Hi there,
since I am new in that forum, I thought I would share some of the experiences I gained in creating MDF molds as a start (as well as the bladder welding machine in the tools section). As a disclaimer I have to say that I only built 2 frames using this method, so there is still a lot to learn.
The reason I am doing MDF molds is that it is cheap and quick. Up to now, I did not optimize for surface quality but for getting my projects done.
It takes about 2 evenings in my workshop to create a mold for the tubes of a bike (unless it is as large as the Lotus, then it takes 4 evenings.)
So the process is as follows (assuming you built or own a CNC mill which fits the mold):
I screw the number of MDF boards on the CNC mill base as needed. For a typical bike frame, 2 MDF boards are sufficient as the resulting mold has 38mm thickness. Adding one board more to increase stiffness seems a good idea in general though. It is important to use a sufficient number of screws to ensure the boards are well bolted down. It is equally important to place the screws in locations which will not be milled later - otherwise the lifetime of your milling bits decreases radically. Sometimes it makes sense to create a milling process which just engraves the outlines of the cavities on the topmost board so you can place the screws right.
Then I mill the mold in a roughing fashion. I leave 0.3mm stock. I choose the mill and stepping to have cusps of less than 0.5mm. This is actually quite rough and permits quick milling. A typical setting is 1.4mm stepover with a 6mm ball mill.
I also mill alignment holes into the molds (of course)
When this is done, I use epoxy to soak the MDF. I do not remove the mold from the mill so I don't have to worry about the accuracy when placing it again into the mill. In order for the epoxy to soak in really deep, I heat the MDF with a heat gun locally really hot - definitely use full protection (breathing mask, ventilation on...) in that process because the epoxy only enters the wood deeply when it is quite hot. As I operate at high temperatures it happens sometimes that I overheat the epoxy and it starts emitting smoke. That's hard to avoid. As I said, use full chemical protection and ensure good ventilation. When done right, the epoxy soaks to a depth about 2-4mm into the MDF, effectively making it plastic-like for further processing.
The next day I do the finishing. Usually I mill with a parallel and contour operation with 0.8mm stepover/stepdown on a 6mm ball nose mill.
I found this is the best compromise between milling time and sanding time later. Sanding with 180-240-400-800 grit paper finishes the mold.
I hope that helps some of you.
here are some pictures:
Build thread Lotus 108 (in German though)Presentation of the Lotus 108 build