Material Matters: Improving Plastic Optics Performance at Every Step

Posted by The RPO Team on Sep 25, 2017 9:08:00 AM

An interview with Robert Mendenhall, materials scientist and Director of Sales and Business Development with Guernsey Coating.

In talking with RPO’s optical design and manufacturing engineers, Robert Mendenhall shared some common challenges Guernsey Coating faces with adhesion to plastic optics, and tips to prevent loss and improve quality.

What are the most common challenges you see in coating plastic optics?

Every step of the optical molding process impacts end quality and yield:

  1. Material selection
  2. Material handling, even before it’s melted
  3. How the mold is made
  4. How the mold is treated
  5. How the mold is maintained
  6. How the mold is cooled
  7. Part handling after molding

It starts at material selection. We always recommend that customers choose a plastic substrate vendor that understands optics. Many consumer electronic and medical device product designers are new to optical engineering, and they turn to major plastics suppliers who are accustomed to design and production of generic plastic parts, not plastic optics. Most generic plastic parts focus on mechanical rather than optical properties.

For example, acrylic. There are a thousand different types of acrylic, each with different properties. Some will absorb more or less water just by sitting out. The differences within each material category can be significant in terms of optical performance and yield, even if their mechanical properties are similar. Your optical molder would be much more experienced in choosing the best material within each type of plastic.

Material handling also has a tremendous impact. If, for example, a company samples plastic in the US, then orders production volumes from the factory in China, the properties can change over time, from lot to lot. Even the time delay due to shipping from overseas can impact the quality of the coating due to moisture absorption over the longer period of time between molding and coating. Treat materials as living things.

Contamination impacts adhesion to plastic optics, which impacts overall plastic optics yield, performance and lifetime. Oil, water, dirt and gas are common contaminants. Many molds are porous, allowing gases to leach into the plastic optic. In fact, gas from the mold itself can leach into the plastic, which leaches out over time.  Most molding operations are not carried out in a clean room environment. The molds can easily be contaminated, which in turn can contaminate optical parts.

Mold maintenance and molding practices also are a common cause of contamination. The use of a mold release is not always correct and with some manufacturers this can vary widely from shift to shift. It may not be noticed until it's too late.

Injection molders who do not specialize in plastic optics often negatively impact quality in molding, polishing, and inspection. Many people in the industry doing plastic optics are not optics experts. We often see a lack of awareness regarding the handling of substrates.Touching the surface of an optical part without knowing how to properly handle the substrate can lead to scratches, rag marks and even fingerprints. The upside? It is easy to identify where the defects came from.

Expertise in how plastic optics perform at low temperature is also critical to end quality. The higher the temperature, the better the adhesion, but in plastics, we have to be careful with high temperatures. Guernsey Coating has worked to address how to achieve superior adhesion at low temperatures.

What are the top considerations when talking with your plastic optics manufacturer and optical coater?

Address the application for the optical part first. For example, the quality of the optic itself matters far more in a non-mirrored optical element such as a beamsplitter where light will be passing through the plastic substrate. The plastic has a job in the end performance, as the light passes through the substrate. This will change over time, depending on how it's molded. The first part may differ from the last, both in striations and index changes.

A good optical partner will understand the requirements for:

  • End usage
  • Tolerances
  • Price sensitivity

There are tradeoffs in:

  • Variability
  • Yield
  • Coating performance
  • Lifetime
  • Environmental performance

What are you seeing a high demand for in plastic optics?

Certainly a lot of work in VR/AR, primarily at the R&D stage right now. Toys are increasingly utilizing optics, and LED/lighting, driven by governmental regulations targeting efficiency gains.

And the biggest challenge in optics right now?

A disconnect between performance and cost requirements, especially in lighting. It’s a non-linear function in price and performance. An efficiency gain in lighting from 85% to 95% is far more than a 10% increase in cost, so a good optical partner will explore your performance and pricing options with you to strike the right balance. There are also big difficulties maintaining the same coating performance between glass and plastic substrates. Most customers don’t want to suffer the cost and time delays due to the development of coatings for plastics, yet the expectation is that the coatings will have the same performance regardless of the substrate changes. Unfortunately, coatings are not considered as much as they should be during the optical design process. The earlier coaters are brought into the design process, the better the overall product cost, performance and lead time becomes.

To learn more about RPO’s plastic optics capabilities, download our free design checklist:

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To learn more about Guernsey Coating, visit  http://www.guernseycoating.com or call 805-642-1508

Topics: Engineering & Design, Plastic Optics