Hydraulic valve choice

[Robin Hewitt]

I really would appreciate it if someone who knows about these things could pass an opinion before I buy the wrong valve. Again

I'm making a plunger type injection molder with 2 rams, one ram closes the mold the other injects the plastic. I want to drive both off one pump.

The pump does 3.5 litres/minute and has a built in 90 bar relief valve.

The mold closing ram wants to hold at the full 90 bar while oil diverts to the injection ram.

If I bleed off less than 3.5 litres/minute the pump relief valve must dump the excess flow, thereby holding 90 bar for the mold closing ram.

I was going to control the bleed off using a needle valve, but thinking about it that doesn't work. When the injection ram meets resistance I lose my pressure drop across the needle, the flow rate drops in exact proportion and everything slows right down.

I have found a nice, cheap, pressure compensated flow regulator (throttle valve?) but when I called the manufacturers they weren't sure if I had the best solution and even more vague when it came to suggesting an alternative.

I called their distributor to get a price and they have 3 in stock, followed by 12 weeks to get more in but the girl said she'd hold one for me 'til the end of the week

The curves look good, here's the link. http://www.sterling-hydraulics.co.uk.../pdf/J02E2.pdf

Forgot to mention, on the other side of this valve is a second relief valve dumping excess flow to the low pressure return. This sets the maximum injection pressure and allows the injection ram to slow down if it must.

[Al_The_Man]

Over here, distributors usually have in-house reps which are knowledgeable enough to look at your system and spec everything in, did you try that tack?
Al.

[One of Many]

If both valves in the circuit were of dual solenoid tandem center configuration, then once the cylinders reach their destination(in/out) and the respective valve is shut off, they will pass oil through return to tank. The main relief could be set at the pump. No need for a second relief unless you need to control pressure at one specific cylinder. Speed could be controlled with directional flow controls\check valves or orifices. If need be, you could block the oil in a passage to maintain its pressure for short durations with an accumulator while the pump is doing work with the other cylinder.

Constant pressure systems do generate a lot of heat and will require several times the circuit oil capacity to keep it cool.

Lots of options. There should be all kinds of examples around the web.

DC

[Robin Hewitt]

When I got no real help from Sterling Hydraulic I called their distributor for price and availability. I'm sure they would have been helpful if I was trying to buy a complete system but all I wanted was one valve.

I'm not trying to control the speed, but I am trying to control pressure at one specific cylinder. (I'm actually trying to imitate a pneumatic ram injector because I know they work)

Nearly all the cycle is opening and closing the mold. The higher pressure mold tight shut/injection phase is relatively short.

There must be other ways of doing it but my valves, gauges and filter are already bought. The flow control valve was obviously wrong, I was just wondering if this valve would achieve my purpose

Forgot to mention the pump has a solenoid valve. It dumps the pump output straight back in to the reservoir until I close it.

[One of Many]

Seeing only one valve, no one could tell you if it will work. Seeing where this valve is placed and the whole circuit should help clarify what you have verses what changes may help get it closer. A description of the cycle steps would also be essential.

In the end, sadly, it won't matter what you already bought if it won't function as connected.

Is this all manual or automated somehow?

DC

[Robin Hewitt]

I really was trying to be brief, didn't want to be boring, thought I had given enough information

Drawing the hydraulic circuit would mean learning a whole new language.

Looks like I just have to build it. I have included two pressure gauges which will soon tell me if I have flopped.

To answer the question, automatic, electronics I can do

[One of Many]

It sounds like you are really needing to comprehend the logic in the circuit. Having a symbol based drawing sure helps, if you know how each symbol reacts. Not much different than electronics IMHO.

Here is one site that has quite a bit of information to browse through.

Sequencing circuits

For inexpensive books, these sure have plenty to offer the non-technical reader for a decent introduction into the world of hydraulics.

Womack's Machine supply


DC

[bluejay_ca]

Much agreed, you need a Hydraulic print. I have 10+ years in plastic injection and am struggling with your description.

[Robin Hewitt]

Start position...
Mold ram set to open
Injection ram set to open (the return stroke)
Pump switch open (no pressure)

Switch mold ram to close
Close the pump switch
Wait until mold shows closed
Open the pump switch

Switch the injection ram to close
Close the pump switch
Wait for injection cycle to complete
Open the pump switch

Switch injection ram to open
Close pump switch
Wait to complete
Open pump switch

Switch mold ram to open
Close pump switch
Wait to open
Open pump switch

[One of Many]

One valve to control 2 cylinders? That seems to be making it harder to control since there will be volume changes in the circuit while the second cylinder is in motion and no back pressure that will maintain the mold closing cylinder pressure.

Also, where you have the flow control, once pressure is coming up in the injection cylinder, the pressure will equalize on both sides of that valve after a slight delay. This is where a sequencing valve can help. It acts like a relief, but instead of internal drain, the drain can feed the down stream circuit.

Maybe you could look at the control valve and draw up its configuration, then label the ports in your drawing?

I found a couple interesting examples in my Fluid Power Directory. The first one looks similar to what you have done. The second is just what you expect.

DC

[Robin Hewitt]

The flow control valve provides the back pressure when operating the second cylinder, all it has to do is flow less than the pump output.

The pressure drop across the flow control does change, which is why I posted the link to a pressure compensated flow control valve.

The pressure drop across the flow control never drops in to the unstable part of the graph because the second relief valve maintains the flow.

That is unless I am misunderstanding something, hence the question

[One of Many]

Robin,

I added some pic's to my previous post. Check those out.

DC