Fixing plastic parts with a 3D pen

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Downunder35m
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Fixing plastic parts with a 3D pen

Post by Downunder35m »

Some time ago I got a cheap 3D pen, made in China of course.
Crappy touch functions to set filament type and temp.
Would have been easier to just provide the temp function and to remember the last setting LOL
Anyway...

Ever had the problem that your kid comes to you with a broken plastic toy?
Most of the rigid ones are made from ABS but of course it is often impossible to know for sure.
In my case it was a broken hinge thingy on a doll house.
A disaster for a small child - trust me LOL
Turned out it did not react at all to a drop of acetone :(
So no ABS or similar.
Di-Chloro-Methane is always my next choice and it failed as well to do more than smear the color off.
Using some good epoxy might have been an option but I noticed that unwilling plastics never provide a long lasting bond.
Plus, keeping some fancy and expensive epoxy around that you only need once a year makes no sense.

For such difficult jobs I prefer to use UV curing resin or glue thats hard a rigid.
But even the thin door that should go back on the house did not let any UV light pass through.
Means I could have only provided some surface adhesion in a filed out crack :(
Since the broken off bit was only about 3mm thick and just over one centimeter long I decided to sand a slight bevel on the edges - either side of course.
Meant I had to screw haft the house apart to reach the areas later with the 3D pen.
Note to self: Never do such a thing with the little one around and watching ....
To "glue" the two pieces together I decided to crank up the temp to max on the pen and tried some clear PETG.
Said on roll 220-245°C...
Pen crapped out at 230 but the stuff came out nice and sticky.
To hold the parts together I use some high tech clamps I found near my washing basket, some might call them pegs ;)

Why and how use PETG for this?
PETG has the "problem" to literally bond to everything, including glass.
And it requires a quite high temperature, making it ideal to bond some unknown plastic or even different ones together.
Problem is the pen...
They use a teflon tip and the filament already cools down on the way out.
Plus at these high temps they really struggle with the heating and transport if you don't have a USB "charger" providing over 3A and that is able to provide the juice in bursts if required.
Means you have to let the thing warm up for a while and every so often push the button to get some filament out so it won't start to overheat on the inside.
I usually allow about 4 minutes here once the thing says it is ready to go.
Speed is critical here!
You want a slow enough flow of PETG to allow the parts to be heated up enough to create a stronger bond by fusing together.
Like when applying silicone chaulk to a corner you want to push the nozzle so the tip creates a smooth surface.
Best to try first with some chopsticks or just carboard to find which angle and speed works best for you.
The sanded bevel should not be much wider than your nozzle diameter, ideally just a bit smaller to create a smooth surface that is even.
For bigger parts you need to work in layers and you might not be able to fill a huge gap without trapping some air here and there.

How to clean it up if there was some overflow or the filling went over the surface?
If you do need to fill a lot and the edges are more or less stright using some painters tape certainly helps.
Not for an easy removal but to prevent the PETG from fusing to parts where you don't need it.
PETG is very difficult to cut.
Sanding works, sort of but the stuff loves to clog up the paper, so wet use it a must here.
Ideally you cut use an ultrasonic blade or a heated one that is very sharp.
Both pose the risk of damaging the parts you joined though.
If the surface ir really bumps I use some 120 to 240 grit paper on a posticle stick to smooth it out a bit.
Then I use a ceramic deburring tool to scrape the surface flat.
They are quite cheap and basically just a piece of ceramic with 90° angles instead of a cutting edge.
Those cheap knife sharpening thingies using a hardened steel piece to do the work can be used as well but are often a bit short and require good hand eye coordination.
Polishing PETG to get it shiny again is basically impossible on a hobby level unless you really want to spend a day sanding and using plastic polishing paste.
But you can cheat with a dremel tool and ABS slurry ;)
Use some rounde tool to grind the filled gap SLIGHTLY below surface level.
Make sure to clean the edges so you have the original plasits slightly exposed as well.
I prefer clear ABS and acetone.
If you need colored ABS please let the mix rest for a day or so to check if pigments fall out.
In most cases you will find a clear and slightly colored liquid layer and something cloudy at the bottom.
You want to seperate a bit of the clear stuff before mixing the rest properly ;)
Use the mixed stuff to fill the crack in layers that you let dry fully before applying the next one.
Try not to go overboard so you won't need to sand again ...
The last two or three layers you use the clear stuff - it will make the cloudy and maybe dull loking surface shiny.
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