How a Straight Ejector Pin Works in Moulds
A straight ejector pin in an injection mould accomplishes three tasks in a few hundred milliseconds:
- Transfers ejector force. The mold's ejector rod pushes the plate, and each pin transfers some of that force to a part.
- Vacuum breaks. The pin point rips the core part and admits air behind it, preventing vacuum lock and distortion.
- Defines parting-line quality. A clean gate-side witness line or a flashing ring that needs further rework depends on pin tip-cavity surface fit.
Straight ejector pins slide through a guide hole in the core with only a few hundredths of a millimetre clearance, so the body must be straight, round, and dimensionally stable at moulding temperature or they gall, bind, and break mid-shift in the hardest to reach cavity.
Straight, Stepped, or Blade: Choosing the Right Ejection Geometry
Mold designers choose between three common ejection pin families. The straight design remains the default for good reasons:
|
Pin type |
Geometry |
Best suited to |
Trade-off |
|
Straight ejector pin |
Uniform shank diameter from head to tip |
Flat or gently curved part surfaces; standard ejection points; easy replacement |
None for most applications - the simplest, most economical, most available option |
|
Stepped (shoulder) ejector pin |
Tip diameter smaller than shank |
Small, deep ribs and bosses where tip force must be concentrated |
More expensive; stress concentration at the step |
|
Ejector blade |
Thin rectangular cross-section |
Very narrow ribs and edges |
Limited force; fragile tips |
A straight ejector pin is the right choice whenever the ejection point sits on a flat or radiused surface and the available diameter is at least about 1 mm. It is also the service-friendly choice: because dimensions follow recognized standards, a maintenance team can swap a worn pin without reworking the mold - provided the replacement meets the original diameter, hardness, and straightness specifications. That consistency is precisely what our grinding and inspection process is built to guarantee.
Tool Steels Selected for Heat, Not Just Hardness
Ejection components fail from thermal cycling as often as from mechanical wear. Every cycle, the pin tip sits against plastic at molding temperature and then cools - thousands of times per day. We therefore select hot-work grades by default:
|
Material grade |
Typical hardness |
Key properties |
Recommended use |
|
SKD61 / H13 (hot-work tool steel) |
46–52 HRC core; surface nitrided to ≥ HV 900 |
Excellent thermal fatigue resistance, good toughness, resists softening at molding temperature |
Standard choice for plastic injection molds, including high-temperature engineering resins |
|
SKH51 / M2 (high-speed steel) |
60–64 HRC |
Very high wear resistance and compressive strength |
Small-diameter pins, abrasive filled resins, long runs |
|
SUS 420 / 440C stainless steel |
50–56 HRC after heat treatment |
Corrosion resistance in aggressive processing environments |
PVC, flame-retardant resins, medical and optical molding |
|
Carbon steel 1045 |
Case-hardened surface |
Economical, good machinability |
Low-volume molds, prototyping tooling, non-critical ejection points |
Vacuum heat treatment is applied before final grinding, and hardness is verified batch by batch. If your mold runs glass-filled nylon or runs hot enough that standard pins lose hardness, we will recommend the appropriate grade and surface treatment during DFM review - before production, not after a failure.
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