Electric Scooter Wheel Hubs Produced by Insert Molding

Electric Scooter Wheel Hubs Producing by Insert Molding

Parts Material	PA6+30%GF, Steel
Application Industry	Tools & Equipment
Manufacturing Process	Mold Making Service & Insert Molding Service
Mold Type	Production Tooling
Tolerance(Accuracy)	Parts: ±0.1mm(±0.004″)
Surface Treatment	Standard Smooth

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Requirements

Details

In 2022, a maker of electric scooters for city commuters came to us with a wheel project. Their scooters, aimed at young urban riders, needed wheels that were both light and tough—so they wanted to use insert molding: a steel core for strength, wrapped in PA6+30%GF plastic for lightness and wear resistance. The big ask? The steel and plastic had to stick together like glue. If they came loose, the wheel could wobble or even fail when hitting curbs or potholes—something no commuter wants. And since they made 3,000 scooters a month (small batches for them), every wheel had to be consistent. No room for “sometimes good, sometimes off.”

The Problems We Ran Into

Getting these two materials to work together wasn’t easy. Here’s what tripped us up at first:

Steel shifting in the mold
The steel core is smooth, and when we injected the hot PA6+30%GF, the plastic would push against it. In the first 500 wheels we made, about 15% had steel cores that slid off-center by up to 0.8mm. When we tested these on scooters, you could feel the wheel wobbling at 15km/h—like riding with a flat spot, but worse. Over time, that would wear out the bearings fast.
Plastic and steel not sticking
PA6+30%GF doesn’t naturally grab onto steel. We pulled test samples: yank the steel core, and it popped out with just 800N of force. But city riders hit curbs—our tests showed that can jolt the wheel with 1,200N. At 800N, the steel would start to loosen after a few months.
Cracks from shrinking
PA6+30%GF shrinks a little as it cools—about 1.2%—but steel barely shrinks at all (less than 0.01%). This mismatch pulled the plastic tight around the steel, and after 500 rides, 12% of wheels developed tiny cracks where the two materials met. Small cracks now, big problems later.

How We Fixed It

We focused on three things: keeping the steel in place, making the plastic stick better, and stopping the cracks.

Locking the steel in position
We added a small groove to the steel core, then carved a matching ridge in the mold. When we dropped the steel into the mold, the ridge fit into the groove—like a puzzle piece. This stopped the sliding. In the next 1,000 wheels, only 1.5% had cores off by more than 0.1mm—no more wobble in tests. We also drilled four tiny holes (0.2mm) in the mold near where steel and plastic meet. These let air escape, so the plastic filled every gap around the steel, no bubbles.
Making the plastic grip tighter
We sandblasted the steel cores with 80-grit sandpaper—roughing up the surface so the plastic had something to “bite” into. Before, the steel surface was smooth (like a polished spoon); after, it was bumpy (like fine sandpaper). Now, when we pulled, it took 1,500N to yank the steel out—way more than the 1,200N from curb hits.
Stopping the cracks
The shrinkage issue needed slower cooling. We kept the mold 10°C cooler than before, so the plastic shrank more gradually. We also held the plastic under pressure 20% longer (18 seconds instead of 15) as it cooled—pushing it tighter against the steel so there was less stress. After these tweaks, only 2% of wheels had tiny cracks after 1,000 rides—most were so small they didn’t affect strength.
Catching issues fast
We set up a camera to check every wheel right after molding. It took 2 seconds per wheel to scan for steel alignment and plastic coverage. If something was off, we fixed it before the next batch. This kept mistakes from piling up, even with small runs.

What Happened Next

After these changes, the wheels held up:

Almost all (99.5%) stayed aligned, so no more wobble.
The steel and plastic stayed stuck through 5,000+ curb hits in tests.
Cracks were rare enough that the client’s warranty claims for wheel issues dropped by 90%.

For a scooter that gets ridden daily on bumpy city streets, that’s a win. It showed that with the right tweaks, insert molding can make parts that are both light and tough—no compromises.

Requirements

Requirement Category	Details
Material Combination	Wheel body: PA6+30%GF (for lightweight and wear resistance); core: Steel hardware (for structural rigidity)
Bond Strength	Steel-plastic interface must withstand ≥1,200N of force (to resist curb impacts in urban use) without separation
Alignment Stability	Steel hardware must remain centered in the PA6+30%GF body to avoid wheel wobble during operation (typical commuting speeds: 15–20km/h)
Resistance to Internal Stress	Minimize micro-cracking at the steel-plastic interface, even after 500+ rotation cycles (to ensure long-term structural integrity)
Durability	Withstand repeated urban use (including uneven roads and curb impacts) over the scooter’s typical lifespan
Dimensions	Confidential (optimized for scooter wheel assembly)
Cost Parameters	Confidential (aligned with production feasibility and market competitiveness)

Frequently Asked Questions

What if my project involves a challenge I don’t see in any case study?webadmin2025-07-14T06:14:41+00:00

What if my project involves a challenge I don’t see in any case study?

No worries—our case studies focus on the usual kinds of problems, but we deal with unique or tricky challenges all the time. Even if your specific issue isn’t featured, the problem-solving approach and technical skills you see in the case studies still apply. Send us a note with what you’re up against, and we’ll walk through how we’d tackle it for you.

Are the techniques and methods shown in these case studies up to date?webadmin2025-07-14T06:14:02+00:00

Are the techniques and methods shown in these case studies up to date?

We keep our case studies fresh—they show the methods we actually use right now. When we start using new tools, materials, or process fixes (like better mold simulation software or greener plastics), we update the case studies to include those too. That way, what you’re seeing matches how we work in today’s manufacturing landscape.

How can I discuss applying similar approaches to my project?webadmin2025-07-14T06:08:51+00:00

How can I discuss applying similar approaches to my project?

If a case study clicks with what you’re working on, just get in touch with our team. Share the details of your project, and we’ll talk through how the strategies highlighted—like material pairing, mold tweaks, or process adjustments—might fit your goals. If needed, we can also craft custom solutions that match what you need.
Feel free to reach out at sales@kingstarmold.com to start the conversation.

Are these case studies based on real projects?webadmin2025-07-14T06:05:23+00:00

Are these case studies based on real projects?

Yes, all our case studies are based on real client collaborations. To protect our clients’ rights, we only share details that have been approved for disclosure—including specific timelines, challenges, solutions, and measurable outcomes, all backed by data from client production records or third-party testing.

How can these case studies help me understand if you’re a good fit for my project?webadmin2025-07-14T06:03:08+00:00

How can these case studies help me understand if you’re a good fit for my project?

Each case study delves deeply into specific issues, such as how to extend the lifespan of components or how to accelerate production speed, and shows you how we solved these problems. Even if your project is not exactly the same as them, these cases can help you clearly understand how we handled the issues: breaking down the problem, adjusting the method, and achieving results. This enables you not only to see the outcomes we have achieved, but also to understand how we think about solutions – so you can roughly judge whether we can meet the requirements of your project.

How does insert molding differ from over molding?webadmin2025-07-14T05:56:20+00:00

How does insert molding differ from over molding?

Let’s start by breaking down what each process actually does.

Insert molding is all about integrating a pre-made component—something like a small metal piece, a ceramic part, or even a rigid plastic piece—into a plastic part during manufacturing. Here’s how it works: you first place that pre-made “insert” into a mold, then inject molten plastic around it. As the plastic cools, it bonds tightly to the insert, creating a single part where the insert adds specific properties the plastic alone can’t provide. Think of something like a USB connector: the metal pins inside are inserts, and the plastic housing forms around them, holding them in place while letting electricity flow through the metal.

Over molding, on the other hand, is about layering materials. It starts with a base part—usually a rigid plastic, though sometimes metal—that’s already been formed. Then, a second material, often something softer like rubber or a flexible plastic, is injected over this base. The result is a part that combines the strength of the base with the properties of the overlaid material. A common example is a tool handle: the core might be a hard plastic for structure, and the outer layer is a soft, grippy material that makes it comfortable to hold.

Now, how do these two differ?

For one thing, the role of the added material is distinct. In insert molding, the insert is like a functional piece that the plastic wraps around—it might add strength (like a metal rod in a plastic bracket) or conductivity (like copper in an electronic part). The plastic here is mostly there to hold the insert in place and give the part its overall shape. In over molding, the base part is already a complete structure, and the overlaid material is there to add secondary traits: maybe flexibility, a better grip, or a seal. It’s less about adding a “working” component and more about blending textures or functions.

The process flow is different too. Insert molding requires careful placement of the insert before any plastic is injected—you need to make sure it stays put while the molten plastic flows around it, which is why molds often have little pins or slots to hold inserts in place. Over molding usually happens in two steps: first, the base part is made, then it’s moved to another mold (or the same mold shifts) to have the second material injected over it. It’s more about building up layers than encapsulating a pre-made piece.

Material compatibility also plays differently. Insert molding often pairs dissimilar materials—like metal and plastic—so you have to make sure the plastic can bond to the insert, maybe by roughing up the insert’s surface. Over molding typically uses materials that work well together, like a hard plastic base and a soft rubber that adheres to it without much extra work.

In the end, it comes down to what you need the part to do. If you need a piece that combines a specific functional component (like a metal pin for electricity) with plastic, insert molding is the way. If you want to blend a rigid structure with something soft or flexible, over molding makes more sense.

What are the common challenges in insert molding and how to solve them?webadmin2025-07-14T05:45:26+00:00

What are the common challenges in insert molding and how to solve them?

Common Problems You May Encounter

One frequent issue is inserts shifting out of place during injection. When this happens, parts often don’t work right—like connectors that fail to conduct electricity. The fix here is to tweak the mold’s positioning setup: adding tighter-fitting clamping slots or small locating pins that hold the insert steady, so it doesn’t move when plastic flows in.

Another problem is weak adhesion between the plastic and the insert. Over time, this can make the insert loosen or even fall out, especially under stress. To improve this, you can rough up the insert’s surface—sandblasting works well—or pick a plastic that bonds better with the insert material. It’s about making sure the two materials “grab” onto each other properly.

Inserts can also warp or crack if the injection pressure or temperature is too high. Sometimes the material just isn’t strong enough. In these cases, dialing back the pressure or heat often helps. If that’s not enough, switching to a tougher material—like using hardened steel instead of regular steel for metal inserts—usually does the trick.

Then there’s the issue of plastic cracking around the insert, mostly from uneven cooling and shrinkage. This tends to happen where the plastic meets the insert. Adjusting how long you hold pressure during cooling can ease the stress, as can softening the insert’s edges—rounding off sharp corners, for example, keeps the plastic from pulling too hard as it sets.

Two Problems We’ve Fixed

Case 1: Shifting pins in USB-C connectors

A client was making USB-C connectors, but 15% of them were faulty because the small metal pins inside kept moving during molding. This meant the connectors often didn’t charge or transfer data properly. We reworked the mold with two tiny locating pins (fitted to within 0.01mm of the pin’s size) and added a quick visual check before injection to make sure pins were seated right. After that, only 0.3% of parts had issues.

Case 2: Loose plastic on sensor housings

A car parts maker was having trouble with their sensor housings—plastic kept peeling away from the aluminum inserts, which risked sensor failure. We suggested sandblasting the aluminum to give it a rougher surface (measuring Ra 1.6μm, which gives the plastic more to grip) and switching to a PA66 plastic that expands and shrinks at a rate closer to aluminum. Tests afterward showed the bond was 40% stronger when we pulled on the inserts—no more peeling.

What should you keep in mind when designing inserts for insert molding?webadmin2025-07-14T05:37:13+00:00

What should you keep in mind when designing inserts for insert molding?

How you treat the insert’s surface matters. Things like sandblasting to create tiny grooves or adding coatings (zinc plating, for example) help the plastic grip better. This stops the insert from pulling away when the part is used.

The insert’s size has to fit snugly with the mold. Keeping dimensions tight makes sure it stays in place when plastic is injected—no shifting, which would mess up the part or even damage the mold.

Sharp edges and corners on the insert are best avoided. When plastic cools and shrinks, these sharp spots can concentrate stress, making the finished part more likely to crack.

The insert and plastic should expand and shrink at similar rates when heated or cooled. If they don’t align, it can create stress inside the part as it cools, leading to warps or cracks later on.

The insert’s shape needs to make it easy to put in the mold and keep it there. Small grooves or bumps, for example, can act like anchors, holding it steady while plastic is being injected.

We have a design team that has handled numerous insert molding projects before. Whether you need assistance in reviewing your existing design or require us to start the design from scratch for you, you can contact us.

What are the common insert materials used in insert molding?webadmin2025-07-14T05:31:35+00:00

What are the common insert materials used in insert molding?

Common insert materials include metals such as copper, steel, and aluminum. Copper inserts are widely used in electronic connectors—think the metal pins in USB ports, where they’re molded into plastic housings to ensure stable electrical conductivity. Steel inserts often find their place in automotive parts, like seat frames, where they reinforce plastic structures to handle repeated weight and stress. Aluminum, valued for its lightweight properties, is frequently embedded in laptop casings or car interior components to balance strength and reduced weight.

Ceramics are another key group, particularly useful in scenarios needing high-temperature resistance or insulation. For instance, ceramic inserts are often used in electrical insulators or small components within engine systems, where they protect plastic parts from heat damage while blocking unwanted electrical flow.

Pre-formed plastic parts also serve as inserts, enabling the creation of complex structures. A common example is tool handles: a rigid plastic core is molded into a softer, grippable outer layer to combine structural support with comfort, seen in items like screwdrivers or power tool grips.

Even fiber-reinforced composites are employed, typically to boost localized strength. They’re often found in industrial gear, such as plastic gears with composite inserts that enhance durability in high-wear areas, or in sports equipment like bicycle frames where they add rigidity without excessive weight.

What are the main differences between insert molding and traditional injection molding?webadmin2025-07-14T05:27:56+00:00

What are the main differences between insert molding and traditional injection molding?

Although both involve cooling the melted plastic into the desired shape, there are still differences between them.

Traditional injection molding produces parts using only plastic: molten plastic is injected into a mold, where it cools and solidifies into a single-material component. Insert molding, by contrast, begins with placing prefabricated inserts—such as small metal components or ceramic pieces—inside the mold cavity. Molten plastic is then injected around these inserts, bonding with them as it sets to form a part combining plastic and the inserted material. This requires careful attention to ensuring compatibility and secure bonding between inserts and plastic, with mold designs needing specific features to hold inserts in place reliably.

Traditional injection molding offers a simpler process with fewer steps, less complex tooling, and faster production cycles—factors that make it cost-effective for high-volume manufacturing. It accommodates a wide range of plastics and avoids issues like insert misalignment or poor bonding. However, it is limited to single-material parts, which may lack the structural reinforcement or functional attributes—such as conductivity or heat resistance—provided by multi-material designs. Additional assembly steps are often necessary to integrate components like fasteners or metal parts.

In contrast, insert molding eliminates the need for secondary assembly. It embeds the inserts (typically metal hardware) directly into the plastic, thereby reducing labor costs and improving the integrity of the components. It can combine the multi-functional properties of the plastic with the unique properties of the inserts – for example, using metal to enhance strength or using ceramics for insulation – making it easier to produce complex and high-performance components. Its disadvantages include the higher initial mold cost due to the need to design special mold features for placing the inserts, the longer production cycle caused by the need to position the inserts, and the risk of insufficient bonding or insert displacement if the material or process parameters do not match.

In terms of practical application, traditional injection molding is highly suitable for simple, single-material components, such as packaging, basic consumer goods (such as plastic containers) or non-structural parts, which are prioritized in terms of cost and production speed. Injection molding technology performs well in manufacturing components that require higher functionality, such as automotive components with metal contacts, electronic devices that need to have both insulation and conductivity functions, or industrial tools with reinforced parts – in these scenarios, the combination of materials can bring value beyond what a single plastic can provide.

What is insert molding?webadmin2025-07-14T05:16:28+00:00

What is insert molding?

Insert molding works by first putting pre-made parts—like small metal pieces or other materials—into a mold. Then you inject molten plastic into that mold. As the plastic cools and hardens, it bonds firmly to those pre-made parts, turning everything into a single solid piece. It’s similar to using hot plastic to “glue” components together, resulting in one strong part instead of having to put pieces together later.

Technically, this injection molding method embeds preformed inserts—usually metals, ceramics, polymers, or composites—into the final plastic part. The process starts with carefully positioning these inserts in the mold cavity before molten plastic is injected. As the plastic cools and sets, it wraps around the inserts, creating mechanical or chemical bonds that form a unified structure. This cuts out the need for extra assembly steps like screwing or gluing parts together, shortens production time, and makes the finished part sturdier. By combining the useful properties of the inserts—such as conductivity or strength—with the flexibility of plastic, it lets manufacturers make complex, multi-material parts designed for specific uses.

Note:
We maintain pre-market confidentiality agreements and sign NDA/NNN with all our customers. Every case you see has been shared with client approval. To protect sensitive information, some details have been blurred or modified. All photos were taken internally by KingStar Mold. Thank you for your support and cooperation.

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