How to Optimize Back Pressure
Pt. 1 || See Pt. 2 below
Back pressure is mostly set by experience of the processor. Most processors use a back pressure value of 50 to 120 hydraulic psi or around 500 to 1200 plastic psi for back pressure values. Does this work? Yes, absolutely! But for those who have taken my course and/or hear me speak, I always say ‘Make data driven decisions!’ and don’t rely upon opinions. So how does one optimize back pressure with data? I discuss this in the article, but there is a question that I ask at the end of the article. We will discuss that in the next post. Please also try to send me answers and I will post the names of those who answered it correctly (with permission).
What does back pressure do?
- Back pressure helps to compress the softened, semi molten pellets and squeeze off all the air or gasses from in between them. This happens in the compression section of the screw.
- Back pressure will also help in compressing the melt to the same consistency, i.e. melt density every single shot. Since melt density is consistent, the fill is consistent, leading to shot to shot consistency improving the overall process capability.
Procedure of finding the optimum back pressure
With the two bullet points mentioned above, it only becomes logical to generate a graph of fill consistency versus back pressure, look at the results and choose the back pressure value. Once the parts are molded with the pack and hold pressures and are inspected for cosmetics, the back pressure may be changed to get rid of any splay.
Following is the procedure:
- Set the pack and hold phases to zero. Add a charge (screw rotation) delay if required.
- Mold parts that are about 75% full by adjusting the transfer position (cut off, 2nd stage).
- Starting with a low back pressure value such as 25 psi hydraulic, mold 10 shots, collect them and weigh the complete shot – parts + runners.
- Repeat this at steps of 25. Collect the data at 50, 75, 100 psi hydraulic.
- Plot a graph of the Range versus back pressure and find the value where the range is the lowest.
Why does the range go back up after it bottoms at a certain value?? Check back soon for Part 2.
How to Optimize Back Pressure Pt. 2In the last email, I mentioned a simple procedure to optimize the back pressure. I have got a few emails with some positive feedback. I do want to mention here that this procedure is a little time consuming. So does one have to do this every single time one is setting up a process? May not be! This can also be an academic study to help one understand the effect of back pressure on the process and part consistency. You may have had enough experience to know where to set the back pressure for the material you are processing and the type of the part you are molding.
Now to the question of ‘Why does the range go back up after it takes a dip?’As mentioned, the back pressure helps compact the melt and squeeze out any gasses in the plastic being collected for the next shot. As the back pressure increases, the plastic melt density increases due to increased compaction. The back pressure is not only working on the front of the nozzle time but also on the front of the flights of the screw that is turning. The plastic material is being conveyed on the flights. There comes a point where, as the back pressure increases some of the plastic starts to slip on the flights and is does not move forward. This is what causes the inconsistency and the range in the graph goes back up.