Last updated: 2026-09-04 · 6 min read

Why Does My Part Fill Only Partway? — Telling a Short Shot from Hesitation and Fixing Each

A part that stops short can fail for two different reasons: the melt runs out of pressure before reaching the cavity end, or the flow front cools and stalls before it gets there. Short shot and hesitation look alike on the bench but come from opposite places — one is a pressure problem, the other is a temperature-and-speed problem — and fixing the wrong one wastes machine time.

Key Facts

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  • Short shot = the cavity is not fully packed because injection pressure cannot push the melt to the end; hesitation = the flow front decelerates and solidifies before reaching the end because the melt cools too fast in a thin section or at a flow-length extreme.
  • The diagnostic starts with where the unfilled region is: short shots leave the last-to-fill area empty with a sharp freeze-off front; hesitation leaves a rounded, thickened lip where the flow stalled and cooled.
  • Boosting injection pressure fixes a short shot; boosting it for hesitation usually makes things worse by packing the front of the cavity while the far end is already frozen.

    • Cycle time — The total time for one injection molding cycle: injection + packing + cooling + mold open/close + ejection. Typical range: 15–60 seconds. Cycle time determines production rate and cost per part — reducing cycle by 5 seconds saves 10–15% annual production cost. The related guide covers adjacent topics.

    How Do You Tell Short Shot from Hesitation on the Bench?

    Four diagnostic signals separate them:

    Signal Short Shot Hesitation
    Where the part is unfilled Last-to-fill region, usually farthest from the gate At a thin section, a sudden thickness transition, or a flow-length extreme
    Shape of the freeze-off front Sharp, clean edge where melt stopped abruptly Rounded, thickened lip where flow decelerated and piled up
    Injection pressure curve Pressure hits the machine limit before the cavity fills Pressure is adequate but the flow front stalls at the same point every cycle
    Response to higher melt temperature Modest improvement — more pressure Significant improvement — flow front stays fluid longer

    The physical difference is what the melt is doing when it stops. In a short shot, the material is still fluid but there is not enough pressure to push it into the remaining cavity — the screw has bottomed out or the machine has reached its limit.
    In hesitation, the melt front reaches a thin section, decelerates because the resistance jumps, and cools below the point where it can flow further — the front freezes in place even though pressure is still available at the screw.

    The practical test: raise the melt temperature by 10–15 °C and see what changes. If the unfilled region fills, the problem was hesitation — the flow front needed more thermal energy to stay fluid through the thin section. If the unfilled region barely grows and the pressure still hits the limit, the problem is a short shot — the cavity needs more pressure , not more heat.

    How Do You Fix Each One Without Creating New Defects?

    Two different playbooks, applied in order:

    1. For short shot: increase pressure , then check for flash. First raise injection pressure and hold pressure in small increments — 5 bar at a time — until the cavity fills. If the machine is already at its limit, reduce the screw transfer position so more material is injected before the hold phase begins, or slow the flow front so the melt does not freeze off before the pressure wave reaches the end. After each adjustment, check the parting line and ejector pins for flash — the same pressure increase that fills the cavity can open the parting line if the clamp tonnage is marginal.
    2. For hesitation: raise melt temperature and flow speed through the thin section. Increase barrel temperature in 10 °C increments, starting at the front zones nearest the gate, and raise the injection speed through the hesitation zone so the melt crosses the thin section before it can cool. If the thin section is severe, consider a mold-temperature increase in that region to keep the melt fluid longer. The risk of fixing hesitation too aggressively is jetting or material degradation — if the melt is too hot or too fast through a restricted gate, it can shoot past the gate land and create a different defect.
    3. For either: verify the fix across 20–30 consecutive shots. A single good shot after an adjustment does not mean the process is stable — run 20 to 30 cycles and check that the fill is complete and consistent, that no new flash has appeared, and that the ejector pattern is clean. Short shots and hesitation are both sensitive to small changes in melt temperature, back pressure and mold temperature, so the fix must hold across the normal variation of the process, not just at one set point.
    4. If neither fix works: check the tool design before changing the process further. A gate that is too small for the flow length, a runner that freezes before the cavity, a vent that traps gas at the fill end — these are tool problems that no amount of pressure or temperature will fix on the press. If the process window is closed no matter what you try, the root cause is in the steel, and the fix belongs in the tool room, not the machine settings.

    Warpage — Part deformation after ejection caused by uneven shrinkage. Amorphous resins warp less (0.4–0.7% shrinkage). Semi-crystalline resins warp more (1.0–2.5% shrinkage). Fix: uniform wall thickness, balanced cooling, optimized gate location, adequate packing pressure. The related guide covers adjacent topics.

    Short shot and hesitation both leave an incomplete part, but one is a pressure problem and the other is a temperature-and-speed problem. Fix the wrong one and you waste a weekend of machine time while the real cause sits unchanged.

  • Key parameter identified — critical variable confirmed against spec.
  • Tolerance within achievable range — commercial ±0.15mm or tighter as needed.
  • Material matched to application — resin grade selected for end-use environment.
  • Process window validated at T1 — parameters locked after first trial.
  • Inspection method defined — CMM, visual, or functional test specified.

    • Frequently Asked Questions

    Can I tell short shot from hesitation just by looking at the part?

    Usually, yes. A short shot leaves a sharp, clean freeze-off edge where the melt stopped, because the material was still flowing when the pressure ran out. A hesitation mark leaves a rounded, thickened lip where the flow front decelerated, cooled and piled up before freezing. Location also helps: short shots leave the farthest point from the gate empty; hesitation shows up at a thin section or sudden thickness change.

    When the visual evidence is ambiguous — a thin-walled part with a long flow path can show both patterns — the pressure curve is the tiebreaker. A short shot hits the machine pressure limit; hesitation does not, because the problem is flow-front cooling, not insufficient force.

    Why does increasing injection pressure fix short shot but make hesitation worse?

    Because the two defects have different root causes. A short shot is a pressure deficit — the melt is fluid but there is not enough force to fill the last region, so more pressure pushes it further. Hesitation is a thermal deficit — the flow front has cooled and thickened, and more pressure cannot re-fluidize frozen material; it only packs the fluid region behind the freeze front, creating flash or stress without moving the front.

    The right fix for hesitation is to keep the front fluid longer — higher melt temperature, faster injection speed through the thin section, or higher mold temperature in that area — so the flow crosses the obstacle before it cools.

    What if neither higher pressure nor higher temperature fixes the fill?

    Then the problem is in the tool, not the process. A gate too small for the flow length, a runner that freezes before the cavity fills, or a vent that traps gas at the fill end are tool problems no press setting will overcome. The fix belongs in the tool room — open the gate, add a vent, or re-route the runner.
    The signal is a process window closed across reasonable pressure, temperature and speed settings; if the part will not fill, the steel is the constraint.

    Key Takeaways

    1. Short shot is a pressure problem — the melt is fluid but there is not enough force to fill; hesitation is a thermal problem — the flow front cools and stalls before it reaches the end.
    2. The diagnostic starts with the shape and location of the unfilled region: sharp freeze-off at the farthest point is short shot; rounded lip at a thin section is hesitation — confirmed by the pressure curve.
    3. Fixing the wrong one wastes machine time: more pressure fixes short shot but packs the gate area in hesitation; more heat fixes hesitation but can cause jetting or degradation if the real problem is pressure .

    An incomplete part is a diagnostic problem before it is a process problem.
    Read the shape and location of the unfilled region, check the pressure curve, and run a simple melt-temperature test to separate pressure deficit from thermal stall. The right fix follows directly from the right diagnosis — and the wrong fix is the most expensive way to find out which one it was.

    Sources

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  • Plastics Technology — Injection molding troubleshooting and defect diagnosis
  • HASCO — Mold design and gating references
  • DME — Tooling components and runner design guides
  • MoldMaking Technology — Process optimization and defect correction
  • SPE — Injection molding process engineering resources

    • Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet