Q&A. Molding expanding foam in recyclable molds

Discussion in 'Construction' started by GregMcFadden, May 20, 2007.

  1. GregMcFadden

    GregMcFadden Facilitator RCWC Staff

    Dec 6, 2006
    I figured that I ought to pass along some of the tips and tricks that I have learned about self skinning expanding high density urethane foam part casting (see the tirpitz picture thread) and provide a single point where people can ask questions about it and add their knowledge and experience.

    Mold materials.

    I use machinists wax for my mold material simply because it is recyclable. I may move to other more permanent media after the molds are proven out, but initially, if a mold is broken or just doesn't work, in the oven it goes, and gets melted down and recast into a new block. A word to the wise. get the wax AND the mold that you will cast the raw stock in to 250F and fully molten. The wax shrinks by 5-10% from its molten state, so if the mold is not hot, the raw cast brick (what we will be machining our finished mold from) will not be flat (or usable) and will be far weaker too. after pouring into the hot mold, cap it with wood and lay a towel or some insulation over it. Let it cool for 24 hours. If it is cool in less than that, you have insufficient insulation, and the wax will be weaker. You can reuse wax a number of times, but after 5-10 it will be discolored and will start separating with gooey little globs of crud in it. At that point throw it out and start over.

    Release Agents.

    I like the Mann series of spray on release agents. (foam will stick to wax very well). Here is what I use:

    First coats, MANN 200 (their general purpose release agent)
    outer coats, MANN 1700 (their urethane foam release agent)

    note that 1700 by itself just does not work. 200 by itself does but the combination usually gives a better finish.


    Smooth On 25iT self skinning urethane foam works quite well and is flexible to boot with a good surface finish.


    Mold making...

    OK folks, now it gets tricky. Generally speaking you will want at least 1-2 degrees of draft angle for every 0.5 inch of depth on a vertical wall. you can SOMETIMES get away with no draft so long as the part is not solid and has a core with a generous draft angle that is easily removable. SOLID PARTS ALMOST ALWAYS REQUIRE DRAFT. WHEN IN DOUBT DRAFT IT!

    The wax itself is not terribly strong, so if you think that the mold is thick enough, it probably isn't. In those cases where the mold is not thick enough, I will pot it in epoxy/fiberglass.

    The wax molds will wear out eventually, and the sharper the inside corners the more quickly it happens.

    Vent holes/paths:

    ALL MOLDS MUST HAVE VENT HOLES AND PATHS. and don't forget that they need to be such that when filled with foam the part can be removed from the mold. Generally, along a seam line if it is high enough I will have .1 inch deep by 3/8" wide vent paths every few inches. If the molded part has a void in it, then you need a vent path there.

    When all other options are ruled out, you can drill holes straight through the top half of a mold, but you will have to clip the excess off before pulling the mold halves apart.


    DO NOT TRY TO ADJUST THE MOLDS FOR SHRINK/EXPANSION OF THE FOAM INITIALLY. you can do that later, however I have found that the flexIT 25 from smoothon mentioned previously will after fully cured not have a significant (for our purposes) expansion/contraction. HOWEVER, you must give the part 2 days to stabilize after mold removal as the part will expand and then contract back to nominal during that time.

    Let the part stay in the mold a minimum of 8 hours.

    Back pressure:

    Yes you need it. You ABSOLUTLY HAVE TO HAVE IT or your parts will not look good. If there is a stringy look to the parts or they seem really low density, your back pressure is insufficient. The causes of this are two many vent holes, no cap, or an insufficient initial charge of mixed resin.

    Keep in mind, that two much resin is very very very bad, as the foam can build up incredible pressure if overcharged. I have had 2" thick walls in a wax mold break out when I overcharged the mold. ALWAYS waste the first few parts by working up a load to proper, NOT DOWN. it will save you in the long run.

    WEAR GLOVES and have good ventilation. (I have all my fans turned on and do my molding on windy days as with a window and door open I get ~10-15 mph winds moving through my workspace, and it is always windy here)

    Please ask questions/comment with your own experience