Grommets made with our technology provide many advantages that traditionally produced grommets cannot match.
December 3, 2019
A wiring harness is an organized set of wires, terminals and connectors running throughout the entire vehicle. They relay electronic information and electric power. Each one has a unique design that takes into account the vehicle’s electrical and geometrical peculiarities.
Harnesses can cost up to €1,000 each due to complex collection of special wires made of special alloys, lighter and more conductive than copper — and can weigh upwards of 30 kg. These special materials are used for each part to resist high temperatures and harsh working conditions.
A protective casing, called a grommet, is placed over the harness and is used to seal the hole through which these components pass; the hole is generally drilled into the steel frame separating the passenger compartment from the engine or trunk depending on the car. Molded grommets, made of soft rubberized 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 poured directly on the harness inside a mold.
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 manufacturing faults and defects.
Cost and weight optimisation of each singular vehicle component has led to requesting harnesses containing only the exact number of cables needed by a car.
This means that, to avoid errors and waste of time during vehicle assembly, the automotive industry’s supply chain requires that a specific harness, made and barcoded according to the unique configuration desired, is sequenced correctly between a number of other harnesses featuring other configurations.
For these reasons, the auto industry demands increasingly automated equipment for this specific technology, with user-friendly controls and smarter molds.
A whole range of tools
The auto industry currently uses complex harnesses and grommets only in medium-high end range, but the trend is moving towards a more generalized use to include trucks and specialized vehicles.
The hybrid vehicle segment is estimated largest in near future for the wiring harness market, by value and volume.
This trend has led to a diversified demand in terms of molding complexity and automation requests.
Dimensional control for grommets, especially after demolding, is a major concern for producers: a slight calibration error in the polyurethane dosing unit or variable humidity in the area surrounding the machine can influence the expansion of high-density foam, generating a grommet slightly wider than expected.
This opens the way for serious problems in the car assembly line: a larger grommet will not fit through the precise hole drilled in the auto body, leading to difficult fitting and last-minute diameter adjustments — a practice not suitable to the precise timing assigned on an assembly line. For this reason, a dosing unit provided with precision metering control and ratio is required, fitted with a complete set of devices ensuring the stringent control over temperatures, avoiding the 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, we offer dedicated turnkey solutions, where sophisticated molds and electronic identification procedures are used to safely apply to the cable harness its own pouring program.
For this application, we supply a special version of the A-Compact 10 dosing unit, with electrically heated rigid and flexible pipes for the components, air drier and stirrers on both 70-liter tanks, a redundant control over 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, and is recommended when formulations are highly viscous or might be contain solid additives: these liquids require special attention. A dedicated barrel-unloading station with a stirrer for the polyol drum guarantees the resin is constantly removed from the tanks walls, avoiding a foam density fluctuations.
Our FPL 7 mixing head is used to inject the formulation into the molds, fitted with a stroke adjustment for the self-cleaning piston to augment mixing efficiency and optimise the foam’s final quality. Generally, the injected polyurethane weighs between 150 and 1,000 grams. The slightly larger FPL 10 version, with hardened internal parts needed when using abrasive fillers, is also available.
Each head is mounted on an ergonomic vertical boom, designed to minimise operator fatigue during injections.
The mold, an intelligent tool
The mold is key to the production system: complex three-dimensional design with numerous “branches” of cables departing from a central core characterizes these long, flexible parts.
We developed an intelligent mold, fully electronically controlled and interconnected with the dosing unit and mixing head. Called MAS (Mould Automation System), this industrial tool provides all the answers to a complex molding process.
The harnesses can contain, in complex cases, hundreds of wires with different diameters that must be pre-grouped to reach properly their individual insertion points through multiple male or female plugs.
These wires, in the portion of harness where grommet molding is required, must be safely housed in different mold cavities, avoiding 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
To ensure this protection, special iris diaphragms are used to drive the different groups of wires to the center of their assigned mold cavity.
Sensors applied to the mold control the mold closing approach and enable its locking, continuously controlling possible harness pinching and verifying that the harness is correctly sealed into its cavity. They are connected to a CPU that verifies correct displacement, giving punctual information when and where the condition is not correct.
A multiple identification procedure is defined to avoid costly errors during production. The mold and harness are individually identified scanning a barcode, QR Code or through RFID. These two IDs define a set injection program that is communicated to the dosing unit and executed.
The operator only handles the mixing head, placing it at the injection point and extracting it afterwards.
At the end of a pre-set polymerisation time, a sequence of signals informs the operator that curing has ended and they can put the mold in extraction configuration: the harness sealing and locking system are automatically disabled, the mold is automatically unlocked using a correct sequence. The mold can be manually opened and the extraction with its grommet extracted.
The operator is also given the correct sequence via a full graphic touch screen HMI for guidance during set-up, minimizing the possibility make mistakes.
Finally, all process data are collected; all information regarding part number, production date/time, mold pressure, temperatures, and all available process parameters are stored on a local database or eventually to the our cloud infrastructure for traceability and data analytics. This configuration is available for single or multi-station fixed molding lines, where foam is injected with mixing heads mounted on pivoting booms.
A Specialised Service
The our R&D specialists have significant experience in developing new grommets, with a dedicated area of the lab available for customer trials and product characterization.
Our complete range of equipment is available to clients — dosing machine, mixing heads, molds, automation, with worldwide technical and spare parts services — and puts us in the privileged position of being a complete supplier for producers of grommets characterised by:
- Excellent dimensional and mechanical properties
- Cost and scrap reduction
- Time saving
- High flexibility
