Threads in Injection Moulding: Moulded Threads vs Inserts vs Self-Tapping

Updated: 2026-10-22

Choose a thread strategy around the joint's job: assembly access, clamp load, maintenance, sealing and failure consequences. A metal insert provides a reusable metal thread, but its retention still depends on the surrounding plastic. A screw driven directly into a boss may be the simpler solution when testing supports the required service history. A moulded thread can be appropriate for a cap even when it would be unsuitable for a highly loaded fixing.

Which route should you compare?

Route	Useful starting point	What must be checked
Thread-forming screw for plastics	Direct assembly with no separate insert	Driving and stripping torque, boss cracking, clamp-load retention and reassembly
Post-mould thermal or ultrasonic insert	A reusable machine-screw connection	Insert-specific hole, installation access, retention and process control
Moulded-in insert	A metal component integrated during moulding	Location, shut-off, loading, flash and the actual sealing/load requirement
Moulded plastic thread	Caps, adjustment features and compatible mating threads	Thread profile, shrinkage, release mechanism, operating torque and wear

Do not choose using a universal number of assembly cycles or a generic pull-out figure. A screw family, insert style, polymer grade, filler and boss design form one system. Compare that system under the temperatures, chemicals and loads the product will encounter. Assembly torque and torque needed to rotate an insert in its boss are different measurements.

Design the boss around the selected component

Select the fastener or insert before freezing the hole. Obtain its manufacturer's drawing and recommendations for the intended resin, including hole diameter and tolerance, depth, lead-in and any required taper. A nominal screw diameter is not enough to specify an insert hole. Check the moulded hole along its full engagement length: draft can make its entry acceptable while the bottom is too tight.

Provide clearance so the screw does not bottom out before it clamps the mating part. Define the screw length together with washer, cover thickness, insert position and thread engagement. Consider the tolerance stack rather than checking each part in isolation. Keep local walls and supporting ribs compatible with the wider rib and boss design; adding a large solid boss may introduce sink or uneven cooling.

Review how the boss fills and where flow fronts meet. Cracking during installation can indicate unsuitable geometry, a weak weld line, installation misalignment or excessive process loading. Enlarging the boss alone does not identify which cause is responsible. Sectioning sample joints can reveal whether the plastic has actually engaged the insert's retention features.

Thermal, ultrasonic or moulded-in installation?

Thermal installation softens the plastic locally as the insert enters its hole. Ultrasonic installation uses vibration and pressure; it requires a suitable insert, resin, equipment and supported part. Specify installed depth and alignment, keep the assembly supported, and establish the installation settings with trials. An insert that looks flush may still have incomplete retention or distorted plastic underneath.

Moulded-in installation moves the positioning task into the moulding cycle. Evaluate positive location, protection of the internal thread, detection of a missing insert and removal of an incorrect part. Neither moulded-in nor post-mould installation is automatically strongest for every design. Nor does moulding plastic around metal establish a leak-tight interface: sealing requires its own design and test.

SPIROL's insert design guide explains why insert selection, plastic properties and installation conditions must be assessed together. For a direct screw connection, use the selected screw manufacturer's guidance, such as EJOT's plastics fastening information, rather than transferring an insert-hole rule to a thread-forming screw.

How are moulded threads released?

An external thread may be divided at the parting line if the design permits the resulting seam and release direction. Internal threads may need an unscrewing mechanism or a suitable collapsible core. Some rounded, shallow profiles can be stripped from a core when the complete part can deform and recover. Material family alone does not establish that this will work; assess interference, wall support, ejection temperature and permanent distortion. See undercut release options.

Check the mating thread, lead-in, end stop and permitted backlash. A metal screw, printed counterpart and moulded counterpart are not interchangeable evidence. Test the intended combination, including any coating or lubricant that changes assembly friction.

Worked planning example: a serviceable enclosure

This is a fictional test-planning example, not a Nordmould result or a prescribed design. Assume a small enclosure has four M3 connections and must be opened 20 times during its intended service history. Compare a plastics-specific screw with a post-mould insert using the same proposed resin and cover. The 20 openings are a project assumption, not an industry limit for either route.

Before producing samples, agree the installation window and required joint loads from the product design. Measure initial assembly behaviour, perform the defined opening/closing sequence, then inspect boss cracks, insert rotation, thread damage and cover seating. Repeat retention checks on separately identified samples after the agreed environmental conditioning. Do not use a successful single joint to approve every cavity or material variant. If one route fails, document the failure mode before changing the hole, component or installation settings; changing all three together obscures the cause.

What belongs in the RFQ and approval record?

Send the component part number, material grade, drawing revision, mating stack, screw length, assembly location and expected access. Define axial load, torque, service cycles, environment and whether leak testing is required. Agree sample quantities, cavity identification, conditioning, test fixtures, failure definitions and the approval owner. Record installed position, process settings and each result, rather than only an average.

Use the RFQ brief builder to assemble those inputs and the sample approval pack to record the agreed checks. A project enquiry should ask for the proposed fastening and installation scope explicitly.