BACK PRESSURE | Technical Tuesdays | HYMID

[@soldsold1]

To understand back pressure here's a simple analogy - if you think of sliding a large wooden box down a stairs... you will need someone below the box pushing up against it to stop it from sliding freely down the steps and getting out of control. So while the box is sliding down the stairs, the person below the box is pushing up against it as if they were trying to push it up the stairs. The box is like the screw going backwards during plasticising and the back pressure is like the person below the box pushing up. The screw goes back and the (very badly named) back pressure pushes against the screw in the forward direction. Back pressure has some positive and negative effects. On the positive side, back pressure squeezes the air that comes in with the plastic granules back up through the hopper so you get less chance of bubbles and splash marks in the parts. Back pressure mixes in colourant better. It adds heat and helps to melt the material. On the negative side, it leaves the melt more compressed in the shot after plasticising so this can cause drool and strings as the mould opens (more decompression distance can counteract this), it also slows down plasticising so if the part walls are very thin and the part doesn't need much cooling you can potentially find the mould waiting for plasticising to finish after the cooling time is finished - giving slower cycle times. The temperature increase from higher back pressure can cause degraded melt if the material is very shear sensitive (eg PVC). Back pressure also affects shot size consistency and it's impossible to guess what back pressure will give the most consistent shots. So to optimise the shot size consistency you should run 10 short shots with different back pressure levels and see which back pressure gives the most consistent shot weight (range or standard deviation) over 10 shots. Back pressure does make the shot bigger by compressing more melt into the shot - typically I would expect to see around 0.3mm extra in the shot size if I doubled back pressure from 50 bar to 100 bar. In imperial that's about an increase of 0.012 inches in the shot if the back pressure is doubled from 750 to 1500 psi. We can see this increased shot size clearly as the melt cushion increases when the back pressure is increased.

[@adambaker9702]

Back preasure puts more material in the flights of the screw not less.

[@patbatey5800]

Back pressure is the amount of pressure the plastic has to exert on the screw to force it backwards, hence the name: back pressure. Increasing back pressure increases the density (the weight) of the shot (molten plastic) at the front of the screw.

[@Stellar2781]

thnx for video BUT i would ask about raising the back pressure u told us in the video that raising it make the part weight less while the density getting higher how is that possible i think both should getting higher Thnx

[@xMrGrimshawx]

Increasing back pressure does increase part weight. I have not been in their facility to see what exactly is going on with the part weight in their particular scenario. I could speculate the back pressure is causing the plastic pressure to exceed the nozzle contact force and drooling between shots. Perhaps the hold time is shorter than needed to completely seal the gate leading to inconsistent part weight, the process could be running pressure limited causing weight inconsistencies with viscosity changes. If it's a hot runner mold the hot tip could have an internal leak. It's a guessing game without thoroughly investigating a root cause.

[@annabsami5326]

GREAT WORK KEEP GOING

[@hymid]

Thanks! We certainly will!

[@tuongtrinh4982]

hope that you make new video about maintenance molding machine

[@hymid]

We have a number of projects that we’re working on! Stay tuned!

[@harshamin1908]

I dont necessarily agree with back pressure and part weight comparision you drawn in your video. Also, the other thing I noticed is the part weight only went down by 0.02grams or something. That's not big enough change to draw any conclusion.

[@monster7363]

I've been molding for 25 years. As far as I know adding backpressure increased the shot density therefor increasing the part weight. Or maybe i've been doing it wrong all along.

[@svci96]

As you are 25 years experienced, I want to speak with you sir

[@Cncguru-wl9hn]

Your right. It’s a compression therefore increases the density

[@nagarajsonu8558]

Hi

[@hymid]

Hi!