Print smaller than stl-file

[kojan2 0]

Hello. I tried to print a stl-model of a bottle. Everything went fine I thought until I measured the inner diameter. 

It was supposed to be 40 mm, but instead it was 37 mm. What can I do to make sure the dimensions are correct?

I used a 0.8 nozzle with colorfabb co-polyester filament. I'm going to try with clear colorfabb HT next time to make it resemble glass more.

Any help would be appreciated. Getting the dimensions correct is really crucial. 

 

 

/CJ

 

[gr5 2,172]

The problem is that PLA (and PLA HT) sticks to itself as it prints, like snot or mucus.  As it prints the inner wall it is stretched like a liquid rubber band (it's tight because the PLA also shrinks as it cools in the first milliseconds out of the nozzle).  This pulls inward and makes vertical holes smaller than desired.

 

With a 0.4mm nozzle the shrinkage is usually 0.4 to 0.5mm (diameter).  I'm not surprised it's bigger with a 0.8mm nozzle.

 

The best solution by far is to just fix it in CAD.  Note that the outer diameter can shrink also but not as much as the rest of the part supports that outer wall from shrinking.

 

All manufacturing techniques (milling, FFF, SLA, injection molding) require that you fudge things like this in CAD.  Some people who do injection molding don't know this as the "factory" takes care of that step for you.

[kojan2 0]

Thank your for help. Guess I have to experiment a bit to get the right size of my model. It's just so frustrating when it takes 22 hours to finish a print. 

[geert_2 557]

1 hour ago, kojan2 said:

Thank your for help. Guess I have to experiment a bit to get the right size of my model. It's just so frustrating when it takes 22 hours to finish a print. 

 

You could do this kind of tests on small test pieces, of only a few mm high. In these tests, provide as much different features as realistic for your typical models: different line-widths, different holes, different extensions (poles), overhangs, curves and roundings, bridges,...

 

If you use bolts and nuts, also model the required hex-recessions into these tests. Idem for dovetail- or snap-fit mechanisms, etc...

 

[XYZDesignPro 70]

I work for a plastic bottle manufacturer.  I print a lot of bottles with various neck finishes.  I work in SolidWorks.  Try adjusting the Horizontal Expansion setting in the Shell menu.  I find a -0.125mm (negative 0.125mm) setting brings most of my neck finish I.Ds to an acceptable accuracy compared to the model.