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Grommets, a time-saving solution

03 Dec 2019

A wiring harness is an organized set of wires, terminals and connectors that run throughout the entire vehicle and relay electronic information and electric power. It has a unique design that takes into account the electrical and geometrical peculiarities of the vehicle.

The harness can cost even 1,000 € each, it contains a complex collection of special wires — made of special alloys, lighter and more conductive than copper —and it can weigh up to 30 kgs. Special materials are used for each single part, suitable to resist to high temperature and extremely harsh working conditions.

Over these wire harness is put a protective casing, a grommet, utilised to seal the hole through which these components pass, drilled in the steel frame separating the passenger’s compartment from the engine area or from the luggage trunk. Moulded grommets, made in rubber soft plastics and expanded polymers are clamped over a pre-assembled harness, allowing the operator to insert the whole kit throughout the car body in a few seconds. Instead, Polyurethane grommets are directly poured on the harness inside a mould.

Cannon Solution for grommets

PUR grommets, the solution

The very large range of different kits that must be produced, working on irregular inserts composed by a variable number of cables or pipes, requires skilled operators and a high degree of visual control to avoid faults in manufacturing.

Differently from the past, the cost and weight optimisation of each single component of a vehicle has led to the request of harnesses containing only the number of cables needed by a personalised car.

This means that, to avoid errors and waste of time during the vehicle’s assembly, the supply chain of the car industry requires that a specific harness, made and bar-coded upon the unique configuration desired by a specific client, is sequenced correctly between a number of other harnesses featuring other configurations of cables. It is easy to understand the value of these harnesses and the cost occurring when something goes wrong.

For these reasons, the car industry demands for this specific technology more and more automated equipment, with very user-friendly controls and smarter moulds.

A whole range of tools

The car industry currently uses complex harnesses and grommets only in the medium-high range of vehicles, but the trend goes towards a more generalised use, also for trucks and special vehicles.

The HEV (hybrid electric vehicles) segment is estimated to be in the near future the largest segment of the automotive wiring harness market, by value and volume.

This trend has led to a diversified demand in terms of moulding complexity and request of automation.

The dimensional control of the grommets, especially after the demoulding, is a major concern for their producers: a slight error in calibrating the Polyurethane dosing unit or a variable humidity in the area surrounding the machine can influence the expansion of the high-density foam, thus generating a grommet slightly wider than expected.

This reflects in serious problems in the car assembly line, because a larger grommet will not fit through the precise hole drilled in the car body, leading to a difficult fitting and to last-minute adjustments of the diameter —a practice not suitable with the precise timing assigned on an assembly line. For this reason, a dosing unit provided with the most precise control of metering and ratio is required, fitted with a complete set of devices ensuring the most stringent control of temperatures, avoiding any undesired influence of external humidity on the chemicals.

The formulations used for the grommets —whose density averages 800 kg/m3 —are characterised by high viscosity at room temperature.

For this application, Cannon offers dedicated turn-key solution, where sophisticated moulds and electronic identification procedures are utilised to safely apply to the cable harness its own pouring program.

Cannon equipment for grommets

Cannon supplies for this application a special version of its A-Compact 10 dosing unit, with electrically heated rigid and flexible pipes for the components, air drier and stirrers on both chemical’s 70-litre tanks, a redundant control of temperatures across the circuit, with sensors placed up to the mixing head and closed-loop control of chemical’s output and ratio.

A version with a piston dosing solution for the resin is available, especially recommended when formulations are very viscous or might be filled with solid additives: these liquids require specific attention. A dedicated barrel-unloading station with stirrer for the Polyol drum guarantees the utmost constancy of supply of the resin side to the machine tank, avoiding a fluctuation of foam density values across the whole working shift.

A Cannon FPL 7 mixing head is used to inject the formulation into the moulds, fitted with a stroke adjustment of the self-cleaning piston in order to augment the mixing efficiency and optimise the final quality of the foam. Usually the injected Polyurethane weighs between 150 and 1,000 grams. Also available is the slightly larger FPL 10 version, with hardened internal parts needed when using abrasive fillers.

Each head is mounted on a very ergonomic vertical boom, designed to minimise the operator’s fatigue during the injections.

The mould, an intelligent tool

The mould represents the key of the production system: complex three-dimensional design with numerous “branches” of cables departing from a central core characterises these long, flexible parts.

Cannon developed an intelligent mould, fully electronically controlled and interconnected with dosing unit and mixing head. Called MAS (Mould Automation System) this industrial tool provides all the answers to a complex moulding process.

The harnesses can contain, in the most complex cases hundreds of wires of different diameters, that must be pre-grouped in order to reach properly their individual insertion points through multiple male or female multiple plugs.

These wires, in the portion of harness where the moulding of a grommet is required, must be safely housed in different mould cavities, avoiding any possible pinching during the closing operation.

Electronic sensors are added in the most critical areas of the mould, to warn the operator in case of a misplacement of one bunch of cables and for other  different operation phases:

  • Harness positioning
  • Mould closing and harness sealing
  • Mixing head connection
  • PUR Injection
  • Curing and part download
Cannon grommets electronic control

To ensure this protection special iris diaphragms are used, to drive the different groups of wires to the centre of their assigned mould cavity.

The sensors applied to the mould also control the mould closing approach, enable its locking, continuously control a possible harness pinching and verify the harness sealing in its cavity.

The sensors are connected to a CPU that verifies the correct displacement of the wires, giving punctual information where the right condition is not matched.

A multiple identification procedure is defined to avoid very costly errors during the production of a grommet.

The mould and the harness are individually identified scanning a bar code, a QR Code or through an RFID.

These two identifications define a set injection program that is communicated to the dosing unit and executed.

The operator only handles the mixing head into the injection point and extracts it.

At the end of a pre-set polymerisation time, a sequence of signals informs the operator that curing ended and he can put the mould in extraction configuration: the harness sealing and locking system are automatically disabled, the mould is automatically unlocked using a correct

sequence, the mould can be manually opened and the extraction of the harness with grommet can occur.

The operator is also given with the correct sequence, by a full graphic touch screen HMI, in order to be guided during the set-up process, minimizing the possibility to incurring into mistakes leading to scraps.

Finally, all process data are collected; all information regarding part number, production date/time, mould pressure, temperatures, and all available process parameters are stored on a local database or eventually to the Cannon Cloud infrastructure for traceability and data analytics. This configuration is available for single or multi-station fixed moulding lines, where foam is injected with mixing heads mounted on pivoting booms.

A Specialised Service

The Cannon R&D specialists in Caronno Pertusella, near Milano, Italy, have matured a significant experience in the development of new grommets, with a dedicated area of the laboratory available for customer trials and products characterization.

The availability of the complete range of equipment necessary for this application —metering machine, mixing heads, moulds, automation, with worldwide Technical and Spare Parts Services —puts Cannon in the privileged position of One-Stop-Shop supplier for the producers of grommets for harnesses, for automotive or other final applications, characterised by:

  • Excellent dimensional and mechanical properties,
  • Cost and scrap reduction
  • Time saving
  • High flexibility for just-in-time and just-in-sequence manufacturing conditions.

Cannon supplied recently numerous complete solutions to major players located in Europe, Mexico, North and South Africa, and welcomes inquiries for complex moulding projects that may require laboratory trials, joint mould development, prototyping and pre-production services.