Page 60 - Injection Molding Advanced Troubleshooting Guide

5.3 Cylinder Sizing 43

hydraulic pressure. Then we calculate the surface area of the bore with 3.14 × R .
2
If we use a 3-inch diameter bore cylinder we take the radius, and calculate 1.5 ×
1.5 × 3.14, which equals 7.065 in of surface area. We then multiply this by the
2
2,000-hydraulic psi being used. This equates to 14,130 pounds of holding force. So
in this scenario a 3-inch bore cylinder should provide a robust condition.
Just as for the cavity pressure, it is very important to know exactly what hydraulic
pressure is going to be used. This is often overlooked or assumed. Keep in mind
that on some machines the core pressure can be set up as pressure limited with a
pressure relief valve. So even though you are changing the pressure on the control-
ler the actual pressure is not going up if the relief valve is set lower. So what your
controller says may not be actually correct. The only way to accurately tell is to put
in an inline pressure gage to measure the pressure. In the case above, if the pres-
sure is limited to 1,000 psi you would blow back the cylinder because we would
only be applying 7,065 pounds of force to counteract the 10,000 pounds the cavity
is applying to the core surface. I have witnessed this many times and therefore had
to upsize cylinders. In one case upsizing the cylinder was not an option; instead we
had to add a second set of cylinders to activate a lock to hold the component in
place, which made the tool more complex with a suicide condition.
One other thing that needs to be considered is the psi rating of the cylinder itself.
If the cylinder is only rated for 1,500-hydraulic psi and you are using 2,000-hy-
draulic psi you will run into cylinder failures with leaks and blown seals.
In a case where your cylinder is not rated for the hydraulic pressure being used,
for example with a cylinder rated for 1,500 psi and using 2,000-hydraulic psi, you
can add an inline pressure limiting valve on the mold to protect the cylinder from
failures. Also in the case where the cylinder may be rated for higher pressure that
is not required you can add this pressure limiting valve to protect the moving
component from failure. Also with some hydraulic valve gates the cylinders are not
rated for high pressures. One other aspect to consider for bore size is that if you are
using a double-ended rod the diameter of the rod will need to be subtracted from
the bore size because it reduces the surface area on the set/forward position.
There are also cases where a self-locking cylinder can be used if you do not have
the real estate for the bore size needed to counteract the cavity pressure. These
types of cylinders have a physical mechanical lock on set that locks the hydraulic
piston in place to prevent blow back. I have many tools with these cylinders and
they are reliable with minimal maintenance if used properly. They do have set/pull
pressure specifications that you need to be aware of when using them. Timing with
your component in the set position is critical for the mechanical lock feature but a
very simple process when installing on the mold.