Injection Molding (944 words)

Injection MoldingInjection 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 1860’s. 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
can’t 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, CD’s, 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 1860’s. 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
Works Cited:
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.

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