Insert Molding | Metal-Plastic Parts

Insert molding combines metal and plastic into one strong part. It puts metal pieces inside plastic to make things work better. You will learn all about it now!

What Is Insert Molding?

Insert molding is when you put metal parts into a mold. Then hot plastic flows around the metal. When it cools, the metal and plastic stick together as one piece.

Insert molding is not the same as overmolding:

Insert molding: Put metal in BEFORE the plastic
Overmolding: Add more plastic AFTER the first layer

This makes parts that are strong, small, and work better. Companies use it to make tools, car parts, and things you use every day.

How Insert Molding Works: Step by Step

Clean the inserts: Wash the metal parts to make them very clean
Put inserts in mold: The metal goes in the right spot
Inject plastic: Hot plastic flows around the metal
Cool down: Everything gets hard
Take out part: The new part is ready to use

Look at this simple picture of how it works: !

Where Insert Molding Is Used

Insert molding makes many things you see and use:

Cars

Sensor boxes
Wire plugs
Dashboard knobs

Doctor Tools

Surgery tools that are easy to clean
Hearing aids that are small and strong
Medical devices that last long

Electronics

USB ports that won’t break
Phone buttons that feel nice
Computer parts that work in hot places

Did you know? 70% of car fluid connectors use insert molding because it stops leaks so well!

Why Insert Molding Is Good

Insert molding has many big benefits:

Saves money: Costs 30-50% less than putting parts together later
Makes stronger parts: Won’t come apart easily
Looks nicer: No screws or glue showing
Lighter weight: Use less metal, more plastic
Faster to make: One step instead of many

One car company saved $0.70 per part by using insert molding instead of screws!

Materials That Work Together

Not all materials mix well. Here are good matches:

Metal Insert	Good Plastic Matches	What They Make
Stainless Steel	Nylon, PEEK, PPS	Hot tools, engine parts
Brass	ABS, Polypropylene	Bathroom pipes, knobs
Ceramics	LCP, PSU	Parts that need heat protection

The best combos stick together and don’t crack when hot or cold. For 5 axis machining of complex metal inserts, special skills are needed.

Making Good Insert Molded Parts

To make good parts, you must be careful about:

Insert placement: Must be in the exact right spot
Temperature differences: Metal and plastic expand differently when hot
Locking shapes: Design bumps and cuts so parts lock together
Clean materials: Any dirt makes weak spots

Common problems include:

Parts not sticking together
Warping when cooling
Metal showing through plastic

For complex shapes, CNC milling helps make perfect inserts that fit just right.

Real World Success Stories

Medical Tool Case Study

A company made surgical tools with stainless steel insert molded into PEEK plastic. This made the tools:

60% lighter
Safe for sterilizing
Easier to hold during long surgeries

Car Sensor Example

A car sensor with brass inserts in nylon 6/6 plastic could:

Work in very hot and cold places (-40°C to 120°C)
Last through 10,000+ temperature changes
Stay waterproof for years
These success stories show why so many companies now use insert molding.

Insert Molding Costs

Insert molding costs more at first but saves money later:

Starting costs: More expensive molds
Running costs: Less labor, fewer parts
Long-term savings: Fewer broken parts, less waste

A real example: A company made automotive parts for $0.50 each with insert molding instead of $1.20 with traditional assembly. They saved $700,000 in one year!

How To Choose A Good Insert Molding Company

When looking for help with insert molding, ask these questions:

“Do you have ISO 9001 quality control?”
“Can you test how the plastic will flow?”
“Have you made parts with my materials before?”
“How do you make sure inserts stay in place?”
“Can you help make my design better?”

Stay away from companies that don’t test parts before making lots of them. For custom precision metal machining, make sure they have experience with your specific metals.

Insert Molding vs. Other Methods

Method	Good For	Not Good For
Insert Molding	Strong parts, electrical connections, tight spaces	Very big parts, simple shapes
Overmolding	Soft-touch grips, water seals	Metal-to-plastic bonds
Assembly with Screws	Taking apart later, fixing	Waterproof needs, small parts
Ultrasonic Welding	Thin plastics, clear parts	Metal-to-plastic, high strength

Insert molding works best when you need strong metal parts inside plastic that won’t come apart. Sinker EDM machining can create very precise metal inserts for the most demanding applications.

Sustainability Benefits

Insert molding helps the planet too:

Uses 18% less CO₂ when using recycled plastic with aluminum inserts
Makes parts that last longer, so less trash
Needs less shipping because parts are already together
Uses less material than all-metal parts

One company switched from all-metal parts to insert molded parts and cut their material use by 22%!

New Trends in Insert Molding

Insert molding keeps getting better:

Micro-insert molding: Making tiny parts under 1mm for medical devices
Smart inserts: Putting computer chips inside plastic
Bio-compatible materials: Safe to put in the human body
Automated insert placement: Robots put inserts in perfectly every time

The market for insert molding grows about 12% every year as more industries see how useful it is.

Questions People Ask A Lot

Can insert molding make very small parts?

Yes! Micro-insert molding can make parts smaller than a grain of rice.

How strong is the bond between the insert and plastic?

Very strong! It uses both mechanical locking (physical shape) and chemical bonding. Parts often break somewhere else before this bond breaks.

Can I use insert molding for small batch projects?

Yes! Companies now offer “rapid tooling” with less expensive molds for smaller projects. This works well with CNC prototype machining for testing designs.

What inserts work best?

Inserts with rough surfaces, grooves, or holes work best because they give the plastic more places to grip.

How long do insert molded parts last?

With the right materials, they can last for decades. Many car parts made this way last the whole life of the car.

Conclusion

Insert molding joins metal and plastic to make strong, lightweight parts in one step. It saves money, makes better products, and helps many industries create amazing things.

From tiny hearing aids to car parts that must work perfectly for years, insert molding solves tough problems. The thermoplastic injection process combined with precisely machined metal inserts creates parts that couldn’t be made any other way.

If you need parts that combine different materials with perfect material compatibility, insert molding might be your answer!