Optics manufacturing & prototyping

Downsides of injection molding optics?

By 2 September 2020 No Comments
injection molded custom optics

There are many optics manufacturing methods, such as CNC machining, diamond turning and injection molding. Injection molding optics does seem like a perfect method, but what are its boundaries and limits? In this article we discover the pro’s and con’s of injection molding optics.

Injection molding optics: the process

Injection molding of optics is a common way to manufacture polymer optics in larger volume. With injection molding it is possible to have spherical, aspheric and freeform surfaces in your optical design.

The mold

The mold that is being used to manufacture the polymer optics has three features, namely the cavity details, the optical inserts, and the housing that holds both the inserts and cavities. The mold is shaped like the negative of the final part. This might impact what you can and cannot do in your design, more on that later.

During the molding process all thermoplastics shrink as they cool. General shrinkage is approximately 0.5 to 0.6%. This should be taken into consideration when determining the final dimensions for the mold.

The material that is being used for the optics is being injected into the mold in its molten state. The clamp mechanism holds the two halves together during the process. Then the injected material cools off and takes the shape of the insert and cavity details, whereby the final optic is created. After cooling, the mold can be opened, and the optic can be removed.

The injection molding process is complex and requires a combination of variables and control parameters. As such, injection molding of optics offers a high degree of accuracy and repeatability. This is thanks to the precision of the press and also the precision built into the mold.

Injection molding your optics?

If you want to have your optics injection molded it is important to take a couple of questions into consideration:

  • In which environment will the optic be used? Will it have to endure extreme temperatures or extreme humidity?
  • When will you need the parts?
  • Will you require prototypes?
  • How many parts will you need?

Some things to keep in mind when choosing injection molding as an optics manufacturing method:

  • Uneven flow characteristics can occur due to extreme variations in part thickness.
  • Thinner optics have less shrinkage-compensation issues and shorter cycle times, which are less costly.
  • A nearly uniform wall thickness is ideal for injection molding.
  • Whenever possible you should add a surface of power on both sides of the optic to prevent flat surfaces from sinking while cooling down.
  • Optics that have thick cross sections will require increased cycle times to mold. They will also present a challenge when it comes to surface figure accuracy.
  • In order to optimize melt flow, it is ideal if the part is mostly symmetrical. As such biconcave, biconvex and meniscus shapes should be avoided as much as possible.

Pro’s of injection molding optics

Traditional injection molding has some advantages for optics manufacturing:

  • High volume series production
  • High precision to cost ratio
  • Highly automated process, low labor costs
  • Integration of optical and mechanical functionality
  • Good product consistency

Con’s of injection molding optics

However, there are some downsides as well:

  • High tooling costs
  • Long lead times
  • Part design restrictions
  • Small batch production can be costly

Let’s take a closer look at these disadvantages of injection molding.

High tooling costs & long lead times

The upfront costs are high due to the design, testing and required tooling. There is the initial design and prototyping (sometimes done through CNC machining), followed by the design of the prototype mold. Only after extensive testing is it possible to injection mold the optical part. This extensive process slows down the project, resulting in longer lead times.

Part design restrictions

Optical parts will need to be designed with injection molding restrictions. In general these include:

  • Avoiding undercuts
  • Avoiding sharp edges
  • Using uniform wall thickness

Some tools are made from aluminum or steel. So taking material away in order to make a larger cavity is no problem. However, adding material back is a challenge and might lead to scrapping (part of) the tool and starting over.

Small batch production will be costly

Setup times for injection molding can be quite lengthy. This is due to the complexity of the tooling and the necessity to get rid of previous material in the machine before starting the next batch production. As such, small batch production has always seemed too expensive to do with injection molding.

Is there an alternative?

However, there is another way…

With Addoptics’ unique process we are able to cost effectively create your required amount of optical parts, whether they are prototypes or finished products. There is no minimum order quantity. There is also no high upfront investment or high tooling costs. Our manufacturing process focuses on precision, high-grade quality material and speed. We can deliver your polymer optics to you within days. This is partly due to our manufacturing method being able to help with both prototyping and manufacturing of your optical parts.

Discover Addoptics’ Smart Manufacturing as a Service:

  • Cost-effective testing of optical designs
  • Full design freedom
  • Iterative process
  • Stay ahead of the competition with rapid prototyping
  • Product delivery in days instead of weeks

Ready to start prototyping with us? Reach out to us to discuss your project requirements. We’d be happy to hear from you!

Sources

Leave a Reply