Extruder Melt, Gear Pumps

STABILIZATION OF EXTRUDER THROUGHPUT TO 0.1% USING A PRECISION MELT PUMP

STABILIZATION OF EXTRUDER THROUGHPUT TO 0.1% USING A PRECISION MELT PUMP

Why use a melt pump?

A screw type pump is used in an extruder, instead of some other type of pump, because it is an inefficient pump. By this we do not mean it is a bad pump -- just that it generates a lot of heat. But how else would you melt a large volume of plastic? If you put almost any plastic in a large pot with a flame under it, the outside would be hopelessly degraded before the inside is even warm. The only practical way to melt a large volume of plastic is to deal with it a little at a time and subject it to frictional heat. The screw type pump is thus ideal for melting the plastic.

With suitable mixing sections, the screw can also do a very fine job of mixing the plastic. See the section on screws on this site for a discussion of mixing in extruder screws.

But...the one thing a screw in an extruder is not good at is producing a uniform output. The output of an extruder used alone is highly dependent on the back pressure. The output is typically down to half at around 4000 psi back pressure -- about 10% variation in throughput per 1000 psi change in back pressure. The output is also highly dependent on the viscosity of the plastic, ths size and shape of the pellets, the temperature of the various zones in the barrel, and a host of other factors.

Some improvement in throughput stability can be had by controlling the melt pressure of the die (see melt pressure control on this web site.) But for extrusion of any precision product, a melt pump should be used after the extruder to meter the plastic delivered.

Control of the melt pump.

As a practical matter, a melt pump cannot be controlled manually. Some sort of pressure control must be used to synchronize the extruder RPM to that of the melt pump. But, while the output of a melt pump with a simple input pressure controller is far more stable than the extruder used alone, there are still variations of perhaps 1% or 2% as the back pressure changes. This is due tothe fact that, while the main body of plastic is metered by the teeth, there are still miscellaneous leakage paths -- by the sides of the gears, around the periphery, and through the bearings.

But, back in 1976, Harrel introduced the answer to this problem -- the differential pressure controller. This takes advantage of the fact the miscellaneous leakages in the melt pump are not determined by the pressure at the input of the pump, but by the differential pressure across that pump. The differential controller does not eliminate the leakages in the pump -- it merely stabilizes them. It doesn't really matter how large the leakages are, as long as they stay constant. That is what the differential controller does -- it keeps the leakages always the same. The overall throughput stability of a Harrel GEARTRUDER (Extruder + Gear Pump), with a good differential pressure controller is typically 0.1%. This compares to 5% to 9% for an extruder used alone!

The PC-620 Melt Pump Differential Pressure Controller.

Differential pressure control is, of course, an important part of the Harrel GEARTRUDER -- a rugged extruder with an integrally mounted melt pump. In the GEARTRUDER the differential pressure controller is built right into the DIGIPANEL controller on the unit. For upgrading existing extrusion lines, however, a separate controller is necessary:

* CONTROLS DIFFERENTIAL PRESSURE:
Melt pumps give give much higher accuracy if differential pressure across them is held constant.
* AUTOMATIC START -UP ROUTINE INCLUDED
Automatically sequences start-up for fastest start and for pump protection -- a boon to operators, particularly inexperienced ones.
* AUXILIARY ALPHANUMERIC SCREEN

Displays cause of last shutdown, set points, calibration data, diagnostic data, etc.
* COMPUTER COMPATIBLE
May be monitored and set from Central Console or computer.

For specification on melt pumps, check MELT PUMPS FOR EXTRUSION. For data on complete upgrade packages check COMPLETE UPGRADE PACKAGES.

Harrel.com