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Fiber Reinforced Plastics Shop Implements
Light RTM to Produce Parts
| Company
|
Phoenix Industries
Crookston, MN |
| Change |
Converted 60
percent of molded part production to closed molding.
Light RTM is used for a quarter of that (15 percent).
|
| Cost |
$10,000 for
new equipment. Cost per part reduced by ten percent.
Payback was less than two years. |
| Benefits |
Reduced 80,000
pounds of styrene emissions over two years. 20,000
of this was due to Light RTM. Cleaner production.
Better material efficiency. |
|
Phoenix
Industries, in Crookston, Minnesota, produces fiber reinforced
plastic (FRP) parts in a 100-person job shop. The parts, produced
by open and closed molding techniques, vary in shape, size
and end use.
Reducing styrene emissions was a priority
at Phoenix Industries for a number of years, primarily driven
by worker exposure and air permitting requirements. The U.S.
Environmental Protection Agency (EPA) classifies styrene as
a hazardous air pollutant (HAP). The National Emission Standards
for Hazardous Air Pollutants (NESHAP) for the reinforced plastic
composites industry became effective August 2001, limiting
styrene emissions from FRP shops.
Converting to nonatomized spray resin application
equipment and using low styrene resins in its open mold process
quickly reduced the company's emissions. Although these efforts
reduced emissions significantly, Phoenix Industries saw closed
molding as an opportunity to further reduce emissions, enhance
the efficiency of material use in FRP manufacturing and improve
part quality.
Closed Molding Replaces Open Molding
Converting from open-mold to closed-mold processes reduces
emissions and optimizes the glass-resin ratio, producing a
higher quality laminate, and allowing both sides of the part
to have a finished appearance. With advancements in FRP materials
in recent years, closed molding has become a viable technology,
finding renewed interest as it demonstrates success. Vacuum
molding is one relatively simple and affordable means for
open molders to move to closed molding.
In two years, Phoenix Industries converted
60 percent of its open molded parts production to closed molding.
The company selected Resin Transfer Molding (RTM) and Light
RTM technology as its vacuum molding systems. The conversion
reduced 80,000 pounds of styrene emissions during 2000 and
2001. Light RTM is used for a quarter of the closed molding
(15 percent overall). Phoenix uses Light RTM when a part is
produced less frequently because it is less costly to use
on a smaller scale than RTM. The company plans to continue
the conversion to closed molding, anticipating additional
significant bottomline benefits.
Light RTM
Light RTM is a vacuum-assisted, low-pressure, resin injection
system. The vacuum draws the resin through the mold, limiting
the pressure needed for injecting the resin. Because limited
pressure is used, the molds do not require extra engineering,
helping to keep costs down. Light RTM results in lower environmental
emissions, improved quality and part-to-part consistency,
and reduced per part cost. Light RTM has nearly universal
application. If a part can be "pulled"—part
configuration allows molds to easily separate—it is a
candidate for Light RTM.
Three major components make up this molding
system: a two-part mold, a vacuum source and a low-pressure
resin injection pump.
|
The general steps to producing
a Light RTM part are: |
| 1. |
Gelcoat
as normal. |
| 2. |
Manually
place the reinforcing media in the mold. |
| 3. |
Bring
together the two halves of the mold and draw a vacuum
to seal their contact areas. |
| 4. |
Inject
resin to coat the part's perimeter. Then apply vacuum
near the mold's center to draw resin through the glass
media towards the vacuum source. |
| 5. |
Cure,
demold and finish the part as usual. |
Jeff Burgess, Phoenix Industry's CEO, was
brought into the company because of his experience with closed
molding. The following information is based on his knowledge
and on his experience at Phoenix Industries.
Equipment Basics
The cost to investigate and use Light RTM on a small scale
can be minimal. Small, simple parts currently open molded
are ideal candidates for testing closed molding and seeing
quicker successes. Starting with smaller parts of a non-technical
configuration allows experience to be gained without major
risk.
Molds
If an FRP shop has internal expertise building open mold tooling,
it can quickly learn to build Light RTM molds. Having in-house
capability to make molds holds down Light RTM mold cost. Most
open molds can be modified into Light RTM molds. Among other
minor changes, mold flanges need to be widened to six inches
so the countermold—the second half or top mate to the
mold—can be securely held in place. Light RTM and open
molding place a similar level of stress on the mold, unlike
conventional RTM which puts the molds under greater pressure
when injecting the resin. This means that molds for Light
RTM have similar strength requirements as open molds, allowing
the same construction materials to be used.
Countermolds can be built using a number
of techniques. One technique uses calibrated wax which comes
in sheets and rolls to help build the countermold. Two layers
of wax are pressed into the mold, matching the part's thickness.
The sheets of the bottom layer are spaced with small gaps
between them to function as vacuum channels. A second continuous
layer of wax is positioned over the first. The original mold
is connected to a vacuum source which pulls the two layers
of wax together, holding them in place while the countermold
is cast over them. Gaskets, gauges, and resin and vacuum ports
are installed to complete the countermold. Because both a
mold and countermold are needed, building tools for Light
RTM costs about two to two-and-a-half times more than open
molding.
FRP material suppliers have videos demonstrating
the basic steps of this and other mold building techniques.
More extensive formal training for mold building and process
training is available. The cost for two operators over five
days is around $10,000.
Positioning resin
and vacuum ports
Injection pushes resin into the mold, but its flow through
the media is due mostly to the vacuum's pull. The level of
vacuum limits the flow rate. Resin injection ports are positioned
on the mold to obtain adequate initial wetting. Good resin
flow through the media depends on properly positioning the
vacuum ports in relation to the resin injection ports. To
ensure that the resin travels through the media at a constant
rate, vacuum ports should be spaced at an equal distance from
the resin injection ports, this distance is measured along
the mold's contour. Multiple resin injection and vacuum ports
may be necessary to achieve this.
Vacuum source
Systems capable of attaining a vacuum of around 30 inches
of mercury are required for Light RTM. A number of low cost
vacuum options are available. If the plant has compressed
air, a venturi vacuum generator can meet the requirements
of small molds. It costs less than $100. For larger molds,
rotary vane vacuum pumps are available for about $300. For
full-scale production, portable vacuum systems rigged to handle
Light RTM are available for $5,000 and higher.
Low-pressure resin
injection pump
A pumping system is required to feed resin to the mold. With
minimal expense, FRP shops may be able to modify existing
resin application equipment for use as a pump while they experiment
with Light RTM. Equipment specific to Light RTM costs around
$5,000.
Implementation
Issues
Good process control
Process control is absolutely necessary to produce consistent,
quality parts. Key factors include: |
| • |
Tightly controlling the
temperature and viscosity of the resin used because they
both affect how the resin flows through the media. Bad
parts can result if these variables fluctuate significantly.
|
| • |
Thoroughly checking the
mold setup for vacuum leaks before injecting resin. Leaks
dramatically impact how the resin flows through the media
and will cause bad parts. |
| • |
Selecting and placing glass
reinforcing media. A conformable, advanced reinforcement
media may be required for complex parts. Improperly placed
media can lead to mischanneling of the resin and poor
mating of the mold and countermold. These both result
in non-wetted areas and a bad part. |
Reinforcing media
for complex parts
Conventional glass reinforcement works fine for parts of simple
configuration, especially if mating the mold and countermold
requires little effort. Because conventional glass materials
do not readily conform to a part's shape, complex parts can
be very tedious and challenging to load. Advanced reinforcing
medias have a sandwich construction with glass on the outside
and a synthetic interior which allows resin to flow easily.
Its conformable “memory” helps control placement.
Its compressibility makes building variations in part thickness
easy. This newer generation of materials reduces the amount
of labor involved with media placement, especially in complex
parts. But, it costs twice as much as conventional reinforcement
media.
Finishing work
In Light RTM, pulled parts require trimming of cured flashing
material, which can be labor intensive, noisy and dusty. In
open molding, although some part designs require cutting of
cured material, cutting away excess uncured flashing material
is relatively simple. Overspray and trim waste can account
for ten percent or more of the materials used in open molding.
These waste costs counter the extra post cure finish requirements
of Light RTM.
Styrene Emissions
Because parts are removed from the Light RTM mold as soon
as they are structurally sound, some styrene may be released
to the environment during final curing of the part. Compared
to open molding, Light RTM releases virtually no styrene because
the entire system is closed and even the gases evacuated from
the mold can be passed through a small carbon adsorption bed
to eliminate any release during part processing.
Using the Composites Fabricators Association's
(CFA) Unified Emission Factors, the emission factor
for open molding is about 11 percent of the resin's available
styrene. The EPA’s Compilation of Air Pollutant Emission
Factors, commonly referred to as “AP-42,” lists
the emission factor for closed molding as one to three percent
of the styrene available. The AP-42 range is based on semi-closed
processes (i.e., marble casting). Because Light RTM is a closed
process, its emission factor should fall in the lower end
of this range. The table below compares open molding using
a low-styrene resin (38 percent) and nonatomized application
equipment against closed molding using resin with a slightly
higher styrene level (42 percent) to benefit from its lower
viscosity. Closed molding shows a ten-fold reduction in styrene
emissions over open molding.
| Emission factor comparison. |
 |
| |
Open molding* |
Closed molding |
| Resin applied |
1,000 lbs. |
1,000 lbs. |
| Styrene in resin |
38% |
42% |
| Emission factor |
11% |
1% |
| |
|
|
Total styrene emissions
from resin application |
42 lbs. |
4.2 lbs. |
|
| * Compliant with the NESHAP's
requirements for low styrene resin and nonatomized application.
|
Note: Light RTM parts are gelcoated in the
conventional manner used in open molding. Styrene emissions
from gelcoating remain a significant fraction of total styrene
emissions.
Costs
and Benefits
The following cost-and-benefit analysis summarizes
the success Phoenix Industries has had with Light RTM. |
| • |
Lower
cost per part. Per part cost reduced 10 percent.
Productivity of the molds increased with shorter cycle
times. Material use improved and the bill of materials
became more consistent. Part quality was enhanced, while
less labor per part was needed. |
| • |
Reasonable
capital investment. Phoenix had a payback of less
than two years. One Light RTM station, including a vacuum
source and resin injection system, cost under $10,000,
excluding the tooling costs. Closed molding experience
can be gained with less than $1,000 if simple and small
parts are addressed first. |
| • |
Quality
improvements. The day-to-day inconsistencies of
open mold operators were minimized. Light RTM enhanced
part consistency, gave better control
over part thickness improving dimensional tolerance, and
offered a two-sided finish. |
| • |
Reduced
manufacturing wastes. Styrene
emissions were reduced ten-fold. Per part material use
was reduced by 10 percent or more because open molding
wastes—like overspray—were eliminated. This
further reduced styrene emissions. |
| • |
Employee
retention. A much cleaner
work setting and a more desirable job reduces worker turnover.
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MnTAP has a variety of technical assistance
services available to help Minnesota Businesses implement
industry-tailored solutions that maximize resource efficiency,
prevent pollution and reduce costs. Our information resources
are available online. Or, call MnTAP at 612/624-1300 or 800/247-0015
from greater Minnesota for personal assistance.
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