You should not scale-up your models. Shrinkage does not affect FFF prints in the same way as it affects injection moulded parts. To illustrate, consider the following thought-experiment. Imagine that the plastic has a huge 50% linear shrinkage (=87.5% shrinkage in volume!). A 100mm cube that is injection moulded would thus shrink down to 50mm per side. Now imagine FFF printing that same cube. The nozzle lays down a line of molten plastic 100mm long for a side. When it gets to the end of the line, do you really think that the line of plastic will have shrunk back and be only 50mm long? No, what happens is that the plastic
progressively shrinks as it cools, but as the length is being maintained by the nozzle position, the shrinkage will affect only the width and height of the extrusion, not its length. The printed cube will be 100mm per side (near as dammit). The shrinkage will affect the gaps between the lines of solid infill. Now imagine what happens to the Z dimension. As more layers are added, do you think the nozzle will get further and further away from the last layer until the top layer is printed in the air 50mm above the model because of the massive shrinking? No, again what happens is that the previous layer will shrink to leave a gap of half the layer height, but the following layer will fill the entire gap - again resulting in a thinner width rather than a lower height. In fact, the combined stretching effect of the XY shrinkage and the bigger gap due to Z shrink will result in an extrusion width that is only 12.5% the width laid down - your part will be the correct size but full of gaps.
The end result is that with FFF printing, shrinkage results in the part retaining its dimensions but being a lower
density than it would have been with no shrinkage. The shrinkage is effectively translated into (bigger) holes in the part (the gaps between infill lines). Compensation for shrinkage is thus made in FFF printing by increasing the extrusion factor rather than by increasing the size of the part, though usually the extrusion error is significantly greater than the shrinkage so there is no point in making such small adjustments.
Shrinkage can however cause a printed part to warp (your photo shows warping on the solid cube). That is a result of
uneven shrinking, caused by a relatively large volume of plastic not all cooling at the same rate. In the solid cube of your photo, the outside of the cube cools faster than the inside - I can clearly see a circle on the base of your print that demarcates a hot core that resulted in the colder edges lifting. There are various ways of minimising warping. To prevent the base or corner curl-up which I can see on your photo, you need to coat the build plate with something that adheres to the 1st layer more strongly. This is sufficient to prevent mild warping such as you have - the uneven shrinking still creates an upward pull but it is not strong enough to pull the plastic off the bed so instead stretches the plastic slightly, keeping the Z dimension accurate. Try coating your bed with wood glue or solvent cement used to join plastic pipes. I use ABS dissolved in acetone. For worse lifting, layer separation, warping in the XY direction or curl-up of overhangs you can cool the plastic as quickly as possible after it has been extruded so that shrinking occurs entirely on the thin extrusion when it is able to be translated to a stretching effect rather than developing a large block of hot plastic which will behave more like an injection moulded part and pull the rest of the part out of shape. That can be accomplished by a cooling fan with air outlets directed close to the nozzle - but a downside is that if the extruded plastic is cooled too quickly it will not bond (fuse) very well to the previous layer or infill creating a weak part that readily delaminates. You can design for minimum warping by trying to avoid large solid volumes of plastic (adopt the same design considerations as are necessary for moulded parts). Avoid sharp corners as well. A heated bed helps a lot, but will only stop warping on prints that are not too tall. Possibly the best method is to put the entire build volume inside a heated chamber so that the whole model remains evenly hot. The chamber should then be cooled slowly after the print is complete to give inner areas time to cool at the same rate as outer areas. That method will result in the whole part shrinking similar to injection moulding - but it will be far less because the temperature change is not so great (it is not being cooled from anything like as hot as its molten temperature).
The vertical lines on your photo are caused by a different mechanism (there are two causes of that effect that I know of). The easiest way to reduce or eliminate them is to print more slowly (usually only the outer perimeter needs to be printed more slowly).
[edit] - I forgot to mention the most common cause of warping - draughts. Even a very slight draft caused by opening a door, or a slight breeze from the chimney of an open fireplace can cause huge warping that would otherwise not occur. This is because the draught causes a very rapid cooling of the outside of the part. When I was unable to eliminate draughts from a room, I put a plastic bin-bag over the printer to keep it draught-free - but be careful that this does not result in your printer overheating.
Dave
Edited 1 time(s). Last edit at 07/10/2015 09:46AM by dmould.