Young R. Kim
Stephen M. Wolfe
Nathan M. Toohey
Zachary A. Slaton
What is Molding?
Molding is the process of copying the interior surface of one object to the exterior surface of another object. The negative (interior surface) of an object is used to make its positive. In order to make a negative without great difficulty, molding requires a pre-manufactured original from a process such as LaserCamm, Stereolithography (SLA), CNC Machining, or some other means of rapid prototyping. The original is used to make the negative by simply pouring a heated solid (with a high boiling point) as a liquid around it. Once the liquid solid cools and hardens, the original is removed. The resulting object is a mold that can be used to create duplicates of the original. A duplicate can be made by simply pouring a heated solid (with a lower boiling point of the material that the mold was made from) as a liquid into the mold (the negative) and allowing the liquid cool and harden. The resulting object (the positive) displays very accurate details of the original pre-manufactured object.
Design Information Transmitted to the Manufacturing Process:
Molding does not require design information transmission to the manufacturing process since the mold and duplicates can be made by hand. The only design information needed is that the people who create the mold and the duplicates know what they are doing. If this is not the case, then they should work with people who have knowledge about molding or find out exactly what they need to know about molding.
Materials Used:
Making a Mold: The first step in casting a mold is to make a mold box. A mold box is anything that can hold the liquid silicone during the pouring. For our mold box, we used a plastic cup that had the bottom cut out of it in order to pour the silicone inside of it. Then the cup was turned upside down and fixed to the contact paper with Kleen Klay Regular. Kleen Klay Regular is a clay that does not react to silicone. Once the cup is secured to the contact paper, the master piece gets placed inside the cup and fastened to the contact paper using double sided tape. In order to prevent the mold from sticking to the master piece, the master piece must be sprayed with release agent before any pouring occurs. Next, two beakers are needed: one filled with a curative and the other with a prepolymer (from Por-A-Mold). The ratio by volume needed for the curative and prepolymer is 1:1. After the proper amounts have been carefully measured, mix the two polymers together (stir for several minutes). Once the mixture is made, make sure it is free of air bubbles and then pour it into the mold box. The mixture takes 15 minutes to harden and 24 hours to cure. After the mold has cured, the mold can be separated from the mold box by using a knife or scalpel.
Making a Cast: To prevent
inpurities on the cast, make sure that there is nothing left inside the
mold crevices (i.e. dirt or dust) before pouring the cast chemicals into
the mold. To keep the cast from sticking to the mold, spray it with release
agent. The next step is to make the mixture for the cast. Just
like with making a mold, a 1:1 ratio of volume is needed with a silicone
compound and a curative (from Por-A-Kast). (NOTE: The casting
compounds will dry and harden in approximately one minute once they are
mixed together so pouring the cast needs to be done quickly.) Once
these chemicals are in the same container, immediately stir them together
for about 30 seconds. Pour the mixture into the crevices of
the mold until the voids are filled up. Once the mold is filled,
use a flat spatula to level out the top of the mold in order to break the
surface tension. The cast will take about 25 to 40 minutes to cure.
Once the cast has cured, simply bend the mold on the edges and around the
corners until the cast pops out. Whalllaaa... now you have a molded
prototype!!!
Limitations on Size and Shape:
Limitations with respect to surface detail are very minimal. This is because the process of molding is very accurate when reproducing details such as engravings or indented designs.
Even though just about any surface detail can be duplicated very precisely using the process of molding, there is a wide range of limitations associated with molding in terms of volume, hollow pieces, pieces with holes, and thickness. Any batch size over a gallon can be made, but with a little more difficulty. When creating something this large, a "mother mold" is needed to surround the mold. Hollow pieces cannot be created using the process of molding since there is no way to hollow out the inside of the positive while it is cooling and hardening. Pieces with holes can be made, but it is probably more trouble than it is worth because it requires a two-part mold. Thin pieces are also much more difficult to produce than thick pieces using the molding process since the positive will actually harden more quickly in its thicker portions than in its thinner portions.
Cost and Manufacturing Time:
Depending on what people are looking for, there is a wide range of cost for molding. Obviously, if people want to make large volumes of molded pieces it will be more expensive than making just a couple of pieces. However, when compared to other methods of rapid prototyping, it is very cheap. The table below lists the prices for some of the material needed for molding.
POR-A-MOLD S 111:
| Volume | Cost |
| 2 quart kit |
$20.00
|
| 2 gallon kit |
$60.00
|
| 10 gallon kit |
$250.00
|
| 100 gallon kit |
$1,925.00
|
POR-A-KAST Mark 1:
| Volume | Cost |
| 2 quart kit |
$22.00
|
| 2 gallon kit |
$60.00
|
| 10 gallon kit |
$260.00
|
| 100 gallon kit |
$2,125.00
|
SYNLUBE 1711 release agent:
| Volume | Cost |
| 1 aerosol can |
$6.50
|
| 1 quart bulk |
$15.00
|
| 1 gallon bulk |
$55.00
|
| 5 gallon bulk |
$250.00
|
| 55 gallon bulk |
$2,500.00
|
Manufacturing time can be tedious. Depending on the material used, making the mold can take up to twenty-four hours. Also, once the mold is made, making the positives can take at least an hour or more. Due to its lengthy process, molding should not be used to make large quantities of parts.
Stephanie L. Pollice <slp+@andrew.cmu.edu>
Stephen M Wolfe <swolfe+@andrew.cmu.edu>
Zachary A Slaton <zas+@andrew.cmu.edu>
Young R Kim <ykim+@andrew.cmu.edu>
Nathan M Toohey <toohey+@andrew.cmu.edu>