Why this matters
Intermittent short shots and erratic screw recovery on T1 samples are often blamed on the mold, but a bridged feed throat produces the same symptoms. Learn how to tell a feed-side problem from a fill problem before anyone cuts steel. A T1 sample with intermittent short shots or light parts sends most teams straight to the mold: gate size, vents, cavity balance. Sometimes that is right, but the same symptoms appear when pellets bridge in the feed throat and the screw stops picking up material. Telling the two apart before a correction is quoted can save a steel change that was never needed, and it makes the next trial easier to read.
What feed throat bridging actually is
The feed throat is the opening at the back of the barrel where pellets drop from the hopper onto the screw. If that zone gets too warm, the pellets soften and stick to each other and to the wall, forming a bridge that stops fresh material reaching the screw flights. The screw keeps turning but picks up little or nothing, so the shot is short, recovery takes longer than it should, or the screw appears to slip. It is a machine and material condition, and it can be present from the first shot or build up after a few minutes of running.
Why it shows up on T1 and gets blamed on the mold
T1 trials are often run on a trial machine, with a new resin lot, at slow or stop-start cycles while the toolmaker adjusts and inspects parts. Long cooling times and long waits mean pellets sit in a warm zone for longer than they will in production, and low-melt or sticky materials such as TPE, TPU and some polypropylene grades are the first to bridge. The result is a short shot or a light part that appears and disappears, and because the mold is new, it is the obvious suspect, even though the cause is upstream of the nozzle.
Signs that point to the feed side rather than the mold
A fill problem in the mold is usually repeatable: the same cavity is short, the same far corner does not fill, and the pattern follows the gate and vent layout. A feed problem is erratic: shot weight wanders from shot to shot, recovery time climbs or jumps, the screw makes a slipping or cracking noise, and the defect is not tied to one cavity or location. Pellets that look fused or glazed at the throat, a hopper that is hot to the touch or a cycle that was recently slowed down all point the same way.
Checks to run before anyone modifies steel
Ask for shot weight and recovery time logged shot by shot, not only the accepted parts. Check the cooling water flow and temperature at the feed throat, the rear barrel zone setting and whether the material and dryer outlet are hot. Look at the pellets in the throat, confirm the resin lot and any regrind or fines, and try a short run at a faster cycle or with fresh dried material. If shot weight stays stable and the same cavity is still short, the problem is back with the mold; if it stabilizes, no steel needed to change.
How to keep the T1 record honest
A T1 report that says short shots on cavity 3 without recording the machine, resin lot, barrel profile and shot weight trend cannot be used to decide on a correction. Record the machine used, the material and drying conditions, the cycle time and any interruptions, and note that the run was stabilized before samples were taken. If the production press will be different from the trial press, say so in the report, because feed and recovery behavior can change with the machine.
Buyer checks
Quote variables
| Quote variable | Why it changes the mold route |
|---|---|
| Resin and grade | Soft or low-melt materials bridge more easily, so the resin choice changes how much margin the trial has. |
| Trial machine | Feed throat cooling and recovery capacity differ between machines, so a clean trial on one press may not carry to another. |
| Cycle time and interruptions | Long cooling and stop-start running raise the heat soak in the feed zone and make bridging more likely. |
| Correction scope | Separating process stabilization from mold corrections keeps the correction quote to real steel changes. |
Common mistakes
How to apply it
Ask the molder for the T1 run log with shot weight, recovery time, machine, resin lot and barrel profile, and ask whether the short or light shots were stabilized before the mold was judged. If the trial was unstable, repeat a short run after the feed zone is corrected before approving any steel change. If the project is early, the guide can show whether the next move is CAD cleanup, DFM review, prototype tooling, production mold planning, export mold handoff or repair review.