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T C SHIELDING LTD.
Unit 2, Ashburton Industrial Estate, Ross on Wye, Herefordshire, HR9 7BW
Phone 01989 563941, Fax 01989 566874
*Please note - the address above is our registered office. Reg no: 2573493*
EMC T C Sheilding mesh fabric electromagnetic compatibility rfi shielding screen radio frequency interference shielding materials electronic shielding electrically conductive elastomers knitted wire mesh emc emi electromagnetic interference emp electromagnetic pulse nemp nuclear electromagnetic pulse screening attenuation cables electromagnetic shielding ventilation panels shielded windows ito coatings gaskets shielded rooms anechoic chambers capacitive coupling be cu fingers aperture absorption loss electrical bonding flame retardant nickle graphite silicones silver aluminium silicones silver copper silicones silver glass silicones conductive fluorosilicones silver silicones doors windows waveguide cabinets magnetic fabric wrap volume resistivity attenuation db decibal transfer impedance shielding effectiveness susceptability permeability emissions plane wave near field far field h&e field h & e field insertion loss impedance z immunity hertz hz ground plane galvanic compatibility silicones fluorosilicones esd e.s.d. elastomer ranges cross talk conductivity capacitive coupling be cu fingers aperture absorbtion loss electrical bonding flame retardant nickle graphite silicones silver aluminium silicones silver copper silicones silver glass silicones conductive fluorosilicones silver silicones T C Shielding T C Sheilding electromagnetic compatibility rfi shielding radio frequency interference shielding materials electronic shielding electrically conductive elastomers knitted wire mesh emc emi electromagnetic interference emp electromagnetic pulse nemp nuclear electromagnetic pulse Company Profile T C Shielding Ltd specialise in the manufacture of conductive elastomers typically used in EMC applications. The product and materials are designed, developed and manufactured by T C Shielding, to ISO 9002 approved standards, at its site in Ross-on-Wye, Herefordshire. PRODUCT RANGE (Conductive Elastomers and EMC solutions) Our products fall into one of four categories: Screen Printing: A novel way of producing gaskets either as discrete components or by depositing a gasket directly to a component hardware. Extrusion: A wide range of standard and special profile product is produced - from simple 'O' ring cord down to 1 mm diameter, to complex (inc. hollow) forms 20mm in section. Extrudate can be supplied in continuous length, cut length, joined or fabricated to form door seals. Moulding: Compression moulding is used to produce complex components of various shapes, sizes and gaskets of three dimensional forms. Other Products: Including fabric wrap, mesh, vent panels, windows and foil tapes. SALES/MARKETING T C Shielding has a wide customer base covering a broad range of markets:- Telecommunication, aerospace & defence, medical, railways and industrial electronics. Our customers include Motorola, GEC Marconi, Westinghouse, Siemens, Nortel, Ericsson, Nokia, Lucent Technologies, British Aerospace and Thomson of France. Some 40% of T C Shielding's business is exported. T C Shielding offers comprehensive technical and sales support direct from its UK manufacturing plant at Ross-on-Wye. By contacting the numbers shown in this document we will be pleased to offer technical advice and prices, or arrange visits to customers' facilities to discuss applications in more detail. T C Shielding customer support activity is further enhanced in the UK and overseas by a network of agents dedicated to service the EMC business section. Printed Gaskets - Design Guidelines Standard connector gaskets produced by printing are readily available and are covered in detail on the standard products information sheets. The flexibility of the printing process makes this method ideal for custom made special designs. l Gasket Variants Pure Print - this particular type of gasket is a very cost effective method of producing relatively small, simple gaskets. The gasket is merely printed to the correct form and design without any punching. Material wastage is minimal. Typical thickness - 0.5mm. Punched gasket - to cater for more complex forms and offer clean edge definition, punching is utilised. However, the method does not generate excessive wastage of material as one would expect with die cutting from sheet. A special printing screen is used to minimise waste. Typical thicknesses of 1.00/1,50mm are possible. Substrate gasket - as certain designs of gasket increase in size and complexity it is often necessary to use a rigid carrier to improve handling and assembly. The printing process is ideal for this method. The substrate can be a variety of materials, e.g., metallic, plastic, glass, etc., provided they are flat (i.e., free from protrusions) and able to withstand the curing temperature of the printing polymer. The substrate will have to be intrinsically conductive or have a conductive coating. The ability to print on substrates lends itself to printing directly on component hardware. This can give major benefits in terms of handling, ease of assembly, serviceability and cost when compared to conventional gasket methods. The physical constraints of the component are 600mm x 800mm x 25mm deep. Traditional flat gaskets can suffer limitations due to uneven contact stress (and hence sealing/shielding) due to the large contact area of the gasket joint. Printed gaskets offer a solution and are unique in that the gasket can be produced with a stress raising bead (typically 2 to 4mm wide). The bead can be configured to provide optimum contact stress with the minimum clamping load. The printing process can also provide the following 1: Additional environmental sealing from a sealing bead in conventional elastomer, protecting the EMC bead from the effects of fluid degradation and corrosion. 2: Compression limiting by printing hard stop pads in relevant areas thus preventing over compression of the sealing bead. l Gasket configuration Printed gaskets can have a variety of forms, from simple pure prints to complex sub-assemblies. However, there are certain rules that apply to the profile and positioning of the bead and its base, namely: a) Beads, due to the meniscus effect, have a distinct relationship between height and width. Therefore, to achieve optimum performance, the bead width should be between 1.5 and 5.0mm. b) Adjacent beads should be separated by at least 1.0mm. When printing on to substrates (or hardware) a minimum clearance of 0.5mm should be applied to the edge of the component (including fixing holes and cut outs). c) It is possible to print a second bead onto a primary print but always allow for a maximum print thickness in the region of 0.3 to 0.5mm. l Design T. C. Shielding Ltd. offer complete technical service in the layout and design of printed gaskets, from simple punched forms to complex multi-gasket sub-assemblies. To gain optimum benefit from printed product design, it is recommended that T. C. Shielding Ltd. be consulted at the earliest stage of any design programme. This will ensure that all aspects of design, cost and function are realised. l Materials A comprehensive range of highly conductive materials are available for the printing process - please refer to the material fact sheet. Extruded Forms - Application Details Standard or special extrudate can be supplied in a variety of forms, cut to a discrete length, continuous length or fabricated. Fabrication involves joining the extrudate to form a continuous seal. The joint is fully vulcanised with a conductive jointing material. Fabrication covers a multitude of configurations from a simple 'O' ring to complex panel flange gaskets up to 1.5 metres in size. l Installation To effect a mechanical and electrical seal the gasket needs to be compressed. Insufficient compression can result in fluid leakage and poor electrical performance. Excessive compression will result in physical and electrical failure of the joint. The best method for controlling compression is by locating the gasket in a groove - typical configurations are shown below. Correct Assembly Type A - Machined or Type B - formed groove cast/moulded groove in sheet material If grooves are not possible, compression control can be achieved by alternative means, e.g., shouldered fixings, washers, spacer plates and in certain cases limiters built into the gasket. l Compression Gaskets have a finite working compression range. Each sectional profile has its own loading characteristic primarily due to its stiffness and shape. Conductive 'O' rings typically have a range between 10 and 25%. Hollow profiles have very low load requirements and are therefore ideally suited for applications where the flange is insufficiently stiff or where low clamping loads are prevalent - depending upon section, hollow profiles have a compression range of 7.5 to 50%. l Groove dimensions When assembling gaskets into fully enclosed grooves it is important to remember that rubbers behave as 'incompressible' fluids. Therefore sufficient allowance has to be given for volume displacement. Incorrect Assembly gasket damage, nibbling Groove overfill condition As a general guide allow a minimum of 5% free volume at extremes of tolerance. l Retention Many components require that the gasket is securely retained in both assembly and service. There are many ways of achieving this involving self retaining forms, pressure sensitive adhesives, or by retaining portions of gasket by clips or fixings. Space limitations prevent us from covering all the above aspects in detail. However, if you require specific details or recommendations on groove sizes, clamping load, etc., please do not hesitate to contact T. C. Shielding Technical Department. Product Range l Conductive / Non Conductive Elastomer Seals & Gaskets in Silicone / Fluorosilicone. Wide materials choice and non standard designs available. l UL 94 - VO , TPE, PU, and Neoprene Foam Profiles wrapped with 'Met Cloth' - Metalised Fabric. (Ref: 1/9) l Oriented Wires in Solid Silicone. (Ref: 1/8) l Conductive Optical Film for Windows. (Ref: 1/10) l Bake & Peel Tape in Tin Clad Copper. (Ref: 1/11) l Die Cut Conductive PTFE Material. (Ref: 1/12) l Conductive EMC Windows - Many options available. (Ref: 1/13) l Aluminium & Copper Tapes with Conductive Adhesive. (Ref: 1/16) l Attenuating Honeycomb Vent Panels / Dust Filters. (Ref: 1/18) l Neoprene & Silicone Foams wrapped with mesh products Stainless Steel / Copper / Monel / Aluminium Mesh. (Ref: 1/19) l Microwave Absorbing Foam Materials. (Ref: 1/21) l Beryllium Copper / Stainless Steel Finger Strips. l Ferrite EMI Suppressors - many options available. l Conductive & Decorative Spray Coating. l PCB Level EMI / RFI Screening Cans. l EMC Cable Glands. Shielding Theory - Basics Electromagnetic compatibility (EMC) refers to the inter-relationship between various electrical/electronic systems within a component as well as the relationship between the component and the electromagnetic environment. There are three causes of problems which may occur individually or together: l Effects of Conducted interference Conducted interference occurs as a result of an unintentional effect on an electrical system of voltage drops, pulses/spikes and high frequency currents. (eg. electric motors) l Effects of Near field The near field of a system is influenced by galvanic, inductive and capacitive coupling resulting in emissions in close proximity to the source. l Effects of Far field The far field of a system is influenced by environmental factors (eg. radio and TV transmitters) which in turn can influence the system. Note: An electrical/electronic system may be both the source and the victim of electromagnetic interference (EMI). The near field and the far field are the most relevant for gasket shielding, we shall therefore concentrate on these aspects. Wave theory Typically a generating source will produce a wave with two components, a magnetic field and an electric field. The relationship between the magnetic (H) field and the electrical (E) field is dependent upon the nature of the source and the distance from that source. The ratio of the two fields is important and is expressed as wave impedance Z. Certain sources generate strong magnetic fields and are said to have low impedance. Similarly a high impedance source generates electric fields. At long distances from the source the components of E and H become equal. The wave is then classed as plane wave. This relationship is applicable to a variety of devices that generate electromagnetic waves, from those whose function it is to do so, for example radio transmitters and microwaves, to those which create it as a by product, for example power cables. Shielding When a wave encounters an object some energy will be reflected, some will be absorbed (converted to heat and create residual internal current flow) and a certain amount will leak through. Many factors govern the proportions of the above relationship; the impedance of the wave and the object is particularly important. The greater the difference, the more energy is reflected. When the wave impedance is low (ie. magnetic field) a greater proportion of energy is absorbed. This is the main reason magnetic fields are difficult to shield effectively. Any internal current flow can create a field on the internal side of the barrier. Therefore, the ideal method of shielding is to reflect the wave energy. Any absorbed energy can create residual current. Shielding Effectiveness. Shielding effectiveness is a measure of the attenuation or reduction of energy across a component or test piece. The unit is a decibel or dB. The relationship is logarithmic, that is: