Page 59 - Injection Molding Advanced Troubleshooting Guide

42 5 Mold Hydraulics

There have been many failures over the years with the root cause of using exces-
sive hydraulic pressures. In most cases this occurs with the pull position with the
attachment point of the cylinder rod and component getting stretched/stressed
whereas in the set position they are compressed. One other thing to consider when
using T-slot couplers is that it is important not to have sharp corners on the T or
the slot, because sharp corners will increase the chance of failure from stress.

„ „5.3„ Cylinder Sizing

When the cylinder is being used to resist plastic pressure, it is critical to under-
stand proper sizing. If the component is using a locking angle with the stationary
half you just need to size the cylinder to move the component. When the hydraulics
need to withstand plastic pressure the math is very simple and no textbook is
required to determine cylinder size.

To correctly size a hydraulic cylinder on a mold follow these steps:
First the mold designer really needs to understand what are the potential maxi-
mum plastic pressures so he/she can accurately calculate the plastic pressure the
cavity surface will have on the component. Many will look at just pack/hold pres-
sure but you need to look at the peak plastic pressure. Also, you cannot just base it
off the material being used; as mentioned earlier there are a few things that can
drastically impact the pressure. If you are unsure, round it up; never round it down
if you want a robust tool and process. Once you have determined the maximum
plastic pressure the rest is easy.

The next step is to accurately calculate the cavity surface that is on the surface
area of the component. Once you have determined the cavity surface area you
multiply that by the maximum plastic pressure we discussed. This result will be
the pressure that you will need to design your cylinder around.
The next step is to determine what bore size your cylinder will be. I typically will
design the bore size so it exceeds the cavity pressure force by 1.5 times to make
sure it is robust. For example, if our expected cavity pressure is 10,000 psi and the
cavity surface area of the component is 1 in we know that there would be 10,000
2
pounds of force acting on the component. If we multiply the 10,000 by 1.5 ratio
we know we need approximately 15,000 pounds of force to counteract the cavity
pressure.
To determine the hydraulic cylinder bore size needed we need the surface area of
the cylinder bore size and the hydraulic pressure being used on the machine or
hydraulic auxiliary pump cart. In this example, let’s say we will have 2,000 psi of