I also have the HF unit. It can make strong welds, but not always pretty.
Here are some tips:
1. Notch the crack in a V shape where you are going to apply the weld (from the back when you can)
2. Keep the extra pieces you cut off and use them in the weld. This is the right kind and color, for sure
3. Try to weld from the back. If the pieces are put together very closely and held well while welding from the back, any front crack will be hardly noticeable
4. Have some type of "spoon" available to push/shape the plastic while still warm. These are more like hobby tools than a table spoon and are different sizes/shapes.
5. DO NOT OVERHEAT THE PLASTIC !!! Better to heat and weld, let cool and do some more. When you over heat, it will burn and/or shrink and make a mess. No burnt marks (bad!)
6. When using the rods, I've had the best success when cutting pieces and laying them into the V notches above, then heating. Sometimes better to place the pieces in the V before heating, sometimes have the pieces cut, heat the work material, then put the pieces in the void and heat. Depends one what and how you are welding.
7. Make sure everything is VERY CLEAN. Just like soldering, oils, dirt, etc will interfere with the bonding and results.
I've used this for repairing broken body panels on ATVs, motorcycles, etc. Along with a good heat gun to heat and take out kinks and bends, it has been a handy item.
Hope this helps!
- JC