English edition

A threaded connection in a metal injection molded component must provide both a usable thread and a structure capable of carrying the assembly load. Choosing a molded thread, a post-sintering tapped thread or a metal insert requires more than the nominal screw size. Required preload, disassembly conditions, dimensional variation after sintering and the remaining metal around the boss must be considered together.

In small housings, covers, hinge brackets and wearable metal frames, a screw can maintain both assembly position and pressure at the joint interface. A clearly defined thread that accepts a screw may still have insufficient effective engagement, or the screw tip may contact the bottom before the cover is clamped. Thread geometry, the load path and the space required for manufacturing therefore belong to the same design problem.

1. Manufacturing routes for MIM threaded connections

MIM combines metal powder and binder into a feedstock, injection molds the shape, and removes the binder before sintering. It can integrate bosses, ribs and walls, but the functional thread does not necessarily have to be completed in the mold. An alternative is to produce the main body by MIM and machine only the threads, bearing faces and reference holes.

Tooling investment and recurring manufacturing cost should be compared together. An unscrewing core may eliminate tapping while increasing mold complexity and the molding cycle. Secondary machining introduces tools, fixtures, cleaning and inspection, but allows functional dimensions to be established from reference surfaces after sintering.

• Molded threads: Consider them when feature integration and reduced secondary machining justify the tooling, and the required thread accuracy can be maintained after sintering. Demolding, parting-line marks and damage to the molded part require attention.
• Cut tapping after sintering: Consider it when thread size and position need to be controlled from final machining references. Pilot-hole variation, chip evacuation, tool access, effective depth and cleaning space remain important.
• Form tapping after sintering: Consider it when the actual sintered material has suitable ductility and microstructure. Chipless processing must be balanced against sensitivity to pilot-hole size, lubrication and forming load.
• Metal thread inserts: Consider them for repeated disassembly, wear resistance, repair requirements or a thread surface different from the parent material. Receiving threads, residual wall thickness and installation space must be included.

Thread milling provides another secondary machining option. With adequate tool access, circular interpolation and positioning accuracy, it can offer flexibility in diameter adjustment and chip control. In small holes, cutter diameter, neck length and stiffness limit its application, so it is not an automatic substitute for tapping.

2. Mold parting and sintering shrinkage in molded threads

External threads and parting-line interference

External threads can be divided between mold members according to the opening direction. A step or flash along the parting line can interfere with the mating thread. A small protrusion crossing the thread profile can affect both the initial turning resistance and the contact distribution after assembly.

Narrow flats near the parting line can help control mold shut-off and witness marks. Their position and width must nevertheless be compatible with the working portion of the mating thread, the required continuity of the profile and the connection load. Dimensions from one mold example should not be applied indiscriminately to other thread sizes.

Unscrewing an internal thread core

An internal thread mechanically engages its molding core, preventing ordinary straight withdrawal. With an unscrewing core, the molded part must be restrained while the core rotates and translates along the thread lead. Core stiffness, drive design, wear and the handling strength of the molded part are particularly relevant for small threads.

A satisfactory feedstock thread immediately after molding is not yet a finished metal thread. Pitch, pitch diameter, roundness and axis direction must be checked after debinding and sintering. A single average shrinkage factor for the overall component cannot adequately describe every local thread dimension.

Support conditions and thickness transitions can influence the final position and inclination of a long boss attached to a thin floor. Mold compensation and sintering control must therefore relate to the final thread references. The benefit of completing the thread in the mold depends on process capability that includes these sources of variation.

3. Post-sintering tapping: pilot holes, chips and material condition

The role of the molded pilot hole

A MIM boss and pilot hole can be produced first, followed by tapping after sintering. Whether the molded hole can be used directly or requires drilling, reaming or another finishing operation is a separate decision. An acceptable average diameter may conceal a locally restricted section, taper, out-of-roundness or an inclined axis.

For cut tapping, a smaller pilot hole increases the material to be removed and the tool load. An oversized hole can reduce thread height and the effective area available to resist stripping. Select the pilot hole with reference to the final thread tolerance, material condition and tool recommendations. The tap should not be expected to correct substantial distortion left by sintering.

Through holes and blind holes

A forward chip-ejection strategy requires a clear exit below the hole. Blind-hole machining requires suitable tool geometry and conditions for removing chips toward the entrance. Even a nominally through hole can behave differently if its exit is obstructed by the fixture. Compacted chips can increase torque and contribute to tool failure.

Effective thread depth, tap travel and total drilled depth are distinct. The incomplete cutting region at the tap lead, the drill-point geometry and chip space must be allowed for. Cleaning and drying also matter: retained chips or abrasive particles can destabilize assembly torque and damage thread surfaces.

Form tapping is different from molding a thread

Form tapping creates an internal thread by plastically displacing the wall of a hole in already sintered metal. It is not the same process as shaping feedstock in a MIM mold. Reduced chip generation is useful only if the actual sintered material can tolerate the required deformation.

Alloy designation alone is insufficient. Sintered condition, the distribution of residual pores, local defects and heat treatment can change ductility and machining load. Conditions established for wrought material or another steel should not be transferred directly to a small MIM connection. Trial machining is needed to establish the applicable range.

The pilot-hole diameter for a forming tap must not be treated as interchangeable with that for a cutting tap. A smaller hole increases the amount of displaced metal but can also increase torque and frictional heating. Material flow may leave a split profile at the thread crest; this must be distinguished from a harmful crack at the root. Cross-sectional geometry, dimensions, machining load and connection performance should be assessed together.

4. Effective engagement and blind-hole depth

Effective engagement length, L_e, is not the total distance a screw enters a hole. It is based on the actual overlap of complete external and internal thread profiles capable of transferring load. Entrance chamfers, incomplete starts, the screw-tip geometry and the tap’s incomplete thread region must be excluded as appropriate.

Boss outside diameter D_b, pitch P, remaining wall thickness t and bottom clearance C represent different design variables. Increasing screw diameter increases its section but removes metal from a boss of unchanged outside diameter. Pitch and pitch diameter also matter, so nominal diameter alone cannot establish connection strength.

Tolerance accumulation in screw projection

For a screw whose length is specified beneath the head, subtract the assembled cover, washer and other relevant stack thicknesses to estimate its projection into the boss. A countersunk screw or another length convention requires the corresponding dimensional definition.

Check minimum engagement using the short-screw and thick-stack combination. Check bottom interference using the long-screw and thin-stack combination. Evaluating only nominal dimensions can miss one of these limits. Washer thickness, coatings and the actual bearing locations belong in the relevant stack.

Increasing torque without adequate clamping

If the screw contacts the hole bottom or an incomplete thread region first, turning resistance may increase before the cover is adequately clamped. A tool can then reach its torque setting without producing the required joint pressure. Assembly evaluation should include seating, screw length and effective thread depth.

Deepening the hole to create clearance also affects the metal remaining beneath the boss and nearby functional surfaces. A drill point approaching a thin floor can create a risk of unintended breakthrough or deformation. Extending thread length should therefore be checked for new thin sections elsewhere in the component.

5. The load path from thread flanks to the boss root

Tightening develops tensile preload in the screw and compression through the cover and bearing surfaces. Load received by the internal thread passes through the boss wall and root into the surrounding floor and ribs. The first limiting location determines the resulting damage mode.

• Screw and engaged threads: Excessive screw elongation or stripping of the parent-material thread requires examination of screw strength, material condition, engagement and thread tolerances.
• Boss wall and free edges: Wall opening or cracking toward an edge requires examination of the minimum machined wall, hole eccentricity and distances to adjacent holes and boundaries.
• Boss root and floor: Eccentric assembly or service loads can increase local bending. Fillets, floor thickness, rib connections and support conditions influence this region.
• Head bearing surface and cover: Local indentation, cover deformation and settling can change preload. Contact area, flatness, material, finish and tightening sequence all contribute.

Engagement length and nonuniform load sharing

Engaged thread turns do not necessarily share load equally. Elastic deformation of the screw and parent material, thread geometry and pitch errors influence the distribution. Doubling the number of engaged turns therefore does not imply twice the connection strength.

Required engagement depends on the relative strengths of the screw and parent material, actual thread dimensions, boss geometry and service loading. A universal engagement of one or one-and-a-half nominal diameters is inappropriate for all MIM materials. Additional engagement cannot by itself correct a boss wall that fails first.

Wall transitions and local integrity

More metal around a hole can increase the supporting section, but an abrupt thickness difference relative to the surrounding floor can complicate molding, debinding and sintering. Ribs and gradual transitions can be considered while preserving the minimum section remaining after machining.

Average density or a material certificate alone does not establish the local integrity of every boss. Suspected defects or demanding assembly loads may require representative sections combined with component testing. Allowable stresses for a sintered material should not automatically be replaced by wrought-material properties carrying the same alloy name.

6. Receiving threads and wall thickness for metal inserts

A metal insert provides a separate surface for contact with the screw. It can address wear, repeated disassembly and repair needs, but the parent material still carries the load. Both the screw-to-insert interface and the insert-to-parent-material interface must be assessed.

Wire inserts and STI receiving threads

A wire insert engages a dedicated receiving thread on its outside while forming the screw thread on its inside. It is not a coil added to an ordinary tapped hole of the final screw size. Use the tap and gauge requirements specified for the insert system.

The enlarged receiving thread reduces the remaining metal in a boss of fixed outside diameter. Assess the minimum distance from the outermost receiving-thread geometry to the boss surface, rather than using the final screw diameter. Position error can make one wall thinner than its nominal value suggests.

Installation depth and entrance geometry must prevent the coil from protruding above the bearing face or obstructing entry. For products with an installation tang, follow the product-specific instructions to determine whether removal is required and how retained material is managed. Tangless products require their corresponding tools and procedures. Free-running and locking versions must also be distinguished. A wire insert does not inherently provide a screw-locking function.

Space required by solid bushing inserts

A bushing insert adds its own wall and external retention structure around the internal thread. Externally threaded products require their specified receiving thread and machining depth. Key-locking products additionally require space for the keys and a compatible parent-material condition. Final screw size alone does not establish interchangeability between insert systems.

Keys that resist insert rotation do not establish the pull-out strength of the parent thread or the strength of the boss root. Installation-tool access, the removal path for replacement and retained debris also require attention. Metal insert installation must not be treated as equivalent to heat-setting a threaded insert into a thermoplastic.

Comparing costs over the connection’s use

Comparing only the insert purchase price with direct tapping omits receiving-thread machining, installation, inspection and cleaning. Conversely, damage to an integral thread may require replacement of the entire housing. Compare initial manufacturing and maintenance costs after defining expected disassembly conditions and the repair unit.

7. Tightening torque, preload and contact conditions

Tightening torque is an input used to generate preload. Much of it is consumed by friction at the threads and beneath the head, so identical torque does not necessarily produce identical preload. Material pairing, coatings, lubrication and changes from repeated use affect the relationship.

Lower friction can increase preload at the same torque. Lubrication or coating changes therefore require more than an assessment of easier assembly. Higher friction can produce the opposite result, reaching the torque setting before the necessary preload develops. Galling in stainless contacts requires examination of material pairing and surface and lubrication conditions, rather than simply increasing torque.

The design must retain sufficient clamping against separation or slip in service while keeping maximum assembly preload within the limits of the screw, parent threads, boss and bearing surfaces. Additional screw load under external loading depends on the relative stiffness of the joint and screw and on where the load enters. Simply adding the entire external load to initial preload is insufficient.

Repeated assembly and different causes of preload loss

Initial and repeated assembly may involve different contact conditions. Coating wear, lubricant changes, local indentation and debris can alter the torque–preload relationship. Products intended for disassembly should therefore be evaluated after the specified reuse sequence as well as at first assembly.

Preload loss does not always mean rotational loosening. Settling and local deformation can reduce preload without screw rotation. Distinguish vibration-induced rotation, contact settling, thread damage and insert movement before choosing a remedy. A locking feature cannot compensate for inadequate bearing stiffness or incorrect screw length.

8. Thread gauging and connection-performance testing

A thread plug gauge assesses conformity with specified thread tolerances. It does not directly measure preload, pull-out resistance or service life. A gauged thread can still fail in assembly because of a thin boss, a local defect or insufficient effective engagement.

Dimensional checks beyond screw fit

Apply GO and NO-GO acceptance according to the relevant thread specification and gauging procedure. Depth, entrance form, axis position and inclination, and the relationship to bearing surfaces need appropriate checks of their own. For an insert connection, evaluate the receiving thread before installation and the final thread afterward using the applicable procedures.

A screw entering the hole does not verify thread tolerances. It is a mating component rather than a calibrated gauge, and one favorable pairing does not establish production interchangeability. If burrs or chips cause resistance, investigate before forcing the gauge into the hole.

• Dimensions and geometry: Check gauging, effective depth, axis location and bearing relationships, with mold cavity, sintering lot, tool-use interval and post-treatment condition recorded.
• Tightening: Observe torque, rotation and seating, and measure preload directly when necessary. Control screw, washer, lubricant, coating, tightening speed and sequence.
• Limiting load: Identify thread stripping, insert displacement and boss cracking. Distinguish axial tensile testing from tightening-to-failure testing.
• Repeated assembly and service: Examine entry behavior, torque change, residual preload and surface damage after the relevant sequence, with reused components explicitly defined.

Test boundaries and failure modes

A test that rotates a screw to a tightening limit is different from pulling the connection axially. The former includes friction and the assembly process; the latter depends on fixture support and load introduction. A fixture that constrains the boss more strongly than the actual product may produce an unrepresentative failure mode.

Record failure location and mechanism alongside maximum load. Separating sintering lots, mold cavities and tool-life intervals helps distinguish material variation from machining variation. Acceptance limits and sample numbers should follow product risk and reliability requirements rather than arbitrary universal values.

The thread-gauging framework of ISO 1502 and torque–clamp-force testing concepts of ISO 16047 can inform verification. Confirm that the actual thread size, material combination and locking features fall within the applicable scope. Citing a standard number alone does not establish the durability of a particular MIM connection.

9. Information to retain in drawings and production specifications

A thread designation and depth allow an initial manufacturing assessment but do not fully describe assembly and service requirements. Functional references and the actual screw–cover combination are needed to align secondary machining and verification.

• Material condition: Specify the MIM alloy, heat treatment, surface treatment and relevant property or microstructural requirements.
• Thread and references: Define designation, pitch, tolerance class, the effective thread region, and hole position and axis direction relative to datums.
• Assembly stack: Compare screw length, cover and washer thicknesses, bearing surfaces, minimum engagement and bottom clearance at maximum insertion.
• Supporting structure: Check the minimum machined wall, edge distances, boss fillets and floor thickness.
• Insert specification: Identify the system, installed length, receiving thread, depth, direction, locking or tang provisions, and inspection requirements.
• Assembly and validation: Connect surface and lubrication conditions, tightening control, reuse limits, and functional, strength and life evaluations.

Process sequence depends on material condition and final function. Machining before heat treatment can reduce tool loading but leave subsequent dimensional changes to manage. Machining afterward can help control final dimensions while changing tool requirements and loads. Coatings on threads or bearing surfaces require renewed attention to both dimensions and friction.

Development should begin with assembly loads and disassembly requirements, followed by the available boss space and a comparison of manufacturing routes. Trial machining and representative assembly testing then establish the conditions to retain for pilot holes, tools, cleaning and inspection. A successful MIM threaded connection combines accurate thread geometry with adequate support throughout the surrounding structure.