Best practice with ABS: no delamination, no cracking, no warping.

[abuharsky]

Hello All.
I spent a couple of weeks, even months in a vain attempt to print anything bigger than 40mm cube from ABS plastic.
There are no problems with printing small models, but with large models having constant problems: you just can not print any big vase without cracks and delaminations!

First, I tried few experiments with slicer(KISSlicer) settings for better adhesion between layers (slightly more flow rate, slightly more hotend temp - 240C, 250C instead of 230C, very low speed - 20-30 mm/s).

Second, I tried to make simple enclosure for printer using cardboard.



Third, I tried to print model inside thin wall to make thermos effect to get uniform cooling of the plastic.



although it did not work out all the details of cracked :X



I wanted to understand why this is happening.


At high temperatures, the plastic expands when low narrows, it creates internal tension. It is necessary to calculate or determine experimentally the value of narrowing:

1) look for the coefficient of thermal expansion ABS: 73.8 * 10 ^ -6 ° C-1 ( http://temperatures.ru/pages/tempera...o_rasshireniya )
2) remember physics 6th grade ( http://goo.gl/fP6CPU )
3) calculate how much less will be the length of the extruded rod 100mm at the bed temperature of 100C and 230C of the nozzle
ΔL = a * L * Δt = 73.8 * 10 ^ -6 * 100mm * (230C - 100C) = 0,96mm - which is almost 1% of the original length.

Upon reaching room temperature, 25 ° C - it is 1,5% (ΔL = a * L * Δt = 73.8 * 10 ^ -6 * 100mm * (230C - 25C) = 1,51mm)


1% - It is very small.

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Accordingly, with this small change in length of the plastic (1%) occurs corresponding to the internal pressure value of which is not too high.


The fact is, if the bonding strength is higher than the internal stresses - problem solved.


Then I tested the strength of bonding layers.
Testing methodology is simple: on the printed cylinder with 2 perimeters I evenly apply pressure with finger on the wall as long as it is not cracked between layers. By the depth pressing I detected the magnitude of the applied strength.


== Print speed affects the adhesion, but not much: I got about the same results for speeds 30-50mm/s;
== Much greater affect temperature of the extrusion: at 220C-260С I got the same bad result (plastic cracked from the slightest finger pressure), but the situation changed dramatically at temperature 270C-280C.
(Also, I have 2 different ABS filaments from different manufacturers, for the second filament I determined the optimal temperature in 260C.)


Finally, a few rules for successful work with ABS:
1) maximum heat for your particular filament (note that not all filaments are the same, they have different properties: viscosity, shrinkage, bonding and etc.)
2) the average print speed (I recommend 30-60mm/s)
3) calibrated heated bed and ABS juice for good adhesion of the first layer; ( http://www.makerbot.com/blog/2011/06...g-and-curling/ )
4) a bit of luck!


UPDATE #1
Most often china thermistors can't show you real temp of your nozzle, when you think its 250C, in real it may only be 220C(or 280C). Don't forget to fine tune your temp sensors.