What Is the Short Shot Method?
The short shot method is a systematic, scientific technique used in injection molding troubleshooting and defect analysis. Its core operation involves intentionally reducing the amount of molten plastic injected into the mold cavity (i.e., shortening the injection stroke) to produce a series of incomplete molded parts—known as "short shots."
By observing and analyzing these short-shot samples—each representing the melt flow "frozen" at a different filling stage—we can visually trace the flow path of the melt, monitor how the flow front advances, and pinpoint exactly where and when defects (such as weld lines, air traps, sink marks, or burn marks) originate.
In essence, the short shot method deliberately does not fill the part completely, creating a sequence of “semi-finished” parts. By examining these, we can reverse-engineer the root cause of molding issues.
Purpose and Benefits of the Short Shot Method
The short shot method is not merely about diagnosing "short fill" problems. Its primary advantages include:
Visualizing Flow Behavior: Makes the invisible internal cavity flow visible, revealing how melt travels from the gate to the farthest regions of the cavity.
Locating Defect Origins: Precisely identifies at which filling stage and location defects like weld lines, air pockets, sinks, or burns first appear.
Assessing Flow Balance: For multi-cavity or family molds, it reveals whether all cavities fill simultaneously—indicating flow balance.
Validating Mold Design: Evaluates the appropriateness of gate locations, runner sizing, and vent placement.
Optimizing Process Parameters: Provides direct, visual feedback to fine-tune injection speed, pressure, temperature, and other key settings.
Note: Refer to the process diagrams mentioned in the original text for step-by-step guidance. If anything seems questionable, keep reading—the explanation follows.
Step-by-Step Procedure
Set Material Dosage and Decompression
In the plasticizing (metering) screen, set an appropriate shot size plus decompression (suck-back).
Example: 80 mm + 5 mm = 85 mm total screw position.
Configure Initial Injection Stage
Set a medium-high pressure and medium-low speed for the first injection stage.
Set the end position of this stage to 0.
Example: 100 bar, 30% speed, end position = 0.
Define Target Fill Level
Estimate the location of the defect or area of interest.
For instance, if you want to know where the melt front reaches when injecting from 85 mm down to 45 mm, enter 45 mm as the V/P (velocity-to-pressure) switchover point.
Set injection time to 4 seconds.
Apply Minimal Holding Pressure to Stabilize Screw Position
Set holding pressure to a low value (e.g., 20 bar), speed to 0, and time to 1 second.
Why not zero out holding parameters?
During screw forward motion, cavity backpressure can push the screw backward before it reaches the target position. By applying a small counter-pressure (20 bar) with zero speed for 1 second, we create a brief stabilizing force that counters cavity pressure—allowing the screw to hold its intended position momentarily without advancing. This ensures the short-shot sample accurately reflects the intended fill level.
This technique works reliably on hydraulic machines under 400 tons. Electric machines don’t require this—they can directly set a precise screw position.
Analyze Short-Shot Samples
For a 2-cavity mold, you might observe flow imbalance due to hesitation effects. Gradually increase the injection stroke in increments (e.g., +5–10 mm per trial). After each shot, collect the part. Arrange the series of short shots from least to most filled. You’ll see a clear “flow front progression map.” Based on where defects appear in this sequence, you can take targeted corrective actions—such as implementing multi-stage injection profiling or adjusting injection speeds.
Factors Influencing Short Shot Results
Material Flowability
Under identical conditions, high-flow materials may fully fill the cavity, while low-flow materials exhibit short shots. Comparisons must use the same material grade and batch—even minor variations between batches or suppliers can invalidate short-shot positioning.
Thermal Stability
Prolonged residence time or excessive barrel temperatures can degrade the polymer, altering its flow behavior. Degradation may manifest as discoloration (yellowing/blackening), bubbles, or abnormal flow-front shapes—distorting analysis.
Temperature
Barrel Temperature: Higher temperatures reduce melt viscosity, improving flow and yielding “fuller” short shots.
Mold Temperature: Higher injection mold temps slow cooling, allowing longer flow distances. Uneven mold temperature causes asymmetric flow (one side flows faster than the other).
Injection Speed
This is the most critical parameter affecting flow morphology:
High speed: Melt flows like a flood—lower apparent viscosity, fills farther, but risks jetting or spray marks.
Low speed: Melt advances like a tide—higher viscosity, smoother flow, but may freeze prematurely.
Crucially: Injection speed must be held constant during short-shot testing. Varying speeds produce incomparable results.
