Extrusion, progress

After two weeks it is time for an update. We found three basic working principles to extrude material: With an extrusion screw, plunger or by using a pressure pump. (which has similarity’s with the second principle). We discussed these three ways of extrusion with our expert, Rolf Koster, this resulted in the following.

Extrusion screw:
This ways of extrusion results in:
– A good heat distribution
– Dense material at the extrusion head
– A better change to get a equal output
– A continue process

Plunger
This results in:
– Easy construction
– Bad heat distribution
– Bad and material
– Bigger change on air bubbles
– Discontinues process

Air or liquid pressure
We decided tot cancel this concept because it becomes to complext for domestic use.

Based on this outcome we chose to try and create the extrusion screw. The problem with this concept is that the screw can be hard to create. In designing this concept we have several options to create a screw.

1. Take a wood auger
This was tried in 2006 by Forrest Higgs.(http://blog.reprap.org/2006/02/initial-experiments-on-filament.html)

Problems that were occurring:
– To wide barrel fitting
– Not the right depth of the thread
– Not a variable thread dept
– A wood auger does not have a center. This cause a leak of pressure

2. buy an existing worm wheel
Problems:
– The are not long enough
– The threads are not deep enough
– There are to many windings per cm
– Expensive

3. Create our own screw,

To avoid the problems that are mentioned above we decided to make oure own screw. In this way we van examine the feasibility of the screw concept concerning the best circumstances. The problem however is that it has to be possible for other people to make the screw as well. To make sure that other people can make the screw also we want to make an file which can be but in a small automatic rotational machining appliance. This kind of appliances are availably in CadFab labs. These labs can be used by everyone.

The Biggest problem to overcome is to design the screw in a way that it can be produces in a CadFab lab. Small machines have limited power and therefore it can be hard to use steel. An other big problem is pitch of the propeller. This is limited tot the speed and the power of the machine. We hope to have some test results later this week. If this way of building a screw does not work out we can try to make it like Adrian Bowyer did (http://reprap.org/bin/view/Main/GranuleExtruder).

Advertisements

2 Responses to “Extrusion, progress”

  1. aka47 Says:

    Guys

    An auger system has to have at least 3 distinct zones or areas to work well. each has a different feed rate.

    1. feed
    2. degassing
    3. metering

    I would guess that the degassing and feed zones are proportional to the chip size, or more specifically to the air or gas space around the chips that feed in through the feed zone.

    I cannot claim to have thought this one up, but having skimmed the book I can recommended it to you for reading and to explain it all.

    Plastics Microstructure and Engineering Applications. (Auth Nigel Mills, Publisher Butterworth-Heinmann (Elsevier), 3rd edition)
    ISBN 0-7506-5148-2

  2. aka47 Says:

    In essence an auger of fixed and constant pitch won’t work very well.

    The pitch (feed rate) has to reduce towards the nozzle end of the feed assembly.

    Basically the feedstock being chipped is less dense and consequently the feed-rate higher at the input end.

    As the feedstock melts and the gas/air bleeds off the feedstock becomes more dense. ie the feed rate is lower.

    I guess it’s a bit obvious really when you think of the physics of it all.

    This is perhaps why a plunger and cylinder assembly cannot have a plunger that fits too tightly (where does all the gas go, you don’t want it in your extruded polymer do you)

    If you can’t buy the book get a look at it, It will save you a bunch wasted time.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s


%d bloggers like this: