Insert Molding Services
RpMolds offers expert insert molding services. Embed threaded brass inserts, structural pins, and electrical contacts directly into your injection-molded parts — eliminating secondary assembly and producing stronger, more reliable components in a single shot.
One Shot. Metal and Plastic Together.
Insert molding is an injection molding process where pre-formed metal components — threaded inserts, pins, shafts, or electrical contacts — are placed inside the mold cavity before the plastic is injected. The molten plastic flows around and mechanically locks to the insert, producing a single integrated part.
The result combines the structural strength and wear resistance of metal with the design flexibility and light weight of plastic — in one manufacturing step, with no secondary assembly operations required.
Benifit of Insert Molding
Eliminates secondary assembly
Inserts are captured in the mold — no press-fitting, ultrasonic welding, or post-molding installation needed
Stronger fastening than plastic alone
Threaded brass inserts achieve 500–1,200 N pull-out force, versus 80–200 N for self-tapping into plastic
Lighter than all-metal parts
Replace machined metal housings with plastic bodies and metal only where needed — typical weight reduction of 30–60%
Reduced part count and BOM complexity
Multiple components consolidated into one molded assembly — fewer suppliers, less inventory, simpler logistics
Reusable threads over thousands of cycles
Critical for products that require repeated assembly/disassembly without thread wear or stripping
Four Categories of Inserts We Work With
Select a category to explore materials, dimensions, retention specs, and typical applications for each insert type we regularly process.
Design note: For optimal pull-out performance, the plastic wall surrounding the insert should be at least 1.5× the insert outer diameter. A well-designed M4 brass insert in PA66-GF30 achieves 700–900 N pull-out. Inserts must incorporate anti-rotation features (knurls, hex body, or grooves) — smooth cylindrical inserts will rotate under torque load.
Design note: Shaft insert diameter should be ≥ the expected load / (allowable shear stress of plastic × engagement length). For rotating shafts, specify the bearing clearance — typically 0.05–0.15 mm for light loads in PA or POM. Grooves or cross-holes are essential for axial retention; smooth shafts will eject under vibration.
Design note: Plastic materials for electrical applications must have appropriate dielectric strength and tracking resistance. PC, PBT, and PA66 are typical choices. Plating thickness affects insert dimensional tolerance — specify post-plating dimensions. Avoid silver plating in high-humidity environments (migration risk). Always specify the maximum mold temperature to verify plating compatibility.
Design note: Customer-supplied inserts must meet dimensional tolerances that allow reliable mold seating (typically ±0.1 mm on seating surfaces). RpMolds can also design and source the metal inserts as part of your project. Provide functional requirements and assembly intent — we'll engineer the insert geometry and the mold pocket together to ensure reliable placement and retention.
Materials for Insert Molding Applications
The plastic substrate must accommodate both the insert’s thermal expansion coefficient and the mechanical loads at the insert-plastic interface. These are our most commonly processed materials for insert molding.
PA6 / PA66
Automotive, power tools, industrial fasteners
PC
Electronics, medical devices, optical housings
ABS
Enclosures, panels, consumer devices
POM
Gears, bearings, precision connectors
PC+ABS
Automotive interior, electronics housings
PP+GF
Consumer goods, fluid handling, housewares
PA+GF30
Automotive structural, industrial equipment
PBT / PET
Connector housings, terminal blocks, relays
6 Rules for Reliable Insert Molded Parts
Insert molding introduces unique stress concentrations that don’t exist in standard injection molding. These guidelines prevent the most common failure modes — cracking, insert pull-out, and rotation.
The plastic surrounding an insert must be thick enough to bear the pull-out load. For a brass M4 insert (OD ~7 mm), the minimum plastic radius from insert center to nearest wall should be ≥ 10.5 mm. Thinner walls will crack under torque or thermal cycling.
Every threaded insert must have mechanical anti-rotation features — knurled outer surface, hex body, or circumferential grooves. Smooth cylindrical inserts will spin under screw-driving torque regardless of the plastic's compressive grip.
Weld lines form where plastic flow fronts meet, creating a bond 20–40% weaker than the bulk material. Gate position should ensure weld lines form in low-stress regions — never bisecting an insert seat or boss structure.
The large CTE mismatch between metals and plastics creates differential shrinkage stress around inserts. Design the boss wall thickness to accommodate this — and avoid materials with extreme CTE mismatch (e.g., POM around large steel shafts) unless the part operates in a narrow temperature range.
The mold seat (the locating feature that holds the insert in position during injection) must be tighter than the insert's own manufacturing tolerance. Specify the required insert positional accuracy in the finished part — we'll engineer backward to the mold seat tolerance and insert procurement spec.
Abrupt wall thickness changes at the insert base — especially at flanged inserts — create stress concentration points. Include a small radius (R0.5 minimum) at the transition between the insert flange and the plastic boss wall to distribute stress across a wider area.
Ready to Quote Your Insert Molding Project?
Upload your CAD file and we’ll return a quote within 24 hours — including a free DFM review that checks insert placement, wall thickness, and retention design before tooling starts.