|
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 significantly reduced the company's emissions. 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 advances in FRP materials, 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:
- Gelcoat as normal.
- Manually place the reinforcing
media in the mold.
- Bring together the two halves of
the mold and draw a vacuum to seal their contact areas.
- 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.
- 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. Computer simulation of resin
flow through a mold can help determine where to place
vacuum and 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 American Composites Manufactures
Association (ACMA) 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 lb. |
1,000 lb. |
| Styrene in resin |
38% |
42% |
| Emission factor |
11% |
1% |
| |
|
|
Total styrene emissions
from resin application |
42 lb. |
4.2 lb. |
|
| * Compliant with NESHAP 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 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.
For More Information
Other MnTAP publications for
the FRP industry:
MnTAP has a variety of technical assistance services available to help Minnesota businesses implement industry-tailored solutions that maximize resource efficiency, prevent pollution, increase energy efficiency, 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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