Injection Molding Essay, Research Paper

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.

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

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