a mold and air is blown into it, causing the pressure to push the plastic
outwards to match the mold. Upon the plastic’s cooling and hardening, the part
is ejected.
Three main types of blow molding exist:
· Extrusion Blow Molding (EBM) first occurs by taking melted plastic and extruding it through a parison with compressed air and into the mold. It features two variations: continuous and intermittent.
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· Blow Molding (IBM) is generally used for small medical and single-serve bottles. It is used to produce large quantities of hollow glass and plastic objects by injection molding a polymer onto a core pin which is rotated to a blow molding station to be inflated and cooled. IBM imposes restrictions on bottle design, only allowing a champagne base to be made for carbonated bottles.
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· Injection stretch molding is suitable for cylindrical, rectangular, or oval bottles and has two main different methods, notably the single-stage and two-stage processes. With the single-stage method, the same machine is used to both preform manufacture and bottle blow the object. This method is highly suitable for low volumes and short runs. In the case of the two-stage process, the plastic is first molded into a “preform” using the injection molding process. The “preforms” are then packaged and fed after cooling into a reheat stretch blow molding machine. While there is a high capital cost and a large floor footprint is required, injection stretch molding can produce very high volumes and feature minimal restriction on bottle design. The bottles can also be sold as a completed item for a third-party to blow.
· Spin trimming is an operation closely related to blow molding. It occurs when a knife spins, or revolves, around a container that has an excess of material due to the molding process. The knife cuts the excess material away and allows it to be recycled to create new moldings.
Casting is a 6,000-year-old
molding process that involves filling a liquid material into a mold of a
desired shape. The liquid goes on to gradually cool and solidify. The
solidified part is called a casting. It is either ejected or broken out of the
mold to finalize the process. Typically, metals or cold setting materials such
as epoxy, concrete, plaster, or clay are used in casting. Casting is the
preferred process for producing complex shapes that would otherwise be too
difficult or costly to make through other methods. A copper casted frog is the
oldest living proof that intricate casting patterns were used as early as 3200
BC.
The two main types of casting are metal and non-metal (such as plaster,
concrete, or resin). Metal casting involves the heating of a metal into its
liquid state and sequentially pouring the liquid into a mold. The mold and
metal are allowed to cool until the liquid metal solidifies, at which point the
casting is recovered from the mold. Plaster, concrete, or resin casting
typically make use of single-use disposable molds or multi-use molds made of
small, rigid pieces such as latex rubber. Topical treatments can be applied to
the surface of plaster or concrete when the surface is flat or lacks
transparency. They can also be used to give the appearance of metal or stone.
Resin is particularly well adopted in the construction of sinks, countertops,
and shower stalls. Adding powdered stone and different colors can provide a
near-realistic imitation of natural marble or travertine.
Fettling is the process of cutting, grinding, shaving, or sanding away unwanted
irregularities caused by seams and imperfections in the molds. Today, the
integration of robotics has been adopted to perform some fettling. However,
“fettlers” have historically carried out this grueling work manually, often
with risks for repercussions to their health.
One way to save costs throughout the entire casting manufacturing phase is to
apply casting process simulation software such as AutoCAST and MAGMa; this
simulation uses numerical methods to calculate quality, solidification, and
cooling, and provides a measurable prediction of the mechanical properties,
thermal stresses, and distortion. It is considered the most valuable innovation
in casting technology in 50 years.
Vacuum molding, sometimes
referred to as vacuum forming, is a straightforward molding process that uses
vacuum pressure to force a sheet of heated and stretched plastic onto a
single-surface mold. The plastic is heated to a forming temperature and the suction
holds the plastic sheet against the mold until the desired shape is achieved.
Vacuum molded components are preferential to complex fabricated sheet metal,
fiberglass, or plastic injection molding for applications such as kiosks,
automated teller machines, medical imaging equipment, engine covers, or for
interior trim and seat components of train wagons.
There is a broad range of possible patterns in vacuum molding. Wood is the most
common mold for vacuum molding, mainly because of its affordability and its
freedom to perform design changes. Recycled objects can also be used as molds
for their sustainability. Despite being costly, aluminum molds can accelerate
the fabrication process because of their effectiveness with shallow draw parts.
Composite molds are more affordable than cast or machined aluminum molds and
offer reliable durability while producing high-quality parts. The most suitable
materials for vacuum molding are thermoplastics while the most common and
adaptable is High Impact Polystyrene Sheeting (HIPS). Acrylic is a suitable
material for vacuum molding, used for its transparency, in applications such as
aerospace, for example, with cockpit window canopies.
Finishing operations are necessary to transform the product into a suitable
state. Common finishing methods include: guillotining, drilling, roller
cutting, press cutting, and CNC (Computer Numerical Control) machine cutting.
Compression molding is a
forming process that heats and softens a plastic material in order to achieve a
desired shape. It entails placing the plastic material, either in the form of
pellets or sheet, into an open, heated metal mold. The mold then softens the
material, forcing it to conform to the mold’s shape as pressure is applied
while it closes allowing the curing phase to take place. Once completed, excess
materials protruding from the mold, called “flashes,” need to be removed to
achieve a good finish.
First developed to manufacture composite parts for metal replacement
applications, compression molding is typically used to make larger flat or
moderately curved parts for the automotive industry including Long Fiber
Reinforced Thermoplastics (LFT) and Glass Fiber Mat Reinforced Thermoplastics
(GMT). Some of these parts include: hoods, fenders, scoops, spoilers, as well
as smaller, more intricate parts.
One main advantage of compression molding is its capacity to mold large,
relatively intricate parts as well as to produce ultra-large basic shapes that
would otherwise be impossible with extrusion techniques. It is also one of the
lowest-cost methods when compared with transfer or injection molding. Plus,
waste reduction is maximized, which is particularly beneficial when working
with expensive compounds. The drawbacks of compression molding include poor
product consistency, difficulty in controlling flashing, and its lack of
suitability for certain types of parts.
Compression molding can manufacture based on numerous materials such as
Polyester fiberglass resin systems (like Bulk Molding Compound (BMC) or Sheet
Molding Compound (SMC), polyamides-imides (like Torlon), polyimides (like
Vespel), PolyPhenylene Sulfide (PPS), PolyEther Ether Ketone (PEEK), phenolics,
thermoset polyester vinyl ester, epoxy, Diallyl Phthalate (DAP) and silicones.
As its name implies, dip
molding is a plastic manufacturing process that takes heated metal molds and
dips them into a PVC liquid called plastisol to form a plastic part. The liquid
can either be heated or at room temperature. The part is then cooled, drained,
hardened, and stripped from its mold to produce the finished product. The molds
can be submerged multiple times to achieve the desired thickness. For certain
materials, a curing process may be required.
Dip molding can produce parts at a fraction of the cost of injection molding
and at an accelerated pace. It is suitable for short runs of prototypes as well
as for high-production orders. Plastisol is an affordable material and is
available in a broad range of custom and standard colors. It is also flame
retardant, UV and mildew resistant and relatively resistant to scratching and
abrasion. In addition to plastisol, dip molding materials include latex,
ieneoprene, polyurethanes, silicones, and epoxy. The main drawbacks include the
time required to produce a part and the difficulty of controlling the thickness.
The range of possibilities with dip molding is vast, however, some common
applications for dip molding include caps and plugs, gasoline nozzle covers,
gloves, protective ax covers, socket holders, and many more.