Welcome!

First, you are correct that your process will be more solid and reliable if it is short during first stage. This is critical to compensating for material variation… which will always save you money over the long term.

Second, your first stage injection pressure will show an increase as the material reaches the end of fill. If you look at the injection pressure integral for the entire cycle, you will find that the energy applied to the polymer will be virtually equal in either case.

You should always approach your processing in a logical manner… regardless of whether the machine has an all-electric, hydraulic, or a hybrid design.

-Andy

note: After further questioning, it was discovered that the quantities of slip, AB, and MB anti-block additives have not changed. Also, they do not currently collect any MFI (melt flow index) or viscosity data from the supplier.

It is very likely that your material supplier has changed material characteristics on you. The efficiency of your additives relies on their ability to ‘bloom’ or migrate to the surface. Additionally… it appears that overall morphology of the polymer, including crystallinity, has changed since the material no longer exhibits the two week drop in COF as it once did.

If a significant change in either molecular weight, or molecular weight distribution has occurred, then it will change the COF of the base polymer as well as the additive’s ability to ‘bloom’ or migrate to the surface.

You should require the supplier to provide certification for each lot that you receive. This should include some basic data including the Melt Flow Index.

note: the same effect occurs with injection molding with molded-in lubricants and internal release agents.

You should consider performing some basic material tests at your facility… this should include the MFI (Melt Flow Index), but may also include the DSC (Differential Scanning Calorimeter) and capillary rheometer.

Some companies actually purchase a benchtop extrusion line to test and understand the processing characteristics of incoming material lots prior to actual production.

-Andy

Regarding the screw – Most screw manufacturers will recommend that you use an outside micrometer. Since the flights do not match up, you should lay a block gauge block across a couple flights on one side of the screw and deduct the thickness of that block from the overall measurement. For consistency, you should note the distance, from the end of the screw, that the micrometer contacts the screw. As a result you should have a table with lengths and corresponding diameters.

Regarding the barrel – Using an inside micrometer, most companies will follow a similar procedure as with the screw. The diameters should be take at specific distances down the barrel and be listed in a table of lengths and diameters.

You should avoid using a surface plate with a height gauge to measure the screw wear… This will often mask the areas of the screw with high wear.

-Andy