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This guide breaks down what preload actually does, the standard preload classes used by major manufacturers like HIWIN, THK, and TBI, and how to match the right preload level to each axis of an injection molding machine.
What Is Ball Screw Preload?
Preload is the internal axial force applied between the ball nut and screw shaft by over-sizing the rolling elements relative to the groove pitch. In simple terms, the balls are slightly too large for the gap, creating a continuous elastic load that eliminates clearance.
Injection molding applications benefit from preload in three key ways:
- Eliminates backlash — direction reversals on injection and ejection axes have zero lost motion
- Increases rigidity — the system deflects less under heavy clamping and injection forces
- Improves positioning repeatability — consistent contact means consistent stopping positions
However, preload also creates rolling friction even at zero external load. This friction generates heat, consumes motor torque, and accelerates wear. The goal is always the minimum preload that meets the application's rigidity and accuracy requirements.
Standard Preload Classes
Major Taiwanese and Japanese manufacturers use a standardized 3-level system:
表格
| Preload Class | Axial Preload Force | Typical Application |
|---|---|---|
| Z0 (Zero / No Preload) | 0% of dynamic rating | Conveying, low-accuracy positioning, transport axes |
| Z1 (Light Preload) | ~2–3% of dynamic rating | General automation, ejection axes, medium accuracy |
| Z2 (Medium Preload) | ~5–7% of dynamic rating | Clamping axes, injection axes, high-rigidity machining |
| Z3 (Heavy Preload) | ~8–10% of dynamic rating | Ultra-high rigidity machine tools, rarely used in molding |
Some manufacturers label these differently — P0/P1/P2, C0/C1/C2, or F0/F1/F2 — but the principle and force ranges are equivalent across HIWIN, THK, TBI, and other major brands.
Preload Selection by Injection Molding Axis
An injection molding machine has multiple ball screw axes, each with very different load and accuracy requirements. One preload level does not fit all.
1. Clamping Axis (Toggle / Direct Press)
Recommended: Z2 Medium Preload
The clamping axis carries the highest forces in the machine — often tens of tons. Mold separation during injection requires maximum rigidity to prevent flash and parting-line mismatch.
Why Z2:
- Eliminates all backlash for precise mold closing position
- Maximizes axial stiffness under tonnage load
- Reduces deflection that causes uneven mold wear
Considerations:
Clamping axes run at relatively low speeds (typically 5–15 m/min), so heat generation from Z2 preload is manageable. Thermal growth is still a factor on long-travel large machines, but the rigidity benefit outweighs it.
2. Injection / Screw Feed Axis
Recommended: Z1–Z2 (application dependent)
The injection axis requires a balance of speed, accuracy, and force. It accelerates quickly, reverses frequently for decompression, and must deliver consistent shot volume.
For standard precision molding → Z1 Light:
Most general-purpose injection machines perform well with light preload. Backlash is minimal, friction is low, and heat generation stays within acceptable limits for 24/7 operation.
For precision / optical molding → Z2 Medium:
When shot weight repeatability must be below 0.1%, medium preload ensures zero lost motion on every injection stroke. Pair with a cooled screw design to manage the additional heat.
3. Ejector Axis
Recommended: Z1 Light Preload
Ejection axes move relatively low mass and require moderate positioning accuracy. They cycle frequently but at low force.
Why Z1:
- Eliminates backlash for consistent ejector pin position
- Lower friction means less heat and longer grease life
- Sufficient rigidity for standard ejection forces
Z0 is rarely used here because backlash would cause uneven pin advance and potential part sticking on the first stroke after direction change.
4. Injection Unit Traverse / Nozzle Touch
Recommended: Z0 or Z1 Light
This axis simply moves the injection unit forward and backward for nozzle contact. Positioning accuracy requirements are modest, and the load is primarily friction from linear guides.
Z0 (no preload) works perfectly fine for nozzle touch. Some machine builders specify Z1 for slightly firmer nozzle contact force consistency, but the difference is minimal.
Common Preload Mistakes in Molding Applications
Mistake 1: "More preload = better machine"
Many buyers assume Z3 heavy preload is a premium upgrade. In reality, it generates 3–4× more friction than Z1, dramatically increases heat, and shortens expected service life by 30–50%. Unless the application genuinely needs maximum rigidity, heavy preload is a net negative.
Mistake 2: Ignoring preload loss over time
Preload decreases as the raceways wear. After millions of cycles, a Z2 screw may effectively become Z1 or even Z0. This is why precision machines develop backlash problems after 5–10 years of heavy use. Double-nut designs (DFU/DFI series from TBI, DFS from HIWIN) allow preload re-adjustment, which is a significant advantage for long-term molding production.
Mistake 3: Specifying preload without considering speed
High-speed axes (injection on fast-cycle packaging machines) generate more friction heat. A Z2 preload that works fine at 10 m/min may cause thermal runaway at 30 m/min. Always cross-reference preload with the screw's DN rating and intended operating speed.
Double Nut vs Single Nut Preload
Preload is achieved through two main construction methods, and this matters for maintainability:
Single-nut preload (oversized balls):
The nut is a single piece with slightly over-sized balls. This is compact, cost-effective, and common in Z0–Z1 range. Disadvantage: preload cannot be adjusted in the field; once it wears, the nut must be replaced.
Double-nut preload (spacer / offset):
Two separate nuts are mounted together with a spacer or angular offset creating the preload force. Common in Z1–Z2 range for high-load axes. Advantage: preload can be re-adjusted or re-set during maintenance by modifying the spacer thickness. This extends the usable life of the screw assembly significantly.
For injection molding production environments where machines run for 10+ years, double-nut preload on clamping and injection axes is a smart investment.
How to Verify Preload on Existing Machines
If you're troubleshooting an older all-electric machine, you can estimate remaining preload without disassembling the nut:
- Backlash test — Mount a dial indicator on the nut housing, apply alternating axial force by hand, and measure lost motion. Any measurable backlash indicates preload is partially or fully lost.
- Drag torque test — Disconnect the motor coupling and measure the torque required to rotate the screw. Compare against factory specification; significantly lower torque indicates preload loss.
- Temperature monitoring — A sudden drop in operating temperature (with same production parameters) can indicate preload has decreased to near-zero.
Conclusion
Preload selection is a balancing act between rigidity, accuracy, friction, heat generation, and service life. For injection molding machines, the optimal strategy is axis-specific:
- Clamping axis: Z2 medium preload (double-nut design preferred)
- Injection axis: Z1 for general use, Z2 for precision applications
- Ejector axis: Z1 light preload
- Nozzle touch / traverse: Z0 or Z1
By matching preload class to each axis's actual requirements, molders get the best combination of positioning accuracy, energy efficiency, and component longevity. And for machines expected to run 24/7 production for a decade or more, choosing double-nut designs that allow preload re-adjustment delivers substantial long-term value.