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| Introduction
to Long Fiber |
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THIS
GUIDE: The PlastiComp™ Material Processing Guide
is intended to be a useful resource for people who have the
responsibility to set up and/or operate injection molding
machines producing parts made from long fiber reinforced materials.
In preparing this manufacturing pocket reference, industry
specialists with decades of experience have focused on delivering
the most helpful information they have gathered regarding
the molding of long fiber reinforced thermoplastic materials.
We recognize that there are a variety of processing manuals
available but we’ve learned that there is widespread
interest in a compact, convenient and easy to use document
that can be carried in a pocket, briefcase or posted at a
molding machine location.
LONG FIBER REINFORCED THERMOPLASTIC
MATERIALS: Simply stated, reinforced thermoplastic
materials identified as being “long fiber” are
those in which the individual reinforcing fibers are of uniform
length and are in parallel alignment with each other. In other
words, the individual reinforcing fibers are aligned with
respect to each other, and are exactly as long (usually 11mm)
as the pellet. In contrast, short fiber reinforced thermoplastic
materials contain reinforcing fibers of various short lengths
that are randomly orientated in the resin/pellet.
ADVANTAGES OF LONG FIBER REINFORCED
THERMOPLASTIC MATERIALS: Injection molded parts manufactured
with long fiber reinforced thermoplastic materials can deliver
predictable superior mechanical performance compared to short
fiber reinforced thermoplastic materials. Product engineers
can literally design the performance characteristics (tensile
and impact strength, dimensional stability, thermal stability,
etc.) of a finished part, knowing that a long fiber reinforced
material can be engineered to deliver the specific desired
mechanical performance to meet the application needs.
QUALITY RESULTS REQUIRE SKILL AND ATTENTION:
The production of high-quality injection molded parts requires
knowledge of the molding process, skill and careful attention
to startup details, and consistent management of the process
throughout the entire production run. The resulting performance
properties of a molded part can be degraded as a result of
inadequate preparation and monitoring of all elements of the
molding process. With good preparation and process. |
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| Material &
Machine Preparation |
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MATERIAL
AND MACHINE PREPARATION: Proper material storage, handling,
drying and machine feed conditions must be maintained in order
to ensure consistency in the manufacturing of high quality injection
molded parts.
MATERIAL STORAGE:
Always store Comple¯tTM materials in a warm, dry location.
Cold temperatures encourage condensation on packaging and pellets
while warm, moist conditions cause polymer moisture absorption.
Internal (for hygroscopic material) and external (for nonhygroscopic
material) pellet moisture levels are a common cause of both
process and part quality problems. Further, high moisture content
in the polymer can degrade the performance properties of certain
materials.
MATERIAL DRYING:
Proper drying of both hygroscopic and non-hygroscopic materials
is essential for molding quality parts. Drying time and temperature
will vary depending upon the specific polymer being used. Please
refer to the “Process Foldout” for specific drying
time and temperature for all Comple¯tTM materials. Do not
attempt to dry the material in an air circulating oven.
MATERIAL HANDLING: A pneumatic
material handling system has proven to be the best method for
safe transfer of injection molding materials. Hose and auger
systems can also be used but have disadvantages of floor space
requirements and interface to hoppers and dryers. Whether using
a pneumatic or auger system, smooth inner walls on hoppers and
transfer pipes, hoses and tubes are essential to minimize pellet
damage. |
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| Machine Operations |
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MACHINE
OPERATION: The overall objective during the molding
of Comple¯tTM long fiber reinforced materials is to get
the pellet from the hopper to the finished molded part with
minimal reduction of the original fiber length. The many performance
benefits of long fiber reinforced material can be reduced
or lost through inattention to a careful machine setup and
closely managed manufacturing process. Proper equipment settings,
smooth flow and watchful process management are essential
to the production of a molded part that features a perfect
surface, correct color, dimensional accuracy and conforms
in
every way to customer blueprint specifications.
PROPER SETUP
• Take time to study the recommended machine settings
• Establish a process monitoring discipline
• Confirm all machine parameters before starting the
run
THE RIGHT SCREW
• General purpose metering screw – L/D ratio 18-20:1
– 40% Feed, 40% Transition, 20% Metering. Mixing screws
should not be used.
[See Figure 1]
• Low compression Ratio – 2:1 to 3:1
• Deep flights – Metering zone 3.5 mm. Feed zone
7.5 mm
• Free flowing check-ring – three-piece ring valve
[See Figure 2]
• Nozzle configuration should be general purpose with
a minimum orifice diameter of 7/32 in. Full taper (nylon tip)
design nozzles are not recommended for use with long fiber
reinforced materials.
[See Figure 3]
SCREW SPEED AND BACK PRESSURE
• Slow – 20-50 rpm
• Adjust to the lower end of indicated speed range
• Adjust to low to moderate back pressure – 25-50
psi
TEMPERATURE MANAGEMENT
• Understand recommended temperature profile for the
specific material being molded.
• In general, getting the pellet into the melt phase
as quickly as possible, at fairly high temperature, increases
success. With this approach, fibers flow more easily and move
safely through the screw with minimum fiber breakage.
• Adjustments may be needed depending on shot size and
barrel capacity
• Quickly heat the rear zone of the machine to minimize
shear and fiber damage
•Temperature Zones |
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Rear – Lowest
Middle – Slightly higher than the rear zone
Front – Slightly higher than the middle zone
Nozzle – Typically same temp. as the front zone
A flat or reverse profile is often used to enhance fiber
length in the molded part. |
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INJECTION PHASE SPEED AND PRESSURE
• Adjust to the lower end of the indicated speed range
– 2-3 in/sec
• Adjust to the lower end of the indicated pressure
range – Medium to Maximum
• Cushion: 1/4 in
USE PREFERRED NOZZLE AND GATE AND RUNNERS
• Gates, spurs and runners will be dependent upon part
and mold configuration. Full round runner systems (1/4 in
dia) are preferred but trapezoidal designs (1.25 x depth of
runner) are acceptable. Gates should be as large as practical
depending upon part size and configuration. Minimum 1/4 in
x 1/8 in Land Length 1/2 in Gate Depth.
• Molds should be vented wherever possible, at parting
lines, runners, ejector pins, etc. (.002 in x .500 in typical).
• Regrind up to 5% may be used without impacting performance
of the finished part.
MOLDING CYCLE INTERRUPTIONS
• It is critical that heated material remain in motion.
If, for any reason, the molding cycle is interrupted for more
than a few minutes, you must reduce the barrel cylinder temperatures
while the machine is idle, or, purge the machine as described
below.
MACHINE SHUT DOWN PROCESS
• When a machine is to be shut down from molding Comple¯tTM
materials, it is essential that the machine be purged with
polypropylene or polyethylene. After the heating cylinder
has completely cooled you may shut down the machine. |
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FIGURE 1
General purpose metering screw – L/D ratio 18-20:1 – 40% Feed, 40% Transition, 20% Metering: Mixing screws should not be used. |
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FIGURE 2
Free flowing check-ring – three-piece ring valve Disassembled view of a three piece screw tip valve assembly showing its component parts. |
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FIGURE 3
Nozzle configuration should be general purpose a minimum orifice diameter of 7/32 in. |
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