July 14, 2022

Injection Molding Machine Electric Discharge

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Grounding is very important for static prevention but also for protection of people and equipment from electrical ground faults due to motor failure, heater failure, etc. The ground at your injection molding machine may be primarily for this purpose. Disturbing this ground and using it for other purposes may be a bad idea. Electrical faults and static can potentially flow thru the multiple connections you make to this ground, creating a greater hazard. You must know what your ground connection is connected to on BOTH ends and what its true purpose is, the sad thing is a lot of “grounds” are not true earth grounds. For static purposes you may want to connect to your building steel grounding grid (hopefully you have one).

We had a similar problem with the injection molding machine, complaints were such that we thought we had an intermittent short to ground, we locked the injection molding machine out immediately tore it to pieces electrically and eventually discovered it was static electricity, we installed rubber isolation mats on floor reducing the ability for the current to run through the employees body to ground and it fixed the problem. Should maybe give it a try on one injection molding machine, see if it works. It is a heck of a lot cheaper than earth grounding that many injection molding machines, and I would bet your maintenance people will love you for it, I have driven a few ground rods in in my times and unless you rent a jack hammer it is not fun. Also consider there are often times underground conduits and drain lines which could get punctured. The isolation mats you just throw them down, they are excellent for the back as well.

The major culprit that I’ve found over the years, for electrostatic discharge during injection molding is caused by the friction of the plastic pellets through the delivery hoses for the hopper. The injection molding machines should definitely be grounded with copper rods in the floor and I believe they should reach nearly to the water table for optimum grounding. I have seen the “lightning bolt” jump about 6 inches between a hose and material handlers head when he was working near the hose while it was loading the hopper with PP. Obviously we weren’t using the grounded hoses that have the internal grounding wire, which must be exposed to grounded metal at the connection to the hopper. Another source of shock for the operators is when molded parts (esp. PP and PE) are conveyed to a plastic tote or bin. You can see the small parts try to “climb” the wall of the container as the charge builds. I’ve attached bolts to a grounding wire to the conveyor and put them in the totes to help dissipate the charge so operators wouldn’t get zapped when they reached into the tote.

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