Specifying Injection-Molded Plastic Optics
William S. Beich
Plastic optics have benefits and drawbacks, and a set of material
properties that requires entirely different specifications. Learn them before you
The use of plastic optics continues
to grow in popularity as optical design engineers discover the many ways these components
can manage light. Plastic optical elements and systems appear in a wide spectrum
of industrial and medical applications, including surgical instruments such as laparoscopes,
arthroscopes and cystoscopes. They also have found application in disposable medical
devices such as blood analyzers.
Other examples of the creative use of plastic
optics encompass imaging systems for displays, night-vision goggles and various
kinds of head-mounted 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 enable mechanical and optical
features to be integrated into one element. This is an advantage to product designs
that impose space and weight constraints. Also, in the case of spinning elements
in bar-code scanners, plastic optics allow the distribution and balance of weight
to be factored into the part itself. Courtesy of NCR.
They also appear in display projectors,
laser bar-code scanning systems, biometric security systems, smoke detectors, automated
toilet flush valve systems, spectrometers, particle measuring instruments and optical
encoders. Plastic optics are useful for certain telecommunications products and
are commonly used to replicate diffractive optical surfaces.
This wide array of applications stems
from several key advantages that plastic optics have over competing glass solutions;
namely, cost, weight and ability to integrate mechanical and optical features.
Tremendous economies of scale are possible
through the use of multi-cavity 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 offer higher impact
resistance. It is well-established that a polycarbonate window of sufficient thickness
can withstand the force of impact of a .22-caliber bullet.
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, such as that of a handheld bar-code
scanner, imposes space and weight constraints. By combining a reflective aspheric
optical surface with integrated mounting features (molded at the same time as the
mirror), designers can reduce the overall product weight and element count in the
However, with these advantages come
challenges. No one material, whether it be glass, metal or plastic, can address
all optical design problems. Polymers present several challenges to the designer,
including thermal effects, stress birefringence and the limited range of available
The thermal differential index of refraction
coefficient for optical plastics is approximately an order of magnitude greater
than that of glass. This means that high-performance lens systems requiring large-temperature-band
operating conditions are better served by hybrid glass-plastic designs.
Most optical plastics also have a fairly
low maximum sustained operating temperature, ranging from about 80 °C for styrene
to 90 °C for acrylic to about 123 °C for polycarbonate and the cyclic
olefin copolymer (Topas) and cyclo-olefin polymers (Zeonex/ Zeonor).
Birefringence is another potential
issue when using polymers, which exhibit residual stress — a problem exacerbated
by poor part design or poor gate location in the mold. It is extremely valuable
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, plastics have lower
refractive indices than glass. The most commonly used optical molding resins and
their indices of refraction.
Like glass, plastic optics can be coated
using physical vapor deposition, except coatings are applied at much lower temperatures
on plastic 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 gold coating, greater than 95 percent from about 700 to
1000 nm. A protective overcoat can be applied to metal coatings to improve scratch
The injection-molding process
Injection molding is a very cost-effective way
to reproduce spherical and complex aspheric plastic optics. The effort to produce
the optical form is confined to the mold insert. Inserts are fabricated in hardened
steel, polished or plated with nickel and then single-point diamond-turned to the
negative shape of the final component surface. During the molding process, replicating
the surface of the insert creates the optical surfaces of the finished element.
Afterward, 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
optics 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.
Formed in multicavity molds, plastic optics allow significant economies
of scale by enabling not only reproduction of aspheric and other complex geometric
surfaces, but also fully integrated assemblies, such as this viewing lens for a
center punch. Courtesy of Lee Valley Tools.
The 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).
Injection-molding techniques can reproduce
optics with a high degree of repeatability. 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.
Rules for specifying
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 molding vendor. These tolerances should be considered state-of-the-art.
As always, the tighter the tolerance demanded, the more costly a part will be.
First, a basic specification is the
optical forms. 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 flow analysis to model
how the part will fill in the mold. This exercise usually is undertaken early on
in the design cycle.
A thinner optic will have fewer shrinkage-compensation
issues and shorter cycle times, translating into less costly parts. Thicker optic
cross sections not only increase cycle times, but also make it more difficult to
hold a tight surface figure. Because flat surfaces have a tendency to sink as they
cool in the mold, you should have a surface of power on both sides of the optic
Other specifications include:
• Focal length: ±0.5
to 1.0 percent.
• Radius of curvature:
±0.5 to 1.0 percent.
• Optical power (for diameters
of up to about 4 in.): 2.0 to 5.0 fringes per inch.
• Irregularity (for diameters
of up to about 4 in.): 1.0 to 2.0 fringes per inch.
• Scratch/dig: 40/20.
• Centration: ±1
• Center thickness (up
to about 1 in.): ±0.0005 in.
• Diameter (for diameters
up to about 4 in.): ±0.001 in.
• Repeatability (lens
to lens over a molding run): 0.3 to 0.5 percent.
Selecting the right vendor
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 must find 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
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.
Meet the author
William Beich is business development manager
for G-S Plastic Optics in Rochester, N.Y.
Rules of Thumb
The design of
injection-molded plastic optics involves more than specifications. The following
are a few guidelines:
• The desired ratio of the diameter
to center thickness is 5:1 or less.
• The desired ratio of the center
thickness to the edge thickness is 3:1 or less.
• For an optic without a flange,
the clear aperture should be no closer to the edge than 1.5 times the edge thickness.
• All walls should be tapered
for a draft angle of at least 0.25°. However, on elements with tight tolerances,
it is not uncommon to see 5° to 10°.
• Gate type should be discussed
with the optical molder. Many configurations are possible.
• Shrinkage varies according
to the material but falls into the range of 0.1 to 0.7 percent.
• Holes and sharp corners can
be a source of localized stress in the part. This is a contributing factor in stress-birefringence
and can be the source of weld lines.
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