Translucent and Clear Plastic Injection-Molded Parts

Manufacturing clear molded parts adds all sorts of considerations to your designs and how we make your parts. The ideal materials for a project are determined by a material’s qualities and its behavior during both manufacturing and the end-user experience. Because transparent injection molding hides nothing, more finesse in design and manufacturing is required than when molding materials that can obscure a less-than-flawless design. The planning and preparation of raw materials, equipment, tooling, and molding processes must be precise.

Also, because clear plastics don’t hide impurities created during the molding process, material storage must ensure the material stays clean.

HDPE resin pellets provide an especially durable option for your molded parts.

Materials Options for Translucent and Clear Injection Molding

Different materials have different advantages in clear plastic injection molding. Here’s a rundown of the qualities of our clear plastics.

Acrylic (PMMA)

Acrylic can be injection-molded into crisply clear and colored plastic products, plus it's non-toxic, scratch-proof, and UV-resistant, making it a popular choice for use in outdoor equipment. Acrylic's accommodation and its variety of colorants make it a good choice for lenses, light fixtures, and objects requiring them. On the downside, acrylic can be inflexible and breakable, so it can't be used in products that must withstand a lot of pressure, and it must be dried after it absorbs moisture, possibly slowing the manufacturing process and causing issues for end-users.

High-density Polyethylene (HDPE)

Like acrylic, high-density polyethylene (HDPE) is UV-resistant, but it is more easily translucent injection molded. HDPE is less brittle than acrylic, and it's cheaper to produce, but it still breaks under high pressure. These qualities dictate its common use in containers, especially bottles, and pipes. HDPE is formed by applying high heat and pressure to petroleum. These transparent injection molding features, along with being inexpensive, are why it's used in mass production.

Polycarbonate (PC)

More expensive than acrylic or HDPE, polycarbonate is an alternative to clear plastic injection molding. It’s as transparent and UV-resistant as acrylic but less prone to damage from extreme temperatures. Polycarbonate's high impact resistance makes it a good choice for products that maintain safety, such as windows, containers, helmets, and safety goggles. However, like acrylic, it must be dried after it absorbs moisture before injection molding can occur.

Polyetherimide (PEI)

In addition to UV resistance, polyetherimide (PEI) is impervious to intense heat, repeated pressure, and acids, making it appropriate for medical and automotive products. It is also often used as a translucent material in aerospace engine components, temperature sensors, and heat shields. PEI is more expensive because a steel mold must be used in manufacturing.

Polypropylene (PP)

Commonly used in textiles, packaging, and containers, the high flexibility of polypropylene (PP) makes it an ideal choice for waterproof fabric for bags and clothing. It also has high electrical resistance, so it is used in electronics, and its nonreactivity with acids and bases makes it a good container for solvents and other corrosive materials. It even works well as a sort of dynamic hinge on non-loadbearing parts.

 

Liquid silicone rubber (LSR) is a biocompatible, two-part thermoset that has great thermal, chemical, and electrical resistance. It also comes in optical quality for excellent translucence.

Optical Silicone Rubber (OLSR)

OLSR is an advanced material that has many properties that make it a preferred material choice over polycarbonate (PC) or acrylic (PMMA) for lighting and clear optical parts. All materials reduce the amount of light flowing through them. When your product requires a clear PC or PMMA part, you can improve light transmission using OLSR, which allows up to 94% light transmission.

Still another advantage is the non-yellowing factor. Thermoplastics without additives are not UV-resistant, meaning the parts could yellow and degrade due to prolonged exposure to light and sunlight. OLSR is non-yellowing, so it's excellent for outdoor fixtures exposed to harsh environments.

In plastic design, thick and thin parts are not easily achieved without sacrifice. That doesn't necessarily ring true with OLSR design. You can achieve thick and thin geometries that often can be as thin as 0.010 in. (0.25mm) and as thick as 2 in. (50mm), all within the same part geometry because you don't have issues with material flow, sink, or voids. OLSR is extremely resilient against the transparent molding issue that frequently creates thermoplastic problems, including the ability to create micro details. You also can design in negative draft angles without the need for expensive tooling that would normally require lifters, collapsible cores, and side-actions.

Most Popular Materials for Injection Molding

So, what are the most popular materials we use to produce clear plastic injection molded parts? Well, first we should say that popularity shouldn’t be the determining factor for your part. One of our application engineers at Protolabs can discuss the best approach, if you’re not sure. Every project is unique, and just because a material is ordered the most often, it doesn’t mean you should use it, too.

1. Acrylic/PC (depending on impact strength required), LSR
2. PETG (polyethylene terephthalate glycol)
3. PP, HDPE, PEI (ULTEM—requires high temperature tooling)
4. MABS (transparent ABS), K-resin

Light Transmissivity Through Popular Materials

Ultimately, the thermoplastic properties of your parts and the expectations end-users have about your part survivability, will influence your choice of optimal material for transparent plastic. Make sure to validate your choice of materials before proceeding with your project. Proper validation will save you time and money from design through manufacturing.

Choose the best Surface Finish for Optimal Clarity

Manual sanding and polishing are best used for simple designs that lack details. Producing a clear product with this finishing method is expensive and time-consuming. The highest finish is SPI-A2, which is often appropriate for prototypes or optical projects with low runs, but if finish isn’t vital when evaluating a prototype, waiting to add it until the production process might save you some money.

For flat or nearly flat and very transparent parts, such as windows or lenses, resin coating is the best option. Because any release agent will display on a part surface, make sure to never use a release agent.

Lead times and costs for surface finishing vary depending on the project.

Design Tips for Clear or Translucent Parts

There are plenty of design tips on the Protolabs website for clear or translucent parts. Here are a few design reminders to wet your whistle.

1. Use uniform wall thicknesses
2. Design gate runners that are wide and chunky enough
3. Set gate location according to the contraction process
4. Ensure that part of the transition is gradual and smooth to prevent any sharp corners. PC products, in particular, must not have gaps
5. Make certain that the mold surface is smooth

Remember that uneven cooling can cause problems, including surface defects and deterioration. Check out the website for many great design tips that will help you with transparent injection molding or clear plastic injection molding.