The fill study (sometimes called a short shot study or viscosity curve study) is the starting point for any properly developed injection moulding process. Get this right and everything that follows — pack, hold, cooling — is built on solid ground.
Here's how to run one.
What a fill study is for
A fill study has two goals:
- Find the transfer position — the point at which you switch from velocity-controlled filling to pressure-controlled packing, with the cavity approximately 95-98% full
- Generate a viscosity curve — to confirm your process is running in the viscosity-insensitive region (where small speed changes don't significantly affect fill behaviour)
Before you start
Make sure:
- Material is fully dried to specification
- Barrel temperatures are stable and have soaked for at least 20-30 minutes
- Pack and hold pressure are set to zero (or near zero) — you're studying fill only
- Back pressure and screw speed are set to your standard values
Step 1 — Short shots
Starting at a low injection volume, run a series of shots, increasing the shot size incrementally until the part just fills without packing.
You're looking for the shot size (or transfer position) where the part is 95-98% full — all features formed, no sink marks, no flash.
Document each shot with a photo and record the peak injection pressure.
Step 2 — Viscosity curve
With transfer position fixed, run the part at a range of injection speeds — typically 5-7 speeds spanning from very slow to very fast.
At each speed, record:
- Fill time
- Peak injection pressure
Calculate relative viscosity at each speed: Relative viscosity = Peak pressure × Fill time
Plot speed vs relative viscosity. You're looking for the flat region of the curve — where increasing speed doesn't significantly change viscosity. Your target process speed should sit in this region.
If no flat region exists, investigate: material drying, melt temperature, or machine condition.
Step 3 — Document and lock
Once you have your transfer position and target fill speed, document both with the supporting data. This becomes part of your process record — not just the final settings, but the evidence that justifies them.
What to do with the results
Your fill study results feed directly into the next stage: pack and hold development. With a stable, documented fill phase, you can isolate packing behaviour with confidence — knowing that any variation you see is from packing, not from an inconsistent fill.
That's the principle at the heart of scientific moulding: study one variable at a time, document what you find, and build a process on evidence rather than instinct.