CNC Machining vs. Injection Molding, Which is the Right Choice?

 

When moving from design to creating your parts, you must decide how to make the parts. It the best case, you had already considered this during the design process and your design is already optimized for one of the processes. Keep in mind, during the development life cycle; the right choice might actually change. Two common methods are CNC machining and injection molding. Both of these have advantages and disadvantages when choosing between them.

 

First let’s look at each process.

CNC Machining:

A process that typically starts with a block of the desired material, CNC machining involves precisely removing material layers at a time. This process allows for many different materials to be used with few limitations. Also, because it is computer controlled, very precise tolerances can be achieved.

Injection molding:

As the name suggests, this process involves injecting material into a mold. This is done by melting the stock material and forcing it into mold at high pressure. The part is then allowed to cool in the mold and then is ejected from the mold. The process is repeated for the next part.

Which process is right for you?

In general, this can be looked at as a trade off between multiple of different characteristics; speed, volume, material, tolerances/surface finish, and design. Each on of these can be a gating facture in using one process vs. the other and might even require changes to the part in order to manufacture.

 

Speed:
This is the simplest. For low volume of parts, CNC machining is the fastest. If you need 10 parts in 2 weeks, CNC machining is probably your only solution. If you need 50,000 parts in 4 months, injection molding is the way to go. Injection molding requires time to make the mold and ensure the parts are in tolerance. This can take anywhere from a few weeks to a few months. Once this is done, creating parts using the mold is a very fast process. The up front time investment of injection molding will pay off at high volumes.

 

Volume:
Let’s look at this independent of time. Instead, the main focus here is price per part. Which is cheaper changes depending on volume. CNC is cheaper when you need a few parts up to a few hundred. While you do get some benefit of volume in the price, it is typically seen between a few parts and a hundred. At larger volumes, you do not get any additional benefit of volume. This is from distributing the setup cost over the number of parts made.

 

The actual cost per part of injection molded parts is significantly cheaper that machined parts. Unfortunately, for injection molding, the creating the mold can be a large upfront cost. This is than spread out across the number of parts made to determine the cost per part. At a certain volume, even with the large upfront cost of the molds, injected parts become cheaper than machined. As more parts are created, the gap in cost becomes greater. This crossover can occur from as little as 100 parts up to about 5000.

 

 

Material:
CNC machining offers a greater selection of materials that can be used to create the parts. This can be very important deciding factor if a high performance plastic or a specific plastic is required. In general, harder plastics are easier to machine than softer materials

 

For injected molded components, the material selection can be more limited. More and more high performing materials are being created with injection molding in mind. Solvay is one of the companies that has a good selection of moldable plastics. There are some materials which would be impossible to machine but you can mold. Rubbers and other flexible materials would fall into this category.

 

Tolerance / Surface Finish:
Once again, the advantage goes to CNC machining. With most materials, a tighter tolerance can be held and a better surface finish can be obtained. One advantage of injection molding is the repeatability from lot to lot. Molds can last for millions of parts with minimal wear. This allows for the parts from one batch to the next to be almost identical. With CNC machining, each part is put on the machine to be created and more variability will be seen.

 

Design:
First question should be; is the design going to change? If so, then you might not want to spend the money to make a mold yet. Molds are difficult and many times impossible to modify for changes in design.

 

But even before that, designing a part for machining can be much different than for molding. In both cases, one process is able to make features that would be difficult or impossible to create using the other process.

 

Machining allows for great flexibility in the design. Many features can be easily made with machining that would be very difficult or cost prohibited in molding. Some of these include, overhangs, large walls with no drafts and variable wall thicknesses.

 

On the other hand, injection molding allows for deep features, square holes and living hinges to be produced easily that could be difficult and expensive with machining.

 

As you can see, the decision between machining vs. injection molding is not straightforward. If you need some of the advantages of both from tolerances, price and design features, you might want to consider a two step operation where the main features of the part is injection molded and secondary machining options are done to perform the final steps. This is common technique to blend the benefits of the two processes together.

 

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