back pressure

By rickbatey
Published: August 25th, 2008
new to injection molding, could someone explain to me about back pressure and what ratio it is

RE: back pressure

Could be less volatiles, less air in the melt, or less unmelted pellets of resin. Rick.

RE: back pressure

When injecting SAN to produce clear Fan Vanes I was having some splay marks . I was thinking the problem was insufficient drying of the resin, but checking the back pressure I did not have enough back pressure. When I increased it the problem dissapeared.Increasing the mechanical work done by the back pressure do I get less volatile matter or have some kind of venting out ?
I appreciate your help. I solved the problem but I would like to have a more deep knowledge of the cause.
Best regards
Bert

RE: back pressure

Pejo,
See Brent's previous reply for a technical description of back pressure, but back pressure is what it is depending on your machine. A good average setting is 1000 psi plastic pressure, clear up to 3000 (I've seen and used more but typically not necessary or advisable) if there is a problem with color mixing. Its main function is to provide consistency from shot to shot.

RE: back pressure

Sorry that's PEDO, sorry again.

Brent

RE: back pressure

Aint Pejo Spanish slang for what us gringos call a fart?

Brent

RE: back pressure

>new to injection molding, could someone
>explain to me about back
>pressure and what ratio it
>is
> what the effect influencing the quality molding

RE: back pressure

Hi Brent;
Thanx for the good reply.
Can U tell me, How much BP can we put in HIPS, Raw Mat. MFI : 10 Gms/10 Min.
In Which Book, can we get the Std./ Optimum BP for HIPS.
Secondly, How BP is responsible for Thrust Bearing Damage? And how to avoid this damages. Pls' Elaborate.
Thanks & Regards;
Shashikant

RE: back pressure

Back pressure is required for uniform plastification of the melt. Check the manufacturers recommendation for the given material and use the recommended back pressure during molding.

Increasing back pressure increases heat in the melt so too much backpressure will over heat the melt.

RE: back pressure

Most machines (hydraulic) will always have some back pressure. A little back pressure will cause better mixing of the resin. It will eliminate voids in your shot. Increase your back pressure if you need better color mixing because you are getting streaking. If you are using most or all your shot size and need longer residence times. Increasing the BP will cause the resin to be "worked over" more and heated more sort of faking longer residence times. I have done this with Acetal when I was getting unmelted particles in the parts.
Bruce

RE: back pressure

Hi Brent,
thanks to educate me your last quote 'use only (BP)when needed'?
isnt it good to apply this BP all the times during injection process?

many thanks

RE: back pressure

BP in hydraulics is oil pressure applied to the injection cylinder(s)that the screw must overcome during retraction. It is often given in hydraulic pressure on hydraulic machines, but is best stated in plastic pressure like electric machines do. You'd figure the PP in hydraulic machines, if not given on the control using the Intensification Ratio (area of screw/area of hydraulic piston) this multiplied by hydraulic line pressure.
To be completely accurate, though you probably should subtract the piston rod area from piston area, since the oil would be on rod side.

Back Pressure slows the retraction (not RPM)of the screw and adds heat to the melt and aids mixing many times. Use only what is needed.
Brent

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