redesigned machine and understanding slow medium and fast

[graemeian]

I have a '91 Mitsubishi machine. I am trying to run it with two smaller pumps using vfd's for flow control. I also replaced the screw motor with an electric motor and gearbox. (17.5 HP rated grearbox and 10 Hp motor to start.)
My screw diameter is 50mm . The manual uses speed values such as slow and medium. Does someone have actual numbers? I am sure it has something to do with surface speed as my 15mm Boy screw should spin much faster rpm wise.
Back to my motor/gearbox. I have three pulley ratios that will allow me to spin the screw at 60, 100, and about 135 rpm respectively. I can add a third VFD if necessary. I will be injecting clear PC. It appears that going on the low side would just slow production to a point to where the resin in the barrel is heated for too long. Possibly too fast would crack the polymers and whip air in the resin. Where can I find information that would give me surface speeds for different plastics? Thank you.

[Suhas]

Hello,
Good observation on the surface speeds changing with the screw diameters. I wish the material manufacturers understood as much as you did!
You are sort of out of luck. They will give you rpms and a range and that is it. As you just found out, a 35 rpm on a 50 mm screw is not the same as 35 rpm on a 15 mm screw. At this point, it has all been based on experience. I wrote the book on the subject but I have researched and found no good answer. Some day I think I will make an R&D project out of this. Sorry again, I do not have a good answer.

By the way, there is a Rule of thumb - rpms should be adjusted such that the screw recovery time should be 2 to 3 seconds less than the cooling time. Please if anyone tell you that let it out of the other ear. Read this: http://fimmtech.com/index.php?id=6&subid=53

Regards,
Suhas

[brentb]

One can purchase a portable (hand-held) tachometer for a very reasonable price. Some presses have a plexiglass "window" over screw coupling, if not use care as to not get too close.

KOM

brent

[graemeian]

Suhas et al.,
Thank you very much.
Where can I find some literature about injection molding? Here is a review of my experience...
I built my first machine from a sch200 pipe, heating band, a funnel and a ram. I could get about three shots before reversing the ram and reloading the barrel.
I now have a small BOY machine and a larger Mitsubishi (200 ton). I know a little about plastics. If I get black marks due to compression combustion (dieseling), I know to reduce injection speed etc.
I bought my own machines because the companies in the area I spoke with made low precision parts and I had the feeling that they would crank up production speed when I was not around. My parts need a coining step and a slow cool time to keep them geometrically stable over time of use (years).
Any good literature sources will be appreciated.
Thank you again.
Regards