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Proto labs

Rapid Insert Molding Speeds Prototypes and Small Volumes of End-Use Parts

Increasingly, digital manufacturing companies that offer molding services for custom prototypes and low-volume production parts are finding their customers are demanding more speed than ever. Insert molding technology helps them trim turnaround time from months to weeks.

Increasingly, OEMs are seeing a change in the timelines of the companies they sell to. Multiyear projects that bring in millions of dollars are waning, while shorter projects requiring “quick turn” manufacturing are on the rise. Many companies are broadening their manufacturing capabilities in an effort to accommodate these faster turnaround projects. Increasingly, insert molding is one of those capabilities.

Product designers and engineers frequently turn to insert molding to improve part strength while trimming part weight and reducing assembly costs, production time and labor: all elements of rapid manufacturing. Insert molding is often favored above traditional overmolding, because the former is a single step process, and the latter is a two-step process requiring one shot for the substrate and another for the overmold. In insert molding, a threaded insert, bushing, sleeve or boss is manually placed into the mold. The mold closes, the plastic flows, the mold opens and the part is ejected. The result is improved part strength and reliability.

Good candidates for insert molding are rotating parts such as fan blades or drive gears, or any part that contains a threaded hole. Knobs, handles, or dials that screw onto a threaded stud are good examples, as are plastic air or hydraulic manifolds requiring additional strength in threaded areas. From a design perspective, rapid injection molding works best when designers maintain proper draft angles, keep wall thicknesses consistent, eliminate undercuts, and avoid unnecessary part features and superfine surface finishes. The process is also ideal for parts that are a combination of plastic and metal, according to Becky Cater, Global Product Manager of Injection Molding for Minnesota-based Proto Labs, which recently introduced a rapid-turnaround service that installs molded-in inserts during the molding process.

“A typical application of insert molding is to include one or more threaded metal inserts in a plastic part when that part is intended to mate to another part in an assembly,” Cater told Design News. “Plastics alone may not have sufficient mechanical properties to withstand the forces required to fasten two parts together. For example, threads in a plastic part can become worn over repeated usage, which can result in a failed part. Metal inserts help reinforce the properties of the plastic and ensure reliable fastening over repeated use. This combination of plastic and metal allows designers to take advantage of the weight reduction of plastics and the strength of metal.”

Increasingly, digital manufacturing companies that offer molding services for custom prototypes and low-volume production parts are finding their customers are demanding more speed than ever. Insert molding technology helps them trim turnaround time from months to weeks.

Rapid insert molding technology makes it possible to produce custom parts in low volume within days. (Image: Proto Labs)

There are multiple ways to install inserts into a plastic part: molded-in inserts that are installed during the molding process, and post-molding installation, where the inserts are installed into a molded part in a subsequent manufacturing step (heat staking and ultrasonic installation are two examples of post-molding insert installation methods). Compared to installing inserts into a plastic part in a post-molding process, the use of molded-in inserts eliminates the need for secondary installation of the inserts, which further reduces project turnaround time and costs.

Newer processes have helped bring down the costs of molded-in inserts, according to Cater, and allow for the production of parts made of multiple materials, including a variety of types of thermoplastic as well as liquid silicone rubber. Historically, insert molding was often expensive and time-consuming, making it practical only for high-volume parts and offering little flexibility for design changes.

“Affordable quick-turn insert molding makes molded-in inserts accessible to a broader range of designers and engineers, even on low-volume products, giving them additional design flexibility,” she told Design News. “Instead of waiting several months for a traditional steel tool for insert molding, with rapid insert molding, you can have parts in-hand in a few weeks.”

Additive manufacturing technologies are an important element to rapid turnaround in injection molding, particularly for the design validation stage.

“The benefit of 3D printing is the ability to get one or a handful of parts to evaluate without having to invest in tooling,” said Cater. “If you plan to eventually scale up to injection molding, it’s wise to consider designing for moldability even in the 3D printing stage. Incorporating best practice molding design guidelines in the design of your 3D printed part will help avoid the need for changes to your design when you transition to molding.”

The lower cost of entry of these processes makes rapid insert molding a cost-effective manufacturing option, even for low-volume parts. The ability to purchase parts on-demand allows customers to minimize their inventory costs and have confidence that when they need more parts, they can get them quickly.

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