Welcome!

I have an older {brand name} molding machine and it has a very large & heavy injection unit. What is the best way to compensate for overshoot?

Once the machine transfers, the ideal result would be for the screw to go directly into packing. Unfortunately, the inertia of the injection unit can cause additional material to be injected during 1st stage. In most cases, you can just transfer from 1st stage to 2nd stage early to compensate. Unfortunately, the inertia creates force applied to the screw resulting in a pressure-controlled situation. Although the slowdown is typically consistent, some high-precision applications need more precise control near the end of fill.

If more control is required, and you cannot change machines, then you can create a slowdown profile at the end of fill. In this situation, the profile should closely match the natural slowdown of the screw using a couple steps. The benefit to this approach is that you have a more consistent slowdown than just allowing the screw to slow down.

This correction should only be used when the natural slowdown of the screw provides inadequate consistency during transfer.

-Andy.

-Andy

Th Melt Flow Index, MFI, is take under pressures and shear rates which do not represent the actual conditions the material will see during processing. As a result, the MFI is not a good representation of the material processing characteristics. For these reasons, many people completely ignore this data, yet it can be helpful for scheduling and the anticipating potential changes.

For example, if you are processing a lot of material which has a MFI of 5, and you have to decide whether to change to a lot with an MFI of 6 or a lot with an MFI of 10… it is more likely that the lot with a 6 MFI is more likely to process similar to the lot currently processing.

Additionally, if you have to change from a lot with an MFI of 5 to a lot with an MFI of 10, the technician can be warned that the next lot is likely to have a lower viscosity.

I have found MFI data to be a moderately helpful tool when no other rheological data is available. Whenever possible, it is helpful to have more relevant rheological data such as that from a spiral flow, capillary rheometer, or a dual plate rheometer.

-Andy