Injection Molding Machine Bearings: Screw, Mold & High-Temp Applications

Injection molding machine bearings are rolling element assemblies that sustain heavy axial loads from the injection screw and thermal stress from plastic melt temperatures reaching 200–300°C, while also enduring impact loads from the clamping mechanism cycling thousands of times per hour.

Injection molding machines operate under continuous high-precision demands: the barrel must seal completely, the screw must generate sufficient injection pressure, and the mold must open and close millions of cycles without dimensional wear. These requirements place enormous stress directly on the bearing system. Wrong bearing selection — or inadequate lubrication — causes sudden shutdowns, mold damage, and costly scrap plastic. Understanding bearing function in each machine subsystem is essential to avoid premature failure.

Definition and Role in Injection Molding Machines

An injection molding machine contains three major mechanical assemblies: the injection unit, the clamping unit, and the drive transmission system. Each assembly places different demands on the bearing selection.

The injection unit houses a screw that both rotates and translates inside a heated barrel. The screw plasticizes material while advancing to inject plastic into the cavity. Injection force can reach dozens of tons on large machines, creating enormous axial load on the screw bearing. The bearing at this position must handle this force without deformation.

The clamping unit uses a toggle mechanism or hydraulic cylinders to close and hold the mold during injection. Clamping force ranges from a few tons (50T machines) to over 2,000 tons (large automotive part machines). The toggle system contains many pivot joints, each using needle roller bearings or plain bushings.

Temperature is unique to plastic processing. The barrel zone reaches 200–320°C depending on material grade. Bearings near this zone need high-temperature lubricant and a cage material stable at elevated temperature — standard polyamide will soften and lose performance above 100°C.

Injection Screw — Thrust Bearings and Radial Guidance

The injection screw is the most technically demanding bearing location. The bearing must handle two simultaneous loads: axial thrust from the injection process and radial load from screw mass.

Spherical Roller Thrust Bearing Series 29300

Spherical roller thrust bearings (SRTB) in the 29300 series are the industrial standard for screw thrust support. These are cylindrical roller assemblies oriented along the screw axis with self-aligning capability to accommodate minor misalignment.

Key characteristics of the 29300 series: the large contact angle (approximately 45°) allows extremely high axial load capacity, while also supporting some radial load. This is why the 29300 series replaced flat thrust bearings in modern molding machines — flat bearings cannot self-align and cannot handle radial load.

Dynamic load data sourced from SKF Plastics Industry Application Guide and FAG Injection Molding Bearings.

Cylindrical Roller Bearings (NU Series) for Radial Support

Behind the SRTB thrust bearing, the screw requires a radial guidance bearing to handle bending loads. Cylindrical roller bearings (CRB) in the NU series are ideal because they:

• Carry higher radial load than ball bearings of the same size
• Allow free axial movement — no thermal stress when the screw expands
• Are simple to remove and replace during maintenance

Common series: NU 318 ECM/C3 (bore 90mm), NU 320 ECM/C3 (bore 100mm), NU 326 ECM/C3 (bore 130mm). The C3 clearance class indicates loose-than-standard internal clearance — essential because the screw heats and expands during running.

The pair 29300 SRTB + NU CRB forms the standard screw bearing assembly. The SRTB carries all axial thrust; the NU CRB carries radial load and accommodates thermal growth. See also thrust bearings and cylindrical roller bearings.

Clamping Mechanism — Needle Roller Bearings and Bushings

The toggle clamping mechanism is a system of many joints operating continuously. On a mid-size 250T machine, the toggle typically has 8–12 pivot joints. The mold closes and opens every 3–5 seconds, meaning each joint performs 12–20 rotations per minute continuously.

Needle Roller Bearings

Needle roller bearings are ideal for toggle joints because of their exceptional load-carrying capacity relative to size — thin rollers (d ≈ 2–4mm, L/d ≈ 5–10) support high radial load in confined space. This is critical when toggle design is space-constrained.

Common needle bearing types in toggle systems:

• NK series (no inner race): mount directly on hardened shaft. Saves space but requires shaft surface hardness HRC 58–64
• RNA series (with outer race only): mount into a housing while the shaft passes through
• HK series (stamped cage): for small joints with low load

Needle roller bearings do not self-align and cannot carry axial load. When toggle joints experience axial load, combine with thrust washers.

Plain Bushings

Some toggle positions use plain bushings instead of roller bearings — especially at guide pins and mold guide bushings. Plain bushings tolerate shock load better in oscillating conditions and sustained static load.

Common bushing materials: bronze or PTFE/bronze composite. The self-lubricating composite suits positions difficult to reach for grease application.

See needle roller bearings for drive transmission applications.

Technical Specifications by Machine Clamping Force

Bearing size and type depend directly on clamping force measured in tons (T). The table below consolidates typical specifications by machine class:

Important: C4 clearance is used for bearings near high heat sources or machines running three shifts continuously. Per ISO 281:2007, minimum calculated life L10h must reach at least 20,000 hours for continuous industrial service.

Equivalent load for toggle bearings is calculated as P = X·Fr + Y·Fa, where Fr is radial load and Fa is axial load. For purely radial needle bearings, Fa = 0 and P = Fr.

Temperature Control and Lubricant Selection

Temperature is the critical factor in bearing life within molding machines. The thermal gradient from the barrel (200–300°C) to the screw bearings creates continuous heat flow. Without control, bearing temperature exceeds 120°C — the threshold where standard polyurea and lithium greases begin to degrade.

Temperature Zones

Zone 1 — Near barrel (bearing 80–120°C): Use perfluoropolyether (PFPE) or synthetic grease with calcium sulphonate thickener. Continuous operating temperature to 180°C. Examples: Klüber Barrierta L 55/2, Molykote EM-50L.

Zone 2 — Screw and thrust bearing (bearing 60–80°C): Polyurea PAO-based greases resist heat without frequent change. Examples: SKF LGWA 2, FAG Arcanol TEMP110.

Zone 3 — Toggle mechanism and slides (bearing <60°C): Standard lithium complex grease with EP additives for shock load tolerance.

Lubrication Schedule

Excess lubricant in a bearing generates heat from viscous drag — more grease is not always better. For SRTB, filling 40–50% of the cavity is optimal.

A 15°C rise above optimum operating temperature halves bearing grease life — the Arrhenius principle applies directly to base oil degradation.

Bearing Brands and Standards — Fanuc, Haitian, JSW and Alternatives

Injection molding machines come from many manufacturers with different bearing requirements. Understanding OEM (original equipment manufacturer) bearing brands enables correct replacement selection.

Fanuc Roboshot (Japan)

Fanuc Roboshot machines use all-electric drive systems. The screw uses NSK or NTN SRTB bearings with tighter tolerance (P5 class). Radial CRB bearings are usually NSK NU series with a specialized heat-resistant cage. When replacing, maintain P5 or P6 tolerance class — standard P0 causes vibration and injection quality loss.

Haitian (China — global export leader)

Haitian is the world's highest-volume injection molding machine brand by unit sales. The Jupiter and Mars lines use FAG or SKF SRTB for the screw, INA needle bearings for the toggle. Haitian specifies replacing toggle bearings every 5,000 hours under full-load three-shift operation — in practice this varies with lubrication quality.

JSW J-ADS (Japan)

JSW Japan Steel Works uses NSK or Jtekt (Koyo) bearings in most positions. A distinctive feature: JSW screw shafts are tapered (conical) rather than cylindrical — the SRTB mounts on a conical sleeve, requiring the correct adapter sleeve when replacing.

ZVL as a Quality Alternative

ZVL (Slovakia) manufactures the complete 29300 SRTB and NU CRB range to ISO/DIN standards. ZVL bearings meet P0/P6 precision grades with steel from Slovakian foundries under strict composition control — suitable as a replacement for SKF, FAG, NSK in standard-tolerance molding machines (not P5-specific applications).

ZVL provides competitive European pricing compared to SKF and FAG in the SRTB 29300 and CRB NU range, particularly attractive for large molding facilities managing large machine fleets and controlling maintenance costs. See ZVL thrust bearings for available part numbers.

Injection Molding Bearing Selection by Machine Class

Correct bearing selection for injection molding machines starts with machine clamping force and screw diameter, not with brand preference. The following selection guide covers the four common machine classes present in Vietnamese industrial parks.

50T–150T Machines (Small Packaging and Medical Components)

Small machines use compact SRTB: 29314 E (d=70, D=120, B=35, C=235 kN) or 29316 E (d=80, D=140, B=44, C=285 kN) as the screw thrust bearing. Toggle needle bearings in this class use HK 2020 (d=20 mm, compact) or HK 2520 (d=25 mm, standard) at each pivot joint. Operating speeds are higher in small machines — the screw rotates faster to maintain production rate. C3 clearance is required for all bearings despite lower injection forces.

150T–400T Machines (General Purpose)

This is the most common machine class in Vietnamese garment accessories and consumer goods manufacturing. SRTB: 29320 E (d=100, D=170, B=42, C=400 kN) standard. Toggle needles: NK 30/20 (d=30 mm, roller length 20 mm) at main cross-head joints. Some machines in this class use hydraulic clamping rather than toggle — hydraulic cylinder guide bearings then use cylindrical roller bearings NU 213 or NU 215 as the piston rod guide.

Screw barrel temperatures in this class: 200–280°C for ABS, HDPE, PP. Zone 1 bearings (near barrel) must use calcium sulphonate grease — standard polyurea fails at extended exposure above 150°C.

400T–1000T Machines (Automotive Parts)

The automotive sector in Vietnam — primarily seating components, bumpers, door trims — uses machines in this range. SRTB requirements increase substantially: 29326 E (d=130, D=220, B=54, C=620 kN) to 29334 E (d=170, D=290, B=75, C=1,000 kN) depending on screw diameter.

At this scale, temperature management becomes a primary engineering concern. Three-shift continuous operation keeps barrel temperatures sustained at 200–300°C for 20+ hours per day. The thermal gradient creates a challenging boundary condition for Zone 1 bearings — PFPE grease (Klüber Barrierta or equivalent) is the only lubricant class that survives indefinitely in this service.

Tie-Bar Bearing Requirements

Tie-bars are the four guide columns that maintain mold alignment during clamping. Each tie-bar has linear bearings or bushings at the movable platen. On machines above 500T, these positions use needle roller and cage assemblies (NCS or RCS series, JTEKT/INA designation) rated for the full clamp force divided across four tie-bars. Tie-bar bearings require relubrication every 250–500 hours — more frequently than toggle bearings — because each mold cycle (3–5 seconds) creates reciprocating micro-motion that expels grease from the contact zone.

Real-World Scenario — Plastic Manufacturing in Industrial Park

At a plastics facility in an industrial park specializing in automotive interior trim, a fleet of twelve Haitian Jupiter 650T machines runs three-shift continuous operation. The maintenance team encountered a recurring problem: SRTB thrust bearings failed after 8,000–10,000 hours instead of the designed 20,000 hours.

Investigation revealed dual causes: (1) the bearing was lubricated with standard lithium grease instead of calcium sulphonate per OEM specifications — the grease burned and formed deposits in the bearing raceway; (2) the relubrication interval was 6,000 hours instead of the recommended 2,000 hours.

The solution: switch to calcium sulphonate mineral-based grease with pour point above 300°C, reduce the relubrication interval to 1,500 hours, and install temperature sensors on four machines to monitor bearing temperature. Results over eighteen months: zero premature SRTB failures, estimated bearing life reached 18,000–22,000 hours.

The cost of upgraded grease and sensors was substantially lower than the cost of unplanned shutdowns — each unplanned failure on a 650T line cost an estimated 15–25 million VND in scrap and lost production.