Calculating Material Usage Requirements

I just received a question which is a common frustration for any process whether it is injection molding, blow molding, extrusion, or  compounding…

I have a problem calculating the material usage on the production floor. When I weigh 10 shots the average weight is 5.2g per shot for an 8 cavity mold. We originally calculated a material requirements of 70.2kg of polypropylene to mold 108,000 parts. When the run was complete, we used more than 70.2kg of material to mold the parts, and all 108,000 parts weighed more than 70.2kg. Can you please advise us what is the real calculation for the material usage?
My Response
Plastics materials have a tendency to exhibit variability. Using this scenario, I will suggest a few factors to incorporate, and a few strategies which may also help increase your accuracy.
Compensate for Startup and Shutdown – All processes have some amount of loss when starting up and shutting down the molding machine. Most companies know the average startup time required to initiate a production run as well as the time to shut down the machine. During this time, material is purged, and scrap parts are being generated. A good starting point for losses is to assume the machine is molding scrap parts throughout this time. Remember, if the machine is scheduled to be shut down and re-started during the run, these processes also need to the considered.
Compensate for Scrap – Since virtually every process creates scrap, you should compensate for the expected scrap rate by adding that loss to your expected amount of material usage.
Compensate for Troubleshooting – When troubleshooting, technicians tend to put more material into the mold as time progresses. For instance, when sink marks occur, the most common action is to increase packing pressure… likewise, when flash occurs, they tend to increase clamp force rather than adjust the transfer position. As a result, the part weight of the last part tends to be higher than the first parts that are produced. Therefor, if part weight is not measured and monitored regularly, I suggest you add a 10% variability factor.
Additional Thoughts
Additionally, polypropylene is a highly semi-crystalline polymer. When the initial shots were measured to calculate the material usage, it is likely that the mold temperature had not yet stabilized. As a result, the mold temperature increased a little, causing more material to be packed into the mold cavity.
Ultimately, the best way to control material usage, and limit costs, is to routinely monitor the shot weight as well as minimize scrap and downtime.

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