The above spreadsheet can be used whenever setting up the injection velocity for any of the three injection strategies below.  Screw position v/p transfer can be used when getting the fill time and fill pressures for the spreadsheet (from the fastest velocity to one over 10 seconds).  Due to the fact that check ring shut-off generally occurs differently at different injection velocities, using the MeltSwitch plus Travel for v/p transfer will give more accurate results.

2-Stage, Pack with 2nd Stage (RJG's Decoupled℠ II)

  1. DISABLE PACK/HOLDOn most machines, this means setting pack/hold pressure and time to zero and setting the screw recovery time to what would be a "long" pack/hold time.  This prevents the back pressure from pushing more plastic into the mold during screw recovery.  It is important to put the screw rotation delay setting back to the original setting when adding pack/hold pressure and time.  If the machine does not have a screw recovery delay timer, then set the pack/hold time to a "long" time.  

  2. GET PARTS 95-99% FULL (JUST SHORT) @ MAXIMUM INJECTION VELOCITY

    • Not pressure-limited

    • Maximum injection time set higher than actual.

    • Be careful not to overpack or run parts too short if there is a possibility of damaging

      the mold.

    • Make sure other parameters are set "reasonably".

  3. OPTIMIZE INJECTION VELOCITY

    • Add enough hold pressure and time to fill out the parts (only if short shots don’t eject easily).

    • Run in automatic, if possible, to keep barrel residence time constant.

    • Record injection pressure at transfer & fill time ( actuals must be less than set) @ 8 - 12 different injection velocities.

    • The slowest fill time should typically be between 10 & 12 seconds.

    • Plot relative viscosity (transfer pressure times fill time) versus relative shear rate ( 1 divided by fill time). Pick point just past the bend where the relative viscosity flattens out (spreadsheet gives velocity).

  4. SET HOLD PRESSURE

    • Before adding hold pressure, make sure that the parts are 95 - 99% full at the optimum injection velocity with pack/hold disabled.

    • Set hold time so that you are sure the gate is frozen (typically 10 -  15 seconds will be plenty). Reduce cooling time as much as hold time is increased ( must still be longer than screw recovery time ).

    • Set hold pressure midway between just filled and where flashing or part distortion is unacceptable. Note:   If the difference between the high and low pressure is more than 4,000 psi, the clamp force may be set too high.

  5. SET HOLD TIME

    • Run with press in automatic, if possible, to keep barrel residence time constant. If running semi-automatic, keep mold open time as constant as possible.

    • Plot hold time versus part weight.

    • Change cooling time as hold time is changed, to keep barrel residence time constant.

    • Pick time where part weight stops increasing and add 1 to 2 seconds.

  6. MINIMIZE CYCLE

    • If cooling time is limited by screw recovery time, set screw speed at 90% of maximum. Then, set cooling time 1 second longer than recovery time.

    • Otherwise, reduce cooling time until signs of ejector "pushing" or part distortion occurs. Then set screw speed so that recovery time is 1-2 seconds less than cooling time.

    • Minimize mold open time.

  7. OPTIMIZE OTHER PARAMETERS

    • Cushion: 5 – 15% of shot size setting (recheck 95-99% full with pack/hold disabled after adjusting cushion)

    • Max. fill time: 10 - 20% above actual

    • Ejectors: Minimum stroke and number of strokes needed to remove parts. Ejectors should move quickly & smoothly.

    • Mold open/close:  Minimum opening for parts to repeatably clear mold.  Movement should be quick & smooth.

    • Max. injection pressure: 200 psi hydraulic (2,000 psi plastic) above the point where filling becomes pressure limited (may be more than 200 psi (2,000 psi plastic) above actual peak pressure).

3-Stage (RJG's Decoupled℠ III)

The above paper discusses the problem with 3-stage when there is any significant variation in check ring shut-off.  It was shown that the MeltSwitch works about the same as v/p transfer by Cavity Pressure and slightly better than traditional Screw Position.  It also discusses a modified 3-stage process in which using the MeltSwitch virtually eliminates the part variation caused by check ring shut-off variation for machines that can limit velocity during the second stage of injection(pack/hold).  The difference in the traditional 3-stage setup and the modified version will be highlighted below.

  1. DISABLE PACK/HOLD:  Same as 2-stage, pack with second stage.

  2. GET PARTS 95-99% FULL (JUST SHORT) @ MAXIMUM INJECTION VELOCITY:   Same as 2-stage, pack with second stage.

  3. OPTIMIZE INJECTION VELOCITY:    Same as 2-stage, pack with second stage.

  4. SET PACK WITH 2ND VELOCITY PROFILE

    • If the machine cannot limit velocity during pack/hold (second injection stage):

      • Typically 10% of optimum fill velocity.

      • Transfer from fill velocity to pack velocity at point where parts are about 95% full (check transfer on injection pressure curve before and after).

      • Set v/p transfer midway between just barely packed out and where flashing or part distortion is unacceptable. Note: If the max injection pressure at transfer is more than 2,000 psi (plastic) above the max pressure achieved to get the part 95% full, the clamp force may be set too high.

    • If the machine can limit velocity during pack/hold (second injection stage):

      • Set v/p transfer at 95% full using the MeltSwitch

      • Set two profiles for the pack/hold stage, with the first profile (pack) set at a high pressure and the second profile (hold) disabled with a "long" hold time.

      • Limit the pack velocity during the first profile of pack/hold to typically 10% of optimum fill velocity.

      • Set the time for the first stage (pack) midway between just barely packed out and where flashing or part distortion is unacceptable.  Note: If the max injection pressure at transfer is more than 2,000 psi (plastic) above the max pressure achieved to get the part 95% full, the clamp force may be set too high.

  5. SET HOLD PRESSURE: Just high enough to eliminate screw "bounce" (May have slight "bounce", depending on the machine response at transfer - screw position before and slightly after changeover to hold pressure should be the same).

  6. SET HOLD TIME:  Same as 2-stage, pack with second stage.

  7. MINIMIZE CYCLE:  Same as 2-stage, pack with second stage.

  8. OPTIMIZE OTHER PARAMETERS:  Same as 2-stage, pack with second stage.

 

2-Stage, Pack with 1st Stage (RJG's Decoupled℠ I)

 

For 2-Stage, pack with 2nd stage, filling the last 1 - 5% with pack/hold pressure can sometimes create problems with surface appearance at the end of fill and with thin areas that hesitate or freeze off when the flow front slows down before the cavity is 100% full. The same is true of 3-stage when filling the last 5% with a slow velocity.  In such cases, it may be best to fill and start the pack at the initial, optimum fill velocity before transferring to pack/hold pressure. When using this method, traditional Screw Position v/p transfer is never recommended due primarily to the problem of check ring variation causing flash.  Cavity Pressure v/p transfer works well as long as the transfer point is far enough into pack so that viscosity variation doesn't cause transfer variation.  This can also limit the velocity that is used for fill as the momentum of the screw may flash the cavity.   MeltSwitch v/p transfer works as well as Cavity Pressure when the transfer point is far into pack and better than Cavity Pressure when it is not.   The differences between 2-stage, pack with 2nd stage, and this method are highlighted below.

  1. DISABLE PACK/HOLD:  Same as 2-stage, pack with second stage.

  2. GET PARTS 95-99% FULL (JUST SHORT) @ MAXIMUM INJECTION VELOCITY:   Same as 2-stage, pack with second stage.

  3. OPTIMIZE INJECTION VELOCITY:    Same as 2-stage, pack with second stage.

  4. SET V/P TRANSFER:  Using the MeltSwitch, set the transfer point midway between where the cavity is full and where unacceptable flash occurs.

  5. SET HOLD PRESSURE:  Generally set high enough to get rid of screw bounce as in 3-stage.  A slightly higher hold pressure may sometimes be needed to minimize sinks and voids.  This should be done with a "high" hold time.

  6. SET HOLD TIME:  Same as 2-stage, pack with second stage.

  7. MINIMIZE CYCLE:  Same as 2-stage, pack with second stage.

  8. OPTIMIZE OTHER PARAMETERS:  Same as 2-stage, pack with second stage.