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Tolerances:
Because extrusions are not 100% contained by metal tooling, tolerances must generally be looser than other molding processes. While specialized tooling can
hold tighter tolerances, generally these are the "normal tolerances" you should expect:
Wall thickness: ±.005
Cut length: ± .062 or more
Width or height: ± .010 per inch of width
Straightness: .045 bow per foot
Materials:
Most thermoplastics can be extruded, including LDPE, HDPE, ABS, polystyrene, polypropylene, acetates, butyrates, polycarbonates and thermoplastic
rubbers, among others. Recently, more vendors are able to work with wood-filled materials, or even glass-filled materials. Some, such as nylon, are so fluid
when molten that they are difficult to control. PGS is unable to extrude nylon because of tolerance controlability. Others such as acetal or butyrate may have
an objectionable odor that makes fewer producers want to run them. Filled materials are available now using fiberglass, carbon fiber, aramid fiber, glass bead,
stainless steel fibers, and PTFE, as well as flame retardants and UV stabilizers.
Part Geometry:
Material gradually melts as it proceeds through the barrel, and out of the die. Since it is a liquid as it leaves the die, wall thicknesses must be uniform, and the
shape symmetrical. Otherwise, the greater pressure on one side will force the profile sideways, creating a "bow", instead of a straight part. A well-balanced
shape allows for maximum running speed, for lowest cost. Any hollow in the profile creates knitlines, where the material separates, and rejoins. Multiple
hollows are complex to run, and should be done by specialists.
Process Variations:
The most common variation on straight extrusion is dual durometer extrusion. Here, a "side machine" (about 1/4 or less the size of the main machine) runs in
tandem with the primary machine, feeding a different material (flexible vinyl with rigid PVC, for example) to the die, where the the streams merge into one
extrusion made of two bonded profiles that are often "two hardnesses", or dual durometers. Another common variation is cross-head extrusion. In this process
the flow of plastic is altered to allow solid material, such as copper wire or fiberglass strands to feed into the melt flow, and become part of the extrusion. Cross
head extrusion is used when such reinforcements cannot pass through the machine's screw and barrel.
Secondary Operations:
Secondaries for extrusion include cutting and drilling, mitering of corners (for gaskets or frames), belling (increasing the diameter at the end), taping (typically
done during extrusion), and cutting to length. This can be done during extrusion (on-line), or after extrusion (off-line). Off-line cutting is more laborious and
expensive, but can hold tighter tolerances. Secondary punching can be done on-line or off-line, but is cheaper on-line. This setup actually has a mini-punch
press as part of the extrusion line. Flexible extrusions are often spooled as a secondary duty, although this is generally done on-line (during extrusion).
Tooling:
Basic extrusion tooling typically consists of a flat plate die, or a round steel plate with a cut opening to the profile shape. Streamlined dies are longer, and have
a gradual transformation from round (at the machine end) to the profile shape at the exiting end. Basic flat plate dies can cost only a few hundred dollars, but
streamlined dies are cut using wire EDM, and run from $1,000 to $20,000 or more (most are under $5,000). Streamlined dies are required for highest
throughput, or for working with materials which burn easily when residing in the barrel too long, such as rigid PVC. As in other plastics processes, more
expensive tooling yields better control, higher quality and cheaper parts.
Vacuum sizers are cut aluminum blocks which completely encapsulate the profile, while a vacuum pulls the hot shape out against it's walls as it cools. The
cooling medium may be either air or water. This "freezes" the profile in the correct shape. Vacuum sizers are used primarily with hollow shapes.
graduated from oversized at first to the final profile shape. As the profile passes through, any tendency for it to move or sag is counter-acted, and it is pushed
back (repeatedly) to it's correct shape. While these devices can cost from $2,000 to $20,000 or more, they help the profile to be run at maximum speed, for
the best piece price. They also give a better defined shape, and thus, a tighter set of tolerances.
Because extrusions are not 100% contained by metal tooling, tolerances must generally be looser than other molding processes. While specialized tooling can
hold tighter tolerances, generally these are the "normal tolerances" you should expect:
Wall thickness: ±.005
Cut length: ± .062 or more
Width or height: ± .010 per inch of width
Straightness: .045 bow per foot
Materials:
Most thermoplastics can be extruded, including LDPE, HDPE, ABS, polystyrene, polypropylene, acetates, butyrates, polycarbonates and thermoplastic
rubbers, among others. Recently, more vendors are able to work with wood-filled materials, or even glass-filled materials. Some, such as nylon, are so fluid
when molten that they are difficult to control. PGS is unable to extrude nylon because of tolerance controlability. Others such as acetal or butyrate may have
an objectionable odor that makes fewer producers want to run them. Filled materials are available now using fiberglass, carbon fiber, aramid fiber, glass bead,
stainless steel fibers, and PTFE, as well as flame retardants and UV stabilizers.
Part Geometry:
Material gradually melts as it proceeds through the barrel, and out of the die. Since it is a liquid as it leaves the die, wall thicknesses must be uniform, and the
shape symmetrical. Otherwise, the greater pressure on one side will force the profile sideways, creating a "bow", instead of a straight part. A well-balanced
shape allows for maximum running speed, for lowest cost. Any hollow in the profile creates knitlines, where the material separates, and rejoins. Multiple
hollows are complex to run, and should be done by specialists.
Process Variations:
The most common variation on straight extrusion is dual durometer extrusion. Here, a "side machine" (about 1/4 or less the size of the main machine) runs in
tandem with the primary machine, feeding a different material (flexible vinyl with rigid PVC, for example) to the die, where the the streams merge into one
extrusion made of two bonded profiles that are often "two hardnesses", or dual durometers. Another common variation is cross-head extrusion. In this process
the flow of plastic is altered to allow solid material, such as copper wire or fiberglass strands to feed into the melt flow, and become part of the extrusion. Cross
head extrusion is used when such reinforcements cannot pass through the machine's screw and barrel.
Secondary Operations:
Secondaries for extrusion include cutting and drilling, mitering of corners (for gaskets or frames), belling (increasing the diameter at the end), taping (typically
done during extrusion), and cutting to length. This can be done during extrusion (on-line), or after extrusion (off-line). Off-line cutting is more laborious and
expensive, but can hold tighter tolerances. Secondary punching can be done on-line or off-line, but is cheaper on-line. This setup actually has a mini-punch
press as part of the extrusion line. Flexible extrusions are often spooled as a secondary duty, although this is generally done on-line (during extrusion).
Tooling:
Basic extrusion tooling typically consists of a flat plate die, or a round steel plate with a cut opening to the profile shape. Streamlined dies are longer, and have
a gradual transformation from round (at the machine end) to the profile shape at the exiting end. Basic flat plate dies can cost only a few hundred dollars, but
streamlined dies are cut using wire EDM, and run from $1,000 to $20,000 or more (most are under $5,000). Streamlined dies are required for highest
throughput, or for working with materials which burn easily when residing in the barrel too long, such as rigid PVC. As in other plastics processes, more
expensive tooling yields better control, higher quality and cheaper parts.
Vacuum sizers are cut aluminum blocks which completely encapsulate the profile, while a vacuum pulls the hot shape out against it's walls as it cools. The
cooling medium may be either air or water. This "freezes" the profile in the correct shape. Vacuum sizers are used primarily with hollow shapes.
graduated from oversized at first to the final profile shape. As the profile passes through, any tendency for it to move or sag is counter-acted, and it is pushed
back (repeatedly) to it's correct shape. While these devices can cost from $2,000 to $20,000 or more, they help the profile to be run at maximum speed, for
the best piece price. They also give a better defined shape, and thus, a tighter set of tolerances.
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