Anthony J. DiBattista, process engineer at Boy Machines Inc. (Exton, PA), presented an informative overview of the basic dos and donâts of LSR molding to LSR beginners earlier this year at LSR InfoTech 2004 in Milwaukee, WI (see "A Lean Trade Show," March 2004 IMM for details). LSR experts at materials supplier Dow Corning Corp. and LSR meter-mix equipment supplier Fluid Automation Inc. collaborated with DiBattista in preparing his presentation. What follows are excerpts even LSR experts might find rewarding to review.
LSR molding problem/solution checklist
Problem 1: Erratic shot size
- Verify that the check ring is properly seated.
- See if the check ring is broken, or if something is holding it open.
- Make sure the screw is retracting properly.
- Check for barrel and screw wear.
- Ensure the static mixer is properly located before the shutoff valve.
- Recalibrate or replace transducer.
Problem 2: Screw feeding goes past the setpoint
- Make sure the meter-mix pressure isnât too high.
- Check for cured material in the shutoff valve.
- Make sure air pressure on the material shutoff valve is properly set.
- Properly locate the static mixer before the shutoff valve.
- Ensure that the mechanical stop is set on the press.
- Check for air in the injection machineâs hydraulic cylinder.
- Increase backpressure.
Problem 3: Screw does not feed
- Verify that meter-mix pump pressure is sufficient.
- Ensure that the pressure-reducing valve is set high enough.
- Check to see if the static mixer cured.
- Make sure the filter pack is not cured or plugged.
- Decrease backpressure.
- Check for cured material in the barrel.
Problem 4: Nozzle curing
- Reduce plant water temperature.
- Use a dedicated chiller.
Problem 5: Nozzle drooling
- Make sure the pin will physically close (electrical switch malfunction).
- Use decompression to reduce material pressure.
- Make sure air is turned on.
- Check for cured material; the check ring may not be seating.
Problem 6: Material leakage between nozzle and sprue bushing
- Check that nozzle is flush against sprue bushing (adjust carriage).
- Radius of nozzle tip should be slightly less than sprue bushing.
- Check nozzle and sprue bushing for chips and cracks.
- Use thin cardboard for slight misalignment.
- Check nozzle O-rings for wear or damage.
Problem 7: Shot size decreasing (taking too long to refill)
- Check that cooling on barrel and nozzle is low enough, and not causing material skin-over inside the barrel.
- Check for plugging of the filter pack.
- Check meter-mix supply pressure.
Problem 8: Shot size increasing
- Verify that the check ring is not damaged; something may be holding it open.
- Check barrel and screw for wear.
Problem 9: Mold wonât close
- Run mold setup program.
- Increase clamp force.
- Recalibrate, replace mold transducer (or move transducer about 1/8 inch).
- Check for flash (or part stuck in mold).
Problem 10: Gray, black material from barrel
- Barrel and screw may be too new.
- Check the type of check ring; if the springâs too stiff, it could cause the screw to deflect.
- Make sure the screw recharge speed isnât too fast.
- Material may be abrasive.
Problem 11: Ejectors wonât move (or retract to .00)
- Make sure counter is set to at least "1," not "0."
- Increase speed or pressure.
- Lubricate and check for binding or trapped flash.
- Check ejector coupling for loose fasteners or misalignment.
Problem 12: Platen wonât move
- Make sure the ejector has retracted (see Problem 11).
Problem 13: Injection unit wonât retract
- Increase sprue speed.