The versatility of plastic optics opens new doors for optical designers who understand the whys and hows of specifications.

As optical design engineers discover the many ways that plastic optics can manage light, applications for the devices continue to grow. Plastic optical elements and systems appear in a wide spectrum of commercial, military and medical applications, including surgical instruments such as laparoscopes, arthroscopes and cystoscopes. They also have found applications in disposable medical devices such as blood analyzers.

Other examples include imaging systems for displays and various kinds of wearable, near to the eye displays. Plastic optics are frequently found in PC peripherals, such as videoconferencing cameras and microscopes and in consumer devices such as compact disc and DVD players.

Plastic optics also appear in display projectors, bar-code scanning devices (both laser-based and image-capture-based systems), biometric security systems, smoke detectors, automated toilet flush valve systems, spectrometers, particle measuring instruments and optical encoders. Plastic optics are useful for certain telecom and datacom products and are commonly used to replicate diffractive optical surfaces.

This wide array of applications results from several key advantages that plastic optics have over competing glass solutions; namely cost, weight and ability to integrate mechanical and optical features. Production economies of scale are possible through the use of multicavity molds. This is especially true for reproducing aspheric and other complex geometric surfaces, which are costly to produce in glass.

For a given volume, plastic optics weigh approximately 2.5 to 5 times less than glass optics and can offer higher impact resistance. Plastic optics also afford designers the opportunity to integrate mechanical and optical features into one element. This is a key advantage if a particular product design imposes space and weight constraints. An excellent example of this is seen in a handheld barcode scanner device (Figure 1). For this imaging-based scanner system, multiple lenses are combined with critical mounting and alignment features into one platform (this is molded at one time in the same shot). Not only is the overall weight of the product reduced, the product designers are able to lower production costs by reducing the number of elements used in the assembly.

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Figure 1. Microscan barcode scanner and polygon mirror. The MS-820 scanner is designed and manufactured by Microscan Systems Inc., Renton, WA.

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These advantages come with challenges, however. No one material, whether it be glass, metal or plastic, can address all optical design problems. Thermoplastic resins present several challenges to the designer, including thermal effects, stress birefringence and the limited range of the index of refraction found in the optical thermoplastics (from approximately 1.49 for PMMA to 1.59 for polystyrene). Ultem polyetherimide has an index of about 1.68, but is probably not suitable for broadband visible use due to its absorption in the blue.

The coefficient of thermal expansion for optical plastics is approximately an order of magnitude greater than that of glass. When a system is designed to operate over a fairly wide temperature range and can be refocused, a plastic optic solution can be employed in the design. If, however, the system cannot be refocused, other techniques must be used to athermalize the system.

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Figure 2. Element design and finished product. The Intersect Laser Level is designed and manufactured by Irwin Industrial Tools.

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Most thermoplastics with optical properties have a fairly low maximum sustained operating temperature, ranging from about 80 °C for polystyrene to about 130 °C for some cyclic olefin polymers (such as Zeonex E-48R). Ultem has a maximum continuous service temperature of approximately 170 °C.

Birefringence is another potential issue when using polymers, which exhibit residual stress — a problem exacerbated by poor part design and/or poor gate location in the mold. Moreover, some optical thermoplastics, such as polycarbonate, have a higher degree of stress in them (i.e., inherent to the polymer itself). Certain grades of Zeonex (COP) have lower stress and may be a better choice where birefringence is an issue. It is wise to involve a competent optical molder as early as possible in the design phase to ensure that these issues are accounted for in the part and tool design.

Although optical glass catalogs offer a wide selection of materials to choose from in terms of index and dispersion, the range of optical-grade plastics is fairly limited. Overall, optical thermoplastics have lower refractive indices than glass. The most commonly used optical molding thermoplastics and their indices of refraction are shown in Table 1.

TABLE 1.
SPECIFICATIONS OF OPTICAL-GRADE PLASTICS

Properties

Acrylic (PMMA)

Polycarbonate       (PC)

Polystryene       (PS)

Cyclic Olefin Copolymer

Cyclic Olefin Polymer

Ultem 1010 (PEI)

Refractive index

NF(486.1 nm) Nd (589.3 nm) Mc (656.3 nm)

1.497 1.491 1.489

1.599 1.585 1.579

1.604 1.590 1.584

1.540 1.530 1.526

1.537 1.530 1.527

1.689 1.682 1.653

Abbe value

57.2

34.0

30.8

58.0

55.8

18.94

Transmission % visible spectrum 3.174 mm thickness

92

85 to 91

87 to 92

92

92

36 to 82

Deflection temperature

3.6 FF/min @ 66 psi 3.6 FF/min @ 264 psi

214 °F/101 °C 198 °F/92 °C

295 °F/146 °C 288 °F/142 °C

230°F/110°C 180 °F/82 °C

266 °F/130 °C 253°F/123 °C

266 °F/130 °C 263 °F/123 °C

410 °F/210 °C 394 °F/201 °C

Maximum continuous service temperature

198 °F 92 °C

255 °F 124 °C

180 °F 82 °C

266 °F 130 °C

266 °F 130 °C

338 °F 170 °C

Water absorption % (in water, 73 °F for 24 hrs)

0.3

0.15

0.2

<0.01

<0.01

0.25

Specific gravity

1.19

1.20

1.06

1.03

1.01

1.27

Hardness

M97

M70

M90

M89

M89

M109

Haze (%)

1 to 2

1 to 2

2 to 3

1 to 2

1 to 2

—

Coefficient of linear exp. cm x 10—5 cm/°C

6.74

6.6 to 7.0

6.0 to 8.0

6.0 to 7.0

6.0 to 7.0

4.7 to 5.6

dN/dT x 10—5 °C

—8.5

—11.8 to —14.3

—12.0

—10.1

—8.0

—

Impact strength (ft-lb/in.) (lzod notch)

0.3 to 0.5

12 to 17

0.35

0.5

0.5

0.60

Key advantages

Scratch resistance Chemical  resistance High Abbe Low dispersion

Impact strength Temperature  resistance

Clarity Lowest cost

High moisture barrier High modulus Good electrical  properties

Low birefringence Chemical resistance Completely  amorphous

Impact  resistance Thermal &  chemical  resistance High index

Like glass, plastic optics can be coated using physical vapor deposition. However, coatings are applied at much lower temperatures on plastic optic substrates. It is possible to specify reflective, antireflective, beamsplitter and conductive coatings for a wide variety of plastic substrates. Antireflective coatings can be multilayer, with an average surface reflectivity of less than 1 percent over a range of 450 to 650 nm; or single-layer MgF₂, with an average surface reflectivity of about 1.5 percent from 450 to 650 nm.

Aluminum, silver and gold metals are used as reflective coatings and can be used to create first- or second-surface mirrors. A standard aluminum coating will provide a surface reflectance greater than 88 percent from 450 to 650 nm, and a gold coating will provide greater than 95 percent from about 700 to 1000 nm. A protective overcoat can be applied to metal coatings to improve scratch resistance.

In addition, it is possible to apply hard coatings to plastic optics. There are several chemistries of hard coating (thermally cured or cured by the application of a UV light source) that can be applied as well as hydrophobic and antifog coatings. These coatings are typically applied in a dipping process and are usually applied in fairly thick layers (>1 μm).

The injection-molding process

Injection molding is a very cost  effective way to reproduce spherical and complex aspheric plastic optics as well as free-form shapes. The effort to produce the optical form is confined to the mold insert. Most inserts are fabricated in hardened steel and are then polished using traditional or deterministic polishing techniques. Inserts can also be plated with nickel and finished using singlepoint diamond turning. Either way, the insert is finished to the negative shape of the final component surface. During the optical molding process, replicating the surface of the insert creates the optical surfaces of the finished element.

All thermoplastics exhibit a shrinkage factor when they cool in the mold. Different for each material, this factor ranges from 0.1 to 0.7 percent. Part design, tool geometry and process considerations affect these shrinkage factors and may require that the toolmaker adjust the mold after completing initial trials. A skilled optical molder will provide assistance in evaluating these issues.

Injection molding produces one or more optical elements per molding cycle through the use of single- or multicavity molds installed in the press. Economies of scale are possible by increasing the number of cavities in the mold.

The optical injection-molding press consists of a fixed and a moving platen, a clamping unit and an injection unit. Plastic pellets fed into the injection unit are plasticized into a molten state and injected into a mold mounted between the fixed and moving platens, which the clamp mechanism holds together. As the material cools and solidifies in the mold, the optic takes on the shape of the insert and cavity detail. After cooling, the mold opens and ejects the optic runner system from which the optics are removed (degating).

Optical injection-molding techniques can reproduce optics with a high degree of repeatability and accuracy. Much of this is due to the precision of the molding press as well as to the precision built into the mold itself. A mold’s construction typically exhibits a tighter set of tolerances than those required of the components it produces. An experienced optical molder, therefore,  should dictate how the mold is to be constructed.

If there is uncertainty about how a part will process, the mold can be built steel-safe. That is, it will be built to smaller dimensions than the nominal final dimensions of the part. This will allow the mold maker to make very fine adjustments once initial molding trials have been performed. Part geometry, part size, the choice of material, the overall mold design, the gate scheme and a host of process issues all play critical roles in the quality of the final product.

TABLE 2.
TOLERANCES FOR STATE-OF-THE-ART
INJECTION-MOLDED POLYMER OPTICS

	Attribute		Tolerance
	Radius of curvature		±0.5%
	Effective focal length		±1.0%
	Thickness		±0.020 mm
	Diameter		±0.020 mm
	Surface figure		≤2 fringes per inch (1λ per inch)
	Surface irregularity		≤2 fringes per inch (1λ per inch)
	Wedge (TIR)		<0.010 mm
	S1 to S2 displacement   across the parting line		<0.020 mm
	Scratch-dig		40-20
	Diameter/thickness ratio		<4:1
	Part-to-part repeatability:   in one cavity of the mold		≤0.5%

                                         Element optimized for injection molding.

                                        • These tolerances are for elements less than 75 mm in diameter.
                                        • Tolerances may vary with part geometry.
                                        • Surface figure and irregularity is expressed in fringes or waves
                                             (where 2 fringes = 1λ) per inch of diameter.

Rules of thumb

Because of the number of variables inherent in the process, it is difficult to speak in general terms about what tolerances are possible. Each job should be approached on a case-by-case basis with a competent optical molder. Nevertheless, the following serves as a starting point for discussion with the optical molder. These tolerances should be considered state of the art (Table 2). As always, the tighter the tolerance demanded, the more costly a part will be.

• A basic specification is the optical form. The ideal plastic optic shape is a nearly uniform wall thickness.

• The overall part design should be as symmetrical as possible to optimize the melt flow in the mold.

• Strong meniscus, biconvex and biconcave shapes should be avoided.

• Extreme variations in part thickness can cause uneven flow characteristics.

• Large, thick or uneven parts may require detailed three-dimensional mold flow and cooling analysis to model how the element will fill in the mold. This exercise usually is undertaken early on in the design cycle.

• A thinner optic presents fewer shrinkage-compensation issues and shorter cycle times, translating into less costly parts. Optics with thicker cross sections not only require increased cycle times to mold, they also present a greater challenge in terms of maintaining a higher toleranced surface figure.

• Because flat surfaces have a tendency to sink as they cool in the mold, one should add a surface of power on both sides of the optic, whenever possible.

Selecting the right optical molder

Injection molding of optics is a complex interaction among the design of the part, the design of the mold tools and the processing of the molds in the press. Designers are well advised to partner with an optical molder who thoroughly understands the engineering issues.  The optical molder should be involved in the process as early on as possible.

It is in the designer’s best interest to visit the vendor’s manufacturing site to check on its capabilities. It is important to realize that the parts produced will be no better than the tools in which they are molded.

However, good tooling alone does not guarantee that good parts will be molded. Complete understanding of the optical molding process is the driving factor in producing precision plastic optics. The optical molding company should have experience with a variety of optical forms and materials.

Finally, it is important that the molder have the metrology capability in-house to perform all of the necessary measurements for the components it manufactures. It is safe to say that you cannot manufacture what you cannot measure.

Injection molding precision plastic optics is a highly specialized discipline requiring a detailed knowledge of optical design, mold construction techniques, state-of-the-art mold processing capability and optical metrology expertise. The design considerations outlined above will provide the designer with the fundamental knowledge required to begin a successful program using precision plastic optics.