Injection Mold Design for Insert Molding and Overmolding Applications
A part can look perfect in CAD and still cause problems once it reaches the test bench. A metal terminal may start working loose after a few hundred cycles. A soft grip may lift at the corner the first time someone pulls on it.
The problem is often not the part itself, but the small decisions made around the insert, material, mould and process. The usual reaction is to add steps. An extra assembly station, a bit of adhesive, a clip, a screw. Another route is to let one mold do the joining.
That is what insert molding and overmolding do. Insert molding places a ready-made component, usually metal, into the mold so the plastic forms around it. Overmolding puts a second material, often a TPE, over a rigid base. The finished part's lifespan depends mostly on injection mold design, and for Dalal Plastics, which has been moulding for 56 years, that is the first thing to check.
Why Two Materials Are Harder Than One?
Working with one plastic is fairly straightforward because you’re dealing with one material and its behaviour. Once a second material is added, things get more complicated.
During injection, the inserts can shift or even bend if they aren’t supported properly. Two different materials also won’t necessarily bond well, even if the design calls for it. Flash can form around the areas where the insert meets the mould steel. There’s also the difference in shrinkage between metal and plastic. A joint may look fine right after moulding but develop stress as the part cools.
Most of these problems can be avoided during the mould design stage. The way the insert is supported, how the materials are matched, and how the mould is set up all need to be considered before the first shot.
Chapter One: Locking the Insert in Place
Start with the insert. It needs to stay exactly where it is during the entire shot.
Support the insert properly: Molten plastic puts considerable force on the insert. Locating pins and pockets help keep it from shifting during injection.
Watch the gate position: If the flow hits a thin terminal or contact directly, it can bend or move it. A better gate position lets the plastic flow around the insert.
Give the plastic something to grip: Smooth metal does not provide much mechanical hold. Knurling, grooves, and undercuts can improve resistance to pull-out and twisting.
Preheat where needed: Preheating the inserts helps reduce sudden temperature changes and the stress that can come with them.
Keep shut-offs clean: Areas around terminals and threaded inserts need proper shut-offs to prevent plastic from getting into unwanted areas.
Match loading to production volume: Hand loading may be practical for smaller quantities. For higher volumes, automated loading can give more consistent insert placement and help maintain cycle times.
Small changes can matter: Sometimes a minor change to a locating feature is enough to stop contacts from shifting during moulding.
Chapter Two: Getting the Second Material to Stay
With overmolding, the main question is simple: will the second material stay where it belongs?
Start with material compatibility: The substrate and overmold need to be selected as a pair. For example, PP, ABS, or PC may need to be matched with a suitable TPE grade.
Choose how the bond will work:
Chemical adhesion: The two materials need to be compatible enough to bond during moulding.
Mechanical interlock: Holes, grooves, and textured areas give the overmold something to hold on to.
Both: Some applications benefit from using chemical adhesion along with mechanical locking.
Choose the right tooling process: Two-shot moulding with a rotary or core-back mould is one option. A two-step process with a transfer may work better for other applications. The choice affects cost, cycle time, and production volume.
Check wall thickness and hardness: These affect the feel of the overmold, sealing performance, and how easily the material fills the part.
Allow air to escape: Proper venting becomes especially important around edges and tight sections where trapped air can stop the material from filling properly.
Control substrate temperature: If the substrate cools too much before the second shot, the bond between the two materials can suffer.
If the grip peels, check the basics first: Look at the interlock design and the material combination before changing the entire process. A better interlock or a more suitable resin pairing may solve the problem.
Proving It Before Cutting Steel
It’s much easier to find and fix a problem before the mould is made. A mould flow study can give a good idea of how the material will fill the part, where pressure may build up around the inserts, and where weld lines or trapped air could occur. After that, trial runs and sample checks help confirm the bond strength and dimensions before the part goes into full production.
That means fewer iterations, lower cost, and a shorter lead time. It also helps when design and molding sit under one roof. Dalal Plastics offers in-house tooling, so teams can make mould and moulding decisions in the same conversation instead of passing them between teams.
Where Dalal Plastics Fits In
Dalal Plastics brings 56 years of injection moulding experience, backed by 75+ high-precision machines ranging from 35 to 400 tonnes. Its customers include electrical and automotive companies, where inserts and overmolds are common. Quality is supported by IATF, ISO 14001, and ISO 45001 certifications, and the company also offers allied services for end-to-end support. To learn more, read about Dalal Plastics or browse the full list of services.
Conclusion
When insert molding and overmolding are done well, nothing dramatic happens once the part reaches production. The contacts hold, and the grip stays. That quiet reliability is the goal.
Both processes work best when the mold is designed around the materials and the problem to be solved, not the other way round.
Do you have a component that combines metal and plastic, or hard and soft materials? Speak with Dalal Plastics about your design. Contact Us
