How Good Are Your Crimps?

Crimping & Crimp Quality - Issue 6

A crimp is of acceptable quality when all electrical and mechanical requirements are fulfilled. What are these requirements and how are they fulfilled? In order to answer these questions, one has to look at the numerous measurable and visual characteristics. The essential requirements of crimp connections are defined in the DIN IEC 60352 part 2. This norm also includes the examination process and the directions for the use of crimp connections.

Numerous characteristics determine the quality of a crimp connection. The following essential conditions must be considered and fulfilled before crimping can take place:

Strip Length and Position of the Conductor in the Crimp

Most crimp connections use stranded wires with an insulating jacket (insulation) in different designs. The wire must be stripped before it is crimped. The strip length and the position of the wire must be selected so that the individual strands are visible on both sides of the crimp zone (crimp barrel). The individual wires should not jut into the plugging or connection area as this can limit the function of, or damage, the terminal.  

The exact positioning of the wire is especially difficult with very small terminals with very short distances both sides of the crimp barrel. High precision is extremely important during the stripping and the feeding of the cable. The insulation should be visible between the insulation holding device (insulation crimp) and the crimp zone (wire crimp). Under no circumstance should it appear inside the crimp barrel (in wire crimp).

Example of stranded wire in a crimped terminal

Perfect Stripping

Perfect stripping of the wire is a must for a good and durable crimp connection. Faults that occur during stripping often remain undiscovered and can have fatal consequences for the crimp connection.

Wire crimp in cross section

For perfect strip quality, the individual strands of the wire must not be damaged or cut off during the stripping process. The insulation must not be damaged. There must be no remnants of insulation on the stripped part of the wire. The individual strands must not be untwisted during the stripping process as this often leads to spreading out of the individual strands, which can cause some of them to fall outside the crimp area during crimping. The crimp connection would thereby be faulty, creating the risk to short circuit. On the other hand, the individual strands must not be over-twisted as this causes an uneven distribution in the crimp barrel and the wire cross-section becomes larger. Therefore, it is especially important to use precise and reliable systems for the stripping process.

Crimp Dimensions with Open Barrel Terminals

One of the most important methods of checking the quality of a crimped terminal is to measure the key dimensions after crimping.  The crimp width and especially the crimp height, determined by the manufacturer, have an important influence on the quality and the long-term behavior of a crimp connection. Also for the insulation crimp, crimp dimensions are defined by the manufacturer (generally as “approximate values”).

Bellmouth

The conductor entry end of the crimp barrel must have a visible radius (funnel-shaped expansion). This entry radius prevents the notching or separation of individual wires and is therefore important for the quality of the crimp connection. A radius on the conductor exit end is permitted but not essential. 

General Condition of the Crimped Terminals

After the crimping process, neither the terminals nor the conductor should show evidence of damage, which could restrict function or influence long-term behavior. Faults can occur with crimped terminals through incorrect handling, incorrect set-up or inappropriate crimp applicators. In practice, the following cases occur frequently:

Tolerance ranges for crimp terminals

Quality Assurance with Crimping

The most important instruments for quality assurance are the visual and dimension checks, optical tests, crimp force monitoring, the measurement of tensile strength and the preparation of a cross-sectional view.

Visual Check

The human eye is still irreplaceable for quality assurance. Many defects/faults can be recognized through a visual check carried out by an expert:

Eventually existing flash height should be less than half of the material thickness

Dimension Check

The measurement of crimp height must be very precise and reproducible. For this measurement, a special micrometer or a special crimp height measuring devices is used.  The crimp height measuring equipment is used mostly in fully automatic processing or where quality assurance has a high priority. Through their high precision and network interfaces, such devices offer the possibility of direct electronic data recording and evaluation. In networks with crimping presses, the crimp height can be automatically adjusted by the press according to the measured values. The crimp width and the crimp dimensions of the insulation holding device are measured with a measuring gauge.

Crimp height measuring

Optical Test Devices

Some characteristics can only be measured optically (e.g. curvature of the terminals, length of the dividing wedges, size of the radii on the crimp barrel). For such checks, measuring or profile projectors or measuring microscopes are used. Also, the aforementioned visual tests are often supported by the use of optical instruments, such as a hand-held or bench-top magnifying glass.

Crimp Force Monitoring

The measurement, recording and evaluation of the crimp force allows crimp faults to be recognized during the running process. This task, known as “crimp force monitoring,” is carried out by crimp force monitors that are either integrated into the crimping press or connected as optional devices.  

The crimp force monitoring system records the crimp force curve at the same time as the press runs. After each crimp, the actual curve is compared to the saved reference curve. Deviations of the curves are evaluated according to different criteria. If the deviation exceeds the defined tolerance limit, the crimp is classed as faulty. 

This procedure allows a 100% test of crimp connections without requiring any additional time. Faults, such as the cut-off of individual strands, wire that is not stripped or fully stripped, or the incorrect positioning of the conductor in the crimp, are found in real time.  Of particular importance is that the crimped samples used to create the reference curves are perfect. If, for example, the reference curve was created with divided individual strands, then divided individual strands will not be recognized as a fault during production . This fault will probably reveal itself once the first perfect examples are finished after the correction (because they are then classified as faulty crimps).

Measuring the Pull-Out Force

In contrast to the previously described test procedures, this test and the tests which follow are destructive tests. The tensile strength of the crimp connections, independent of the cable cross-section, is not allowed to fall below certain values. These values are normalized or defined by the terminal part manufacturer. For the measurement of these values, corresponding pullforce measuring devices are used. These devices come in different sizes and designs. It is also possible for network access here, direct data recording, evaluation and archiving. Possibly occurring deviations from the ideal values allow faults in the crimp process to be recognized at an early stage and enable the introduction of corrective measures.

Preparation of Ground Cross-Sectional View

The preparation of the ground cross-sectional view largely serves as the assessment of the degree of deformation and the symmetry of the crimp. A ground cross-sectional view also allows any occurring ridges to be measured and to recognize possible cracks in the material. 

To create a ground cross-sectional view, the crimp zone is cut horizontally. After that, the cut surface is ground, polished and etched. The prepared samples are then visually checked under a microscope and assessed. 

Ground cross-section of wire

A good crimp connection shows the following characteristics:

Marked deviations from this ideal condition often suggest errors in wire and terminal selection, selection of the proper crimp applicator, incorrectly set crimp height, poor quality stripping or wear and tear of the crimp tooling.  However, the crimp quality also depends largely on the specialist competency of the employees, on the material quality (terminals and wires) and on the quality of the crimp applicators and machines.

Conclusion

The electronic quality assurance systems in the crimping process, such as crimp force monitoring, crimp height and pull-force measuring and the statistic recording and evaluation of the collected measurements, offer a high level of security. Crimp force monitoring allows faults to be recognized during production.  These systems offer support and ease with the assessment of crimp quality. However, they cannot replace specialized and experienced employees. At the end of the day, the optimal interplay between people and technology determines the quality of the crimps.