Printing of designs - faint lines or low quality?

DSM free base version users:

In your workspace, select the set of curves which are appearing faint on printing or during preview.

With the curves selected, navigate to the Display tab-> Style group. The correct layer should be automatically chosen. Now, drop-down the lineweight list and choose a higher line thickness you want to print in.

The next important step is with the curves still selected, toggle the layout setting in selection properties to True. 

Now, go to File-> Print Preview and you should see the chosen layout curves now have a thicker lineweight. You can try printing as a PDF to test it out first.

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DSM paid add-on users:

In most cases, you will be setting a custom lineweight which usually means thicker edges or curves. However, you may notice that once printed on paper or as a PDF the lines start to look faint. This can be corrected easily by tweaking a few print settings.

The line weight itself is a vector graphic property and the default print settings only print the existing design view as a raster image.
1. Head over to File -> Print Preview
2. Under settings, duplicate the highlighted options. The important one here is the overlay vector edges, which if left disabled does not load the correct lineweight and defaults to the 0.35mm (thin) setting.
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You can see in the bottom right corner of the image, that the edges are thick as per the initial design setting of 2.11mm.

If you'd like to print as an image but find the output is blurry, try increasing the resolution setting to 200dpi. To change the current entry, double click on the field 'automatic' and type in 200 or a higher number (in dpi - dots per inch)