Venting a Silicone Mold to Eliminate Bubbles In Finished Casting
Jerry Ross, President of Ross Concepts in Dover, NJ, had to mold and cast a precise prototype of an LCD mounting platform. The application called for low long term shrinkage in the mold and very high tensile strength in the finished casting which will have a wall thickness of about 1/8" (.32 cm). Mr. Ross was selected Mold Max® XLS, Shore 30A tin-catalyzed silicone that exhibits very low long-term shrinkage. He selected TASK® 4 as the casting resin because of low viscosity (abiity to flow), strength when cured and low shrinkage.
This particular prototype design posed challenges in getting the casting resin to flow to critical mold areas. Mr. Ross pre-planned to punch air vents into the cured XLS mold with brass tubing and an exacto knife. The vents would allow air to escape and minimize bubble entrapment in the finished TASK® 4 casting.
The following sequence chronicles Mr. Ross's process in successfully reproducing this model using an advance mold making technique.
1 - A mold box that will accomodate the original model is prepared. Minimum 1/2'' (1 cm) clearance is factored into the mold dimensions.
2 - 1/2'' (1 cm) sulfer-free clay segments will be used to elevate the model.
3 - Clay segments will elevate the model 1/2'' (1 cm) from the mold box base board.
4 - The model is secured onto the clay perimeter base.
5 - Holes in the model are filled with clay.
6 - Modeling clay surface and edges are leveled and smoothed.
7 - Acorn nuts are pressed into the clay around the perimeter of the model. They will create registration points for the mold halves.
8 - The bottom half is ready to have the first layer of rubber applied.
9 - 100 Parts of Mold Max® XLS Part A is weighed.
10 - 4 Parts of Mold Max® XLS Part B is weighed.
11 - Parts A and B are combined in the mixing container.
12 - The two components are mixed together with a drill followed by a hand mix.
13 - To ensure proper mixing, the rubber is poured into a second, clean mixing container.
14 - The XLS® silicone is mixed again in the second container.
15 - Then the mixture is then vacuum at 29 inches of mercury.
16 - The XLS® is poured into the lowest corner of the mold box and allowed to rise over the model.
17 - The XLS® rubber is allowed to cure for 16 hrs.
18 - The next day the mold is tested to make sure it's fully cured.
19 - The mold box is turned over for demolding.
20 - A drill is used to loosen the mold box screws.
21 - The baseboard is removed.
22 - The clay is removed from the perimeter exposng the back of the model.
23 - The clay is carefully removed from the holes of the model. The acorn nuts are removed from the cured silicone.
24 - Once the clay is removed, the model is cleaned with alcohol.
25 - Ease Release® 200 release agent is sprayed onto the model and cured rubber.
26 - Ease Release® 200 is brushed over all surfaces.
27 - More Mold Max® XLS is mixed and vacuumed to pour the second half of the mold.
28 - The XLS® is poured over the model, filling the mold box.
29 - The model is then left to cure overnight.
30 - The next day, the mold rubber is cured. The screws are removed.
31 - The mold box sides are removed.
32 - The two halves of the mold are carefully separated.
33 - The mold half is removed.
34 - The original model is separated from the mold.
35 - And removed.
36 - Following demold of the second half of the mold, both mold halves are post cured in a hot box for 4 hours at 200°F/93°C to drive off residual alcohol and water from the cured rubber. This step will prevent cure inhibition between the rubber and the TASK® plastic.
37 - The end of a brass tube is sharpened using an exacto knife.
38 - The brass tube is carefully punched through a pre-selected point on the top half of the mold. This point was selected because it is ove of the highest points in the top half of the mold.
39 - The brass tube is slowly forced through the rubber until it comes through the opposite side.
40 - A core of Mold Max® XLS is removed as a result of punching through.
41 - The mold halves are assembled, making sure that the mold keys are aligned and locked.
42 - TASK® 4 Part A is dispensed by weight.
43 - TASK® 4 Part B is dispensed by weight.
44 - The TASK® components are mixed together thoroughly.
45 - A syringe is used to inject the liquid plastic into a vent hole. The mixture is seek its level in the mold. When the liquid plastic becomes visible at the top of each vent hole, that is your indication that the mold is filled.
46 - The TASK®4 is allowed to cure over night. The mold halves are separated revealing the finished casting.
47 - The casting is a perfect reproduction of the original model. Flashing around the casting perimeter is removed with an exacto knife.