Page 79 - Injection Molding Advanced Troubleshooting Guide

8.1 Load Sensitivity 63

„ „8.1„ Load Sensitivity

Load sensitivity or load compensation is a test that measures the ability of a ma-
chine to react to viscosity change. In general, it is a well understood fact that all
materials will have viscosity variation over time. Load sensitivity tests the ability
of the molding machine to provide a consistent fill time even if viscosity varies.
To conduct a load sensitivity test follow these steps:
1. Run a standard fill only shot without second-stage pressure on. This fill only
shot should be 95–98% full at transfer.
2. Record the pressure at transfer and fill time. These will be FTm and FPm.
3. Back the barrel away from the mold. The next shot needs to be an air shot run at
process with the barrel backed off. For some machines the barrel forward switch
must be moved to accommodate this new position; some machines allow turning
off the carriage whereas others require the carriage to be forward. On machines
where the barrel must be forward against the mold use an RJG purge plate to
allow an air shot to be run in semi-automatic operation.

4. Cycle the press allowing the machine to shoot into the air.
5. Record the pressure at transfer and fill time for the air shot. These will be FTa
and FPa.

Then the load sensitivity is calculated according to Equation 8.1.

      FTm− FTa  


  

FTm
                 FPm− FPa   ×100 = % load senstivity (8.1)



  

1000

Note: use 1,000 for the denominator when using hydraulic pressure; use 10,000 for the denominator when
using plastic pressure.
The accepted standard is that this number should be close to zero. If the value is
greater than 5%/10,000 psi the machine will not be capable of maintaining con-
sistent fill velocities as conditions vary. Machines that are load sensitive will not
provide consistent fill times, which means that the process will not be repeatable
over time. If fill times are not consistent all other parts of the process will also be
affected. See Figure 8.3 for an example of a load sensitivity form.