Plastic Injection Molding

Injection Molding vs CNC Machining

When moving from design to creating your parts, you must decide how to make the parts. Two common methods are CNC machining and injection molding. Both of these have advantages and disadvantages when choosing between them. Which is the Right Choice?

Let’s look at each process:
CNC Machining is a process used in the manufacturing sector that involves the use of computers to control machine tools. Tools that can be controlled in this manner include lathes, mills, routers and grinders. The CNC in CNC Machining stands for Computer Numerical Control.

Injection molding process produces large numbers of parts of high quality with great accuracy, very quickly. Plastic material in the form of granules is melted until soft enough to be injected under pressure to fill a mould. The result is that the shape is exactly copied.

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.

Volume:
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 of the mold can be a large upfront cost. This is 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.
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.

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.

For injection molding, the material selection can be more limited.

Tolerance / Surface Finish:
The advantage of 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:
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. You need consider the advantages of both from tolerances, price and design features to perform the final desicion.

Jingwei industry has served the injection molding and CNC machining industry for over 15 years and has a team dedicated to providing you with the most cost effective solutions for design and manufacturing your new products.To know more,Please visit us at :jweimolding.com or via info@jweimolding.com.

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