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/HOLD:  On 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).