What Makes Shoulder Bolts Different
A shoulder bolt (also called a stripper bolt) has three distinct sections: a hex socket head, a precision-ground cylindrical shoulder, and a threaded end. The shoulder is the defining feature — it is ground to h6 tolerance (±5-8 μm) and provides a smooth bearing surface for components that slide along it.
In contrast, a standard hex socket bolt has a threaded shaft with no precision ground section. It can clamp, but it cannot guide. Shoulder bolts do both.
Primary Applications in Injection Molds
| Application | Function of Shoulder | Shoulder Length Determines |
|---|---|---|
| Stripper plate guide | Guides the stripper plate along the shoulder during ejection | Maximum stripper plate travel distance |
| Ejector plate retainer | Holds the ejector retainer plate to the ejector plate while allowing axial movement | Maximum ejector stroke |
| Slide core travel limit | Limits how far a side-action slide can travel on opening | Maximum slide travel |
| Spring retainer | Holds a spring in compression while allowing the adjacent plate to move | Available spring compression distance |
How Shoulder Bolts Guide Stripper Plates
In a stripper plate mold, the shoulder bolt passes through a clearance hole in the stripper plate and threads into the mold base. When the mold opens, the stripper plate slides along the shoulder portion of the bolt:
- During molding: The stripper plate is held closed by injection pressure. The shoulder bolt is clamped but the stripper plate is free to slide.
- During ejection: The ejector system pushes the stripper plate forward along the shoulder. The part is stripped off the core by the plate's forward motion.
- Travel limit: When the clearance hole reaches the bolt head, the stripper plate stops. The shoulder length equals the maximum travel distance.
- Return: Return springs push the stripper plate back to its closed position.
Shoulder Bolt Specifications
| Specification | Requirement | Reason |
|---|---|---|
| Shoulder tolerance | h6 (0 to -8 μm for 10-18mm) | Bearing-quality surface for sliding contact |
| Surface finish | Ra 0.4 μm (ground) | Low friction for smooth plate movement |
| Hardness | HRC 38-45 | Wear resistance at the sliding contact surface |
| Material | SCM435 alloy steel (Q&T) | Strength + machinability + heat treatability |
| Thread class | 6g (standard metric) | Standard thread for secure clamping |
The shoulder tolerance follows the ISO 286-1 tolerance system. The h6 grade is one grade tighter than the h7 used for dowel pins because the shoulder bolt must provide both clamping AND sliding guidance.
Common Mistakes with Shoulder Bolts
- Using standard bolts as substitutes: Standard bolts have rough, unground shanks that score the clearance bore during plate movement. This creates friction, sticking, and inconsistent ejection — problems that worsen with every cycle.
- Wrong shoulder length: If the shoulder is too short, the stripper plate hits the bolt head before reaching full ejection travel. If too long, the plate never fully closes, leaving a gap at the parting line that creates flash.
- Insufficient clearance hole diameter: The clearance hole in the moving plate must be 0.5-1.0mm larger than the shoulder diameter. Too tight and the plate binds; too loose and the plate wobbles during ejection.
- Over-tightening: The shoulder bolt's thread only needs to clamp the bolt body to the base plate. Over-torquing can stretch the bolt, reducing shoulder diameter at the thread transition zone. Follow the recommended torque specifications.