Introduction
After designing an improved extruder drive for the Ultimaker-1 3D printer: http://www.vinland.com/UM_Extruder_Bearing.html, and then a procedure for measuring spring constants to use with the upgrade. http://www.vinland.com/Spring-Testing.html, and measuring how well the extruder drive was working: http://www.Vinland.com/Extruder_Drive_Force.html,
I started to look for ways to measure the actual pressure of the molten plastic. Since I did not see any simple way to achieve that, I settled for measuring the actual force driving the filament into the melting chamber.
Please note that this is not intended to be a DIY article, just to give ideas what can be done and a starting point for experimenting.

Loadcell Mounting
The loadcell is mounted to the Ultimaker's extruder housing with a screw into a small bracket (the black object in the picture) that is clamped to the housing with two screws into the top plywood section. The other end of the loadcell is located right over the heated melting chamber in the extruder head.

That end of the loadcell has the mounting hole enlarged and threaded for 1/4"-28. The finer thread, 28, instead of the standard 20 threads/inch will not cut deeply into the Bowden tube. The Bowden tube has a matching thread. Note: The Bowden tube ends in the loadcell.

The driving end of the Bowden tube is similarly constrained in the extruder. I have been using this type of mounting for several years without any problems and it is very solid without any movement in the joints.

Bowden Drive End Mounting Block

There is a short piece of Bowden tube that is going into the extruder head as normal and the filament is entering the extruder "hot end" as before. By cutting the Bowden tube as described, I am able to measure the actual force on the filament being pushed into the melting chamber.

Force Display

The 7-Segment Display

The display is actually a self-contained digital voltmeter that measures 0 to 200mV and shows the result on the 7-segment displays. It is separately powered by a 5V adapter. There are many similar displays, both LED and LCD, available on eBay and regular distributors like Digikey.

The display is mounted in a laser cut frame which is attached to the front of the Ultimaker.

Electronic Information
The loadcell is a 20kgf rated unit bought from Phidgets in Canada. Their pricing is good. Phidgets.com The loadcell model number is 3134_0. I have also used Phidgets' bridge amplifier model number 1046_0. It is a very useful interface to different sensors and since its output is USB, it needs to be connected to a computer. It is an easy and good way to start with sensors since Phidgets have interface information available.

I wanted the force measurement to be self-contained so I needed an analog signal that could be interfaced to the analog measurement display. The output signal from a loadcell is a very low voltage, just a millivolt or two so the signal needs to be amplified before connecting the signal to the voltmeter display. I am using a custom amplifier that I designed for a customer and it was a great match for this application.

Vinland's Custom Bridge Amplifier

Summary
The display is calibrated in kgf but basically by moving the decimal point it could be in Newton. The typical printing force is between 2 and 3 kgf. It is interesting to watch the display and I get a feeling for what is happening inside the extruder head. For example, if the initial layer height is too low, the pressure builds up and acts as a warning to adjust the layer height.

For more 3D printer information and electronic troubleshooting see Vinland's blog: www.Vinland.com/blog

Bertho Boman
Vinland Corporation
11600 NW 20^th Street
Fort Lauderdale
FL 33323
(954) 475-9093
Email: boman33 at Vinland dot com
www.vinland.com