Crimp die selection and pull-out strength

The die and the barrel are one system

A crimp die is not a generic press tool. It is designed for a specific barrel geometry and a specific conductor range. The die cavity shape, the crimp depth and the locator all belong to that combination. Mixing a die from one supplier with a barrel from another is a design change, even when the part numbers look compatible.

The barrel geometry includes the wall thickness, the inner diameter, the bellmouth and the material. The die has to compress that barrel onto the conductor without cutting it, cracking it or leaving voids. A die that is right for a copper barrel can be wrong for an aluminium one of the same nominal size, because the two materials flow differently.

This is why the die reference belongs on the drawing and in the work instruction. An operator who has a box of dies and a box of lugs will find a combination that closes, but closing is not the same as crimping to specification.

Die types and crimp geometry

The common geometries are the hex crimp, the indent crimp and the nest or applicator type used on automated presses. A hex die closes the barrel into a hexagonal section, which gives a large contact area and a predictable crimp height measured across the flats. An indent die presses one or more points into the barrel, which suits smaller lugs and thinner walls.

The geometry affects the inspection. A hex crimp is measured across the flats with a micrometer or a go and no-go gauge. An indent crimp is measured at the indent depth. A four-indent crimp is checked for depth and for symmetry, because an uneven indent can bend the barrel.

The die also sets the bellmouth. A small flare at the conductor entry relieves the stress concentration where the strands leave the crimp. A die that closes the bellmouth flat can create a sharp edge that cuts strands under vibration.

Conductor class and the die range

Conductor class changes the die requirement. A Class 2 stranded conductor and a Class 5 flexible conductor of the same nominal area have different strand counts and different bundle behaviour. The flexible bundle compacts more, so the same die can over-crimp it or leave it loose depending on the barrel.

The lug datasheet normally states the conductor range and the die for each range. Treat that table as the starting point. If the actual cable is compacted, tinned or made to a different strand pattern, make a sample and test it rather than assuming the nominal range applies.

Aluminium adds another variable. The oxide layer is hard, the metal creeps, and the barrel is often filled with an inhibitor. The die and the crimp height for aluminium are specific to that combination. A copper die used on aluminium is a common source of loose or cracked joints.

Crimp height and pull-out strength

Crimp height is the distance across the crimped barrel after the die closes. It is the process variable that best predicts the joint. Too high and the barrel is under-crimped, with voids and low pull-out. Too low and the barrel is over-crimped, with cracked walls or damaged strands.

The relationship is not linear. Pull-out strength rises as the crimp height falls, up to a point, then falls sharply when the barrel or the strands are damaged. The target window sits between those limits, and the die maker sets it for the barrel and conductor range.

Measure the crimp height on every setup and at a defined frequency during production. A micrometer across the flats is enough for a hex crimp. Record the value, the die number and the operator. If the height drifts, the die, the press or the material has changed, and the joint should be re-qualified.

Running a pull test that means something

A pull test applies a tensile load along the conductor axis while the lug is restrained. The peak load is recorded, and so is the failure location. The acceptance value comes from the product standard, the drawing or the customer requirement.

The failure location is the diagnostic. A conductor pulling out of the barrel with clean strands suggests under-crimp or a barrel that is too large. A barrel that splits suggests over-crimp or a wall defect. A conductor that breaks outside the barrel is usually the best result, because the joint outlasted the wire.

Sample size and frequency should be stated in the control plan. A common approach is a test at setup, then a defined sample per shift or per lot. For a critical joint, add a cross-section at the same frequency to check voids and strand damage, because a pull test alone can pass a joint with internal defects.

Setting up a die

Start with the correct die and barrel pair, then confirm the conductor range against the actual cable. Set the locator so the conductor sits at the specified depth, and set the crimp height to the middle of the window rather than the edge. Make a sample and measure it before running production.

Run a pull test on the first sample and record the result with the die number, the crimp height and the cable identification. Keep the sample and the record together. If the joint later fails, the setup evidence is the first thing an investigation will ask for.

Check the press. A worn press, a damaged ram or a misaligned die can produce a crimp that measures correctly on one side and is loose on the other. Look at the crimp symmetry as well as the height.

• Die and barrel from the same system, with the die number recorded
• Conductor range checked against the actual cable
• Locator set for the correct insertion depth
• Crimp height set to the middle of the window
• First sample measured and pull tested
• Press and ram checked for wear and alignment

Common failures and what to change

A loose joint with low pull-out usually points to the wrong die, a crimp height above the window or a barrel that is too large for the conductor. Check the die number first, then the height, then the conductor range. Do not fix it by crimping twice, because a second crimp on an already work-hardened barrel can crack it.

A cracked barrel points to over-crimp, a die with a sharp edge or a barrel that is too thin for the die. Check the height against the lower limit and inspect the die for wear. On aluminium, a crack can also come from a copper die or from too little inhibitor.

A joint that passes a pull test but runs hot points to a resistance problem at the interface, often from a contaminated barrel or a plating defect. Add a resistance check or a cross-section to the investigation. Pull strength and electrical quality are related but not identical.

What to put in the work instruction

Write the die number, the barrel part number, the conductor range and the crimp height window. Add the insertion depth, the pull test value and the sample frequency. Include a photograph of a correct crimp and a rejected one, because a written description of a bellmouth is less useful than a picture.

For a new programme, ask for the die qualification record and a sample joint made with the production press. That record should show the crimp height and pull test across the conductor range, not just at one size.