Introduction: Plastic Molds- an Intro

OK- Let me try this again. I was posting with specific reference to the full belley project (Peanut Sheller) but apparently there is substantial interest in plastic molds.

Injection molding comes in several different varieties,but can basically be described as forcing liquid plastic into a metal mold under high pressure. Most injection molding is done with high temperature molten plastic, and then it quickly cools inside the mold. The part is taken out of the mold, and then the process is repeated. The benefit of this technology is that thousands or millions of components can be made very quickly, and inexpensively. The downside of this technology is that the mold is usually costly. (lots of variables here- design life of mold, cycle time, etc) Usually you would use this technology to make thousands, or tens of thousands of parts- this makes the cost of the mold very small on a per part basis.

There is another technology called RIM (reaction injection molding). These molds are less costly than the other, and in this process, cool liquid plastic is injected into the mold, where it sets up chemically. (like epoxy or fiberglass, only a polymer plastic)

To begin along either one of these paths, you need to start with a product that is designed with molding in mind as the production process. There are many product details that may make a part easy or difficult to mold.

If you have a basic product concept or design, then you need someone to develop it into a full 3D CAD model. I have attached an example CAD model for you to see.

Beyond this point, the costs and direction you should take will vary widely, depending on the size and nature of your product.

Dwayne Esterline

GI Engineering Co.


I was browsing previous comments, and thought I would give my 2 cents.

If there is really interest or demand for significant quantities of molds, I can help with developing an injection mold and producing plastic versions of the fiberglass molds.

There are various high strength polymers available, but I bet we could produce mold sets for under $10 per set. We would have the capability of producing them in the hundreds or thousands or tens of thousands. High quantities would be even less expensive.

For comparison, think of the 5 gallon plastic buckets and plastic lawn chairs available. The manufacturing process is the same, and the equipment is readily available where I live.

Any thoghts?

Step 1: Develop a 3D CAD Model

The first step for injection molding is to develop a 3D CAD model of your product. (Computer model)

In this process, a designer takes your concepts, sketches or prototype, evaluates it for moldability, and then draws it inside of a computer environment.

Why? In modern mold manufacturing, all of the machine equipment is driven by computers. Your design can be evaluated by computer and the mold will be cut by computer operated machining centers.

Through this process, an experienced designer will advise you on the changes that may be needed in order to make the molding process as inexpensive as possible.

A good CAD designer can work from your sketches or drawing, or even a rough prototype.

Many companies can provide this service. (Do an internet search for product designers)

I have been designing tools, equipment, and manufacturing processes for many years. I use SolidWorks and AutoCAD design software.

You may learn more about my company on our website

Dwayne Esterline

Step 2: Manufacturing Process Selection

The next step is to select the most economical manufacturing process.

Every manufacturing process has strengths and weaknesses. The objective is to find the right balance between precision, speed of production, and cost.

Simply put, faster production and greater precision generally equal higher cost. (as a general rule)

To select the process that best fits your project, you need to establish some clear objectives.

Answer the following questions:

1. How many parts am I going to make? 10's, 100's, 1,000's, 10,000's, 100,000's etc

2. What production rate will I need? (how many parts per week, da, hour, year, etc)

3. What material do I need to make it from? (What kind of plastic? You may need help from a pro here)

4. What is my target cost per component?

5. How precise are the part tolerances?

6. How large is the component?

7. What kind of features are on the part? (Holes, snaps, thin ribs, textures??)

After you answer the questions above, the answers will help guide you in selecting your manufacturing process.

For the sake of this instructable, let's assume that you would like to make a simple part that is about 4 inches square.

The part is a consumer item, made from ABS plastic, we would like it to cost less than $10 per piece, and we want to make 1,000 per year, and we expect to make a total of 10,000 parts.

Dwayne Esterline