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Frequently Asked Questions
About Runner Lock Pins

Find direct answers to the technical and buying questions mould designers, tool rooms, maintenance teams and procurement professionals ask about runner lock pins. This guide covers runner retention, three-plate mould sequence, tip and undercut selection, H11 and H13 materials, installation, failure analysis, custom manufacturing, price, MOQ and delivery.

Function & Mould Sequence

A runner lock pin is a fixed mould component with a working tip designed to grip or retain the cold runner during mould opening. It helps pull the runner from the sprue or keeps it in the intended plate until the opening sequence separates it from the moulded component. We select the pin from the runner layout, resin, plate movement, undercut form and required release force.
In a three-plate mould, the runner may remain in a separate stripper or runner plate where there is no direct ejector system to remove it. A correctly designed runner lock pin retains or pulls the runner so the planned plate movement separates the gate and releases the runner at the right stage. The pin must work with the complete opening sequence, not independently.
A sprue puller is generally located at the base of the main sprue and pulls the sprue out of the fixed half. A runner lock pin can be located anywhere along the runner branches to ensure the runner network stays attached to the correct plate and does not break or shift prematurely. Both require a pulling undercut and a later release mechanism, but their locations and loads may differ.
Most runner lock pins are fixed components that retain the runner until plate movement forces the runner off the pin. They do not normally have their own independent forward ejection stroke. If the pin must both hold and eject, the mould may use a moving puller pin attached to an ejector plate, but the terminology and function are different. We check the mounting plate to confirm the function.
The pin itself holds the runner. Release occurs when the mould opening mechanism pulls the stripper plate or adjacent plate past the pin, stripping the runner off the undercut. If the plate movement, stripping force, or undercut geometry is incorrect, the runner may remain stuck on the pin or break. The pin geometry must match the release method.
It controls where the runner is held during opening and when it is released. If the pin grips too strongly or the stripper plate does not travel far enough, the runner will not drop. If it grips too weakly, the runner may fall early and become trapped between moving plates. The pin must provide controlled holding followed by clean release.
Yes, a complex runner system may use a main puller pin at the sprue and smaller lock pins along the branches to ensure the entire runner network remains stable during separation. The pins must have compatible undercuts so they release the runner network simultaneously when the stripper plate acts. We review the full runner layout before selection.
Without a runner lock pin or an equivalent machined undercut in the mould, the runner may stick to the wrong plate, fail to break at the gate, break into pieces, or fall unpredictably during mould opening. This can cause severe mould damage on closing and requires continuous manual intervention from the operator.
The pin is a passive component within the mould. Its holding and release functions are driven entirely by the mechanical opening sequence of the mould plates, not by direct hydraulic or electrical control from the machine. The machine simply pulls the mould open; the mould mechanism and the pin must handle the sequence.
Send the runner and mould assembly drawing, plate sequence, required pin length and diameter, preferred tip geometry, moulded plastic material, shrinkage allowance, operating temperature, available mounting space, stripping method and quantity. Complete data allows the pin’s undercut to be matched to the resin and extraction force.

Tip Geometry & Product Selection

Common geometries include Z-shape, reverse taper, conical undercut, grooved or mushroom head. The official page does not state which profiles are standard. The tip shape determines how strongly the pin grips the plastic and how easily the stripper plate can push the plastic off the pin. We must verify the required tip shape from the customer's drawing.
A Z-type or stepped-cut pin is often used for rigid or brittle plastics. It provides a strong, positive mechanical lock during pulling but allows the runner to snap or release cleanly when stripped. The depth of the Z-cut must match the runner diameter and resin properties to prevent the runner from breaking prematurely.
A reverse taper is often considered for flexible or semi-rigid materials (like elastomers or soft polyolefins) that can stretch and pull over a smooth continuous undercut during stripping. If used on rigid materials like polycarbonate or glass-filled nylon, the runner may not stretch enough to strip and will break or jam on the taper.
Rigid materials need different undercuts from flexible materials because they have different shrinkage rates, notch sensitivity and elastic limits. A deep undercut may hold a rigid plastic too tightly, preventing stripping. An inadequate undercut may fail to hold a shrinking flexible plastic. We require the exact resin grade before confirming tip geometry.
Diameter should be selected from the available runner volume, required pulling force and plate space. A pin that is too thick for the runner will leave a thin wall of plastic that tears easily. A pin that is too thin may bend or pull through the plastic. The diameter must be proportional to the runner cross-section.
The length must ensure the tip projects the correct distance into the runner channel while the head or mounting section is fully seated in the backing plate. The working length must also align with the stroke of the stripper plate to ensure complete release. Provide the required overall and working lengths from the mould assembly drawing.
The official page states a length range of 5–15 mm, which may apply only to the working tip or a specific small component, as a total length of 15 mm is usually too short for a standard plate stack. Complete dimensions, including body diameter, head and full length, must be confirmed from the drawing rather than the displayed range.
The undercut depth and angle control the gripping strength. A deep undercut holds strongly but is hard to strip; a shallow undercut strips easily but may fail to pull the runner. The drawing must dimension the undercut profile completely, referencing the resin properties and stripping method. We follow the customer’s specified geometry.
Machining a standard ejector pin removes its hardened surface, potentially exposing a softer core that will wear quickly or deform under stripping loads. Heat generated during machining can also make the tip brittle. We recommend ordering a purpose-manufactured runner lock pin with the correct geometry and heat treatment applied after forming.
Breaking occurs if the pulling resistance (e.g. from tight sprues, deep gates or poor draft) is greater than the strength of the plastic runner at the lock pin, or if the pin's undercut creates a sharp notch that weakens the plastic. The pin geometry, runner size, cooling time and extraction force must be balanced.

Materials, Hardness & Finish

The official specification table lists High Speed Steel (HSS), Hardened Steel and Stainless Steel options such as SS 304, SS 304L, SS 316 and SS 316L. The correct material depends on the required strength, wear resistance, operating temperature, corrosion exposure and tip geometry. We confirm the material condition and heat treatment before supply.
Stainless steel is selected when the mould operates in a corrosive environment, uses aggressive cooling, or processes materials that release corrosive gases (e.g. PVC). However, basic stainless grades may not offer the same wear resistance or hardness as tool steels, so the grade, hardness and stripping load must be verified for the application.
Hardened tool steel or HSS is preferred in high-volume, abrasive or demanding applications where the pin tip and shank must resist significant wear, bending forces and repeated stripping impact. Tool steel provides superior edge retention for Z-cuts and complex undercuts compared to softer stainless options.
The official page displays a hardness of 40 to 45 HRC. This should be treated as a general reference for some stainless or pre-hardened options. HSS or specific hardened steel pins may require a higher hardness (e.g. 55+ HRC) depending on the tip geometry and wear requirement. We confirm hardness against the selected grade and function.
The tip must be hard to resist wear from abrasive plastics and repeated stripping, but the shank must remain tough enough to absorb the pulling force without snapping. If the pin is over-hardened and brittle, the undercut tip or the head may break off under load. Material and heat treatment must be balanced for toughness.
The page displays a finish of 0.3 micron to 0.04 micron. Since it does not state the parameter (Ra, Rz) or the specific surface, buyers should specify the finish required for the working tip and the mounting shank separately. A very smooth shank ensures accurate plate mounting, while the tip finish must support the required grip and release action.
If the tip is too rough, the plastic runner may grip it too strongly and resist being stripped off. If it is too polished, it may not hold the runner during the initial pulling stage. The finish must be controlled to match the designed undercut and the plastic material’s shrinkage behaviour.
Yes, a suitable surface coating can reduce sliding friction during stripping, improve wear life and resist chemical attack from corrosive resins. The coating must be compatible with the base material, the operating temperature and the required holding force. If a coating is needed, the requirement must be stated before quotation.
The official page includes annealed-head wording. An annealed head provides toughness, preventing the head from cracking or shattering under the pulling force or when clamped in the plate. It may also allow for minor machining adjustments during installation. The requirement for an annealed head should be confirmed on the drawing.
Depending on the order, buyers can request dimensional inspection reports, material grade certificates, heat-treatment or hardness reports, and surface-finish verification. For custom tip profiles, specific profile checks can also be agreed. Requirements for documentation must be established before manufacturing begins.

Installation, Customisation & Replacement

It is typically mounted in a counterbored hole in the fixed or runner plate. The head must seat fully against a flat backing surface, and the shank must be a close transition fit in the bore to prevent lateral movement. A loose fit will cause the pin to tilt or break during pulling.
If the tip has a directional undercut (like a Z-cut), it must be aligned with the runner channel or stripping mechanism to function correctly. If the pin rotates in its mounting hole, the undercut orientation will change, altering the gripping force or preventing release. The pin head must have a flat or dowel hole for anti-rotation location.
You cannot cut the working tip because that would destroy the engineered undercut profile and hardening. If the pin has a long shank designed with a cutting allowance, the shank can be trimmed from the head end, provided a new head or retention method is machined. It is generally safer to order the pin to the finished overall length.
Yes. The official page states that customisation is available. We can manufacture non-standard lengths, diameters, head styles, anti-rotation features and specific tip geometries from a complete dimensioned drawing. The operating environment and plastic material should be supplied so the custom design is evaluated accurately.
Clean the component and measure overall length, shank diameter, head dimensions, and the geometry of the working tip (undercut depth, angle, and position). Because a worn or broken tip will not show its original dimensions, it is better to provide the mould assembly drawing or the original part number for replacement.
Yes, we can review an imported component using the original drawing, catalogue code, measured sample, mould section, and material data. We compare the mounting, dimensions, tip profile and material before proposing a replacement. Provide the plastic type and pulling conditions so we can verify the profile suitability.
Verify the pin against the drawing, ensure the tip profile is correct for the resin, clean the mounting bore and counterbore, check the anti-rotation feature if required, and ensure the pin seats fully. Measure the projection into the runner channel. Cycle the mould slowly to confirm that the stripper plate clears the pin without interference.
If the pocket is uneven, too deep, or the backing plate does not press firmly against the head, the pin will have axial play or sit at an angle. This causes the pin to tilt under the pulling load, bending the shank, altering the tip position, and potentially breaking the head off the pin.

Failure, Troubleshooting & Maintenance

The runner may stick if the pin’s undercut is too deep or sharp, the plastic is too rigid, the cooling time is insufficient, or the stripper plate does not travel far enough to push the runner completely past the tip. Review the pin geometry, material shrinkage, and stripper stroke.
The runner will break if the pulling resistance from the sprue or gates exceeds the strength of the plastic runner at the pin, or if the pin’s undercut acts as a sharp notch, weakening the plastic. The problem can also result from a runner that is too small for the required pulling load or excessive cooling making the material brittle.
Early release occurs if the pin's undercut is too shallow, worn out, or the wrong shape for a high-shrinkage or flexible plastic. It can also happen if the runner is ejected while the plastic is still too hot and soft to hold its shape around the pin. Check cooling time and tip wear.
Breakage or bending is usually caused by excessive pulling force, a misaligned stripper plate hitting the pin, a loose mounting fit allowing the pin to tilt, or an unsuitable, brittle heat treatment. Examine the fracture location, check the stripper-plate clearance and verify the runner resistance before replacing the pin.
Plastic buildup, or galling, occurs when the tip is too rough, the mould runs too hot, or the resin contains aggressive additives. This buildup changes the undercut shape, increasing stripping friction and potentially causing the runner to stick or break. The pin may require polishing or a specialised coating.
Inspect the tip for wear, deformation, chipped edges or plastic buildup. Check the shank for straightness and ensure the head is still seated securely without axial play. Verify that the anti-rotation flat or dowel is intact. Record the condition and replace pins that no longer offer a defined undercut.
Replace the pin when the undercut profile is significantly worn, rounded or chipped, when the shank is bent, or when it is loose in its mounting bore. Polishing an undercut to remove wear or galling usually alters its dimensions, destroying its ability to hold and release the runner correctly.

Quality, Price & Ordering

Provide the required pin diameter, overall length, head style, anti-rotation detail, required tip geometry (e.g. Z-cut, reverse taper), material, hardness, quantity, and delivery location. Supplying a dimensioned drawing or standard part reference ensures the correct component is quoted. Include the plastic type if profile advice is needed.
Pricing depends on the pin diameter, overall length, complexity of the tip geometry, selected material (e.g. HSS vs Stainless Steel), required hardness, surface finish, customisation, order quantity and destination. We provide an accurate quotation against the exact verified drawing.
The live product page states a minimum order quantity of 25 pieces. This typically applies to standard dimensions. Specific commercial terms or minimum batch quantities may apply for custom sizes or specialised tip geometries. Please confirm the MOQ and availability when requesting a price.
The page states carton packaging or packaging according to the client’s requirement. Because the working tips are precision-machined and can be damaged by impact, pins should be packed in protective sleeves or compartmentalised boxes with rust-preventative oil to ensure they arrive in perfect condition.
Common diameters and lengths with standard tip geometries are usually maintained in stock for rapid dispatch. Custom sizes, complex undercuts or specific non-standard materials require a manufacturing lead time. Request current ready-stock confirmation before scheduling urgent mould repairs.
Yes. Vardhman Dies & Mould Tools supplies precision components from Maharashtra directly to toolrooms and mould makers across India. We also handle international export orders, providing the correct protective packaging and commercial documentation for secure delivery worldwide.
Use the website enquiry form, email or contact number. Provide the required dimensions, tip geometry drawing, material, quantity and delivery address. Review the quotation, technical specification and lead time before authorising the purchase.

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