Design Tip

Designing Strength to Parts with Insert Moulding

A combination of both plastic and metal, insert molding can add strength and durability to parts

Plastic injection-moulded parts are tough and strong, able to withstand heavy loads, moderately high temperatures, and the corrosive effects of acids and alkalis. In some cases, plastic even competes with metal, and is lighter and usually less expensive. Sometimes, however, plastic needs a little help. The bore in an all-plastic pulley will quickly wear. Tapped holes in moulded handles and knobs are prone to stripping. But by bolstering these areas with metal—a brass bearing journal, for example, or a stainless steel threaded insert—a best-of-both-worlds solution is achieved.

Say Hello to Insert Moulding

Protolabs offers rapid insert moulding. Instead of doing assembly after the fact, many sizes and types of metal inserts can be placed directly into the mould before plastic injection, thus eliminating secondary operations and providing robust mating of metal and plastic. Before redesigning your parts for insert moulding, though, there are a few things you should know, which this tip will cover:

  • What is insert moulding, and how does it compare to overmoulding?
  • Will my part work with insert moulding?
  • Which plastics can be used?
  • What inserts are available, and where can I buy them?
  • What about part design considerations?


overmoulded part
Insert moulding allows you to build added strength into parts with prefabricated components like threaded inserts.
Insert Moulding 101

Insert moulding shares some similarities with overmoulding. Both incorporate disparate materials into the same part, thus improving its properties. Both processes drive up the cost of moulded parts slightly, but this price delta is typically offset by the elimination of manual post-moulding operations. And the lead-time for overmoulded and insert moulded parts is 15 days, a bit longer than Protolabs’ single-part injection moulding turnaround, but still much faster than traditional insert moulding.

But where overmoulding is a two-step process, requiring one shot for the substrate and another for the overmould, insert moulding is, as an industry saying goes, “done in one.” The threaded insert, bushing, sleeve, or boss is manually placed into the mould, the mould closes, plastic flows, the mould opens, and the part is ejected. The result is improved part strength and reliability.

In addition, another benefit of using insert moulding, in many cases, is a reduction in product weight. Weight would be trimmed when a plastic part, with metal inserts, replaces an all-metal part. And, as mentioned previously, overall part costs are frequently lower with insert-moulded parts than those that journey on to the assembly area.

insert moulding illustration
A threaded insert is placed atop a mould core where plastic is moulded over it to form the final component.
What's Suitable for Insert Moulding?

Simply put, insert moulding should be evaluated for any part that contains a threaded hole. An electronics housing unit or medical instrument case with a bolt-on cover are good examples. Knobs, handles, or dials that screw onto a threaded stud are also examples, as is a plastic air or hydraulic manifold requiring additional strength in threaded areas. In fact, almost any plastic component that attaches to a mating piece with screws or bolts will benefit from moulded fasteners.

A rotating part such as a fan blade or a drive gear is another good candidate for insert moulding. Here, a bronze or brass bushing is moulded into the part and serves as the bearing surface, greatly improving product life over non-bushed parts. A metal surgical saw that calls for a plastic handle could be insert moulded, size permitting. Wheels and sprockets, pulleys and augers, most any plastic part that requires greater strength or wear characteristics in certain areas is quite likely a match for insert moulding.

The Right Stuff

Which thermoplastics can be insert moulded? There are few limitations. Stiff, engineering-grade plastics such as Delrin or nylon are good options, as are polyethylene and ABS, but even elastic liquid silicone rubber (LSR) parts can be used. The largest constraint comes from those plastics with very high moulding temperatures, such as PEI (Ultem) or PEEK. Since inserts are hand-loaded at Protolabs, and because the quick-turn moulding process uses non-cooled aluminium tools to reduce part cost and speed turnaround time, placing an insert into a mould that’s been heated to several hundred degrees is a like reaching into a hot oven for a freshly baked cookie—if you’re not careful, you’ll burn your fingers.

Choosing an Insert

Protolabs does not supply inserts. We can, however, point you to companies that do. Suppliers like Tappex produce inserts for moulded plastic parts. Many of these are knurled on the outside, or have special shapes that hold the insert securely in place. Inserts made of brass are common, but steel and stainless steel inserts are also available. If you can’t find what you need with one of these companies, Protolabs welcomes custom inserts (and would be happy to quote them for you as well). Whichever way you go, we currently accept metal inserts only. Also, be sure to ask your supplier for a CAD file, as this must be uploaded along with the part design and assembly drawings during the quoting & process. 

Design Considerations

The rules of insert moulding are no different from those of standard injection moulding. Maintain proper draft angles, keep wall thicknesses consistent, eliminate undercuts, and avoid unnecessary part features and superfine surface finishes whenever possible. This is all good advice, and applies equally to insert moulding.

However, that little chunk of metal does present a few additional considerations. As alluded to earlier, Protolabs hand-loads all inserts. This means the operator reaches into the mould to deposit the insert, and since the mould is usually quite warm, gloves must be worn. Inserts smaller than 3.17mm across can be problematic, as are those too big to fit comfortably in one hand. Nor should the insert location be too deep inside the mould—a 2:1 hand clearance-to-insert depth ratio is a good rule of thumb..

And be aware that Protolabs may need to orientate the part differently to accommodate placement and retention of the insert. The mould halves slide together horizontally; if the insert falls out during closure, damage to the mould may occur. Inserts should also be consistent in size—we recommend a tolerance of +/- 0.05mm on locating surfaces to avoid flashing. 

Finally, knit lines occur when molten plastic flows around an object and rejoins on the far side, a phenomenon that is quite possible when inserts are present. These lines are usually cosmetic, but can lead to crack formation later on. Glass-filled materials help alleviate this problem, as does beefing up the walls around these areas (within reason). Slightly thicker walls or ribs in the area around the insert are a good idea to support the insert and prevent movement. As always, these and other concerns or suggestions will be clearly spelled out in the design for manufacturability (DFM) analysis supplied with your quote. Feel free to contact one of our applications engineers with your questions at +44 (0) 1952 683047 or [email protected].