Many products include components that must be optically clear to facilitate useful functions:
- Visual inspection. Medical devices are often molded of clear plastic so that a problem can be more quickly noted and resolved.
- Indication. A light guide transmits colored light from the interior to the outer surface of a handheld device.
- Protective Barrier. Goggles, face-shields, and windshields protect people. The display cover on a handheld electronic device is there to protect a delicate component.
- Optics. Many applications require light to be focused or diffused with a lens or prism.
When volumes and technical requirements allow, injection-molded parts are preferred. Identifying a transparent polymer appropriate to specific requirements becomes critical. Whether you are doing the work yourself or engaging a product design company, you need to understand the strengths, weaknesses, and costs of the different transparent polymers. Experienced product development engineers should already be familiar with a few of these options:
- Polycarbonate (PC)
- Acrylic (PMMA)
- Polyethylene (PE)
- Polyethylene Terephthalate (PET)
- Polystyrene (PS)
- Polyvinyl Chloride (PVC)
- Styrene Acrylonitrile (SAN)
- Polyphenylsulfone (PPSU)
- Polyetherimide (PEI)
In the last decade, the demand for polycarbonate exceeded global production. That is because polycarbonate is one of those materials that represent an attractive balance of properties. It is not superior at anything, but it is pretty good at almost everything. It is the “bar” against which other transparent polymers are evaluated.
- PC is easy to mold, with good dimensional stability.
- Strong and tough. Ductile, so it will not shatter.
- High service temperature. Some grades can be autoclaved.
- You can find FDA-approved grades for food contact and medical device applications.
- Good optical clarity. Not quite as good as acrylic, but near the top of the list.
- Susceptible to UV degradation, so not appropriate for outdoor applications.
- Poor chemical resistance to some common substances, such as ammonia.
- Increasingly expensive due to its popularity.
- Birefringence, which produces an odd rainbow effect when viewed through polarized sunglasses. This limits its use in certain applications.
When your highest priority is optical clarity, your product development firm will recommend acrylic. This polymer has the highest achievable optical clarity of any of the clear plastics. It also outperforms polycarbonate in two other ways:
- Does not exhibit birefringence.
- Excellent long-term UV resistance.
These strengths make acrylic a common choice for eyewear, windshields, glare-shields, and electronic display covers.
- The excellent clarity and sunlight resistance mentioned above.
- Enormous range of available grades.
- Easy to fabricate with a laser-cutter.
- FDA approved, even biocompatible grades for medical device product development.
- Not as dimensionally stable as polycarbonate
- Easily and immediately damaged by ammonia-based cleaners
PE, often referred to be the trade-name “Polyester,” is the most widely used plastic in the world. Most of that is in flexible raw materials including film, foam, and thread. Your product development firm may try to steer you away from molding clear parts from PE due to relatively poor molding, optical, and mechanical properties. It is only recommended when chemical resistance is paramount.
- Excellent chemical resistance. PE can be used in fuel systems, or in contact with strong acids and bases.
- Some grades have good cold-temperature toughness.
- Everything else. If chemical resistance can be achieved in another way, heed the advice of your product development company and look elsewhere.
Polyethylene Terephthalate (PET)
PET is widely used in disposable applications like beverage containers. It is cheap and lends itself to numerous methods of fabrication, including injection molding, extrusion, and blow-molding.
- Good chemical resistance, though not quite as good as PE.
- Acceptable dimensional stability. Much better than PE.
- Easily recyclable; an increasing priority in new product development.
- Widely approved for food contact.
- Somewhat brittle
- Mediocre strength and stiffness
- Mediocre optical clarity
PETG is a close cousin, incorporating some molecular “tweaks” to make a less brittle polymer. This results in a slight degradation of chemical resistance and recyclability.
There are many types of polystyrene. A very common grade for inexpensive clear parts is High-Impact Polystyrene or “HIPS.” It is cheap, moldable, and tough. One common application is disposable picnic flatware.
- Resistant to many strong acids.
- Susceptible to UV degradation.
- Mediocre mechanical and optical properties.
Polyvinyl Chloride (PVC)
Clear PVC is an excellent material that would be used in many applications except for one obstacle: Molding PVC can release toxic chlorine gas. That means there are fewer molding facilities willing to use it. Those that do must employ safety precautions and remedial measures that drive up cost. A product development company with experience in transparent parts will likely guide you away from PVC. If PVC is a must then consider using it as an easily-machined sheet, rod, or pipe.
- Strong and stiff
- Resistant to attack by many acids and corrosive agents
- Available in many FDA medical and food safe grades.
- Cost (if injection-molded)
Styrene Acrylonitrile (SAN)
SAN is often found in kitchen-and-bath consumer applications. Your product development firm will evaluate the mix of service temperature (e.g., dishwasher), chemical resistance, and moldability to short-list SAN as a cost-effective choice for many applications.
- Highly moldable with good dimensional stability.
- Service temperature near 100°C.
- Approved for food contact.
- Resistant to many household cleaners.
- Will yellow with UV exposure.
- Mechanical and optical properties are only mediocre.
PPSU, commonly called Radel, has the highest operating temperature of any thermoplastic: over 200°C. Radel is expensive, but no other clear plastic exhibits the same combination of temperature and chemical resistance.
- Incredibly temperature resistant. Can be steam-cleaned or autoclaved.
- Excellent resistance to many acids and hydrocarbons.
- Very expensive.
- Clarity is mediocre, and parts will have an amber tint.
- Somewhat susceptible to UV radiation.
PEI, also called Ultem, has many similarities to Radel. The amazing thing about Ultem is that, unlike other polymers, Ultem maintains excellent mechanical properties even at high temperatures. It is one of the elite materials known as a “metal-replacement polymer.” The fact that it is transparent is just a bonus.
- Unequaled dimensional stability, capable of holding some of the tightest tolerances of any thermoplastic.
- PEI is extremely “sterilizable,” with excellent resistance to heat, UV, and gamma radiation.
- Incredible strength, even at high temperatures. Yield strength of 110 MPa.
- Like Radel, Ultem parts will have an amber tint and mediocre clarity.
- Do not use Ultem unless you really need those astounding properties.
A good design engineer or product development company is expected to consider all the goals and requirements when selecting a polymer. If this is done poorly then the product will be more expensive or less capable than it could have been.
There will never be one perfect choice when it comes to choosing a clear polymer for your product design and development. Your goal, and that of your product design firm, must be:
- Enumerate your goals and priorities
- Generate a shortlist
- Rank that list according to priorities. (e.g., cost, chemical resistance, clarity)
If possible, work with a product engineering firm that has experience with the design of optically clear parts.