Rapid injection molding works best for on-demand production, bridge tooling, pilot runs, and functional prototyping. There are several options for injection molding, with aluminum a popular choice as a material for the molds.

Probably one of the first decision points is to decide whether you want to validate your design or you need flexible, on-demand production of end-use parts. The answer will help determine which injection molding option works best.

Then, look at numbers, as in how many parts must be created? This technology can provide a range of part volumes from as few as 10 to more than 10 million. Protolabs focuses on the range of 10 to a bit over 10,000. And, consider whether you need part production for more than a year.

In most cases, the mold price is lower in prototyping needs than in production needs. But the part price can be higher for prototyping. The more parts produced, typically the lower overall cost.

Protolabs offers three types of injection molding service options.

–Plastic injection molding

In plastic injection molding, resin in a barrel is heated to a molten state then shot into a mold to form a final production-grade thermoplastic part. The Protolabs process uses aluminum molds with no heating or cooling lines running through, which means cycle times are a bit longer. The longer time, though, enables mold operators to monitor fill pressure, cosmetic concerns, and other aspects to ensure the quality of the parts. The use of aluminum material for molds offers another benefit of lower cost tooling.

Plastic injection molding is used to produce custom prototypes and end-use production parts in 15 days or less. Protolabs has more than 100 different thermoplastic resins in stock and can also accept customer supplied resin.

To ensure a successful mold, it’s important to follow a few key design tips. Draft on vertical faces should be 0.5 degrees. Keep in mind that the maximum mold size is 18.9 x 29.6 by 8 in., and volume is 59 cu.in.

The maximum depth should be 4 in. from parting line, but can be up to 8 in. if the parting line can pass through the middle of the part. Plastic injection molding holds a tolerance of +/- 0.003 in. (0.08 mm) with an included resin tolerance that can be greater than but no less than +/- 0.002 in/in. (0.002 mm/mm) Wall thickness varies by material. For more design guidance, consult with Protolabs engineering team.

–Liquid silicone rubber

The Protolabs liquid silicone rubber (LSR) molding capability is a thermoset process that mixes a two-component compound together, which is then heat cured in the mold with a platinum catalyst to produce a final LSR part.

This process also produces custom prototypes and end-use production parts in 15 days or less. Machined aluminum is typically used for the molds for cost-efficient tooling and accelerated manufacturing cycles. Protolabs stocks various grades and durometers of LSR materials.

Liquid silicone rubber produces flexible parts, which are usually manually removed from the mold. Therefore, designers do not need to include ejector pins in the mold design.

The LSR-specific injection molding press is precision geared for accurate control of shot size to produce the most consistent LSR parts. Silicone rubber is a thermoset polymer. Once it is molded it cannot be melted again like a thermoplastic.

A few key design tips include keeping part sizes to a maximum of 12 x 8 by 4 in., maximum volume of 13.3 cu.in. Part depth should be no greater than 2 in. from any parting line; deeper parts are limited to a smaller outline. Tolerances are held to +/- 0.003 in. (0.08 mm) with a linear tolerance of +/- 0.025 in/in. (0.025 mm/mm). The draft angle on vertical faces should be 0.5 degrees, and most situations handle 2 degrees. Wall thickness can be as thin as 0.010 in. (0.254 mm). Rib thickness should be from 0.5 to 1.0 (127 mm to 25.4 mm).

Protolabs third processes are overmolding and insert molding, which are two-part injection molding processes where one material is overlaid onto a second substrate part or metal insert to create a single multi-material component. Custom prototypes and on-demand production parts are done within 15 days.

Overmolding begins with the production of the substrate parts, which is achieved in a standard injection molding process with no heating or cooling lines running through the mold machine. Molders monitor fill pressure, cosmetic concerns, and the quality of the parts.

When the total run of substrate parts is molded, then overmold tooling is assembled to the press. The substrate parts are placed by hand into mold where each part is overmolded with either a thermoplastic or liquid silicone rubber material.

Insert molding is a process similar to overmolding, but uses a preformed part. This preformed part is often metal. It is loaded into a mold where it is then overmolded with plastic to create a final component.

As with the other injection molding process, a few design guidelines help ensure a successful production run. These processes handle part sizes to 8 x 16 by 8 in., with a maximum volume of 59 cu.in.

Part depth is a maximum of 4 in. from the parting line, and up to 8 in. if the parting line can pass through the middle of the part. Tolerance is held to +/- 0.003 in. (0.08 mm) with an included resin tolerance that can be greater than but no less than +/- 0.002 in/in. (0.002 mm/mm). Draft should be 0.5 degrees, but many situations can handle 2 degrees.

The Maximum side core dimensions are <_ 8.419 in. W, <_2.377 in. H, and <_2. 875 in. Wall thickness varies by material.

Protolabs engineers are available to answer any questions to ensure a successful injection molded part.