Eric Utley, Applications Engineer, Proto Labs, Inc.

Engineers have several options for their prototyping needs; machining, injection molding, and additive manufacturing. Each has benefits, drawbacks, and cost metrics. Of the multiple additive technologies, one of the more common ones used for prototyping is Stereolithography (SL).

SL is used to create tangible 3-dimensional objects from CAD designs. In a layer-by-layer additive process, photo-reactive polymers are instantly hardened when exposed to an ultraviolet light source, typically a laser.

One of the key reasons to use SL is its ability to print objects with very fine details. The size of such details is controlled by the size of the diameter of the UV light source. The ability to build objects with very fine details is one of the reasons SL is often used for prototyping medical devices, hearing aids, hand-held surgical devices, and other detail-rich designs.

When developing a design, designers should be aware of “negative space,” or space between features, as it can affect the quality of details. Basically, the chemical reaction caused by the laser can bleed into small spaces, reducing the sharpness of a line. Once you get below a millimeter, about 40 thousands of an inch, the fine detail drops. Protolabs can 3D print small, microfine detail, down to about a quarter of a millimeter, ten thousandths of an inch, but any smaller can lose the detail.

Protolabs uses several SL machines in its services. It uses 3D Systems ProJet 6000, ProJet 8000 and Viper and ViperPro for SL needs. The ProJet machines can handle build sizes to 10-in.³. The ViperPro and the ProJet 8000 offer a build size of 25 x 29 x 21 in.

In addition to fine detail, SL allows amazing geometry flexibility. If a design can be molded or made through traditional subtractive machining, an SL system can build it. But if a design has internal structures, or requires channels or other features not machinable, then an additive process like SL should certainly be a consideration.

So the choice of when to use SL versus other prototyping options involves a few key points:

–SL handles design features down to quarter of a millimeter

–Within a design, make sure supports are accessible for removal.

–Wall thickness should be kept to no less than ten thousandths of an inch, or quarter of a millimeter. If smaller thicknesses are required, other options are available.

Protolabs offers several materials for its SL machines. These include:

–ABS-like materials, which offer accuracy, durability, and impact resistance

–Digital Photopolymer materials that deliver durability, flexibility, and transparency

–Nylon materials feature rigidity, strength, and temperature resistance

–PC-like materials, such as Somos PerFORM, 3D Systems Accura 5530, 3D Systems Accura 60, MicroFine Green, Somos WaterShed XC 11122, which handle needs for accuracy, stiffness, and temperature resistence.

–PP-like materials, such as Xtreme White 200 and DSM Somos 9120 to handle needs for accuracy, durability, and flexibility.

–TPU materials for abrasion resistance, flexibility, and impact resistance

Protolabs engineers can offer guidance on the right material choice for a prototype, as well as tips on improving a design for SL. Protolabs services deliver the finished prototype in about three business days, and turnaround can be even faster if needed.

Choosing between SL and injection molding

Both injection molding and a number of 3D printing technologies use resins as the build material. Choosing between these processes rests on a couple of factors.

–Part quantity. The breakeven point for molding is usually about 25 copies and up. Anything less is more cost effectively done using additive technologies. There can be a gray area at quantities between 25 to 500 parts where an engineer could choose SL or another process. This is where price per part comes in. In an apples-to-apples comparison, injection molding will deliver the better quality part. The price of a 3D printed part is pretty well locked in.

–Part longevity. With metal additive processes, parts will last as long as injection molded parts. With resins, the photosensitivity materials used in SL will affect your prototyping choice.

–Designing a mold. SL can be a powerful tool for mold design. It allows designers to iterate multiple versions of a design at low cost.

Post processing

SL objects can benefit from post processing, but that depends how long a designer needs the object.

Post processing involves cleaning excess resin, typically using a solvent bath. Then, the somewhat tacky-to-the-touch part should go through a UV cure cycle, which is like a microwave device that covers the part in UV, curing it and bringing it to its final material properties. From here, parts may undergo sanding or blasting to develop a uniform matte finish.

Protolabs also offers custom finishing, which includes sanding, painting, clear coat or something else a customer wants, such as painting or an epoxy finish. Color finish can also be an option at Protolabs.

Stereolithography is typically used for prototyping, but it is possible to use it for end-use parts. The finishing steps above will help prepare a part for end use, as will secondary processes such as painting or other finishes are needed to protect a part from additional exposure to light and humidity. Electroplating can seal parts and a urethane clear coat works well if a part will be used for a long time.

For any questions on which prototyping or production needs, Protolabs can deliver the answers, visit protolabs.com to learn more.