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Issues Archive

May/June 2013 Vol. 17
No. 3
 Custom cable assembly  
  with Primum and  
  Millennium connectors  
  with both amber and black  
  PUR 
 BIRNS precision mould for  
  high performance EOM  
  cable assembly 
 Custom EOM cable solution  
  for a complex subsea  
  system with rear-exit  
  mechanical termination. 
 J-STD-001 Class 3  
  certified technician  
  prepares cable assembly  
  for overmoulding  
 PUR mixture is inserted  
  into mould with injection  
  gun under pressure 
 Deflashing BIRNS Primum  
  connector overmoulding   
  note the absence of voids  
  or bubbles in the  
  overmould 
 Custom-built Kikusui  
  16-simultaneous channel  
  computerised electrical  
  tester  
Advanced moulding solutions  

By Amy Brown, director of corporate communications,  
  BIRNS, Inc., California, USA
  

Demanding protocols of overmoulding and moulding custom cable assemblies

Today’s subsea connector industry calls for increasingly advanced solutions for moulding and overmoulding complex cable assemblies. The overmoulding for connectors and custom cable assemblies is a critical component of the viability of any such assembly to successfully withstand harsh marine environments, mechanical abuse and repeatability. Properly executed, the precision overmoulding process seals the connector/cable interface, provides strain relief, precludes discontinuous bending and adds elastomeric mechanical strength to the junction. Materials used for overmoulding can include epoxy, polyurethane, polychloroprene and a wide range of other substances, and custom moulding facilities are now capable of producing tailored solutions for a diverse suite of highly specific environments and applications.

BIRNS, Inc., an ISO 9001:2008 certified global leader in the development and manufacturing of high performance connectors and custom cable assemblies, has been innovating subsea technology since 1954. All BIRNS’ Quality Assurance (QA) personnel and production technicians are certified to both J-STD-001 (Requirements for Soldered Assemblies) Class 3 and WHMA-A-620-A (Requirements and Acceptance for Cable/Harness Assemblies) Class 3. The classification is important: Class 3, the highest class possible, is defined as for high performance electronic products, which are “those products where continued high performance or performance-on-demand is critical, equipment downtime cannot be tolerated, end-use environment may be uncommonly harsh, and the equipment must function when required, such as life support or other critical systems”.

The company includes a world-class NAVSEA S9320-AM-PRO-020 certified moulding facility (for fabrication of submarine outboard cables) – one of only seven such commercialorganisations in the world. BIRNS has perfected the process of taking cable assemblies through the rigours of overmoulding for a variety of demanding applications: from 2.5 kilometres in polar ice, to six kilometres in the ocean; from high voltage to electro-opto-mechanical terminations.

In connector manufacturing, overmoulding is a precision injection moulding process in which one elastomeric material (e.g. polyurethane) is moulded onto a second material (cable stock and/or metal connector shells), forming a powerful chemical bond able to withstand the punishing subsea environment. The process of overmoulding is carried out after the connector has been terminated, and the resulting solid elastomeric adhesion radically extends the life of the cable assembly. The bonding substance is injected into a heated barrel, mixed, and forced into a custom engineered mould cavity to cool and solidify. BIRNS is renowned for overmoulding techniques, including those requiring widely used commercial polyurethane, designed to perform mechanical functions requiring exceptional toughness under high compressive and tensile loading1. For example, BIRNS regularly bonds polyurethane to glass for underwater use under extremes of thermal stress, temperature gradient and nuclear radioactivity, so completely that the bonding precludes electrical leakage current even as low as four milliamperes. BIRNS also has deep expertise in overmoulding jobs specifying NAVSEA PRO-020 certification that require clear/amber military grade polyurethane: Conathane EN-1556. This military grade material has higher tear strength and is highly resistant to oils, gasoline and seawater, while providing outstanding protection against corrosion2. The processes for any polyurethane overmoulding are stringent, as any minute contamination, or failure to adhere to curing and timing details, can result in costly mistakes.

CASE STUDIES

BIRNS specialises in custom cable assemblies that are required to perform flawlessly under severe marine environments, for everything from high voltage to hybrid electro-opto-mechanical terminations (EOM). On a recent project that was to be operated several kilometres deep in ice, BIRNS was asked to develop a cable assembly with a high-powered BIRNS Primum connector, with four 2AWG (American wire gauge) contacts to power a hot-water drill pump at depths of 1.5 kilometres to 2.5 kilometres in the polar ice cap, and two BIRNS Millennium 3G connectors for temperature and pressure sensors for the project. The overmoulding was developed with both amber polyurethane overmoulding for the massive BIRNS Primum connector and with opaque black polyurethane for the BIRNS Millennium connectors.

Another recent BIRNS project included the development of an EOM cable assembly for a complex subsea system. This hybrid cable assembly featured a Kevlar strength member and a rear-exit mechanical termination designed by BIRNS. The system required an un-armoured tow cable with a breaking strength of 8000 pounds (3629 kilograms). BIRNS’ engineering team developed a unique termination design to help give tighter tolerance to the sub-cable length, and utilised rapid-prototyping CNC (computer numerical control) to make the mould for the overmould, which made the unit very streamlined, as well as sealing off areas that could trap and hold seawater.

OVERMOULDING PROCESS

Prior to the actual overmoulding, comprehensive termination inspections, insulation resistance (IR), continuity and Hi-Pot testing are all performed to verify specific requirements. The overmoulding process begins with wire terminations being encapsulated with epoxy adhesive after being cleaned with isopropyl alcohol. The cable jacket is then abraded, as are all surface areas to be overmoulded, and all glass and metal surfaces are sandblasted. All surfaces are chemically cleaned and then dried with clean, filtered and dehumidified compressed air. Specific primers are then applied to all non-urethane surfaces, including neoprene cable jackets and metallic surfaces of connector shells. The polyurethane overmoulding must be applied within eight hours of primer application or the entire priming process must be repeated. The overmoulding station must be well ventilated, temperature controlled and free from contaminants – especially silicone in any form, as the molecular structure of silicone prevents the adhesion process and is very difficult to remove completely if it contaminates a work area.

The two-part polyurethane mixture is measured and then heated in an oven at 145 degrees Fahrenheit (63 degrees Celsius) for 15 minutes, then the technician slowly hand mixes the two segments, taking extra care not to introduce excessive air into the material. Polyurethane can solidify if stored or exposed at length to temperatures below 75 degrees Fahrenheit (24 degrees Celsius), so care must be taken to keep the material at its proper consistency. Then the formula is poured into the injection cartridge into respective chambers, which is then de-gassed with a vacuum chamber and then loaded into the injection gun. The mould is painstakingly filled and then heat cured at 212 degrees Fahrenheit (100 degrees Celsius) for a minimum of 30 minutes. The technician then removes the part from the mould and post cures it in an oven for eight to ten hours.

Visual inspection (for PRO-020, in accordance with MIL-C-24231) includes comprehensive visual examination of the assembly to ensure that no voids, gas bubbles, flash (ridges on the exterior of the moulding larger than one-thirty-second of an inch/0.7938 millimetres) or other imperfections exist. If applicable, IR testing continuity testing is done again, as well as non-destructive bond testing, which tests the bonding success of the overmould about the circumference of the connector. Seventy-two hours later, hydrostatic pressure testing can be conducted on the overmoulded cable assembly, followed by a final Quality Inspection.

TESTING PROTOCOLS

BIRNS testing capabilities include a state-of-the-art, high performance hydrostatic pressure testing system, with a range of vessels rated to 20,000psi, 10,000psi, 5000psi and 1000psi.The hyperbaric device has a unique control panel for diverse, comprehensive testing on up to three circuits and in six chambers simultaneously, allowing several different tests to be run on multiple products. The system also has a robust, high volume helium testing capability, exceeding stringent ABS/DNV requirements for man-rated penetrator testing.The sophisticated system provides an enhanced, efficient means with which to test a wide range of products for both rigorous safety and demanding performance requirements. Electrical testing capabilities include EIA/ECA-364 (formerly MIL-STD-1344) using a custom-built Kikusui 16-simultaneous channel computerised electrical tester.

This extreme level of testing protocol is designed to push custom cable assemblies and other connector systems to the limit of performance capabilities, to ensure that the finished cable assembly can withstand pressure and duress without degradation of any sort. The solid overmoulding techniques are intrinsic to that successful final testing, and the performance of the part in the field.

 

1. Chase Corporation Fluid Polymers Product Data

2. CYTEC Industries Bulletin P181(d)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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