MDF Positive Vacuum Mould Form for Fiero GT Headlight Covers

Create mirror parts in MDF of positive moulds of Fiero GT Headlight Covers for an acrylic vacuum forming process.

Estimated Duration: 5 hours
Actual: 6 hours
Costing Accuracy: 82%
Work Breakdown:

  • 2 hrs CAD
  • 2 hrs setup & general labour
  • 1.5 hrs VMC machine time
  • Cutter consumption - negligible

Material: MDF

This project involved keeping mould material costs low - very low, to the tune of $12.66 for 2 panels of 5/8" x 12" x 8' MDF and $4.00 worth of carpenter glue. The project took two hours of GCode modelling, 1.5 hours of cutting, glue-up and jig creation, and 1.5 hours of machine time. Time to clean up the giant mess afterward was not anticipated and therefore not included in the quotation. Cutter wear was not considered for such a small job, though in retrospect, carbide would have been a better choice.

Starting out as a set of 12x12 boards glued up 5 at a time, the block was drilled and screwed from the bottom, mounted, and milled in two passes - a roughing pass using a 1/2" 4-flute endmill at 4000 RPM to remove the bulk of material using a waterline algorithm, followed by a two-part finishing pass with a 1/2" ball mill at 4000 RPM using opposing lateral profiling passes. This job required a test pass in scrap MDF (notice the right-hand side of the jig mount) to see if a suitable finish could be achieved at top speed (180 IPM) to keep machine time to a minimum. Starting at 25IPM and incrementing by 25IPM for each test pass, the ball cutter made really nice, clean cuts right up to the max with a little edge lifting regardless of which speed.

The first pass was precarious - the cutter tore up the edges of the MDF as it circled around at each stratum and left fluffy edges on the roughed passes. For the first run, the fluffy edges were no problem since the finishing pass with the ball-end cutter cleaned up the mess very nicely. The second pass incremented in 0.05" steps with a 0.005 scallop height constraint. First pass took 30 minutes, second passes 9 and 6 minutes each.

A mistake was made while gluing up the second block - it was left to cure overnight in the unheated garage resulting in a white crystalline precipitate instead of the usual smooth transparent solid. Upon machining the first 1/2", the top board quickly and entirely pulled away as if only tacked in place. The next form, like the first, was cured indoors at room temperature.

The next block went without a hitch, but was precarious at one moment. The roughing pass created fluffy edges as before, but the cutter pulled up a large chunk of MDF during the most aggressive outlying cuts - I had adjusted the roughing pass to cut 75% instead of 90%, figuring it would heave less - I think it became worse. Nevertheless, the part that lifted was to be entirely cut away, so it did not hamper the final outcome.

To anyone machining MDF on their milling machine, a word of advice: DON'T. Unless you have a router spindle and a brush guard with integrated vacuum attachment, it throws fine particulate (which is made even worse with smaller chip-sizes) onto everything in your shop leaving you with a thick layer of fine sawdust in every nook and cranny. And when it mixes with the oil on your ways, the cleanup time is double because vacuums and air guns are no longer effective. A colleague warned me, but I had to learn the hard way. The second time around, I was able to reduce the amount of fallout by following the cutter with the shop-vac which took away all but the largest debris that easily swept up with a broom.

Finally, a sharp cutter and proper surface speed is essential otherwise you risk pulling up on the material causing it to fail (delaminate). If you must cut without a high-speed spindle, get your surface speed much higher than I did, or keep your feed low, and use less agressive cuts. I ran this job with a 1/2" cutter at 4000rpm, cut at 75% of cutter diameter, 100% depth at 523 SFM and 180ipm which proved a little too aggressive for this material. The fluffy edges may have been solved with a downward cutter, but the budget was already tight. Perhaps someone with more experience in this area could opine.

The customer who contracted this project is in the midst of developing a vacuum-moulding process to produce a run of headlight covers with these moulds and will sell them online at Chris will finish the mould form by fine-detailing the surface and applying a high-temp surface coating to both perfect the final product surface, and increase its durability. Keep checking his website for updates and progress. Good luck with the rest of your project, Chris. Can't wait to see what they look like on your car!

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