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Pressure Forming of Multipart Assemblies Lifts Thermoforming
to New Levels of Quality and Close Tolerance |
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One of the oldest situations in manufacturing
is when customers demand that the technology of an industry be pushed beyond
its current capability to meet their needs. Once that new level is accepted it
is again pushed even further.
So it is with pressure forming of thick thermoplastic sheet. From simple bezels
and machine control panels, the leading pressure forming companies of the world
have accepted ever-increasing challenges to build more and more complex parts.
As the complexity of the parts has increased, so has the demand for closer
tolerances in both formed and trimmed dimensions.
Thick sheet pressure forming is similar to old fashioned vacuum forming, in
that a mold is introduced into a hot thermoplastic sheet, and vacuum is used to
draw the resistant air from between the mold and the sheet. Ambient air
pressure then forces the hot sheet to conform to the shape of the mold and the
sheet cools to the rigid shape. In pressure forming, a pressure box mates with
the periphery of the sheet on the opposite side from the mold. When the vacuum
comes on, compressed air is introduced into the chamber, and hammers the hot
sheet onto the mold surface. Common pressures used are 35 PSI to 60 PSI (241
kPa to 414 kPa). However, higher pressure may be required on large parts of
higher hot strength materials. The material will pick up every detail of the
mold, such as sharp, crisp corners, logo and letter details, and even overall
textures, both light and heavy. The surface details will be indistinguishable
from injection molded parts.
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| There are many advantages over injection molding to pressure
forming parts and enclosures:
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1. Lower initial
investment for modest quantities of parts
(less than 2,000)
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| 2. Shorter mold turnaround time - 8 to
12 weeks |
| 3. Lower mold costs - 10 to 20% of
injection molds |
| 4. Parts are relatively stress-free |
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| The only disadvantages are financial considerations: |
1. The cost of
processing resin into sheet and the use of an oversized
sheet, to allow for clamping during heating and
forming, adds to
material costs. |
2. Single surface detail requires
additional labor to add standoffs and
details to backside of parts when needed. |
3. Trimming of excess material used for
clamping, and of all apertures
require additional labor costs beyond molding time. |
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| A typical pricing comparison between injection
molding and pressure forming is for a "computerized court reporter's steno
machine". A three-part enclosure consisting of a base, a cover, and a small
access door. The customer estimated the maximum requirement would be 1000 to
1200 assemblies. An accelerated lead time was required for pre-production
showing of product.
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There is NO formula to be used to calculate
part size x material x etc. to compare injection molding to pressure forming.
Each project needs to be costed and compared.
The most commonly used thermoplastic materials for pressure forming are usually
rated as fire retardant for electronic enclosures. Material examples are:
FRABS, PVC/Acrylic alloys, modified polyphenylene oxide, polycarbonate, PVC,
and ABS/Polycarbonate blends.
The larger the part, the greater the savings due to the mold pricing. The cost
of pressure forming molds does not increase proportionately to the mold size.
You can often double the size of a part with a mold increase of only 25% to
30%. Injection molds, on the other hand, increase in cost disproportionately to
the increase in size.
Pressure forming molds will always be built of aluminum, because of the heat
transfer properties, which is very important to cooling the formed sheet.
Aluminum is also economical to cast and/or machine and will readily withstand
the pressures generated on the mold.
Pressure forming molds are temperature controlled by building fluid channels
into the back side of the mold surface. Temperature controlled water or a
glycol-water solution is run through the channels and carries away the heat
that flows from the hot formed sheet into the mold. Since pressure holds the
sheet, for at least part of the cooling cycle, in intimate contact with the
mold, the heat will flow faster and more uniformly than in simple vacuum
forming.
The faster cooling cycle is important to the economics of the part cost, and
the uniform cooling is extremely important to building low residual stress in
the finished part. The hotter the mold, during forming, the greater the
shrinkage of the plastic during forming and cooling. Thus, by carefully
controlling a consistent overall temperature in the mold, we will control a
more uniform finished part size, and eliminate post-forming warpage.
Pressure formed parts can be more expensive than vacuum formed because of the
high reject factor due to increased cosmetic requirements and tighter
dimensional tolerances. |
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Consideration must be given to the second
surface, or the side of the part opposite from the detailed mold surface. To
create pressure formed parts that are competitive in design with injection
molded parts it will frequently be necessary to add mounting posts, ribs,
standoffs, and such. These inserts may be machined, screw machined, injection
molded, or thermoformed and can be added by bonding or ultrasonic welding.
Whether to paint the parts or texture the mold, and use material with
homogenous color or cap-coated with select color on a random base sheet is a
matter of customer-desired appearance or a financial consideration of
quantities of parts.
There is no formula to use, just common sense calculations. For example:
MOLDS + ADDL. COST OF COLOR-MATCH SHEET) / QUANT. OF PARTS
versus
PAINTING COST + ADDITIONAL HANDLING COSTS
Another factor can be considered when pressure formed parts are to be mated
with metal painted parts. It may be desirable to have all of the parts painted
to effect a good color and texture match on the complete assembly.
In pressure forming it is not unusual to have undercuts or negative formed
sections in the vertical walls of the parts. They are used by designers to
effect close tolerance fit in the assembly of parts.
In order to remove the formed part from the mold it is necessary that one or
more sections of the mold move back into the mold to allow the part to be
removed. This motion can be achieved by pneumatic or hydraulic cylinders, or
electric screw drives built into the mold.
Another option is that undercut mold sections overhanging the open rim of a
negative cavity mold can be hinged to fold back out of the way as the part is
removed from the mold. One more idea is the overhanging section of the mold can
come out with the formed part, and then be removed and set back into place in
the mold before the next hot sheet arrives for forming. |
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Undercuts will complicate the mold design and
increase the initial cost, as well as create ongoing maintenance expense.
However, this expense will be a fraction of the cost of creating the same
detail in injection molds.
There are no industry-wide established dimensional tolerances since they are
determined by the machinery sophistication and practices of each individual
pressure forming manufacturer.
Those companies who step up and say, "Yes, we can hold very tight tolerances."
Will need to have modern computer-controlled forming and trimming machinery and
very tight control over the tooling fabrication.
A company well-equipped and experienced to do such a job is Profile Plastics
Corporation in Lake Bluff, Illinois, north of Chicago, in the USA. At the SPE
Thermoforming Conference in September, 1998, they won an award for the best
"Multipart Medical Enclosure" of the year. The winning assembly of parts is the
housing of the Ergodyne ON 3.
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A company developed
a machine that assists nurses in transferring patients from their hospital bed
to a gurney, and the reverse, thus reducing the exposure to back injuries
associated with heavy lifting and pulling.
The machine required a housing assembly, and the developing company wanted the
crisp, high-tech look. The high costs and long lead time of injection molding
tooling sent them looking in other directions.
Profile Plastics accepted the challenge to develop and manufacture the
enclosure. Six |
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pressure formed parts were designed to
assemble, by way of a series of undercuts, into a complex housing.
The pressure form molds were machined from aluminum billets, temperature
controlled, and chemically etched with a soft matte finish texture. The
temperature control held tight forming dimensional tolerances, low residual
stress to prevent warpage and maintain material properties, and maintain
uniform gloss control.
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Three of the parts
had undercuts, one of them, the horizontal cover, had severe 1 inch returns on
both sides of a part 8 inches wide and 10 inches deep.
The material selected was an ABS modified with PVC for fire retardency. The
extruded sheets were pre-colored, and to guarantee ongoing color match, each
incoming production run of material is checked by spectral photometer.
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The horizontal cover material is 0.325 inches (8.2 mm) thick before forming,
yielding a part with a reasonably uniform wall thickness of 0.100 inches (2.5
mm). Other parts with less severe forming ratios utilized thinner starting
sheet. Extraordinary consistency of forming, trimming, and gloss level is
maintained in the manufacturing of these parts. |
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There seems to be no limit to what creative,
aggressive pressure formers can achieve.
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| Art Buckel |
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| About
the Author: |
| Art is an Associate of McConnell Co. Inc, which is a consulting company that is based in Fort Worth, Texas. |
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| About
McConnell Co., Inc.: |
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The company was founded by Wm. K. "Bill" McConnell, Jr., and now consists of
three associates, Arthur Buckle, Robert Browning and Donald Hylton, who has
just joined and is a very well known chemist and polymer scientist.
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