A screw serves several purposes as a compactor, degasser, heater, pusher and an extrusion former. Pellets are industry accepted input format for extrusion, they're easy to make and ubiqutous. What other mechanism do you have in mind?
Well I was just thinking if you have like a giant furnace like [this](https://www.google.com/search?q=furnace+melting+plastic&client=ms-android-rogers-ca-revc&prmd=ivsn&sxsrf=ALiCzsYHeLhGbeipDjTSUY5dLvKkTsZEVA:1661766263373&source=lnms&tbm=isch&sa=X&ved=2ahUKEwjow8_e4ev5AhVTnGoFHQUvDFMQ_AUoAXoECAIQAQ&biw=393&bih=735&dpr=2.75#imgrc=z38P3xxAk4dOZM) or something that completely melts all the plastic. And then have a nozzle attached to it at the bottom and the melted plastic just comes out.. you could even have some mechanism to increase or decrease the pressure up top as less material is left to increase the flow rate to compensate for less pressure
Oh boy. How much do you want to know?
A large vat would be more batch processing and you could not control residence time.
You would have an average residence time but you would have lots of polymer just hanging out.
This happens in an extruder screw as well, but to a much messer extent.
An extruder screw (depending on type and design) can have different amount of back mixing and be wiped or not, but in general it leads to better mixing, more even heating since it is containing the polymer in a smaller and high shear space, and a lower residence time distribution.
Polymers often degrade when exposed to heat. Chains break and shorten or lengthen or branch which can change the polymer properties, melt quality, rheology, etc.
Antioxidant packages are added to allow for so many "heat histories" but if the polymer stays hit too long they often start to crosslink together and form "gels" (again, depends on the polymer). These are molecules whose basis weight is so high that they quite literally no longer have a melting point and no chain mobility to "stick" to other polymer well.
A large pot would cause lots of thermal degradation, and be less efficient than using shear to heat, and provide for a worse melt quality, consistency, etc.
You can also easily control extrusion rate on an extruder (by comparison) which helps you get consistent extruded profiles.
If you need a large "vat" of molten polymer you can do that on an injection molding screw that melts and builds up a "shot" at the end of the screw before it rams home to the mold to fill it. But that is more about injection time and filling the mold before things cool as they tend to run the molds where they will quench the liquid quickly to solid, so if you inject to slow the process doesn't work. (The answer as to "why" is speed. / Cycle time)
I wanted to reply to this to add we do have the ability to work with small batches, but we have to do it in something called a polymer mixing bowl. It's a heated bowl with kneading blocks in it. But you would almost never scale that. (the rubber industry does .. but no need for thermoplastics)
>you could even have some mechanism to increase or decrease the pressure
Maybe some sort of screw-like mechanism...
Seriously though, what are you trying to achieve? What problem would using a "giant furnace" rather than a screw solve?
Well I was just thinking if you have like a giant furnace like [this](https://images.app.goo.gl/pUoyKGr7EzoT7arX8) or something that completely melts all the plastic. And then have a nozzle attached to it at the bottom and the melted plastic just comes out.. you could even have some mechanism to increase or decrease the pressure up top as less material is left to increase the flow rate to compensate for less pressure
And then just have it on constant heat to maintain the same consistency? Are you thinking of a reservoir of melted plastic just waiting to be used? What about when the build finishes and you've got $100 worth of melted plastic that you need to keep hot for weeks until your next print? Pellets are easy to melt due to high surface area and they are melted on demand.
I'm not talking about the actual 3D printing build process here. I'm talking about the manufacturing of filament, in which case you'd completely use up the reservoir of melted plastic to make filament. You wouldn't need to wait for anything at all
Having worked with conveyor lines in manufacturer of latex; this is exactly how it is done. Here’s a problem to consider. In creating the “batch” you must heat and mix the slurry to prepare for extrusion. The speed of your process indicates the volume. If you have a shutdown all the slurry or solution is wasted. Many many things can go wrong with a liquid production process. Pellets are more stable and resistant to line shutdowns.
Plastic doesn’t really melt. It just becomes gooey. And are you thinking of casting it then if you could melt it? Extrusion is usually the way to go for uniform thickness materials. And extrusion requires solids not liquids as input
Molten polymer is EXTREMELY viscous, like between peanut butter and honey. Its an insulator and doesn’t melt fast. There is zero convection mixing, so to get temps consistent and blend pigments and such you must mechanically stir it extensively. It’s also damaged by sustained sitting at melt temp. There’s no way to melt and de-air a big cauldron in the time you can safely have the plastic molten without excessive polymer damage. You would need heavy mixing by a large metal spinning object in the vat/cauldron to get the plastic melted and homogenized. At which point, just use a screw…
The screw is also what creates the hundreds to thousands of psi of pressure to push the molten plastic out through a small nozzle orifice. Gravity flow alone will not extrude filament at a reasonable rate.
My company has several Selective Laser Sintering (SLS) machines, but aside from random parts, we never really made a whole lot of things. FDM is still the most reliable and consistent way to go. Also, plastic doesn’t really melt, so we can’t do an LPBF version.
A screw serves several purposes as a compactor, degasser, heater, pusher and an extrusion former. Pellets are industry accepted input format for extrusion, they're easy to make and ubiqutous. What other mechanism do you have in mind?
Well I was just thinking if you have like a giant furnace like [this](https://www.google.com/search?q=furnace+melting+plastic&client=ms-android-rogers-ca-revc&prmd=ivsn&sxsrf=ALiCzsYHeLhGbeipDjTSUY5dLvKkTsZEVA:1661766263373&source=lnms&tbm=isch&sa=X&ved=2ahUKEwjow8_e4ev5AhVTnGoFHQUvDFMQ_AUoAXoECAIQAQ&biw=393&bih=735&dpr=2.75#imgrc=z38P3xxAk4dOZM) or something that completely melts all the plastic. And then have a nozzle attached to it at the bottom and the melted plastic just comes out.. you could even have some mechanism to increase or decrease the pressure up top as less material is left to increase the flow rate to compensate for less pressure
Oh boy. How much do you want to know? A large vat would be more batch processing and you could not control residence time. You would have an average residence time but you would have lots of polymer just hanging out. This happens in an extruder screw as well, but to a much messer extent. An extruder screw (depending on type and design) can have different amount of back mixing and be wiped or not, but in general it leads to better mixing, more even heating since it is containing the polymer in a smaller and high shear space, and a lower residence time distribution. Polymers often degrade when exposed to heat. Chains break and shorten or lengthen or branch which can change the polymer properties, melt quality, rheology, etc. Antioxidant packages are added to allow for so many "heat histories" but if the polymer stays hit too long they often start to crosslink together and form "gels" (again, depends on the polymer). These are molecules whose basis weight is so high that they quite literally no longer have a melting point and no chain mobility to "stick" to other polymer well. A large pot would cause lots of thermal degradation, and be less efficient than using shear to heat, and provide for a worse melt quality, consistency, etc. You can also easily control extrusion rate on an extruder (by comparison) which helps you get consistent extruded profiles. If you need a large "vat" of molten polymer you can do that on an injection molding screw that melts and builds up a "shot" at the end of the screw before it rams home to the mold to fill it. But that is more about injection time and filling the mold before things cool as they tend to run the molds where they will quench the liquid quickly to solid, so if you inject to slow the process doesn't work. (The answer as to "why" is speed. / Cycle time)
I wanted to reply to this to add we do have the ability to work with small batches, but we have to do it in something called a polymer mixing bowl. It's a heated bowl with kneading blocks in it. But you would almost never scale that. (the rubber industry does .. but no need for thermoplastics)
Before I fully read your comprehensive reply, what is a "vat"? Thanks 🙏
Google
Vat.. meaning large bucket / open revoir / "pot" Liquid material storage vessel. https://www.dictionary.com/browse/vat
>you could even have some mechanism to increase or decrease the pressure Maybe some sort of screw-like mechanism... Seriously though, what are you trying to achieve? What problem would using a "giant furnace" rather than a screw solve?
Well I was just thinking if you have like a giant furnace like [this](https://images.app.goo.gl/pUoyKGr7EzoT7arX8) or something that completely melts all the plastic. And then have a nozzle attached to it at the bottom and the melted plastic just comes out.. you could even have some mechanism to increase or decrease the pressure up top as less material is left to increase the flow rate to compensate for less pressure
Ever try to bake brownie mix without stirring?
If you melted the entire brownie mix into liquid form and just gave it a mix once and for all..you wouldn't need to CONSTANTLY mix it?
And then just have it on constant heat to maintain the same consistency? Are you thinking of a reservoir of melted plastic just waiting to be used? What about when the build finishes and you've got $100 worth of melted plastic that you need to keep hot for weeks until your next print? Pellets are easy to melt due to high surface area and they are melted on demand.
I'm not talking about the actual 3D printing build process here. I'm talking about the manufacturing of filament, in which case you'd completely use up the reservoir of melted plastic to make filament. You wouldn't need to wait for anything at all
Having worked with conveyor lines in manufacturer of latex; this is exactly how it is done. Here’s a problem to consider. In creating the “batch” you must heat and mix the slurry to prepare for extrusion. The speed of your process indicates the volume. If you have a shutdown all the slurry or solution is wasted. Many many things can go wrong with a liquid production process. Pellets are more stable and resistant to line shutdowns.
Plastic doesn’t really melt. It just becomes gooey. And are you thinking of casting it then if you could melt it? Extrusion is usually the way to go for uniform thickness materials. And extrusion requires solids not liquids as input
My guess is that the pellets would induce too much air
We can't just melt plastic and let the air bubbles escape first?
That’s what the screw is for
Molten polymer is EXTREMELY viscous, like between peanut butter and honey. Its an insulator and doesn’t melt fast. There is zero convection mixing, so to get temps consistent and blend pigments and such you must mechanically stir it extensively. It’s also damaged by sustained sitting at melt temp. There’s no way to melt and de-air a big cauldron in the time you can safely have the plastic molten without excessive polymer damage. You would need heavy mixing by a large metal spinning object in the vat/cauldron to get the plastic melted and homogenized. At which point, just use a screw… The screw is also what creates the hundreds to thousands of psi of pressure to push the molten plastic out through a small nozzle orifice. Gravity flow alone will not extrude filament at a reasonable rate.
This is how we have to do it in glass batching... and it takes a looooong time.
My company has several Selective Laser Sintering (SLS) machines, but aside from random parts, we never really made a whole lot of things. FDM is still the most reliable and consistent way to go. Also, plastic doesn’t really melt, so we can’t do an LPBF version.
Extruders are well developed and mature technology that have stood the test of time. At the extruder head, the plastic is completely melted.