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Introduction: (Initial Observation)

What do the following products have in common?
Plastic products, Aluminum Products, gold products, candles, soaps, pills, chocolate.

The answer is they are all made by the molding process. Molding is a process that gives a fine shape to the final product.

Although molding is mostly done using advanced manufacturing machinery, the actual process is so easy that it can be done at home. Molding can be an interesting Science Project that also involves art, craft, and technology. Perform experiments to show all the steps of this process including making the mold itself.


This project guide contains information that you need in order to start your project. If you have any questions or need more support about this project, click on the “Ask Question” button on the top of this page to send me a message.

If you are new in doing science project, click on “How to Start” in the main page. There you will find helpful links that describe different types of science projects, scientific method, variables, hypothesis, graph, abstract and all other general basics that you need to know.

Project advisor

Warning: Working with plaster can dry your hand. Wear goggles and rubber gloves for protection. Adult supervision is required. The experiments can be very messy.

Information Gathering:

Study about molding. Find out how molding is used to replicate sculptures and other objects. Read books, magazines, or ask professionals who might know in order to learn about different materials that may be used to construct a mold. Also find out about material that can be used to cast a replica. Keep track of where you got your information from.

The following are samples of information that you may find.

What is Molding?

Molding is the process of making a replica of a sculpture or an engraved object. Making a mold is the first step of this process where a hollow form or matrix is being made. This hollow form is the exact negative of the original model. The second step is pouring a fluid substance in the mold and let it solidify. This solidified cast will be a replica of the original object.

Molds can be made from plaster, resin (plastic, rubber) and gelatin. Fluid can be plaster, molten metal, liquid resin (plastic, rubber). To ensure easy separation of cast from mold, a lubricant such as mineral oil or silicon oil can be used to cover the mold.

Making a plaster cast is a two step process. First you have to make a hollow mold that imitates the shape of the object you want to duplicate, a mold that has the surface details on its inner surface. Then you have to fill the mold with liquid plaster of Paris and stabilize the mould until the plaster solidifies.

This is what I found about plaster casting from an online encyclopedia:

Plaster Casting as a sculpture process, is of three kinds. One employs a waste mold, another a piece mold (both plaster of paris), and the third a gelatin mold; all reproduce the original clay or wax model executed by the sculptor. The waste mold is chipped away (wasted) to free the hardened cast, which was poured in as liquid plaster. The gelatin mold, being pliable, may with care be sprung from the cast and removed intact and used for replicas. The piece mold also may be used again, being so divided as to be readily drawn away from the undercutting of the cast without damage to either. Plaster casts are used not only for the creation of new sculptures, but also for the numerous replicas of famous marble or stone statues. The ancient Egyptians used models of plaster taken directly from the human body. The Romans cast in plaster many thousands of copies of Greek statues. In another sense of the term, plaster casting refers to the surgical technique of encasing in a plaster-of-Paris cast any part of the body in which bones are broken so that the bones may set smoothly without interference by motion, jarring, or physical shock.

Source: www.Encyclopedia.com

Tiles with varieties of designs are among the products that are generally made by casting cement, concrete or plaster of paris in rubber molds.

Rubber molds are made of two part synthetic resins. When you initially mix the two parts, the result is still liquid so you can pour it over the models. It will then harden in about 20 minutes depending on the type of resin.

Why Use Plaster For Mold Making?

Here are a few points to help you understand the purpose of a plaster mold:

  • Plaster molds are an inexpensive way of making a mold compared to silicon and rubber molds.
  •  Making plaster mold is quick.
  •  Plaster molds are easy to handle and move around.
  •  Last, but not least…plaster mold making is fun!

In this project I will use a plaster mold to make a plaster cast.

Question/ Purpose:

What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.

The purpose of this project is to experiment and learn about the molding process. I will make a mold using plaster of paris and then use that mold to make a plaster cast replica of my original model.

My science project requires a question, defining variables, hypothesis, and research using a scientific method. I can study on the ratio of water/plaster or the temperature of water and find out how they may affect the setting time or hardness of cast.

During my initial experiments I noticed that variations in water temperature will affect setting time and can cause difficulty in the control of mixing time. For example, when I mix cold and warm water, parts of slurry hardens faster than other parts. Because of this observation I decided to study on the following specific question.

How does the temperature of water affect the setting time of plaster slurry?

*Slurry is the other name used for water/plaster mix.

Identify Variables:

When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.

The Independent variable (also known as manipulated variable) is the temperature of water used to make plaster slurry.

The dependent variable is the setting time. Setting time is the time it takes for slurry to solidify.

Controlled variable is outside temperature. I will make sure that the room temperature will not have a sudden change during my experiment.

Constants are the ratio of water/ plaster in slurry. Size of each batch. Type of plaster and water. Container shape, type and size used to make slurry.


Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.

As water temperature increases, the setting time decreases. My hypothesis is based on my observations during initial trials.

Note that the hypothesis does not have to be true. You may correct or reject your hypothesis after performing experiments and analyzing the results.

Experiment Design:

Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”

Experiment 1: Making a plaster mold


1.Decide what you want to replicate. Think about nice objects that can easily be removed from the mold. This can be something that you make yourself from clay or by engraving wood. Or it can be nice objects that you may have at home. I have seen people use fruits such as a banana as their model. For this experiment, I purchased some refrigerator magnets from the local dollar store.

2.Make a wooden frame to surround your fluid plaster mix (slurry) while making your mold. Using a frame is optional; however, it is very helpful in making a strong mold.

3.Determine the best ratio of water and plaster by reading the instructions on the bag of plaster that you purchase. Plaster manufacturers recommend that you always weigh out the materials to achieve what they call the use consistency ratio. That is the best way to get the optimum results from the plaster you’re using. I used 2 parts of plaster and one part of water for my mix. Plaster is always added to water and never the contrary. You should be using a clean bucket and clean tap water at a constant temperature, for mixing plaster. The plaster should be sprinkled slowly and evenly across the entire surface of the water. The time taken to do this will vary between 1 and 3 minutes depending on the size of the batch. If the plaster is sprinkled too quickly, dense and dry lumps can form that will not disappear during mixing. When all the plaster is in the bucket, let the mix soak for one to three minutes before mixing.

What if I can’t weigh the plaster and water?

If you don’t have a scale, use the island method to determine the proper ratio of plaster and water. This is how it works:

Estimate how much water it will take to fill your mold. With your dry hand reach into your plaster bag and scoop up a hand full of plaster. The plaster should feel smooth and free of hard lumps. Wiggle your fingers and shake your hand to make the plaster fall through your fingers into the water in the bucket below. This process prevents large clumps of plaster from falling into the water.

Keep sifting the dry plaster into the water with your hand. Work quickly and spread the plaster all around the surface, evenly.

Do not mix the wet plaster or disturb it in any way. Keep sprinkling plaster into the undisturbed water. Eventually you will notice little islands of plaster forming above the water surface. When the islands stay at that level and get damp and do not sink below the water, you have the proper amount of plaster in the water for general purposes.

Let the mix slake (soak) for one to three minutes before mixing.

4.The soaking time may vary between 1 and 3 minutes, depending on the size of the batch. Sufficient soaking ensures that each plaster crystal is surrounded by water, and that air bubbles can escape from the mixture. If plaster is not allowed to soak for long enough, air bubbles may remain in the finished plaster mold. Soaking for too long gives rise to a shorter setting time, earlier hardening and a grainy surface.

5.The mixture should be stirred vigorously with clean utensils which are free from set plaster, to obtain a homogeneous lump-free consistency. The optimum stirring time is dependent on the size of the batch and the dimensions of the stirring device. If plaster is not stirred for long enough it will not be evenly mixed, and if stirred for too long the mixture will become too thick, in both cases this has a negative effect on product characteristics. Longer stirring times generally lead to: greater strength, reduced absorption capacity and shorter setting time.

6.Prepare your molding table (I used a flat piece of wood), place your frame on the table and decide which items are being used as model for making replicas. Note that you should leave sufficient space between each two pieces if you are making mold of multiple pieces.

7.Decide what lubricant you want to use. I used mineral oil (from my local pharmacy). Lubricant simplifies separation of mold from models and from the table. Optionally apply some lubricant to the table and frame (not too much).

8.Apply lubricant or separating emulsion, in as fine a layer as possible using a brush. Wipe with a cloth afterwards to avoid remnants and structuring. Every model must be coated with lubricant.

Incorrect or excessive use of lubricants may cause mildew, and lead to problems or defects during drying and lifting.

9.Set the models in the frame before your plaster mix is ready. When your mix is ready, start pouring your slurry over the models slowly. Note that if the slurry is too soft, models may rise and float.

10.Continue adding the slurry until all models are covered.

11.After all the models are covered, see if your slurry is still fluid, wait for a few minutes. If it is getting hard, continue adding slurry to the the mold so it will have enough thickness.
Let the plaster stay in the frame for about 30 minutes to harden.

12.Tap the frame gently with a hemmer so the mold can separate from the table. Turn it over and use a knife to remove any excess plaster on or around the models. Tap the models with hammer from different directions and then lift each model and remove it. Also remove the mold from the frame. Mold is fragile at this time and you should be careful not to break it.

13.Plaster moulds must be carefully dried in order to achieve even and optimum physical characteristics. Ideal drying temperatures are: 40-45°C or 105-115ºF. Also use a brush to remove loose plaster and dust from the mold. In room temperature, it will take a few days for mold to dry.

Experiment 2: Making a plaster cast


  1. If you need to keep the molds, it is good to paint it with some oil based paint and let the paint dry for at least 24 hours before proceeding with this experiment.
  2. Use a brush to cover the mold with a thin layer of a lubricant such as silicon oil or mineral oil.
  3. Prepare plaster mix as you did in the previous experiment and fill up each mold with plaster mix that should be like a soft paste at this time.
  4. Use a tapping knife to flatten the top if needed. You can insert magnets at this time if you want to use your replicas as posting magnets.
  5. Insert two tooth picks in about 45º angles so you can use them to lift your cast when it is fully hardened.
  6. Let the plaster set and harden for about 30-60 minutes. Carefully tap the mold and start lifting the casts. Note that you should not tap the plaster mold directly with a hemmer. This can easily brake the mold. You should put a piece of wood on the side of the mold and then tap the wood instead.
  7. Let the casts dry.
  8. Paint the casts using water based or latex paint. Cut or remove toothpicks when they are no more needed for handling the cast.

After you pour the plaster in the mold, do not touch, push or move it in any way. This may delay the drying process and make it hard or impossible to remove the cast from the mold.

This a s sample cast after painting. Plaster casts can be painted with latex or oil base colors.

Experiment 3: Effect of temperature on setting time


Using water at different temperatures, I will make plaster mixes and observe the setting time. I will use clean cups and make sure the same amount of water and plaster is used in each test. Plaster is considered set when it is not fluid and we cannot make a dent on that by pushing the surface using our finger.


  1. Prepare some water at 50º Celsius. To do that use tap water and then adjust the temperature by adding some ice water or some hot water while stirring
  2. Add 50 ml water to a clean Styrofoam cup
  3. Add 100 grams plaster of paris to the water in the cup. Gradually, sprinkle the plaster on the surface of water
  4. Record the time as soon as all plaster is poured out
  5. Let the mix sit for one minute
  6. Stir the mix for 2 minutes
  7. Wait for 5 minutes, then once every minute touch the surface of mix with your finger to determine if it is set or not. If it is set, record the time. If it is not set, wait for one more minute and try again.
  8. Repeat steps 1 to 7 for different water temperatures from 60ºC to 150ºC
  9. Record your results in a table like this:
Water Temperature Setting time in minutes

10.Use your above results table to draw a graph for setting time.

Materials and Equipment:

  • Plaster of paris (from art store, hardware store or construction material distributor)
  • Water
  • Varieties of objects as models
  • Mineral Oil (from pharmacy) or silicon oil from hardware store
  • Brush
  • Styrofoam cups

Results of Experiment (Observation):

Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.


No calculations is required for this experiment.

Summery of Results:

Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.

Learn what you have learned about the process of molding.

You must make a graph that shows how the slurry temperature effects setting time. Following is a sample graph. Please note that your graph may be different depending on your results.

Water temperature variations impact slurry temperature immediately. Since compressive strength of set plaster increases as setting time decreases, a controlled-set condition resulting from using water at a uniform temperature produces the best gypsum mold or cast.


Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Possible Errors:

If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.

If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.