Mold Maintenance Tips
Contributed by: Brian Young ,  Assistant Professor of Engineering, Plastics Department, Penn State Erie 

When most people think about mold maintenance, they think about the mold sitting on a cart or table in some state of disassembly being cleaned, polished, or examined. In some cases, that may be all of the maintenance that the molds receive, but (hopefully) your molds only spend limited time on the cart and they spend a lot of their time in the presses making you money. 

A lot of the time spent on the carts and in the tool shop could be reduced if more time and attention were spent on the molds while they are in the presses running. Routine cleaning of parting lines serves several purposes. Keeping the parting lines clear of buildup or ‘trash’ can help prevent hobbing of the parting line. Hobbing is permanent deformation of the steel and it occurs when the steel has exceeded its yield limit. The cyclic loading of the mold opening and closing will hammer any buildup into the steel if it is allowed to accumulate. This deformation of the steel can lead to flashing or even rolled parting lines which can cause drag marks on the parts or in some cases, sticking in the cavities. Having a technician clean the parting line once a shift should be sufficient in most cases to prevent this, but some molds like those with deep textures or tunnel gates may need more frequent cleaning. 

While the technician is cleaning the mold, have him or her check the leader pins for lubrication and the slides for free movement. Catching a slide that is starting to gall, can save a lot of time if you catch it before it seizes. Just having a technician in the mold regularly raises the chances of finding problems and possibly fixing them before they turn into big dollar repairs. 

Clearing the buildup from the parting line will help prevent hobbing, but another way to reduce the chances of steel damage is to lower the clamping pressure. I teach seminars at Penn State Behrend and one of the questions I always ask the participants is, “How many of you always run your machines at full clamp tonnage, not what the part requires, but what the machine can produce.” I would say that less than 5% of the participants actually reduce their clamp tonnage to what is required to run the parts. 

Running higher than necessary clamp pressures increases the forces on the mold which can lead to the hobbing mentioned previously as well as fatigue failures in the tooling. Although the presses are rated to run at those tonnages, running at lower pressures will reduce the wear and tear on the machines in addition to using less energy. Typically at the college, we teach the students to find the clamp tonnage necessary to eliminate flash and use 10% more as a safety buffer to deal with variations in material viscosity. If a lower viscosity lot comes through, you can always increase the tonnage if necessary, but it should be lowered as much as possible when the next lot is used. 

Testing to find the required tonnage to run a mold could be done regularly at start-up to prevent using higher than required tonnages. Reducing the clamp tonnages can also allow parting line vents to work more efficiently, making it easier to evacuate air and gasses from the cavity. I have seen a lot of older molds where the vents are compressed into the other side of the tooling. This restricts the air flow through the vents and makes it harder to fill the cavity. 

There was one instance when I was still working in industry where our larger 220 ton presses were all booked up and we managed to fit the mold into a 130 ton press (barely). The mold ran fine in the 130 ton press at 130 tons of clamp force, but when the 220 ton press opened back up, the mold was back to running at 220 tons of clamp force. That is almost double the necessary force to hold the mold closed against the injection pressures of the plastic, and what’s the benefit? 

Contact Brian Young at Penn State Erie , The Behrend College , Phone (814) 898-6149 or by email at bay101@psu.edu.