Outdoor-rated enclosures incorporate numerous technologies to stand up to the harsh conditions they must face. These technologies include waterproof keypads and membrane switches, custom silicone gasketing, and highly engineered raw materials such as weatherable ABS/PC blends that can be injection molded creating the custom enclosure shape.
Injection molding is a widely used manufacturing process that enables the mass production of complex thermoplastic parts with high precision and efficiency, making it a cornerstone of modern manufacturing for waterproof enclosures. Injection molded plastics are used in almost every industry ranging from high-volume automotive, to ultra-low-cost electronics, to biocompatible medical devices, to just about every consumer product in your local department store. Certain types of injection molded plastics are highly weatherable and able to be used for decades outdoors with no ill effects.
Since many user interface enclosure designs are used outdoors, there are dozens of material options available that can stand up to harsh outdoor conditions including extreme heat and cold, UV exposure, and excessive moisture.
Common Materials for Injection Molding
Injection molding is highly versatile and is compatible with various materials, each offering unique properties that make them suitable for different applications.
Materials commonly used in injection molding include a wide range of thermoplastics that include ABS (Acrylonitrile Butadiene Styrene), Polycarbonate (PC), Polyethylene (PE), and Nylon (PA), with each selected based on specific properties like strength, flexibility, and thermal resistance.
Complex machinery is required to injection mold plastics.
With all the different types of plastics and polymers available, there is no shortage of options for engineers to consider. Still, tried and true materials such as ABS, PC, and blends of ABS/PC remain some of the most used today because of their widespread availability and relatively low costs.
ABS (Acrylonitrile Butadiene Styrene)
ABS is a popular thermoplastic polymer known for its excellent mechanical properties and ease of processing, making it one of the most commonly used materials in injection molding. ABS is composed of three monomers: acrylonitrile, butadiene, and styrene, which combine to give ABS a balance of properties.
These properties include:
- Impact Resistance
- Rigidity
- Strength
- Cosmetic Surface Finish
- Chemical Resistance
- Temperature Resilience
ABS is widely used in consumer electronics, automotive components, household appliances, and even toys. Everyday enclosures for devices like computers and televisions, vehicle interior trim and dashboards, and children’s toys like LEGOs are all made from ABS. Its combination of strength, impact resistance, and a cosmetically appealing surface makes it ideal for products that require both durability and aesthetic qualities.
Pelletized plastic materials are colored and molded into shape.
PC (Polycarbonate)
Polycarbonate (PC) is a high-performance engineered thermoplastic known for its exceptional toughness, clarity, and heat resistance. It is a widely used material in applications requiring high-impact strength and optical clarity. Key properties include high impact strength, transparency, heat resistance, dimensional stability, and flame retardancy. PC can be formed into thin sheets and used has a hard coated graphic overlay material. PC is also able to be injection molded into custom shapes and forms.
Just like ABS, PC is used in a variety of demanding applications, including automotive parts, electronic components, construction materials, medical devices, and safety equipment. Its impact resistance and clarity also make it a preferred choice for eyewear lenses and protective screens.
ABS/PC Blend
ABS/PC blends combine the best properties of both ABS and Polycarbonate, resulting in a material that offers improved toughness, heat resistance, and aesthetic qualities. The blend typically consists of a mix of ABS and PC in varying ratios, depending on the desired properties for the final product. Because of the demand for such properties in low cost manufacturing, raw material manufacturers have included pelletized ABS/PC blends as another catalog offering for injection molding vendors.
Weatherable Plastics
Many injection molded plastics are chosen for outdoor rated user interface enclosures because they are inherently tough materials that can stand up to years of exposure. But not all plastics are deemed weatherable. Some material blends may succumb to the effects of UV and breakdown prematurely experiencing symptoms like yellowing, fading, cracking, swelling, and overall material degradation. If there are specific UL requirements pertaining to weatherability, you must choose a material with the appropriate outdoor ratings.
Outdoor-rated plastics are engineered to withstand a variety of harsh environmental conditions that can degrade the enclosures over time. These conditions vary by location across the globe with numerous military specifications being written that define each. As a result, UL has created a widely recognized material specification on outdoor suitability titled UL 746C that can be applied to weatherproof enclosures.
- High Temperatures: In hot environments, plastics are exposed to elevated temperatures which can cause thermal expansion, softening, or even melting in extreme cases. Plastics with higher glass transition temperatures (Tg) and melting points, like polycarbonate (PC), are better suited to withstand high temperatures without significant loss of mechanical properties. However, prolonged exposure to heat can lead to thermal degradation, resulting in discoloration, embrittlement, or a reduction in strength.
- Low Temperatures: In cold environments, plastics become more brittle, increasing the risk of cracking or shattering upon impact. Materials like ABS and certain ABS/PC blends can retain their toughness at lower temperatures, although even these materials may experience a reduction in impact resistance as the temperature drops. Plastics designed for outdoor use in cold climates often incorporate plasticizers to maintain flexibility when cold.
- Thermal Shock: As plastics are quickly heated and cooled, they expand and contract, which can lead to internal stresses. Different materials within a composite structure or assembly may have different coefficients of thermal expansion, leading to mechanical stress at the interface eventually leading to a failure. Outdoor-rated plastics are often formulated to minimize these effects, maintaining dimensional stability and mechanical integrity even after repeated temperature cycles.
- UV Degradation: UV radiation can break down the chemical bonds in plastics leading to a phenomenon called photodegradation. This process can cause discoloration (yellowing), surface chalking, loss of sheen, and a reduction in mechanical properties such as tensile strength and impact resistance. While some additives can actually hurt the flammability rating of the material, these UV stabilizers are needed to make the material outdoor-rated.
- Water Absorption: Some plastics are hygroscopic, meaning they absorb moisture from the environment. Water absorption can cause swelling, dimensional instability, and a reduction in mechanical properties. Materials like polycarbonate are less prone to water absorption, making them more suitable for outdoor applications. However, in cases where water absorption is a concern, coatings or treatments can be applied to improve resistance.
- Mold and Mildew Growth: In humid conditions, the combination of moisture and organic materials can promote fungal growth. Some outdoor plastics include antimicrobial additives to resist microbial growth. This is a common test for numerous military and aerospace specifications such as Mil-STD-810 and RTCA/DO-160.
- Abrasion and Surface Wear: Dust and sand particles carried by wind can cause surface abrasion, leading to a roughened surface, and loss of luster. Outdoor-rated plastics often have enhanced surface hardness or protective coatings to resist abrasion.
- Salt Exposure: In coastal environments, salt from seawater and marine air can be wildly corrosive to metals. While salt itself doesn't directly degrade most plastics, it can dry the material causing stress cracks, particularly in materials that are already under mechanical stress. As salt corrodes the surrounding materials, iron oxide can cause rust stains on outdoor plastic enclosures around hardware, lids, and hinges.
Injection molded plastic enclosure for human-machine interface.
What is UL 746C?
UL 746C is a standard developed by Underwriters Laboratories (UL) that provides testing methods and criteria for evaluating the suitability of plastic materials for outdoor use. It is part of a broader series of UL standards covering various aspects of material requirements for electrical enclosures.
The focus of UL 746C is on the weatherability of plastics, assessing their resistance to environmental factors such as UV radiation, moisture, temperature extremes, and other conditions encountered outdoors. The standard includes tests that simulate these environmental exposures to determine how the material performs over time.
One key aspect of UL 746C is UV light exposure testing, which evaluates how a material's physical and mechanical properties change after prolonged exposure to ultraviolet light. Materials that demonstrate good performance under UV exposure receive favorable ratings under this standard.
The standard includes F1 and F2 ratings to classify materials based on their outdoor suitability. An F1 rating indicates that the material has passed tests for UV resistance, water exposure, and extreme temperature conditions, making it suitable for prolonged outdoor use. In contrast, an F2 rating suggests the material is suitable for outdoor use but may have more limited resistance to one or more environmental factors compared to F1-rated materials. While the F2 rating can indicate the test specimen was partially tested, it typically means the material has met one of either UV resistance or water exposure requirements, not both.
UL 746C is widely applied in industries where plastic components are exposed to outdoor conditions, such as electrical enclosures, outdoor lighting, automotive parts, solar farms, and telecommunications equipment. Products or materials that meet the requirements of UL 746C can be certified by UL, providing assurance that the material has been rigorously tested and is suitable for outdoor use. This certification is often essential in industries where safety, reliability, and regulatory compliance are critical.
UL 746C plays a vital role in ensuring that plastic materials used in outdoor rated enclosures can withstand the environmental stresses they will encounter, helping manufacturers prevent premature failure of components, avoid costly maintenance or replacements, and ensure the safety and durability of their products in challenging outdoor environments.
Plastics with UL 746C Outdoor Rating
ABS, PC, and ABS/PC blends are available with a UL 746C F1 rating, but it depends on the specific formulation and grade of the material.
While standard ABS and PC materials may not typically have an F1 rating, some specialized grades are formulated to meet the requirements for outdoor use and may achieve an F1 rating. These grades are usually modified with additives that improve UV resistance and other outdoor durability factors.
Manufacturers typically provide data sheets that specify whether a particular grade of ABS, PC, or ABS/PC blend has a UL 746C F1 rating. It's important to check these data sheets or consult with material suppliers to ensure that the chosen material meets the requirements for outdoor exposure.
Summary
There are countless favorable reasons to use injection molded plastics for outdoor-rated enclosures. Plastics are economical while being resilient to the conditions faced outside including extreme temperatures, salt exposure, and blowing sand and dust. Still, the risk of UV exposure and moisture is enough to warrant tremendous industry attention, triggering UL specification UL746C. It’s clear that not all outdoor applications necessarily need the UL746C F1 or F2 material rating.
Engineers must bear in mind that these specialty materials do come with additional risk in terms of added weeks of potential schedule impacts, and up to 20% or more of cost increases. Choosing an outdoor-rated material from the beginning may be worthwhile- as long as the flammability ratings are sufficient.
