In injection molding production, core-pulling and threading mechanism is a core component of mold, mainly used to form special structures such as side holes, side recesses, internal and external threads. Its operational stability directly determines product molding quality, production efficiency, and mold lifespan. In actual production, core-pulling and threading mechanisms often experience problems such as jamming, stuckness, poor thread forming, incomplete core pulling, and mechanism wear. These problems not only cause product scrap and mold damage but also extend production cycle, increase production costs. This article analyzes causes of common failures in core-pulling and threading mechanisms, provides systematic solutions, preventative optimization measures, offering technical reference for mold debugging and production maintenance.

Core-pulling mechanisms are divided into three categories: hydraulic core-pulling, mechanical core-pulling (oblique guide pillars, gear racks, bent pins, etc.), and pneumatic core-pulling. Among these, oblique guide pillar core-pulling and hydraulic core-pulling are the most widely used and have the highest failure rates.

1. Causes of Problem
Clearance between mold guide components (slanted guide pillars, guide grooves, pressure strips) is too small, or machining accuracy is insufficient, surface roughness does not meet standards, and there are burrs or incomplete chamfering; Foreign objects such as plastic flash, metal shavings, and dust enter mold during production, jamming into guide sliding surfaces and aggravating friction and jamming; Core pulling slider and inserts expand due to heat, and insufficient clearance leads to jamming at high temperatures; Inadequate lubrication, lack of lubricating oil on guide surfaces, dry friction causing excessive resistance; Insufficient hydraulic/pneumatic core pulling pressure, or blockage in oil or air circuits, causing malfunction of oil/air cylinder; Incorrect mold opening and closing sequence, causing interference between core pulling action and ejection and mold opening/closing actions.
2. Solution
Re-grind mating surfaces of guide components to ensure a roughness Ra ≤ 0.8μm. Adjust mating clearance appropriately; generally, clearance between guide groove and slider should be controlled at 0.02-0.05mm, clearance between inclined guide post and slider should be 0.03-0.06mm. Clean foreign objects from guide surfaces. Disassemble slider and inserts, thoroughly clean them with alcohol or a dedicated mold cleaner to remove burrs and debris. Repair any scratches. For high-temperature expansion issue, recalculate mating clearance, allowing 0.05-0.1mm based on molding temperature of plastic material. For thermal expansion gaps, replace with mold steel with one offering better heat resistance; Establish a regular lubrication mechanism, using high-temperature mold-specific grease, adding it at least once a week to ensure continuous lubrication of guide surfaces; Inspect hydraulic/pneumatic system, checking for leaks and blockages in oil and air circuits, adjusting pressure to rated range (hydraulic core-pulling pressure is generally 8-12MPa), repairing or replacing faulty oil/air cylinders; Optimize mold action sequence, adding limit switches and delay valves to ensure mold opens only after core-pulling action is completed and closes only after resetting to correct position, avoiding action interference.

1. Causes of problem
Insufficient core-pulling stroke; unreasonable design of inclined guide post length and cylinder stroke, failing to reach preset core-pulling position; Failure of slider limit device; wear of limit block and loose screws, causing slider positioning deviation; Pressure leakage in hydraulic pneumatic system; insufficient core-pulling power, preventing slider from fully reaching its position; Wear of mold parting surface and slider contact surface, creating gaps, allowing molten plastic to seep in and form flash, thus hindering core pulling.
2. Solutions
Recalculate core-pulling stroke, lengthen inclined guide pillars or replace with a longer-stroke hydraulic cylinder to ensure core-pulling distance meets product's demolding requirements, leaving a 1-2mm safety margin; Inspect limit device, replace worn limit blocks, tighten fixing screws, and add locating pins if necessary to ensure accurate slider positioning; Repair hydraulic/pneumatic sealing components, replace aged sealing rings, adjust system pressure to prevent pressure leakage, and ensure sufficient core-pulling power; Repair worn parting surface and slider contact surface by grinding, welding, then precision machining to eliminate gaps. After mold closing, check fit to ensure there are no gaps or leaks.

1. Causes of Problem
Long-term high-speed reciprocating motion leads to frictional wear on guide surfaces; insufficient hardness of mold steel (HRC≤45) results in poor wear resistance; Excessive pressure at product's glue level causes slide to bear excessive lateral force, leading to deformation and displacement due to prolonged stress; Excessive impact during mold closing causes inclined guide pillars to strike slide, resulting in chipping and deformation of inserts; Cooling system malfunction causes localized overheating of slide, softening of material and accelerating wear.
2. Solutions
Slider, guide groove, and inserts are made of high-hardness wear-resistant steel (such as Cr12MoV, SKH51), with heat treatment increasing hardness to HRC48-52, and surface nitriding to enhance wear resistance; Optimize mold structure by adding support pillars and wear-resistant blocks to distribute lateral pressure, prevent slider from bearing force alone; Adjust mold closing speed to reduce high-pressure mold closing impact, add buffer pads to contact surfaces of inclined guide pillars and slider to reduce impact damage; Inspect mold cooling water circuit, clean scale, ensure smooth cooling water circulation, control slider operating temperature below 60℃ to prevent overheating and deformation.

Reaming mechanisms are mainly used for forming internal and external threads on products. They are divided into three categories: manual reaming, mechanical reaming (gear and rack, worm gear), and servo reaming. Servo reaming has the highest precision and is the most widely used. Faults are mostly concentrated in thread forming, mechanism rotation, and reset stages.

1. Causes of Problem
Insufficient machining precision of auger core, resulting in tooth wear, chipping, and substandard surface roughness; Mismatch between auger speed, direction of rotation, mold opening/closing speed, causing thread to start rotating before it has fully cooled, leading to tooth deformation; Poor flowability of plastic raw material, or insufficient injection pressure or holding pressure, resulting in insufficient material or incomplete filling at threaded area; Insufficient auger stroke, causing the thread to not fully disengage, resulting in stripped teeth due to forced demolding; Poor mold venting, causing air trapping at threaded area, resulting in insufficient material, bubbles, and burrs.
2. Solutions
Re-machine or replace auger core, ensuring complete tooth profile and meeting precision standards. Polish surface to Ra≤0.4μm. Severely worn cores should be scrapped and replaced. Adjust auger parameters. Set a reasonable rotation speed for servo auger (generally 10-30 r/min). Ensure threaded area of product has cooled to ejection temperature (usually below 80℃) before initiating auger operation, with rotation direction consistent with thread direction. Optimize injection molding process. Appropriately increase injection pressure, holding pressure, and holding time. Select raw materials with moderate flowability to improve filling effect of threaded area. Calculate auger stroke to ensure sufficient core rotations and complete thread ejection from product cavity, avoiding forced ejection. Add venting grooves and venting pins to threaded cavity, controlling depth to 0.01-0.02mm, to eliminate air entrapment and improve thread forming quality.

1. Causes of Problem
Insufficient clearance in gears, worm gears, or drive shaft, or inadequate coaxiality during installation, resulting in excessive rotational resistance; Tucked gear core is stuck in cavity, or product threads shrink and tighten around core due to cooling, preventing rotation; Worn or broken teeth in transmission components, or foreign objects entering, causing jamming of rotation mechanism; Lubrication failure, insufficient oil in gearbox, leading to dry friction and a surge in resistance; Insufficient servo motor power, or incorrect inverter parameter settings, resulting in insufficient power output.
2. Solutions
Recalibrate coaxiality of transmission components, adjust mating clearance, and control gear meshing clearance to 0.03-0.05mm. Repair or replace worn or broken transmission components. Optimize cooling process, extend cooling time, or add cooling water channels inside coiled gear core to reduce product shrinkage clamping force. If necessary, adopt a core-pulling followed by coiling sequence. Disassemble gearbox and transmission mechanism, thoroughly clean foreign objects, repair scratched transmission surfaces, and ensure smooth component rotation. Regularly add gear-specific lubricating oil, check oil level, and change lubricating oil monthly to prevent lubrication failure. Calculate load power, replace with a higher-power servo motor, readjust inverter parameters to ensure power output matches coiled gear load and avoid overload.

1. Causes of Problem
Faulty reset limit switch, resulting in signal transmission errors and premature mold closing before trunking tooth is fully reset; Excessive clearance in transmission components, causing lag and misalignment during reset, leading to inaccurate core positioning; Excessive mold closing speed, causing core to collide with cavity before reset action is completed; Fatigue or breakage of reset spring, resulting in failure of mechanical reset mechanism.
2. Solutions
Inspect limit switch and sensor probe, replace faulty components to ensure accurate reset signal transmission, and add double limit protection; Replace worn transmission bearings and gears to eliminate clearance, ensure accurate trunking tooth core reset; Reduce mold closing speed, set a mold closing delay, and confirm trunking tooth is fully reset before high-pressure mold closing; Replace fatigued or broken reset springs with high-strength springs, and inspect mechanical reset mechanism to ensure reliable reset action.

1. Solution: Replace aged seals and gaskets in hydraulic/pneumatic system; tighten pipe joints; investigate leak points; regularly replace hydraulic oil and filter elements to ensure cleanliness of oil and gas.
2. Preventive Measures: Check sealing components weekly; test hydraulic oil/air circuit pressure monthly; address leaks promptly.

1. Solution: Clean cooling water circuit; install a water/oil temperature controller to accurately control mold temperature and avoid localized overheating.
2. Preventive Measures: Check cooling system before production; monitor mold temperature in real time; keep water circuit unobstructed.

1. Regular Maintenance: Check core-pulling and tightening mechanism's operation, lubrication status, and limit devices daily before production; clean guide surfaces and transmission components weekly, add lubricating oil; disassemble and inspect transmission mechanism and sealing components monthly to check component wear.
2. Process Control: Strictly adhere to mold parameters when setting injection temperature, pressure, cooling time, and reamer speed to avoid overload operation; monitor product quality in real time during production, and immediately stop machine for repair if any abnormalities are detected.
3. Spare Parts Reserves: Stock up on vulnerable parts (sealing rings, limit blocks, gears, reamer cores, inclined guide pillars) in advance, and replace them promptly in case of failure to minimize downtime.
4. Personnel Training: Strengthen training for operators and mold repair workers, ensuring they are familiar with mold structure and operating principles, master troubleshooting and maintenance techniques, and standardize operating procedures to avoid mold damage caused by human error.

Case 1: Hydraulic Core Pulling Slide Block Jammed, Severe Flash on Product
Problem Description: In a plastic shell mold, hydraulic side core pulling mechanism jammed after producing 500 molds, preventing core pulling and resulting in excessive flash at side hole area of product.
Cause Investigation: Blockage in cooling water circuit, overheating and expansion of slide block, insufficient clearance, and plastic flash stuck in guide groove, causing jamming.
Solution: Clean cooling water channels and guide grooves of foreign objects, grind and repair guide surfaces, readjust fit clearance, increase lubrication frequency. Fault was eliminated after production.
Case 2: Servo threaded retractor slippage, high scrap rate of threaded products
Problem Description: In a bottle cap thread mold, servo threaded retractor mechanism frequently slipped during production, resulting in defective threads and a scrap rate of 15%.
Cause Investigation: Retractor speed was too high, product cooling time was insufficient, thread was not fully cured before rotation, and core surface was worn.
Solution: Reduce retractor speed, extend cooling time, replace retractor core with a new one, and optimize venting system. After adjustments, scrap rate dropped to below 1%.

Failures in injection mold core-pulling retractor mechanisms are mostly due to four core factors: unreasonable structural design, insufficient machining accuracy, improper process parameters, and lack of routine maintenance. During production, fault diagnosis logic should be followed: "First check external causes (process, lubrication, foreign objects), then check internal causes (structure, components, accuracy)," problems such as jamming, poor molding, and abnormal reset should be addressed accordingly. Meanwhile, establishing a routine maintenance mechanism, optimizing process parameters, improving operational standardization can significantly reduce failure rate of core-pulling and thread-locking mechanism, ensure continuous and stable mold operation, improve product molding quality and production efficiency, and reduce production costs.
For core-pulling and thread-locking molds with different structures, solutions need to be flexibly adjusted based on product structure, raw material characteristics, and production volume. Complex faults can be diagnosed through joint consultations with mold design, injection molding process, maintenance personnel to address root cause and extend mold's lifespan.