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Injection Molding
What Is Injection Molding And Why
It Is Important
By Todd
Johnson
Injection molding is a
manufacturing process widely used for producing items from toys and plastic
trinkets to automotive body panels, water bottles, and cell phone cases.
A liquid plastic is forced
into a mold and cures - it sounds simple, but is a complex process. The liquids
used vary from hot glass to a variety of plastics –thermosetting and
thermoplastic.
History
The first injection molding
machine was patented in 1872, and celluloid was used to produce simple everyday
items such as hair combs.
Just after the Second World War, a much-improved injection molding process -
'screw injection' was developed and is the most widely used technique today.
Its
inventor, James Watson Hendry, later developed 'blow molding' which is used for
example to produce modern plastic bottles.
Types of plastic
The plastics used in
injection molding are polymers - chemicals - either thermosetting or
thermoplastic.
Thermosetting plastics are
set by the application of heat or through a catalytic reaction. Once cured,
they cannot be remelted and re-used - the curing process is chemical and
irreversible.
Thermoplastics, however, can
be heated, melted and re-used.
Thermosetting plastics
include epoxy, polyesterand phenolic resins, whilst thermoplastics include
nylon and polyethylene.
There are almost twenty
thousand plastic compounds available for injection molding, which means that there
is a perfect solution for almost any molding requirement.
Glass is not a polymer, and
so it does not fit the accepted definition of thermoplastic - though it can be
melted and recycled.
The Mold
The making of molds has
historically been a highly skilled craft ('die-making').
A mold is usually in two main
assemblies clamped together in a press. Making a mold often requires complex
design, multiple machine operations and a high degree of skill.
The tool is usually steel or beryllium copper which is used for mold making
requires heat treatment to harden it.
Aluminum is
cheaper and easier to machine and may be used for shorter run production.
Nowadays,
computer controlled milling and spark erosion ('EDM') techniques have enabled a
high degree of the automation of the process of mold manufacturing.
Some molds are designed to
produce several related parts - for example, a model airplane kit - and these
are known as family molds.
Other mold designs may have
several copies ('impressions') of the same article produced in one 'shot' -
that is, one injection of plastic into the mold.
How Injection Molding Works
There are three main units
which make up an injection molding machine - the feed hopper, the heater
barrel, and the ram.
The plastic in the hopper is
in granular or powder form, though some materials such as silicone rubber may
be a liquid and may not require heating.
Once in hot liquid form, the
ram ('screw') forces the liquid into the tightly clamped mold and the liquid
sets.
More viscous molten plastics
require higher pressures (and higher press loadings) to force the plastic into
every crevice and corner. The plastic cools as the metal mold conducts heat
away and then the press is cycled to remove the molding.
However, for thermosetting plastics, the mold will be heated to
set the plastic.
Advantages of Injection Molding
Injection molding enables
complex shapes to be manufactured, some of which might be near impossible to
produce economically by any other means.
The wide range of materials
enables almost exact matching of the physical properties required by the
article, and multi-layer molding enables tailoring of mechanical properties and
attractive visual appearance - even in a toothbrush
In volume, it is a low-cost
process, arguably with minimal environmental impact. There is little scrap
created in this process, and scrap that is produced, and is re-ground and
re-used.
Disadvantages of Injection
Molding
The investment in tooling -
making the mold - typically requires high volume production to recover the
investment, though this does depend on the particular article.
Producing the tooling takes
development time and some parts do not readily lend themselves to a practical
mold design.
The Economics of Injection
Molding
A high-quality mold, although
of relatively high cost, will be capable of turning out hundreds of thousands
of 'impressions'.
The plastic itself is quite
inexpensive and despite the energy required to heat the plastic and cycle the
press (to remove each impression), the process can be economic for even the
most basic items such as bottle caps.
Cheap injection molding has
led ultimately to disposability - for example
of razors and ballpoint pens.
With several hundred new
plastic compounds being developed each year and modern mold-making techniques,
injection molding is certain to continue increase in use over the next fifty years.
Although thermosetting
plastics cannot be recycled, their use, particularly for high precision
components, is also set to grow.
Todd Johnson
· Regional
Sales Manager for Composites One, a distributor of composite materials.
· B.S. in Business Management from University of
Colorado Boulder's Leeds School of Business
· Business Development Manager for Ebert Composites
Corporation
Experience
Todd Johnson
is a former writer for ThoughtCo, who wrote about plastics and composite
materials for 2-1/2 years between 2010 and 2013. He is a Regional Sales Manager
at Composites One, a composite materials distributor in San Diego, CA. Johnson
provides support to the Greater San Diego manufacturers of fiber reinforced and
polymer products. He regularly attends composite industry trade shows including
JEC, ACMA, SME, and SAMPE. In 2008 he presented at the Global Pultrusion
Conference in Baltimore, MD. Previously, Todd spent six years as the Business
Development Manager for Ebert Composites Corporation.
Education
B.S.,
Business, Management, Marketing, and Related Support Services - the University
of Colorado-Boulder's Leeds School of Business; attended Griffith University in
Queensland, Australia.
Todd Johnson
ThoughtCo and Dotdash
ThoughtCo is
a premier reference site focusing on expert-created education content. We are
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