November 25, 2022

Injection Molding of Optical Components

The use of precision polymer optics is becoming a necessity for increasingly sophisticated products. Polymer optics is an important technology that facilitates the successful development of many different devices, as well as optical components with complex geometric surfaces.

Injection molding of optical components is one of the most efficient mass production technologies for manufacturing polymers with high precision and in complex and detailed geometries.^[1] The manufacturing process can still be a challenge, however, since optical components require high quality and high replication degree with precise surface contours.

Injection Molding Process

Injection molding is a common way to manufacture polymer optics, especially in large quantities.

The Mold

The mold that’s used to manufacture polymer optics has three key features: cavity details, optical inserts, and housing for both. The mold is a negative impression of the final component, which may impact what the design can include.

Thermoplastics shrink as they cool, roughly by 0.5 to 0.6%. This is important in determining the final dimensions of the mold. The material that is used for the optics must be injected in its molten state, and a clamp holds the two halves together. Once the injected material cools with the shape of the mold and the cavity details, the mold can be opened, and the optic can be removed.

The Molding Machine

The optical injection molding machine, also known as a press, has both a fixed and a moving platen, a damping unit, and an injection unit. The mold is put into the press with one half mounted to the fixed platen, and the other half mounted to the moving platen.

The thermoplastic pellets are then plasticized into a molten state and injected into the mold. The clamp holds the two mold halves together during the injection process. Once the polymer cools and solidifies in the mold, the material takes the shape of the insert and cavity details to create the final product.

After cooling, the mold is opened and ejects the finished optic product, which is attached to the runner system.

The Process

The combination of equipment used for injection molding is complex, with multiple variables and control parameters. Without a robust process in place, even the most precise mold and operators will experience drift, which may be from changes in ambient conditions, tool wear, and other fluctuations. This is why it’s vital that the manufacturer be skilled in scientific molding techniques.

Optical injection molding can reproduce optics with accuracy and repeatability. This is largely due to the precision of the molding machine and the precision of the mold itself. An experienced optical molder is necessary to ensure that the mold has a tighter set of tolerances than what’s expected of the components it will produce.

Geometry, size, material, mold design, and process issues can dramatically impact the final product’s quality.

Considerations for Injection Molding

There are several questions to consider when injection molding, including:

The environment in which the optic will be used, such as extreme temperatures or humidity
The manufacturing timeline
The prototyping
The total quantity of optical components intended for manufacturing

Several design details or limitations can impact the injection molding process. For example, thinner optics have fewer shrinkage compensation issues, and shorter cycle times, so they’re less costly overall. Uneven flow can also occur if there are variations in the component’s thickness. A nearly uniform wall thickness is ideal.

Advantages of Injection Molding Optics

Injection molding has several advantages, including:^[2]

Complex Geometries with Tight Tolerances

Injection molding allows for high volumes of uniform, complex parts. It’s important to pay attention to corner transitions, weld lines, wall thickness, and more to achieve precision parts. Injection mold can easily achieve repeatable part tolerances of 0.500 mm, and in some cases, up to 0.125 mm, which produces parts that are accurate enough for most applications.

High Compatibility

There are over 25,000 engineered materials that are compatible with injection molding, including thermosets, resins, thermoplastics, and silicones. With all of these options, it’s possible to achieve the optimal balance of physical, mechanical, and chemical properties for a final product.

Some of the commonly used materials include:

Polystyrene
Polyethylene
Acrylonitrile butadiene styrene
Polypropylene

Materials may be combined to produce a part with the strength, impact resistance, and stiffness required of the project, such as adding glass fibers to thermoplastic to create a durable composite. There are numerous color options as well.

Efficiency

3D printing or CNC machining a single part can take minutes – or hours – to complete. Injection molding is a fast process that takes between 10 and 60 seconds. If you have complex geometry, it could take around 120 seconds to mold, but you can include small parts in one larger mold to achieve the desired result.

This efficiency allows hundreds of identical components in an hour at a low cost, creating new possibilities for low-cost, mass-produced optical components with a rapid time to market.

Repeatability

Polymer injection molding has high repeatability. Once a mold is created and refined, it can be used to produce thousands of identical components before it needs maintenance. An aluminum mold can last between 5,000 and 10,000 cycles, while a full-scale steel production mold can last around 100,000 cycles.

Reusability

Injection molding generates less post-production waste than other manufacturing processes, but there will be excess. Because injection molds rely on molten thermoplastics, the excess can be ground, melted, and reused to save on material costs.

Applications for Injection Molding

Any application that requires an optical component is a candidate for polymer optics. Polymer optics are a key component in grocery store bar-code scanners, LIDAR, and medical applications, including surgical retractors.

Laboratories often use polymer optics for spectrometers, imaging equipment, and cleanroom particle counters. Telecommunications products that require microstructured surfaces like microlens arrays and diffractive optical elements require polymer optics.

Other examples of polymer optics include LED illumination applications, imaging systems, PC peripherals, video conferencing cameras, microscopes, and consumer electronics like smartphones, CD players, and DVD players.

With many of these applications, the key advantages come from the properties of polymer optics over glass. The advantages directly correlate to the possibilities with the injection molding process and its use of molten thermoplastics for complex designs, which isn’t achievable with glass.