How to Run a Viscosity Study

A viscosity study is one of the first structured experiments in scientific moulding.

It answers a basic question:

How does this material behave in this machine at this temperature?

That answer matters because injection speed, fill behaviour, and transfer stability all depend on melt viscosity.

Without a viscosity study, speed selection is often guesswork — or copied from a previous job that used a different lot, machine, or thermal condition.

This article outlines a practical approach to running a viscosity study that produces usable data, not just a chart filed and forgotten.

What a Viscosity Study Is — and Is Not

A viscosity study measures how fill behaviour changes across a range of injection speeds at fixed melt temperature — with fill conditions controlled consistently enough that peak pressure comparisons are meaningful.

It is not:

• A full process development exercise
• A substitute for gate freeze or pack/hold optimisation
• A one-time activity that never needs repeating

It is:

• A characterisation of material flow response in the current setup
• A basis for selecting an injection speed in the stable region of the curve
• A reference point when material, machine, or thermal conditions change

The study does not tell you the perfect process.

It tells you where the process is likely to be repeatable.

Prerequisites

Before starting, confirm:

Machine and material

• Material is dried and conditioned per specification
• Melt temperature is stable and verified
• Shot size is appropriate for the screw diameter
• Decompression and back pressure are set to values that will remain fixed during the study

Tooling

• Tool is at production temperature
• Vents are clear
• No known mechanical issues that will distort fill behaviour

Measurement

• A consistent method to evaluate fill — short shots at fixed transfer, or cavity pressure where available
• Shot weight or fill pattern recorded repeatably
• Machine injection speed displayed in a meaningful unit (mm/s, in/s, or calibrated percentage)

Process state

• Short shots only — fill evaluation without pack influence
• Hold pressure minimal or zero during fill-only evaluation
• Cooling time sufficient that shots are comparable
• Transfer approach defined before starting (see Step 1)

If these conditions are not met, the study will characterise instability — not viscosity.

Step 1: Fix What Must Not Change

During a viscosity study, injection speed is the primary variable.

Hold constant:

• Melt temperature (front zone and measured melt if available)
• Mold temperature
• Shot size (metering and cushion target)
• Back pressure
• Decompression
• Material lot (where possible)

Transfer position: fixed or adjusted?

The goal is to compare peak injection pressure at different speeds under equivalent fill conditions — not to let end-of-fill or pack influence distort the reading.

Two approaches are used in practice:

Option A — Fixed transfer (simplest)

Transfer position stays the same for every speed step.

Set transfer so that even the fastest speed in the range produces a true short shot — no pack, no flash, no hard end-of-fill coupling. Slower speeds will typically show less fill (~80–90% is common). That is acceptable.

This works well when inertia effects are modest and the speed range is not wide.

Option B — Adjust transfer to hold fill state (often necessary)

As speed increases, cavities often fill further at the same transfer position — inertia and melt compressibility see to that. A transfer that gives ~85% fill at the slow end may overfill, flash, or couple fill and pack at the fast end.

If that happens, adjust transfer position at each speed step to maintain the same target fill state — same visual short-shot level, or the same part weight at transfer if you are weighing.

Shot size (metering) stays fixed. It is transfer that moves, not the amount of material conditioned per cycle.

Record the transfer position used at each speed in your results table.

This is not a failure of the study. It is a practical way to keep fill equivalent so peak pressure reflects material response to speed — not resistance from a cavity that has already filled and packed.

What to avoid: changing speed and transfer randomly without a fill target, or allowing high-speed shots to flash or pack while low-speed shots remain short. That compares unlike conditions and produces a misleading curve.

Document your chosen approach before the first shot.

Any unplanned change mid-study invalidates comparison.

Step 2: Select the Speed Range

Choose a range of injection speeds from slow to fast enough to show a clear change in response.

Typical approach:

• Start at a conservative low speed
• Increase in equal increments through the usable range
• Include enough points to show the curve shape — usually five to ten speeds

Avoid speeds that:

• Produce incomplete fill at the fixed transfer
• Cause flash, ejection damage, or vent blow-by
• Exceed machine control stability

The goal is the normal processing envelope — not the extremes of machine capability.

Step 3: Run Short Shots at Each Speed

At each speed:

1. Purge and settle if needed after the speed change
2. If using Option B, set transfer to achieve the target fill state at this speed before collecting data
3. Run a minimum of three consecutive shots
4. Record injection speed, peak injection pressure, transfer position (if adjusted), fill pattern or shot weight, and any visual notes
5. Confirm fill is consistent across the three shots before moving on

Use short shots — fill only, no pack — so the measurement reflects injection behaviour, not hold compensation.

If cavity pressure is available, record peak cavity pressure at a reference location where the application warrants it.

Step 4: Plot and Interpret the Curve

Plot injection speed against peak injection pressure (or cavity pressure if measured).

The typical curve shows:

• Shear-thinning region — pressure changes significantly with speed
• Transition region — slope begins to flatten
• Stable region — further speed increase produces relatively small pressure change

The stable region is the usual target for production injection speed.

Why?

Because small variations in speed — from machine response, material lot, or thermal drift — produce smaller variation in fill behaviour when operating on the flat portion of the curve.

Selecting speed in the steep part of the curve makes the process sensitive to every minor change.

Step 5: Select a Production Speed

Choose a speed within the stable region that:

• Achieves complete fill at the intended transfer approach
• Leaves margin below speeds that cause instability or quality risk
• Supports the cycle time requirement where possible

Document:

• Selected speed
• Expected peak pressure range at that speed
• Reason for selection

This becomes part of the process development record — not just a number on the machine.

Common Mistakes

Changing transfer without holding fill state Adjusting transfer at each speed to maintain the same short-shot fill is acceptable — and often necessary. Changing transfer without a defined fill target, or letting high-speed shots overfill while low-speed shots stay short, compares unlike conditions.

Confusing shot size with transfer If cavities fill further as speed increases, move transfer position — not shot size — to restore the target fill. Shot size (metering) should remain fixed for repeatability.

Studying with hold pressure active Hold masks fill variation. Fill-only shots are required.

Too few shots per point One shot per speed is not data. It is an anecdote.

Ignoring thermal stability If barrel temperatures are still drifting, or the tool is not at temperature, the curve reflects warm-up — not viscosity.

Running once and never revisiting New material lot, different machine, or significant maintenance can shift the curve. The study is a reference, not a permanent truth.

Selecting speed for cycle time alone The fastest speed that fills is not always the most stable speed. Cycle time matters. Repeatability matters more.

When to Repeat the Study

Revisit viscosity characterisation when:

• Material lot or grade changes
• Melt temperature setpoint changes significantly
• Machine plasticising components are serviced or replaced
• Transfer difficulty appears without obvious tooling cause
• Process is moved to a different press

In medical and other validated production, confirm whether the change triggers revalidation or documented re-evaluation per your quality system.

Medical Production Considerations

Medical moulding does not require a different viscosity curve.

It requires that speed selection be evidence-based and recorded.

Regulated environments benefit from viscosity studies because they:

• Support defendable process development
• Provide baseline data for deviation investigation
• Reduce reliance on operator judgement for speed selection

The study should be referenced in process development records and available during audit.

"We always run fast on this material" is not a control strategy.

"We selected speed X based on viscosity study ref Y" is.

What to Record

Minimum documentation:

Store with the process development file for the tool.

Conclusion

A viscosity study is not advanced science.

It is disciplined observation.

Fix the variables that should not move. Vary speed systematically. Keep fill conditions equivalent — fixed transfer or adjusted transfer with a clear fill target. Measure response. Select production speed in the stable region of the curve.

Done properly, it replaces guesswork with a defendable basis for fill behaviour.

Done once and ignored, it is paperwork.

The difference is whether the data is used when the process next drifts — or rediscovered from scratch under production pressure.