NOT GETTING UNIFORM CUT LENGTHS
Things to Check
Slippage in puller.
- Check to ensure that the extrudate cannot be pulled, with hand pressure, through stationary puller belts. If it can, the nip pressure should be increased. This condition can cause the extrudate to slip backwards during the cutting process.
- If the nip pressure to hold the part starts to distort the part shape, check the condition of the belts to make sure they are not worn. If they are, replace them.
- Make sure the extrudate is not wet or slippery. If it is, eliminate the source.
-A longer puller may be required to keep the nip pressure down and the pull force up.
Puller belt speed variation.
- Make sure voltage to motor(s) is uniform.
- Don’t operate pullers with DC or AC motors at edge of their range due to speed fluctuations. Servo motors will operate over the entire speed range consistently.
Is the extrudate stretching?
- Stretching can be seen before and after the puller. If the puller is set correctly for nip pressure and is of the correct length, the cause can be too long of a distance between the die and puller and may require a second puller in a master/slave setup. If the stretch is after the puller it may be cutting speed, bushing sizing and material and blade design.
Extrudate may be too tight or too loose in bushing.
- Bushing fit is critical to achieving a good cut. Some products are more forgiving but most will cut better with a bushing that is just loose enough for the product to move in the bushing but not so loose to allow it to move during the cut cycle. The interior of the bushing should fit the profile snugly but not so tight as to cause the material to bunch up. For sticky products, the bushing may have to be relieved to allow smooth movement. Some applications work best with a lined bushing (Teflon or Delrin). If the bushing is too loose, the material is typically not supported properly during the cut cycle and this generally affects the cut quality, (Squareness, flagged edge, tolerance, etc.). The knife blade should be freely moving between the bushings but with a very small gap (some materials will work best with no gap on the upstream side.
Extrudate not freely exiting the bushing after being cut which causes compression of incoming material.
- Typical solution is to relieve the exit of the bushings to allow free movement of the cut extrudate. If this does not work a blast of air after each cut may do the trick.
- A take-away conveyor may also help.
POOR CUT QUALITY
Things to Check
Check & adjust blade gap
- Turn the flywheel by hand to make sure the bushing is as tight to the cutter blade as possible without contacting the bushings. Some extrudate may require the blade to contact the bushing for good cut quality. .
Is the blade sharp?
- Check the blade sharpness. Also check for nicks, bends, etc. If the blade is not perfect, replace it. Check blade width also for interrupt influences.
Are the bushing edges sharp?
- Make sure the bushing edges are clean, sharp and are not rounded. Interior rounding of the edges can cause flagged cut surfaces. The faces of the bushing should be at a 90° angle to the extrudate bore at the point of cut. The bushing should have a taper of 10-15 degrees to allow the blade to enter the space cleanly. Intersection of the taper to square face should be approximately .125” or greater from the bore. This will depend on the material being cut.
Extrudate/blade friction may be too high.
- Lubricate the blade by adding distilled water (with or without a few drops of dishwashing liquid) or isopropyl alcohol to the lubrication reservoir. Some cutters include a blade wipe to ensure that excess liquid is removed from the blade.
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