May 2001

5/14/2001

5/14/2001

This part hasn't been easy, but in some ways it's been easier than expected.

I now have in my possession a complete car-set of polyurethane body mounts. This consists of three different sizes: the 'standard' size (12 of the 16 sets) with a 0.75in thick upper cushion and 1.375in frame-hole pilot, the pair for the front of the rear-axle kickup is 0.50in thick and has a 1.125in frame-hole pilot, and finally the rear crossmember set is 0.75in thick but uses the small 1.125in frame-hole diameter. Had I noticed the frame holes were different sizes before the frame was powder-coated, I'd have ground the smaller holes out to 1.375in. Regardless, I now have what I need and they all fit their specific locations correctly, unlike the repro set I'd purchased.

The molds were made from a stack of four pieces of HDPE (high-density polyethylene) sheet of varying thicknesses - basically plastic cutting-board material. The center spacers are 1/2in Schedule 40 PVC pipe, approximately 13/16in OD, providing for a nice snug fit on the hat-section steel mount shell. Total material cost for each mold about $5.00, including the handful of 1/4-20 bolts to hold them together.

The urethane material used is Poly 75-80 80A-durometer two-part rubber from Polytek. It mixes easily (2-to-1 ratio by weight) and cures at slightly above room temperature. The 8-pound trial package appears to have enough rubber to do two complete vehicles' worth of mounts. 80A durometer is pretty hard stuff - it's about the hardest you can get and still have flex detectable with finger pressure. 65A would probably be closer to OEM mount consistency.

Big caveat here: this rubber is sold primarily for moldmaking purposes. It isn't sold for use in making any sort of isolator. There may be something fundamentally wrong with using it for such purposes, but so far I haven't found it: a solid rubber body mount is not a particularly high-tech or close-tolerance part. My test protocol to date consists of subjecting sample pieces to 48 hours' immersion in motor oil (no softening or swelling detected), 12 hours in gasoline (ditto), 48 hours compressed to 50% of its relaxed thickness in a bench vise (recovered to full thickness shortly after release.) So I'm comfortable with using it, but as they say, your mileage may vary.

Early test pours produced mounts with a large number of small air bubbles in them. Maybe usable, but not what I wanted. It proved necessary to vacuum-degas the rubber after mixing. The apparatus I've used for this so far consists of a Rubbermaid bowl with a snap-on lid, some bits of plumbing, and a hand-operated Mityvac pump. Seven or eight minutes at 20 in Hg produces good results, though it takes a lot of pumping to get there and the bowl doesn't much like this kind of treatment. If I were to set this up again, I'd find an old thrift-shop pressure cooker and one of the Harbor Freight $10 air-operated vacuum pumps and painlessly pull 25+ in. Hg.

I'd spent no small amount of time looking for some carbon black to mix into the urethane, for coloring, UV-resistance, and surface toughness. In the end, I gave up, and bought a bottle of black resin-coloring pigment. Each 18 ounce (by weight) pot of urethane gets 1/4 tsp of the pigment. And two drops of the pricey Polytek UV inhibitor which, given that body mounts live a fungus-like life under the car to begin with, is probably turd-polishing. The drum- sanded surface of the inside of the molds leaves a matte, very slightly ridged, machined look to the visible surface of the mounts.

On the topic of sanding drums. You want a 2in diameter, 1/4in shank sanding drum at Orchard Supply? $22.00. The local industrial-tool-supply shop wanted $14, if they had one in stock, which they didn't, nor did they have any other useful size. McMaster-Carr to the rescue, again. Seven bucks. Of course, if you want to buy the grit sleeves from them you have to buy in lots of 50 but at $0.18 or so per sleeve that's not too bad. I have no idea how much of their business is Web-driven, but outfits like this are going to be the salvation of E-commerce. Of the stuff I've recently ordered from them, only once was the order not on my doorstep within two days. Only downside is that their site can't provide shipping costs, but nothing I've ordered has ever cost more than four bucks in shipping.

The layers of the HDPE sandwich that form one of the molds.
1/2in SCH40 PVC pipe as center hole spacer. I made the holes in the mold too big and had to wrap the pipe with electrical tape to make it fit... The two-part urethane is mixed by weight; note the highly specialized and accurate weighing apparatus. When that scale was made, by the way, first-class postage was $0.06 for the first ounce. It's not quite as old as the car, but close.
The elaborate and costly vacuum-degassing apparatus for the mixed rubber. Save yourself some pain and find a thrift-shop pressure cooker instead of the plastic bowl. Injection molding. Mixed rubber, ready to go.
The full mold, ready to be put atop the refrigerator for the night. A complete mount set. The outer surfaces have a grained, 'machined' look to them - the urethane retained the imprint of the 120-grit drum-sanded surface of the inside of the mold.

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