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Technical Articles
 
Home >> Technical Articles >>The Plastic Extrusion Process and Design considerations
The Plastic Extrusion Process and Design considerations
Time: 2008-12-20
 

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.
For the insulation of wire
 
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.
 
Materials
 
Most thermoplastics can be extruded, including LDPE, HDPE, ABS, polystyrene, polypropylene, acetates, butyrates, nylons, polyphenylene sulfides, acetals, polycarbonates and thermoplastic rubbers and polyesters, 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. 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. Our database lists numerous affiliates with experience in these materials.
 
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
 
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, although this is generally done on-line (during extrusion).
 
Tooling
 
Basic extrusion tooling typically consists of a flat plate die, (see "A") or a round steel plate with a cut opening to the profile shape. Streamlined dies (see "B"and "C") 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.
OPERATION OF STREAMLINED DIES
 
A Calibrator is a long assembly, being either a continuous piece, or many pieces, through which the profile passes. The openings are cut to the profile shape, 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.
This simplified underwater CALIBRATOR may consist of 4 to 40 plates, at intervals along a steel rod frame. More plates at closer intervals mean much tighter tolerances. Profile opening starts oversized, and reduces to the final shape. Calibrators maximize running speed and accuracy.
 
Compare to other processes
 
Often, extruded parts may be injection molded or thermoformed, depending on their shape. The attraction of extrusion is the low tooling cost, and shorter lead times. For high volume parts, such as sawed-off short clips, injection molding may be a lower cost option. While the piece price may be lower, design changes are required to create draft angles, and tooling is much higher. A "trough" shape can be vacuum formed, but, if it can be extruded, it will be cheaper, as the vacuum formed material has been extruded into sheet already. In profile extrusion, you convert raw resin directly to a finished part.
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