Working with Acrylic

43 Pages·1999·0.96 MB·English

by  Fry

Working with Acrylic
from the series
Speaking of
Plastics Manufacturing
By Bill Fry
Technical Advisor: Phillip Barhouse
Society of Manufacturing Engineers
Dearborn, Michigan
Copyright © 1999 by Bill Fry and
Society of Manufacturing Engineers
987654321
All rights reserved, including those of translation. This book,
or parts thereof, may not be reproduced by any means, in-
cluding photocopying, recording, or microfilming, or by any
information storage and retrieval system, without permission
in writing of the copyright owners.
No liability is assumed by the publisher with respect to use
of the information contained herein. While every precaution
has been taken in the preparation of this book, the publisher
assumes no responsibility for errors or omissions. Publica-
tion of any data in this book does not constitute a recommen-
dation or endorsement of any patent, proprietary right, or
product that may be involved.
International Standard Book Number: 0-87263-504-X
Additional copies may be obtained by contacting:
Society of Manufacturing Engineers
Customer Service
One SME Drive, P. O. Box 930
Dearborn, Michigan 48121
1-800-733-4763
Booklets in this Series
Working with Acrylic
Working with Cellulosic
Working with Nylon
Working with Polyethylene
Working with Polystyrene
Working with Vinyl
Additional booklets are planned
SME staff who participated in producing this book:
Millicent Treloar, Senior Editor
Rosemary Csizmadia, Production Supervisor
Kathye Quirk, Production Assistant
Frances Kania, Production Assistant
Printed in the United States of America
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Introduction................................................................1
Chemically Speaking..................................................8
Typical Applications ..................................................12
Production Methods
Injection Molding............................................................... 14
Casting ............................................................................. 15
Extrusion ........................................................................... 17
Thermoforming (Vacuum Forming) ..................................... 19
Glimpse of the Future ...............................................22
Physical Properties (Explained)...................................23
Specific Gravity.................................................................. 24
Tensile Strength.................................................................. 24
Impact Strength ................................................................. 25
Hardness: Brinell System .................................................... 25
Compressive Strength ........................................................ 25
Flexural Strength................................................................ 25
Modulus of Elasticity........................................................... 25
Thermal Expansivity (Coefficient of Expansion)..................... 25
Specific Heat ..................................................................... 26
Volume Resistivity............................................................... 26
Power Factor...................................................................... 26
Dielectric Constant............................................................. 26
Dielectric Strength.............................................................. 26
Thermal Conductivity ......................................................... 27
Refractive Index ................................................................. 27
Moisture Absorption........................................................... 27
Flammability...................................................................... 27
Arc Resistance.................................................................... 27
Chemical Resistance .......................................................... 27
Glossary ..................................................................29
Some Common Acrylic Trade Names ........................37
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Acrylics make up a large family of varied thermo-
plastic materials. This is a 20th century material that each
day is proving its worth by performing thousands of tasks
in the home and industry.
The term “Acrylic” is used to denote this entire group
of resins rather than a single type of formulation. The
types of Acrylic materials range from clear, free-flowing
liquids, to very hard molding materials in all degrees of
transparency, including complete opacity.
It was in the early 1900s that Dr. Otto Rohm of Ger-
many and his associates produced a satisfactory com-
mercial process for manufacturing Acrylics. About 1931,
the first use of Acrylic resin in the United States was for
laminating safety glass. Soon afterwards other processes
were developed. One of the great achievements was the
development of Methyl Methacrylate. This liquid-form
resin is used for the production of cast sheet. It is pro-
duced as pellets for injection molding and extrusion.
Methyl Methacrylate is also used as a binder for wood,
paint, glass, textiles, rubber, and other materials.
The Acrylics used in injection molding and extru-
sion are usually supplied as pellets to the processors in
Polyethylene-lined cardboard drums of approximately
200 lb (91 kg) capacity. Lined drums are used because of
the hygroscopic nature of the material. By this method
of packaging, moisture absorption is kept to a minimum.
Some processors predry the material before using, while
others have developed techniques in which predrying is
unnecessary. It is emphasized that covers or lids should
be kept on the containers at all times, as well as on the
hoppers of the machines, not only because of moisture
absorption, but also because of contaminants.
2 Working with Acrylic
Liquid Acrylic resins are usually shipped to large
quantity producers of cast sheet in tank cars. The basic
material, Methyl Methacrylate monomer, is highly vola-
tile and has a tendency toward polymerization at room
temperatures. Extreme precautions are taken in the han-
dling and preparation before casting. Because of the dif-
ferent methods of casting and the different end uses of
this product, it is natural to assume that the actual tech-
niques of this operation are very diverse and, in almost
all instances, special to each processor.
The variety of products made from cast sheet of vari-
ous grades range from highly critical aircraft windows
and domes to such articles as shoe
trees or salad bowls.
The manufacturing
of aircraft windows,
windshields, and
bubbles has become a very
complex and highly technical industry. With the advent
of aircraft that can travel at supersonic speeds, it became
a prime requisite that the windows and windshields meet
fantastic requirements. The heights of progress in this
field to which the plastics industry was compelled to
reach was, and still is, comparable to every new devel-
opment that has come forth from the aircraft industry.
In World War II, probably every airplane in the United
States Air Force was equipped with an Acrylic wind-
shield. Today, all airplanes,
whether military or civilian, are
similarly equipped. The win-
dows on the large jet passen-
ger planes are made of
specially designed and fab-
ricated stretched Acrylic
sheets that can withstand
Working with Acrylic 3
tremendous pressures. These windows are, for all prac-
tical purposes, indestructible.
Another very important product is the Acrylic boat
windshield. The ability of Acrylics to withstand the tur-
bulent sea, rain, wind, and sun, has proven to be greatly
advantageous. With simple cleaning, the windshield will,
in most cases, last as long as the boat.
Besides using
casting types of
Acrylic resins
for sheet
stock, Acrylics
are cast into blocks,
from which parts are machined,
or rod stock is used for machining or forming. Of
particular interest in the field of casting is the
preservation or embedding of live tissue, glands, or
other parts of the human anatomy for medical and
surgical research and study.
Acrylics have had a wonderful part in helping to
preserve a person’s well being. It is always a comfort-
able feeling to realize the cosmetic effects of a set of
new dentures, or an eye, or even a plastic contact lens.
Cast Acrylics serve mankind in many ways, be it per-
sonally or in industry.
In the field of injection
molding, Acrylic items are
produced in great quantities.
Most of these products are
parts that must hold close tol-
erances and be free of any flaws,
4 Working with Acrylic
particularly instrument lenses and prisms. Acrylics
can be extruded in almost any design or shape, such
as rods, tubes, or profiles. One of the really fascina-
ting features of Acrylics in this form, is their ability to
pick up light and to transmit it unseen around curves
and show it only at the opposite end. This method of
lighting is used extensively on instrument panels in
electronics and aircraft.
As with many of the other thermoplastics, occasion-
ally a product, for either economical or practical reasons,
must be produced in two or more parts. With Acrylics,
methods of welding have been developed wherein the
weld line is only barely visible, and the weld itself is usu-
ally as strong as any other part of the product. In a molded
part, it is always a good idea to try to design the part so
that the weld line is part of the overall design. There are
many techniques of welding, such as solvent welding,
spin welding, hot air welding, and others. Each of these
methods is particularly well adapted for specific pur-
poses. In the event that a weld line or other marks affect
the appearance of a part
made of Acrylic, such un-
sightly marking can be
cleaned by buffing and pol-
ishing as with metals.
Cast, molded, or ex-
truded Acrylics all have
good optical qualities. The
material can be obtained in
either a state of crystal clear transparency or complete
opaqueness, colored to most any shade or hue. It is be-
cause of its good optical properties and coloring ability,
combined with its exceptional stability and weathering
qualities, that this material has become the leader in
many fields using plastics. Its use as skylights in indus-
Working with Acrylic 5
trial plants and school buildings is widespread. This is
because of its clarity and ability to diffuse light so effec-
tively. The same holds true for the lighting industry,
where it is used in tremendous quantities for fluorescent
lighting fixtures of all types and sizes and for decorative
and cove lighting. The decorative lighting
field, using high-quality Acrylic sheets, is
an important outlet. While it is under-
standable that Acrylics used in light-
ing are desirable for their many
outstanding mechanical and
optical qualities, the ability
to work with it for ornamen-
tal uses, such as lighting ef-
fects, shape, and color, is of
utmost importance.
The outdoor sign indus-
try uses the Acrylics family as one of its foremost mate-
rials. It is able to withstand the ravages of all types of
weather. Besides this, many methods employed for deco-
rating signs of other materials are easily adapted to large
flat sheets of Acrylic. Thermoformed Acrylic letters of
one color, solvent welded to an Acrylic sheet of another
color, make extremely eye-appealing signs.
Acrylics have good dimensional stability. There are
formulations that have a heat distortion point of over
240° F (116° C), which can be immersed in boiling water
for extended periods with no ill effects. Even though
Acrylics generally have excellent impact resistance, there
are modified Acrylics that have extremely high impact
resistance. Special techniques of casting and stretching
make this material shatterproof. One application is its use
as covers or guards on machinery. Besides having very
good impact resistance, its clarity enables the operator to
watch and control machine operations more closely.
6 Working with Acrylic
Acrylics, when molded, extruded, or cast, can be
decorated by many methods. Silk screening is the most
common method, used primarily in the sign industry.
Hot stamping with gold, silver, or colored foil, is often
used for decorating novelty items. Using stencil-type
masks, spray painting is also a method used with good
results on Acrylics. Many products are decorated by this
method. Vacuum metallizing is a method of decorating,
which when done properly, produces an exceptionally
beautiful product, such as an emblem or nameplate.
Usually items of this nature employ both spray painting
and vacuum metallizing done on the underside of a clear
part. An example of this is the emblem usually displayed
on the bug shield of your automobile.
Because of the very lus-
trous,hardsurface finish of
Acrylic materials, extreme
care must be exercised in
handling. Usually sheet
stock is supplied with a
covering of masking pa-
per for protection against
surface scratches. This, of
course, does not necessarily apply to
molded or extruded parts. Cleaning or removing surface
blemishes on Acrylic parts should be done carefully. It
is recommended that washing be done with lukewarm
water, detergent, and a soft cloth. On removal of mask-
ing paper, any adhesive that remains can usually be re-
moved by the adhesive backed paper itself or with a
commercially available cleaner. Solvents should not be
used for cleaning because of subsequent crazing. Craz-
ing does not always occur immediately.
Static electricity on Acrylics is a problem usually con-
fronted when decorating or printing. There are many
Working with Acrylic 7
methods available to the fab-
ricator to help eliminate this
condition. The suppliers of raw
materials are the best source of
information on this type of
problem, because of the various
formulations and their knowl-
edge of effects of the
different methods of
destaticizing.
Acrylics are com-
bustible, though classified
as slow burning, and should never be held over
an open flame.
There are other industries, outside
of the plastics industry, which use Acrylics
in vast quantities. For example, in synthetic
rubber formulations, the addition of Acryl-
ics provides an end product with outstanding heat and
oil resistance. These formulations
have found wide acceptance
as gaskets in the automo-
tive industry. We are all
quite aware of the miracle
materials used in textiles.
Acrylic fibers and other
formulations of Acrylics
blended with wool, rayon, and other
plastic materials give us ever newer
types of fabrics, clothing and carpeting.
Acrylics are also an important part of paints. Because
they possess high luster and are quick drying, they are
widely used in the automotive field. Similarly, because
of durability, lack of odor, and fast drying qualities, acrylic