Tips for small shot sizes

I've just graduated school and started working as a process engineer for an injection molding company. They have a lot of jobs that have small shot sizes on large machines, often a screw stroke of 10 mm or less on a machine with a max stroke of 200+ mm. All sorts of materials (PEI, ABS, Long glass fiber urethane [isoplast], lots of filled nylons). Almost all Toshiba machines, some old hydraulics and some newer electrics.

Any tips for developing robust processes in these scenarios, other than telling them to purchase a smaller barrel/screw?

Yes, Reduce the rear barrel zones (30F - 50F, or more, from typical settings) so the resin does not experience high heats for extended period of times........ then ramp up from there to front zones in a mostly linear fashion,

Joel

Downsize equipment is best approach. General rule of thumb is shot size should be 20%-80% of barrel capacity and 1-5 min. residence time. 5% is a good cushion size not shot size. Reducing the rear zone temp. can help to a point, but if you go too low the resin will not start to melt until it gets into the compression zone of the screw. This can cause side-loading of the screw which wears out a screw in a hurry. With these many shots in the barrel there is not going to be much homogeneous mixing and the majority of the heat driven into the resin will be from the barrel heaters. This is because the screw feed time and revolutions are so short. This will make repeat-ability difficult.

It's most likely that you cannot depend on your machine's monitoring alarms to detect things like short shots or inconsistent process. Instrumenting the mold with sensors would pick this up though.

If you have to run this way, the only recommendation I have is to use an extended nozzle tip with a thermocouple and heater. This is because at that small shot size the entire shot is in the nozzle body and the heater on the nozzle body normally would be used to control tip temperature. But, with such a small shot the nozzle body is basically an extension of the barrel and the temperature should be set to the same temp. as Zone 1.

Excellent recommendations above!
-Suhas

Not great control with that small of a shot.. Machine is likely still accelerating by the time you transfer so you probably don't reach your injection speed anyways. Let alone decelerate into holding.
With this restriction you're better off transferring at say, 50% shot size so the screw 'momentum' continues flow to 98% when you transfer into hold. Just imagine a large plunger filling a small cavity

Also offsetting the cushion (while not desired ofc) should help you achieve a more homogeneous mix. And may even help with screw control.
For example increase your shot size from 10mm to 20mm so your end cushion is 10mm. Or go even further if you can manage. Obviously a large cushion isn't desired because of theoretical variation in material lots but it may be an option for you in this case.
Just be careful not to flash the mold with a large shot size!

A lot of the above goes against scientific molding principles but in my experience molding those extremely small shot sizes it does help

Thanks for the insight everyone, I'll have to try the extended nozzle idea and see how it goes.
I've been dropping the rear zone temps as suggested with unfilled materials but was wary to do so with filled or shear sensitive materials.