How to Calculate Tension Link Stroke for Three-Plate Mold Design
Key Takeaway: The tension link stroke calculation is straightforward: Required Oval Hole Length = Desired Stroke + Bolt Diameter + Safety Margin (2-3 mm). Oval hole lengths are configurable in 5 mm increments. For three-plate molds, use a two-oval-hole tension link to independently control both PL1 and PL2 strokes with a single component.
The Core Formula
Tension links limit plate travel using an elongated oval hole through which a dedicated bolt passes. As the mold opens, the bolt slides within the oval hole. When the bolt reaches the end of the hole, the plate stops. The fundamental calculation is:
Available Stroke = Oval Hole Length − Bolt Diameter
To determine the required oval hole length, work backwards from the desired stroke:
- Step 1: Determine the minimum plate separation needed (runner clearance + drop space for PL1, or part ejection height for PL2).
- Step 2: Add a safety margin of 5-10 mm to account for thermal expansion and manufacturing tolerances.
- Step 3: Add the bolt diameter (typically M10, M12, or M16 depending on link size).
- Step 4: Round up to the nearest available oval hole length (configurable in 5 mm increments).
Worked Example: Standard Three-Plate Mold
Given: A three-plate mold with 30 mm runner height, M12 tension link bolts, and 80 mm part height.
| Parameter | PL1 (Runner Parting) | PL2 (Main Parting) |
|---|---|---|
| Minimum separation distance | 30 mm (runner height + sprue puller clearance) | 80 mm (part height + ejection clearance) |
| Safety margin | +10 mm | +10 mm |
| Desired stroke | 40 mm | 90 mm |
| Bolt diameter (M12) | +12 mm | +12 mm |
| Required oval hole length | 52 mm | 102 mm |
| Nearest configurable (5 mm increments) | 55 mm | 105 mm |
| Actual available stroke | 55 − 12 = 43 mm | 105 − 12 = 93 mm |
Choosing the Right Tension Link Configuration
Four tension link configurations are available, each designed for different stroke requirements:
| Configuration | Stages Controlled | Best For |
|---|---|---|
| Long Stroke (single oval hole) | 1 stage only | Two-plate molds or single-stage control where the machine handles the other stage |
| Stepped | 2 stages at different heights | Three-plate molds where PL1 and PL2 are at different vertical positions |
| Two Oval Holes | 2 stages independently | Standard three-plate molds — independent stroke control for PL1 and PL2 |
| Two Oval Holes with Land | 2 stages + intermediate stop | Molds requiring a specific plate position between the two opening stages |
The two-oval-hole configuration is the most common for three-plate molds because it provides independent stroke control for both parting lines using a single link body.
Bolt and Retainer Selection
Tension link bolts are purpose-designed and not interchangeable with standard hardware bolts. Key specifications:
- Bolt material: Alloy steel, heat-treated to withstand repeated impact loading at the end-of-stroke contact.
- Urethane wrapping: Urethane-wrapped bolts absorb impact shock at the end of stroke, reducing noise, vibration, and bolt wear. Recommended for strokes >50 mm or cycle rates >15 cycles/minute.
- Retainers: Single-flange retainers for standard applications. Double-flange retainers for high-load applications or when bolts must resist pull-through forces.
Multi-Stage Stroke Design Tips
- PL1 stroke should complete before PL2 begins. Set the PL1 oval hole short enough that the bolt reaches end-of-stroke before parting locks at PL2 release. This ensures clean runner separation before part exposure.
- Account for urethane bolt compression. Urethane-wrapped bolts compress 1-2 mm at end-of-stroke impact. If precise positioning matters, account for this in your calculation.
- Verify guide pin engagement length. The total opening stroke (PL1 + PL2) must not exceed the guide pin engagement length minus a safety margin per injection mold design standards.
- Consider thermal expansion. Mold plates expand when heated. A mold running at 80°C with 300 mm between link mounting points will expand approximately 0.3 mm. This is usually negligible, but for tight-tolerance applications, add 1-2 mm to the safety margin.
Frequently Asked Questions
What is the formula for tension link stroke?+
Available Stroke = Oval Hole Length − Bolt Diameter. To size: Required Oval Hole Length = Desired Stroke + Bolt Diameter + Safety Margin (2-3 mm). Oval hole lengths are configurable in 5 mm increments from the manufacturer.
What bolt diameter should I use?+
Use only dedicated tension link bolts — standard hardware is incompatible. Bolt diameter is M10-M16 depending on link size. Larger bolt = less available stroke for the same oval hole length, but higher load capacity.
Can one link control two stages?+
Yes. Two-oval-hole tension links have independent oval holes at different heights — each controlling a different stage. This is the standard approach for three-plate molds.