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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?

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.

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.

-Andy

In the prototype phase, the quickest, and least expensive way to do this is using either rotational molding or blow molding.

Rotational Molding: The material clamped within a mold, it is rotated about multiple axis while it is heated to melt the material and cover the circumference of the interior. The mold is then cooled and the part is eventually removed. This process is very slow, but is great for very low production prototypes.

Extrusion Blow Molding: This process uses an extruded, tube shaped, parison which is clamped within the mold and blown to size using pressurized air. The mold is more expensive than a rotational mold, but you are able to manufacture many more parts than with rotational molding.

You can learn more about Extrusion Blow Mlding with our blow molding series of training:

You can also produce the part using injection molding along with an assembly process such as ultrasonic welding… but this seems far too expensive for a prototype of a basic decorative consumer part.

-Andy

Note: High RPM grinders typically rotate a set of blades at a very high speed. When the rotating blades pass by the stationary bed, they cut the part giving the familiar rat-tat-tat sound. The motors on these grinders tend to be over sized… and they often consume a significant amount of energy relative to the amount of material they consume since they have to maintain a very high RPM to work effectively.

In most cases, I prefer the low RPM grinders because they consume less energy than high RPM grinders… especially if it is running constantly. Low RPM grinders use a heavier set of stepped blades which operate more by maintaining the momentum of the blades to steadily chew up the plastic. Over time… theses grinders tend to be less expensive to operate and maintain… especially if they are in constant use.

Most grinders are best used in applications where the amount of material being fed into the grinder is close to the maximum amount it can consume. A grinder which consumes only 10-25% of it’s capacity can waste a large amount of energy over time.

-Andy