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in this last lecture on polymers I want
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to give a really broad overview of
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synthesis and processing techniques that
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are that are used for polymers so let's
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begin with synthesis synthesis and
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there's basically two forms of synthesis
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synthesis that I want you to be aware of
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the first is addition polymerization and
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we've talked a little bit about this
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before
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but all it is is the process of adding
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monomers one at a time to form some
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linear chain and it consists of three
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phases the first phase is the initiation
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phase and what you see is there's our
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monomer and it's gonna join up with some
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reactive group it's gonna break the
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double bond and then form to the chain
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and leave a reactive carbon region at
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the end so so now this can go bond with
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another monomer so that's the initiation
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phase the propagation phase is where
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what's formed then the initiation phase
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actually goes out and does bond with
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more monomers so it forms this chain and
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so it's important to remember that
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addition polymerization is gonna
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typically form linear chains it's not
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what's going to be used for network
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polymers or heavily cross-linked
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polymers like we talked before so we end
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up with this chain formation and you can
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imagine this goes on for many many
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chains that process is relatively rapid
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so it might take a hundredth or a
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thousandth of a second to form let's say
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a 1000 units during this phase and then
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finally we have to terminate this
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process so there's a termination phase
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and and all that's happening here
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there's one of the one of the chains
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that have formed in the propagation
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phase there's another and there's
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basically two options they could form
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that they could bond here and form this
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terminated combination here or they
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could do what's called
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disproportionation and in this case one
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of the hydrogens is going to bond in
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this this region and then a double bond
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is going to form with this carbon and
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that's what you see here so those are
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your two options to terminate the
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addition polymerization or the chain
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reaction polymerization reaction so
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that's one type of polymerization a
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second well
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me back up the the relative rates of
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initiation propagation termination
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that's how we can control the molecular
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weights of the polymer that we're going
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to produce okay second type of
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polymerization is the is called a
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condensation polymerization reaction or
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a step reaction and this is the
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formation of polymers by stepwise and
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molecular chemical reactions that sounds
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confusing so it's easiest to talk about
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a example so here's an example of the
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formation of nylon 6-6 so we have the
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monomers here the first one is a hexa
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methyl diamine and the second is adipic
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acid and what you see is those react to
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form nylon 6-6 plus water so a couple
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features are important in this type of
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reaction the first is that unlike in the
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addition polymerization reaction the
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reactants that is the the participating
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components do not have the same formula
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as the repeat unit so write hexa methyl
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diamine and adipic acid are neither one
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of those are nylon 6-6 okay the other
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thing and the reason that we call it a
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condensation reaction is that it
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typically condenses out a byproduct in
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this case water so we form the reaction
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between the hydrogen and the hydroxyl
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and we create water because of the
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reaction so there's our condensate the
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feature to be I guess aware of and this
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type of reaction is that it's going to
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be typically a slower reaction than the
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addition polymerization so it takes a
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longer time this is the kind of reaction
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that we would expect to see when we're
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taught when we're forming epoxies and
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other sort of network polymers okay so I
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want to say about polymer synthesis now
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let's move on and talk about the
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processing of polymers and I'm I'm not
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going to cover every possible processing
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type just that the kind of the major
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ones so the first I want to I want to
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talk about is compression molding and
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this type of processing technique can be
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used for both thermoplastics and
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thermosets and I'm showing you the
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process here but I'll just give it to
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you in words
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first you're gonna place the polymer in
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some mold cavity and so that there's
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there there's your sort of pink polymer
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in the mold cavity then you're going to
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heat the mold and apply pressure that's
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going to basically liquefy the the
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polymer and allow it to form in the in
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the same shape as the mold and then when
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you're all when you're all done you're
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going to have the the cooling of the
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polymer part and then you have your
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molded pieces it's fairly simple okay
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now let's talk about injection molding
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in this case there's plastic pellets
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that are put into a hopper so here's
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your hopper with the plastic pellets
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then there's a ram a hydraulic ram that
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forces the pellets into a heating
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chamber and so it's now putting the this
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heated these heated pellets which
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actually become a liquid under pressure
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and then this molten plastic is going to
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be forced under pressure in G so it's
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injected into the mold cavity and forms
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the part so here's a little animation
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kind of showing that process you can see
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that the RAM creates the pressure
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squeezes the polymer into the mold then
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the mold cools the polymer cools becomes
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a solid part and there you have it the
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final a conventional form of polymer
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processing I want you to be aware of is
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extrusion and this is typically only
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used for thermal plastics and it begins
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in a similar way to the injection
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molding we put plastic pellets in a
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hopper and then they're dropped onto
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what's called a turning screw and as the
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screw turns it moves the the polymer
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forward and there's a the there is a
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heating elements along the way so the
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plastic pellets are melting as the screw
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pushes them along and then finally it
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gets through the gets to the end and
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it's forced through a die under the
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pressure of whatever the screw is
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driving and it creates this extrude
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eight
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that comes out of the the die so another
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little animation to highlight that you
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can see the green polymer being squished
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through a die and forming the extrude 8
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and I'll show you that one more time the
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screw turns forces the polymer through a
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die the die then is is creating the
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shape of the polymer as it comes out so
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that's the extrusion process the final
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process I want to talk about is 3d
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printing and it's probably a little bit
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more modern than the other processes
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there are a variety of ways that's where
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you print I'm gonna basically break them
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into two different types and I guess I
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in my mind all the other types or some
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form of subcategory or combination of
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these types so the first is fused
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deposition modeling or FDM and in this
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case what happens is that you have some
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polymer filament
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here's your polymer filament it's fed
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into a nozzle the nozzle is heated to
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the somewhere above the glass transition
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temperature and the the so the filament
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is then extruded and it lays down a bead
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so to speak of polymer and it can do
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whatever shape the the it wants
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depending on how the extruder and the
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build platform move typically this is
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only done with thermoplastics okay
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another technique is what's called
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stereo lithography and I'm grouping it
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with digital light processing DLP
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because they work under the same
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principle so in this case we load a
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polymer resin so unlike in this case
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where we have a solid polymer filament
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here we pour a liquid resin into a bath
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and we pass UV light through the bottom
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of the tank the build platform comes
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into the tank and the polymer is cured
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on the build platform and then as it
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moves up we continue to cure successive
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layers onto that part typically this is
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going to be a
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the resin that's used is going to be a
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thermoset it's not going to be a
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thermoplastic because it's not going to
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be heat driven it's going to be a
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reaction cure
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that's initiated with with UV so let me
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give you just some some animations of
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what these look like so this is a Prusa
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3d printer showing you the FDM approach
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printing some some form of a creature
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here and you can see that the build
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plate moves and the ik and the extruder
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moves above it and that's how you can
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get the the full XYZ motion of the of
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the filament okay
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I also want to show you the SLA printing
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in this case this is the formlabs
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printer and this light you can see is
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actually the laser the UV laser curing
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every layer and this is obviously
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time-lapse so we're drawing the part out
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of the resin bath we have a whole suite
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of these in and actually we have a suite
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of these as well in our makerspace in
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the engineering building and the only
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difference between this SLA process and
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the DLP process is that SLA is using a
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UV laser and DLP is going to use
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basically a UV screen
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that like a TV projection screen
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underneath to generate that the UV the
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the regions where you want the UV to
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cure so those are those are sort of the
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two I would say primary methods for 3d
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printing polymers and in kind of the
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highlights of each so hopefully you
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found that interesting again I know it's
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just very cursory we're not going to go
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into great detail in each of one of
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those but I want you to be aware at
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least as engineers how how we handle and
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process polymers
