Welcome!

In a recent webinar, I received this short question…

Does the use of in-mold labelling have a significant effect on polymer flow?

Basically, there should be no significant change in the behaivor of the polymer. Since the polymer exhibits fountain flow, the polymer touching the label surface is stagnant, while the moving polymer passes through the center.

Many injection molders forget that the polymer does not flow through the mold with a straight plug flow as water does. In fountain flow, the polymer comes in contact with the mold surface and begins to freeze off.

The polymer behind the flow front passes through the center of the channel until it comes in contact with the mold surface. As a result, All the polymer that fills the mold comes from the center of the flow channel as is passes by the polymer it distributes onto the mold surface.

-Andy

Conveying in the feed zone will only occur if the plastic grips the barrel and slides on the screw.  Whenever it grips the screw surface and slips on the barrel, it will not move forward. Consistent feeding requires a consistent balance of friction between the barrel and the material, between the screw and the material, and between the particles of material themselves. Because a consistent balance of friction is difficult to achieve, many problems with surging, uneven output, and size variations actually begin in the feed zone.

Feeding and conveying take place a little differently in a grooved feed extruder. With a grooved feed throat, slippage between the plastic and the barrel is effectively zero. It must move forward as the screw turns, regardless of any variations in friction between the plastic and screw.

Therefor, companies who purchase grooved feedthroats to improve material conveyance. Please discuss the specific application with the machine manufacturer to ensure the feedthroat grooves accommodate the size of your pellets and regrind. 

One interesting variation on this is with a micro-extruder. Many of the small bench-top extruders will taper and groove the feedthroat since it is the only way for the pellets to fit into the barrel and convey down the screw to be properly melted.

Another area of interest for grooved feed throats is in the newer, high capacity shot-pot style injection molding machines. Many of these use the shot-pot two-stage configuration to increase melting capacity… manufacturers are testing more traditional extruder configurations including twin screw extruders and grooved feedthroats.

-Andy

In a recent e-mail, I received this question from one of our customers…

Every check ring drifts. Tests such as the Dynamic Check Ring Repeatability Test test consistency during fill. A 1.3% variation implies that your check ring is wearing very evenly and your process should remain consistent. If the amount of drift makes you uncomfortable, it may be time to inspect and/or replace. But, a long as the drift is consistent, you should be able to maintain a robust process.

There are a variety of check ring designs available to the molding industry… and all of them leak. A properly functioning check ring will leak consistently. If you are running a material with a very low viscosity, you may want to ensure the check ring you are using is appropriate to the applicaton and properly sized for your barrel.

-Andy

In a recent webinar regarding Process Documentation, I received this question regarding process controls…

note: For those new to the industry, open loop process control uses a set pressure to force polymer into the mold during first stage injection – adversely, closed loop control uses a velocity setpoint to control the filling of the mold.

Obviously, if your molding machine is limited to open-loop control, the you can perform the test, but make a note regarding the process control type. Since the screw speed drops as the mold fills, the apparent shear rate you calculate is less accurate since it represents an average of the shear the polymer encounters.

A more realistic representation of apparent shear rate the polymer encounters results from closed loop process control. This is because the screw speed is controlled, representing a more realistic picture of the shear thinning behavior of the polymer. This also gives you much more control over the process over the long term since changes in material viscosity will cause virtually no change to the first stage fill time.

There are some advancements in machine technology which have little to no benefit, these include: back pressure profiling, hold pressure profiling, and cushion control. On the other hand, closed loop process control for 1st stage mold filling is one of the most beneficial advances in injection molding technology over the past 50 years.

For free multimedia presentations on process documentation including in-mold rheology, please visit: http://www.traininteractive.com/free/webinar/player/

-Andy