How to Choose Between Urethane Washers and Springs for Mold Opening Force
Key Takeaway: Urethane washers provide cushioning and spacing between plates — they absorb impact but generate minimal separation force. Urethane springs provide active separation force — pre-compressed between plates, they push plates apart when clamp pressure releases. Use washers for vibration dampening, springs for plate separation.
Different Components, Different Functions
| Property | Urethane Washer | Urethane Spring |
|---|---|---|
| Primary function | Cushioning, spacing, vibration absorption | Active force generation (push-apart) |
| Force output | Low — reactive (absorbs force) | Medium — active (generates force) |
| Pre-loaded? | No — sits at natural height | Yes — compressed between plates during clamping |
| Shape | Flat washer (low height-to-diameter ratio) | Cylindrical (height ≈ diameter) |
| Shore hardness | 60A-80A (softer) | 80A-95A (harder for force generation) |
| Typical use | Between ejector plates, on guide pins | Between mold plates at parting surface |
When to Use Urethane Washers
- Ejector plate cushioning: Absorb impact when ejector plates reach their forward limit.
- Guide pin spacing: Center and space components on guide pins with minimal play.
- Vibration dampening: Reduce transmitted vibration between plates during high-speed cycling.
- NOT for plate separation: Washers do not generate significant push-apart force.
When to Use Urethane Springs
- Initial plate separation force: Overcome adhesion and vacuum when clamp pressure releases — the spring pushes plates apart before the machine opening stroke engages.
- Runner plate separation: Push the runner plate away from the cavity plate to ensure clean separation.
- Supplement to puller bolts: Provide additional separation force beyond what puller bolt springs deliver.
Select urethane spring hardness based on required force: 80A for light force (small molds), 90A for medium force, 95A for maximum force (large molds with adhesion-prone resins).
Frequently Asked Questions
Can urethane washers provide plate separation force?+
Minimal. Washers are designed for cushioning, not force generation. Their low height-to-diameter ratio and soft durometer (60-80A) generate insufficient push-apart force. Use urethane springs for active separation.
What Shore hardness should I choose for urethane springs?+
80A for light separation force (small molds, commodity resins). 90A for medium force (standard molds). 95A for maximum force (large molds, adhesion-prone resins like TPE and silicone). Higher durometer = more force but less deflection range.
How long do urethane springs last?+
Typically 500K-1M cycles at 20-30% compression ratio. Life decreases with higher compression ratios and elevated temperatures. Replace when spring height has permanently decreased by more than 10% (compression set).