Low Volume Injection Molding

The predominant method of making high-volume, low-cost components is injection molding. There are simply no other technologies that are capable of making parts as accurately, quickly or affordably. Injection molding is usually associated with plastic components; however, it can even be used to rapidly make metal components like gears, housings, linkages, etc.

With the power of injection molding, there is one trade of: tooling is expensive and complex. The cheapest mass run CNC machined molds start around $10k and can balloon from there. Add in small detail features or multiple injection points and the tools can become huge investments. The injection machine is also an expensive, highly specialized piece of kit. All this means that it is hard for a start up to take advantage of this production technique.

What we want to talk about today is how injection molding works, and how we could build some prototype tooling to test a proof of concept, or even just make a small batch of parts. In other words, how can we bring injection molding to the small guys!

What is Injection Molding:

Injection molding is the process of injecting a molten material into a cavity, or die, leading to the molten material solidifying as a physical object. The process is most known for making complex plastic pieces quickly and cheaply. Think plastic cutlery, clam shell assemblies for electronics packaging, textured automotive interior pieces, etc. Basically, if the part is plastic, it was likely injection molded. Metal pieces are less common, but most low-cost gear trains or mechanical cases in power tools are made using injection molding.

The core machine used in injection molding needs to take solid plastic pellets, melt them, remove all air, and pressurize the molten plastic for injection into a machined mold. The plastic then flows through various paths, called sprues, which feed the part cavity with the molten material. By changing the size, shape and quantity of voids to fill, one can alter batching, material requirements and many other facets of the design.

Why is Injection Molding so hard:

The process of injecting a molten plastic or metal into a die, letting it cool, then removing the finished product from the machine seems simple. In reality though, maintaining the ideal pressure to inject the material at, proper flow channels to successfully fill the dies, and timing part cool down for removal is an exacting science.

The material the dies are made from also have to be able to withstand the extreme pressure and temperature over and over again. For instance, injecting molten aluminum to rapidly cast a gear, would require an advanced, and incredibly robust tool steel die. The geometry of the die is equally critical, as any material which solidifies before filling a part feature can block the material pathway, leading to quality issues.

Improper die design can lead to improperly filled parts and even catastrophic die failure. Due to the heat and pressure requirements, these dies often crack at stress risers. Unnoticed cracks propagate until the die is entirely destroyed.

Improper process control can also lead to unfilled parts or warped parts. Pulling a part too quickly that hasn’t cooled often results in wavy parts. Too small of an injection pin could punch a hole in the part without even removing it properly. Inadequate heating of the material will not even make a part.

Getting started in injection molding:

Often times in product development, parts are selected and designed around injection molding. It is a logical way to provide a long-term, high-volume path for product manufacturing. Unfortunately, that can make it incredibly difficult to build a prototype that is truly representative of the final product. It also makes it difficult to start on a small scale, as a $45k injection tool will hang around a young company’s expense line, delaying profitability.

Unfortunately, this tool development cost will have to be paid every time a design change too. This means it is best to only move to injection molding once the part design is finalized for mass manufacturing.

What can we do to help:

While we see the need for injection molding later on, we like to develop prototypes which have other avenues to be manufactured. 3D printing is often used; however, we find it rarely translates one-for-one from prototype to finished good. So, for our first prototype we might 3D print parts, but after that we like to use urethane casting. This can give the feel, strength and repeatability of injection molding, without the excessive tooling cost. It also serves as a great test to see what parts of a design aren’t ready for molding.

We have also developed some injection molding tools using a high strength 3D printed plastic to make a low volume/test mold. This allows us to again test a part for manufacture before sending a client to invest huge resources in a process that might never work for them.

Both the urethane casting and plastic injection molding tools reduce cost at the expense of long-term usability. They also have much longer cycle times. This means these techniques are great for prototyping, but will not work for long term, mass production.

Conclusion:

As you can see injection molding is a critical technology for many products. It, like many, also is surrounded by pitfalls. We strive to help our clients avoid these errors and allow them to start selling products from a sound footing.

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