Mold qualification Pressure Drop
Definition: Mold Qualification. 1) To qualify for use 2) a set of procedures/steps which lead to the approval for production of a mold Pressure drop: 1) loss of pressure from one point to the next 2) the drop in pressure as material flows through a runner, across the gate or across the cavity of a mold. 3) Used in mold flow software to help predict flow into a mold.
A pressure drop test on a new mold and or old mold yields information about the flow of materials through the machine nozzle, runner, gate and part. It is run similar to a short shot test but instead of just getting parts out of the mold and weighing them for comparison to percent full actual data is retrieved from the press (if capable) while performing a short shot test. (see Vol. 7 no 3 for details)
These data points are the injection pressure readings at transfer for the filling of the parts. For example when one makes a short shot, at say 90% full and was to read the pressure reading on the machine at transfer, this equals the pressure required to reach this point in fill. Or stated another way the pressure required to fill the part /cavity to this point equals that reading. Since the part is short the pressure inside the cavity at this point is zero, thus that pressure stated is what was used to reach that point.
Now if using an electric press which reads out in plastic pressure this is the correct pressure to use for further calculations. If it is a hydraulic press than the hydraulic pressure reading should be converted to a plastic pressure reading by multiplying the hydraulic pressure by the intensification ratio. This result gives the plastic pressure used to fill to the point in the mold.
The steps involved are identical to running a short shot test, except that there are specific points which need to be checked and also the pressure at transfer need be recorded. The points to check minimally are:
1. Fill to 95% full.
2. Fill to 5% full or just past the gate
3. Fill to just before the gate, thus filling only the sprue and runner but not going through the gate area.
4. Purging of the machine. Note that this data point may not be exact due to safety or machine setup. Taken while in midst of purging gives most accurate.
The math or calculations involved are as follows:
1. Fill to 95% minus fill to 5% equals pressure used to fill part cavity
2. Fill to 5% minus fill to just before gate equals pressure through gate
3. Fill to just prior to gate minus purge equals pressure to fill runner system
4. Purge is purge
Now ideally these number used would all be in plastic pressure, again if electric a direct readout from the machine or if hydraulic and computer does not convert within software of machine it must be calculated by multiplying by the intensification ratio of the press.
The resulting math gives an actual pressure loss to flow across the various points within the material flow path system. Thus for example if the machine itself only generated say 20,000 Plastic pressure per square inch(PPSI), and if filling to 95% it showed that this pressure reading was 19,000 PPSI, this may be too close to maximum of machine to be consistent. Note that in most cases only 80% of maximum may want to be used so that addition pressure remains for safety or a process window, which in this example is 16000 PPSI.
Now given that the example is using too much pressure, working through where pressure is lost, it could be determined if the gate, runner system or nozzle could be opened up. The reason being is that the wall thickness of the part, and resulting pressure loss through the part usually cannot be changed. The gate should not be changed, but a runner and nozzle may be easily changed, if they themselves are high in pressure loss.
The gate could be changed but only after running various other tests to see its effects on the shear of material and where the gate freeze time is.
If for example the highest area of pressure loss is the cavity/part than it may be that the press itself is not capable of filling and packing the part.
By performing the pressure loss test in conjunction with a short shot test (a bit more data collection) the data collected becomes much more relevant to what is going on. This little bit of data can then be used not only for this tool but in building and designing further tools due to knowing what happens in pressure loss though this design of a runner, gate and part. It may also lead to an understanding of machine capabilities necessary to fill the mold /molds and when acquiring equipment help in the specifications as far as pressure requirements.
The following is an example of how a chart might look.
|
Pressure Loss data |
|
|
|
|
Intensification Ratio |
|
10.00 |
Units |
|
System Hydraulic Pressure |
2,000 |
psi |
|
|
|
|
|
|
|
| | |
Hydraulic Pressure |
Plastic Pressure |
|
Pressure to fill 95% |
1,900 |
19,000 |
|
Pressure to fill 5% |
1,000 |
10,000 |
|
Pressure to fill prior to gate |
500 |
5,000 |
|
Pressure to purge |
300 |
3,000 |
|
| | | |
|
|
Actual pressure used |
Hydraulic |
Plastic |
|
Cavity |
900 |
9,000 |
|
Through gate |
500 |
5,000 |
|
to gate (runner system) |
200 |
2,000 |
|
purge |
300 |
3,000 |
|
System pressure available |
2,000 |
20,000 |
SLSILVEY
09022013
www.silveysplasticconsulting.com
