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. 2
In 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.
Because changing back pressure changes the density of the material. Any time back pressure is changed, a transfer shot needs to be checked, and adjustments made to shot size and/or transfer position to make sure the part(s) are still 95%-98% full.
Yes, the adjustments to get the same fill need to be made. But why the does the range go up? I am posting my answer in the blog.
I think the range go back up again after it bottoms at a certain value, because back pressure became too high so the molecular chains of the melt are broken by extended exposure (too long dosing time) to high shear and heat, generated by screw rotation. Once molecular chain scission happens ‘randomly’, some shots would have more amount of polymer (higher density) and some would have less. Hence, there will be more variation in shot weight.
In some cases, you are correct that the degradation can happen and there can be an increase in the range. However, that will happen in extreme back pressures. At the back pressures you see in the graph, we will not see much degradation.
By range I presume you mean the range in weight across the cavities in one shot?
Yes. A shot = All the parts + the runner. We are looking at the weight of all the plastic being injected into the mold.
If I am reading this correctly I think its from shot to shot within the 10 shots. If there is variation across the cavities I think there is a balance issue that has nothing to do with back pressure.
That is correct Robert. Shot to Shot.
Due to max. possible density of the melt is achieved & after that the melt in front of the screw did not accept more material & it “misses” to accept the material resulting in more fluctuation from shot to shot at values higher the optimum back pressure of 60 psi
confuse… increase back pressure will increase the injected semi-melt material density which should cause shot weight going up, but how could the graph shows at oppsite side???
That is because the melt starts to slip on the screw and the inconsistency increases.
but why the melt starts to slip on the screw ? too much of density caused too much of the friction then resist melt forward ???
and one more question, the range means the weight different in between individual shot? ??