AY Honors/Plastics/Answer Key

1. Make a list of the materials and equipment used in making small castings in plastic molds.

Plastic resin
Catalyst
Eye dropper (for adding the catalyst)
Plastic disposable cup
Craft stick (for stirring)
A plastic mold
An embeddable object
Newspapers
Goggles, gloves, and a smock (or old clothes)
Mold release

2. Know how to clean and properly take care of plastic molds.

Be careful not to scratch your molds, as even a small scratch will be transferred to the cast. Once the resin has cured and the cast has been popped out, the mold can simply be wiped clean. You can also use acetone for cleaning up, but be forewarned - acetone is flammable and (like plastic casting in general) should only be used in a well-ventilated area, well away from an open flame (such as a water heater, furnace, pilot light, or gas stove).

Store your molds in a separate container by themselves when not in use. Do not store them with other objects such as tools, as these can easily scratch the molds.

3. What safety precautions should be used when working with plastics?

Go over these safety rules with your Pathfinders before opening any chemicals.
Wear protective gear, including safety goggles, gloves, and an apron or smock.
Read and heed all manufacturer warnings.
- Know what to do if the chemicals come into contact with a person (skin, eyes, or if ingested) before opening them.
Use only in a well-ventilated area, preferably outside.
Use only under adult supervision
Do not pour resin down a drain. It may cause an expensive clog.

4. Tell how to mix resin for casting and colored layers.

Pour the correct volume of resin into a plastic cup. Do not use a waxed paper cup for this, as the wax will ruin the resin. Then use an eye dropper to add the correct amount of catalyst. Read the directions to see how much catalyst you need for the amount of resin you are using. Thoroughly mix the resin and the catalyst with a craft stick by using a cutting motion - this will minimize the introduction of air bubbles into the resin (which you will want to avoid as much as possible).

Resin can be mixed with either pigments or dyes made for that purpose. Pigments will create opaque plastic, and dyes will create transparent plastic. Only use dyes and pigments formulated for the exact type of resin you are using. Opaque plastic is generally limited to the base layer only - otherwise, the embedded object will obviously not be seen.

5. Why are the following used?

a. Catalyst

The catalyst causes the liquid resin to undergo polymerization.

b. Surface hardener

The resin that comes into contact with the mold will cure to a hard, crystal clear finish. However, the exposed surface (i.e., the top of the mold) may not. The last layer of resin should be mixed with a surface hardening agent so that it will cure completely.

c. Pigments

Pigments are used to color the plastic to add interest to the casting.

6. What is meant by polymerization?

Polymerization is a chemical reaction in which monomers are assembled to form long chains or three-dimensional networks. In short, this is the chemical process that converts the liquid resin into the solid plastic.

7. Prepare and embed two nature items suitable for embedding. Nature items may be embedded in one or more castings.

The first step is to choose the nature items to be embedded. Popular choices include dead insects, leaves, flowers, rocks, seeds, nuts, and shells. Once the items have been selected, it is time to prepare a work area.

Cover the work surface with newspapers or plastic bags. You can also use an old pizza box or a large piece of cardboard as a work surface. Once this has been done, put on your protective gear (goggles, apron, and gloves).

Fill your mold with water, and then dump the water into a measuring cup. This will tell you exactly how much resin you will need, though you will want to divide that up to form multiple layers. For example, if your mold has a volume of six ounces and you want to pour two layers, you will need to mix three ounces in the first batch, and three more in the second.

Carefully dry the mold and examine it for imperfections (such as scratches). Any imperfections in the mold will be transferred to the cast item. If you find an imperfection, your only options are to either live with them, or use a different mold. Once you are satisfied with the mold, spray it with mold release. Some people do not use mold release in order to save time, or money, but this is a false economy. Mold release greatly increases the number of times you can reuse the mold, and the time it takes to spray it into the mold is a tiny fraction of the amount of time it will take you to get the casting out of the mold once it cures. Hence, we highly recommend using mold release.

Mix the resin in accordance with the general instructions given in requirement 4, but be sure to read the mixing instructions that came with the resin for specifics.

For a two layer casting, fill the mold with resin to the half way point. Then let the resin cure for about 25 minutes so that it forms a gel. You make probe the resin with a craft stick to make sure it's the right consistency, but do not use your finger for this. Doing so risks spoiling the resin.

After the first layer has gelled, carefully place the embeddable object (or objects) on the surface of the resin. You may wish to do this with tweezers. Then mix a second batch of resin, noting the instructions on the container. The second (and successive batches) of resin usually has a different ratio of resin to catalyst. Once it is properly mixed, pour it over the embedded object. Place the mold some place where it will be out of the way for the next 24 hours and let it cure for at least that amount of time. After 24 hours, you should be able to pop the cast out of the mold (you did remember to use mold release, didn't you?).

Place a very fine grade sand paper on a flat surface (a piece of plate glass works wonderfully), and then sand any rough spots on the cast item by gently rubbing it over the sand paper. Then buff it with a soft cloth.

8. Make three castings with at least one item embedded in each casting.

These embeddables do not need to be nature items. You can cast any small item you like. Many people cast coins or small toys. Follow the instructions given in requirement 7.

References