Plastic Molds- an Intro




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



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    23 Discussions

    I have a product I would like to mold myself and add some changes to original design to make it more durable. Is there instruction on how to infuse pieces of metal into plastic mold for extra strength? Also what type of plastic can be used for a very high hardness level? Any help would be appreciated. Does the 150 injection machine handle plastics that have a high hardness?

    How about a relatively inexpensive system that can take plastic bottles(PET) and regrind them to be molded into useful item items in the third world???? the alternative now being they fill the dumps end up in the lakes and streams.... We can have crudely ground PET for about 8 cents (US) a pound perhaps less....

    Hi DEsterline,

    Could you please let me know more details? We produce custom-developed injection mold China.

    You can check our website,

    our injection mold More Details.

    If you really have the interest or demand a significant amount of mold, I can help develop the production of plastic injection molds and fiberglass mold release.


    I just started using solidworks, and bought the SolidWorks 2011 book which covers beginner to advanced topics, and is very comprehensive. I often use the book as a reference to go through.
    You can get it @

    Hi DEsterline, Typically, this is the type of thing that would get removed because it has no instruction. However, it looks like there's a fair amount of interest in plastics molds. How about you use this as a place holder for publishing an Instructable on making plastic molds (maybe unpublish this one, edit it, and republish?) We're working on tools that will make this sort of conversation easier without having to post it in an Instructable.

    4 replies

    Hi Boss, I started making my own injection molds about 25 years ago. They are made specifically to run on my injection molding machine. The process of making a mold is deceptively simple. Machining is just one aspect of this process resulting in a usable part. It is a fusion of design relating to function and cost effectiveness. Designing the mold is related to designing the part, as all parts must be designed to be manufacture-able. I may be interested in writing a series of articles relating to the topic with the novice in mind. I learned how to make a mold from a fellow mold-maker, ...referred by the manufacturer of the molding machine, followed with a thorough reading of a book entitled: 107 Injection Molds. The book looked at 107 different problems and mold design solutions. All molds are not alike, and while some are hand placed and pried open, others are just slightly less complex than a submarine. The temperatures and pressures involved are dangerous and a mold could potentially explode if not correctly designed. The question here really becomes... what part to design and instructable about? Any thoughts? Keep it small, about the size of a lighter and I might be interested in molding a few parts should anyone actually complete a mold. -Frank


    Frank, I would love to know where you are located as I have some ideas that I could use some help with.


    Hi Cat,

    I'm just south of Tumwater Washington USA.

    Looking forward to your project.


    Sorry if I misused the instructables forum. This was my first post, and am unsure of what content is acceptable. Yes, I would be glad to publish an instructable on making plastic molds. Dwayne Esterline

    As you can see if you look up my "bite-valve hydration system" posting, I have a long-term interest in learning injection molding. I'm sure I can make molds myself, as I have access to a machine shop, but I guess that my design will have to go through several iterations, as I have only made my part on the lathe so far. I have access to 3D CAD, but could use a pointer to a "how to design simple injection-molded parts." Also, I'd like to either find an injection molding company here in the SF Bay Area of California, or find a vendor who will allow me to try several small runs of a design once given a deposit on a larger order. Do you have any advice in this regard? TIA.

    In the example for the plastic molding process that didn't require high pressure molds, the only items listed are soft plastic products. Is it possible to make products with a more rigid harder plastic as well? Like the material keyboards are made or mouses, monitors, laptops etc?

    1 reply

    How about a simple 3-4 part assembly instructable?

    Sure, you can contact me at I would be glad to give any help I can. Dwayne Esterline

    I'm interested. I have a project that I didn't know how to get moving. More information welcome.

    1 reply

    What type of project are you considering? -Dwayne

    Yes, I would be glad to help. To develop a set of injection molds for any project, you need to start with a good set of product drawings. Have you developed any drawings?

    There really needs to be forums here..