[Jewelry making - Article 00089] - Re: [Orchid] Vent holes in rubber molds

[Ron]

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Andrew,

>     inconsistent results in the wax mold that are produced no vent
>     holes in the mold at the moment 

    I make lots of rubber molds and often face the challenge of full
    fills. So, let me start by saying that a number of factors could be
    causing the problems: 

    * Volume of the injector compared to the volume required in the mold 

    * Injection pressure regardless of plunger pressure by hand or air
    pressure filling 

    * Type of wax used 

    * Temperature of the wax 

    * Complexity of the object 

    * Yes, also vents 

    Some questions to ask yourself: 

    * Does it always fill (or not fill) in the same area 

    - if not always the same, look for another cause other than vents 

    * Is the quality of the filled area what you are expecting or is it
    unacceptable 

    - if not acceptable, look for a pressure problem 

    * Do you get everything from No Fill to Flash with injections 

    - if yes, consider how the mold is held during injection. Too much
    pressure can cause partial fills 

    - too little pressure can cause flash 

    - I use spring clamps and 2 pieces of 1/4 inch plastic to establish
    consistent pressure while injecting 

    If everything you have considered leads you to believe that the
    filling problem is trapped air: 

    ASSUMPTION ON MY PART -- I am working with a fully CUT mold that is
    in separate parts, usually 2 halves 

    1) Select an area and mark a point for your vent that is the best
    mix of the following four characteristics: 

    - within the area that doesn't fill 

    - furthest from the injection nozzle 

    - on the portion of the mold that will hold the thickest volume of
    wax 

    - will be the easiest to refinish IF the surface is disturbed 

    2) Now draw a line from that point to the edge of the mold. I never
    vent to the top or bottom because it is covered by the compression
    plates during injection. 

    3) Take the half of the mold that you have drawn the line on and
    fold or bend is such that the crease is on the line 

    4) Take your scalpel or cutting instrument and draw it along the
    line to form a shallow SMOOTH cut. The rougher the cut the more
    likely you will release air and create flash. A proper vent IMHO
    releases the air and creates no flash. 

    5) Take a very small (I use a flat dental tool) and pick up a very
    small amount of talc, bend the mold again to open the cut. Deposit
    the talc in the cut near the cavity for the pattern 

    6) Allow the mold to relax and blow off any excess outside of the
    cut 

    7) Inject wax as you would normally do it. 

    NOTE A -- I usually add additional talc every 10 or 12 injections. 

    NOTE B -- If you use Silicone spray it will tend to clump the talc
    so I normally do the silicone spray, let it dry, then clean the cut
    and re-add the talc 

    Regarding your second comment -- > don't have my own injector but I
    am looking at getting a new wax injector (hand pump) any
    recommendations? 

    When I started making molds, I used the equipment at the Rockhounds
    Club for my injection. Since I live a very long way from the
    facility, it was very inconvenient. Soooooooo, I went to the local
    pharmacy and bought several sizes of plastic syringes the largest was
    and the smallest was 12cc. They are dirt cheap. Buy the cheapest
    double boiler ($4-6) that you can find and use it to melt the wax let
    the syringe tip stay in the melted wax and be warm. Pull up the wax
    and press it against the mold. Wala, home made injector. You may have
    to trim the end of the syringe for a good round fit to the molds. I
    still use this method today for several different waxes (color and
    compounds vary) for things out of the ordinary. My normal everyday
    wax is Plast-O-Wax. 

Regards Caster
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