Part Weight

[Jshoe]

Does anybody track part weights? I was recently tasked with lowering part weights for our products. We mold almost exclusively TPE and seem to have trouble keeping our part weights in control. I have been tracking injection pressure on a specific job and noticed that when the pressure goes up so does the part weight I have been waiting for a drop in pressure to see if the part weight follows but that hasn't happened yet. Is the pressure increase and part increase possibly an indication that the material itself is getting heavier? I have a fear that the variance is in the raw material, and not the molding process but I need to prove that out before I can dig deeper. Any ideas on what else I can do to find if it is indeed the material or not would be very helpful.

[MTUHusky]

Hey JShoe,

At my shop, we check the weights of every job at the beginning of each run. If those parts are not within a certain tolerance, we stop the press until we figure out why.

In injection molding, most (if not all) plastics are compressible in the molten state. So it makes perfect sense that the more pressure you apply, the heavier the parts become. You are increasing the parts density by compressing more plastic into the same volume.

One important question to ask; "is the part weight changing from shot to shot?". If it is, then you might not have gate seal and plastic may be leaking back into the barrel. If that is the case, you should run a gate seal study. If your part weight is consistent and you are looking to lower the weight, then applying less pack pressure would be the way to go as long as you can achieve gate seal without losing plastic back into the barrel.

Hope this helps,

Husky

[rickbatey]

Not sure who wants to lower part weights but remember you don’t get something for nothing. You can make the parts lighter BUT I would offer that your dimensions will change. Maybe not enough to be bad parts but you may end up with part dimensions that are always fluctuating.
Not sure what you’re molding but soft resins don’t need much hold time. I might even suggest you’re running Santoprene resin. I saw serious variations in that brand when over molding parts for MB in the past. Other brands were MUCH better lot to lot.
Rick

[Jshoe]

Not sure who wants to lower part weights but remember you don’t get something for nothing. You can make the parts lighter BUT I would offer that your dimensions will change. Maybe not enough to be bad parts but you may end up with part dimensions that are always fluctuating.
Not sure what you’re molding but soft resins don’t need much hold time. I might even suggest you’re running Santoprene resin. I saw serious variations in that brand when over molding parts for MB in the past. Other brands were MUCH better lot to lot.
Rick

Thanks for the response Rick,
Our parts have very wide dimensional tolerances but they have tight visual restraints. As I said we mold pretty much only TPE and the material is actually made in house through our own extruder, this is the reason I was questioning the weight before we even mold it. I would love to fill you in on exactly what I am molding but I am new with this company so I would rather not share everything until I know that they will be ok with that. I should also add that we are molding everything through valve gates. This is new to me and I am learning everything I can about them as fast as possible. From the little bit I have already learned so far I am actually now starting to think that the valve gates could be used more efficiently and that could also be part of the issue.

[rickbatey]

Hot runners and valve gates MUST be designed upfront for soft resins and the chemical composition can require further adjustments.
Rick.

[ukfan4life]

Hit reply by accident and can't edit. You should also check for air leaks in the hoses. That could increase pressure