Feature Stories Archive
Machining & Forming Tips
Getting the most out of polycarbonates
includes learning the best methods for sawing as well as
forming and mold design.
There are few thermoplastics that have the outstanding
engineering properties of polycarbonate. Developed
commercially in 1957, it is one of the pioneering members of
the family of engineering thermoplastics, created to compete
with die-cast metals.
Polycarbonates are high-molecular weight, amorphous
engineering thermoplastics that are characterized by a
combination of outstanding impact strength, superior
dimensional stability, glass-like transparency, remarkable
thermal resistance and good electrical properties. In
addition, they have a ductility that is normally associated
with softer, lower-modulus thermoplastics. Polycarbonate is
distinguishable from other engineering thermoplastics by its
freedom of design; it allows engineers selecting a
combination of these properties to meet specification
requirements for a broad range of application needs.
Polycarbonate sheet is extruded in a number of standard
widths and lengths and is available in gauges from 0.030
inch to 0.500 inch thick in monolithic sheet and 0.375 inch
to 1.250 inches thick in laminated sheet. Grades include:
general purpose, UV enhanced, abrasion resistant, aircraft ,
flame inhibiting, bullet resistant, forced entry resistant,
FDA and sign grade. Polycarbonate is also manufactured in
machine grade and is available in plate, rod and tube.
Polycarbonate has good thermoforming characteristics.
When formed, fabricated, machined and finished according to
recommended techniques and procedures for secondary
operations, the final result will be a high quality
Fabricating Polycarbonate sheets
Circular sawing: A table saw or overhead panel saw is
typically used for cutting polycarbonate sheet. A circular
saw blade with carbide teeth, utilizing the "triple chip"
tooth design, is the preferred method. Blades for cutting
3Ú32 inch and thicker material should have three to five
teeth per inch and the hook or rake angle should be
10-degrees to 15-degrees. Circular saws should be run in the
speed range of 6,000 to 8,000 feet per minute.
When sawing thin gauge polycarbonate, it is important to
have a good supporting edge on the saw table with minimal
gap between the saw blade and table-supporting edge. Care
should be taken to ensure that tabletops are smooth and free
from projections that might scratch or mar the sheet.
Band saws: Band saws are useful for trimming formed parts
or irregular shapes. Bands saws should be run at 2,500 to
3,000 feet per minute, and have eight to 12 teeth per inch.
Coarser (larger) blades perform better with thicker gauge
polycarbonate sheet. Proper support of the part to be
trimmed is important because vibration may induce cracking
if the cut is not smooth.
Routing: Routing produces a smooth edge on polycarbonate
sheet and may also be used to cut curved or irregular
shapes. Routers with at least 1-horsepower motor and speeds
of 20,000 to 25,000 rpm are preferred, used in conjunction
with 1Ú4-inch to 1Ú2-inch diameter straight (fluted 2 or 3),
carbide-tipped or high speed steel router bits.
The stock feed must be monitored closely as feeding
polycarbonate sheet at excessive rates can cause vibration
and cracking. It is important to feed the sheet against the
rotation of the router bit and to provide a fence for sizing
when making straight cuts.
Shearing: Die cutting polycarbonate sheet in gauges up to
0.080 inch thick is normally achieved by utilizing the
following steps to calculate the required press tonnage:
F = (P x A)/2,000
where F= required force in tonnage of the press, P=10,000
psi (shear strength of polycarbonate sheet) and A= the
sectional area to be cut.
Steel rule dies mounted in a press provide good results.
Use 3PT-thick (0.042-inch) steel to fabricate steel rule
die-flush, or center bevel ground provides a clean cut.
Facet ground steel rule is used to cut thicker gauge sheets,
above 0.60 inch. Be sure the platens are parallel and that
the backup pad is in good condition. Backup pads can be made
from a variety of materials such as nylon, HDPE, etc.
Drilling: Polycarbonate sheet is easily drilled using
ordinary high speed steel drill bits. Fabricators are urged
to regulate the pressure and speed until a continuous
spiraling chip is observed. If needed, use air or water as a
coolant; using oils may cause crazing. Be extremely careful
if using taps or self-tapping screws; tapping creates
notches which, in a notch-sensitive material like
polycarbonate, can result in stress cracks. The recommended
drill speed is 350 to 1,750 rpm.
Thermoforming: Polycarbonate sheet can be thermoformed on
standard equipment, with vacuum forming, free-blown forming
and line bending the most extensively used processes. While
most standard forming techniques can be used, critical
process modifications specific to polycarbonate sheet are
Polycarbonate sheet must be pre-dried prior to
thermoforming and heating cycles need to be accurately
controlled for uniform product quality. The thermoforming
machine should be capable of generating and maintaining
sufficient vacuum pressure throughout the thermoforming
cycle. A minimum vacuum of 20 inch Hg throughout the entire
vacuum cycle is necessary to retain part integrity.
Most commonly used vacuum forming machines with infrared
heating elements perform well for polycarbonate sheet
forming. Rotary and shuttle designs with automatic or
automatic controls are the most suitable because of their
timer control accuracy, uniform heating sources and
sufficient vacuum power. Single-sided heating has proven
effective for polycarbonate sheet gauges up to 0.177 inches.
For thicker gauges however, it is recommended that
dual-sided heating ovens be used for effective radiation
Mold materials & mold design: Polycarbonate allows
the use of a variety of mold materials such as wood, filled
and unfilled polyesters, epoxiers and metals. Molds for
vacuum forming need only 14 psi, so there is little wear on
the tooling due to the low pressure of the material against
the mold surface. Use of standard mold design practices and
mold materials should be observed.
Molds: Male molds are normally more suitable for vacuum
forming. However, other factors such as part size, finish
and shape dictate the mold design. Molds are constructed of
materials that relate to cost effectiveness for the length
of the run of a given product.
Pre-drying: Polycarbonate sheet must be pre-dried before
thermoforming because it absorbs moisture at a high rate.
Trapped moisture forms vapor above 250F and the vapor
expansion creates bubbles in the sheet. Sheets should be
placed in a dehumidifying air circulating oven for
pre-drying, with approximately 1-inch separation between
sheets. Oven temperature should be 250F and monitored with
Polycarbonate sheet begins absorbing moisture immediately
upon removal from the pre-drying oven. The rate of
absorption is dependent upon the ambient dew point. For this
reason, it is crucial to transfer the sheet directly to the
forming machine as quickly as possible.
Heating cycle: Heating polycarbonate sheet for vacuum
forming requires a heat penetration range of 360F to 410F.
The ideal forming temperature is 375F.
When polycarbonate sheet reaches forming temperature,
uniform "sag" occurs. The amount of sag depends on the size
and thickness of the sheet. Once the uniform temperature has
been achieved, timers can accurately reproduce the
condition, with part-to-part consistency maintained.
Polycarbonate sheet "sets up" quickly compared to other
thermoplastics, and can be removed from the mold in a short
period of time.
Cold bending: Polycarbonate sheet can be cold formed into
circular shapes for walkways, etc. The rule states that the
radius of the curvature must be at least 100 times the
Strip heating: Strip heating or line bending is commonly
used for producing localized angular bends in polycarbonate.
Generally, pre-drying is not required for a material
thickness of 0.118 inch or less. On thicker gauges,
pre-drying can be avoided by back-routing or V-grooving the
sheet to 1Ú8 inch or less thickness.
Drape forming: Simple contours can be achieved by drape
forming polycarbonate sheet. The sheet should be pre-dried,
then brought to a forming temperature of 325F to 340F in the
oven. Parts are then removed and placed immediately over a
male mold covered with felt. This method is utilized to
manufacture face shields or any other part requiring a
simple radius curvature.
Brake forming: Brake forming polycarbonate sheet is not
recommended. Brake forming imparts localized stresses, which
can exceed the elastic limits of polycarbonate, resulting in
stress cracking or crazing.
Mechanical fastening: Aluminum rivets and machine screws
may be used to join polycarbonate sheet to other materials.
To do so, drill oversized holes and use washers to
distribute and cushion localized stress. Fabricators should
be sure to consider the differentials in expansion factors
for dissimilar materials. Also, be sure drilled holes are
smooth and free from cracks.
Joint-planing: A standard woodworking jointer/planer is
an excellent edge finishing machine for polycarbonate sheet.
Blades must be made from carbide or high speed steel.
Fabricators are advised to avoid removal of too much stock
on each pass; trying to remove too much material may result
in a rough edge or a cracked sheet. Typically, you will find
that 1Ú64 inch or less stock yields the cleanest edge. If
smoother edges are required, wet sanding with a fine grit
#200 sandpaper is recommended.
Edge finishing: Normally, edges may be finished by
planing and sanding, which provides a smooth, matte finish.
Flame polishing polycarbonate is not normally an acceptable
Polished edges may be achieved by solvent polishing.
After sanding to remove all the tool marks, dip a cloth in a
solvent such as methylene chloride and carefully wipe the
sheet's edges. Do not allow the solvent to drip on the sheet
face as discoloration will occur.
Karl Wiecking is the marketing manager
for DSM Sheffield. Located in Sheffield, MA, the 47-year-old
company manufactures a wide range of polycarbonate sheets as
well as other high performance sheet plastics. For more
information, Wiecking can be reached at (413)
John Raynor is the marketing manager for
Charlotte, NC-based Piedmont Plastics. Established in 1968,
Piedmont distributes and fabricates polycarbonate as well as
other types of materials. For more information, Raynor can
be reached at (704) 597-8200.
Plastics Machining & Fabricating
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