Injection Molding

     Injection molding is a process used to form products from plastic. The process
requires a mold, clamping component, injection unit, and some sort of plastic.

As time has advanced so has injection molding by developing new techniques and
new products to aid in the manufacturing of the injection molded parts.

Injection molding was used as early as the 1860ís. It can be used to form many
different products. Whether the products are small, large, complex, or simple
they can be produced. Injection molding has derived from metal die casting.

However, the polymer canít just be poured into a mold, it has to be forced
into the mold cavity. The polymer is forced into the mold and pressure is held
on it to avoid shrinkage in the mold cavity as it cools. Injection molding is
capable of producing a large number of parts with very high precision. All
thermoplastics except polytetraflouroethylene (PTFE), polyamides, and some
aromatic polyesters can be used by the injection molding machine. Some
thermosetting plastics can also be used. The typical fabrication process can be
done by one of two different types of injection molding equipment. Either a
plunger, or reciprocating screw type machine can be used. The process starts by
melting the polymer resin. Once the resin is melted, a mold is placed in the
clamping unit. The clamping unit is to hold the mold together. The plunger or
reciprocating screw then force the polymer resin into the mold. In the plunger
operated machine, the plunger is hydraulically operated. This forces the plastic
through a heated area, where it is then spread into a thin layer by the torpedo.

Then the melt comes to the nozzle and is injected into the mold. The
reciprocating screw rotates, this moves the polymer resin forward for injection.

As the screw rotates it acts to melt, mix, and pump the polymer to prepare it
for injection. The reciprocating screw machine is the most widely used of the
two machines. Once the polymer resin is injected into the mold cavity, the mold
is allowed to cool. The mold has a gate, which limits back flow and directs the
flow of the melt into the mold cavity. Once the mold has cooled and the polymer
has solidified the mold can be removed and the part can be ejected. When the
gate freezes, the screw begins to rotate again and the part is ejected. This
completes the cycle time. Cycle times range due to the amount of time the
polymer needs to cure or solidify. This is called the hold time. Some advantages
of injection molding are high production rates, design flexibility, low
tolerances, can process wide range of materials, low labor, little or no
finishing, and scrap is held to a minimum. However, some disadvantages are high
startup and running costs, part must be designed for effective molding, accurate
cost prediction is difficult, and machine cost is very high. The high tooling
costs come from the molds being built to a high level of precision. The molds
are usually constructed of hardened tool steel, and aluminum or other soft
metals when tooling life is not an issue. Tooling costs can range from $5,000 to
$100,000. However, there are some parts that can not be formed by any other
method of processing except injection molding. These parts typically become
feasible around 1,000 pieces. To go with the high tooling costs there are a
large number of variables that go along with it. Injection molding machines may
require special plant services that other equipment does not. As technology
advances so must the industry to keep up production. One way injection molding
is keeping up is by becoming automated. Usually, operators are placing parts
into molds, and then taking the parts out. Now, robotic devices are being used
to place inserts before molding and remove parts after molding as well as a host
of other operations as well. Not only does the robotics speed up the process,
but makes it much more cost effective. Another way industry is trying to keep up
with technology is by using computer software. The software is called "Mold

Adviser," which is a mold design and analysis package that can be used to help
speed up operations while reducing tooling costs. Using the past standard
operation of designing molds a company could easily waste six to twelve weeks
and anywhere from $30,000 to $40,000 on fixing a mold that has a problem with
filling correctly. The new software will detect these problems up front before
production begins. It is used with a CAD program, and is really divided into two
parts. The Part Advisor and the Mold Advisor. The Part Advisor can test the
parts design and analyze the part to see if it will fill correctly. Once the
part has been analyzed, users begin the Mold Advisor. The Mold Advisor will help
setup gates and runner systems, predict plastic flow, shot size, and cycle time.

Users of the new software have reported that the software is easily learned and
user friendly. Injection molded products include TV, VCR, CDís, CD players and
computer housings. So, one can see the impact that the products of injection
molding have on our every day life. Injection molding has been used since the

1860ís. Products of all sizes and shape have been produced. However, with
advancing technology and the high cost of molds it will be interesting to see
how much further injection molding will advance and how it will advance in the
future.

Bibliography

Harris, Laura. "Injection Molding." 30 Nov 1999. http://www.engr.uconn.edu/cheg/polymer/injmold.htm

"Injection Molding." 30 Nov 1999. http://www.endura.com/injmold.htm

Mapleston, Peter. "New approaches show promise in large-part molding."

Modern Plastics Nov 1999. Snyder, Merle R. "Automation is gaining in vertical
infection molding machine market." Modern Plastics Aug 1996. Taylor, Gerry.

"One-two punch knocks out injection-molding problems. Machine Design 17 June

1999: 80.