Annealing involves the heat treating of the core steel to reduce core losses. Annealing does this by relieving stresses induced from processing. All wound cores are annealed after winding unless otherwise specified by the customer. Annealing is also used to remove coil set from laminations and to help wound cores to hold their shape. To be annealed, a core must not contain any compounds which may contaminate the core steel or otherwise be damaged at the high temperature. Silicon steel items must fit within a cylinder 36" in diameter by 38" tall.

Varnish Impregnation

Varnish impregnation or "bonding" refers to the process of gluing the laminations of a core together. This adds rigidity to the core and is necessary, when cutting across a lamination stack, to prevent lamination separation. Bonding will increase losses in cores by physically restricting movement of the steel which is caused by the effect known as magnetostriction. The bonding is normally applied with a vacuum/pressure impregnation process followed by a heat curing cycle at 450°F. Operation near or above this temperature may cause the bonding to soften and decompose which may lead to physical distortion and separation of laminations. This process can not be performed before annealing as annealing temperatures will destroy the bonding.

Edge Bonding

Edge bonding is a weaker but less restrictive bonding process when compared to varnish impregnation. Here the laminations of a core are bonded together at the outer edges of the lamination stack. Edge bonding will add rigidity to a core with a lesser increase in losses when compared to varnish impregnation. However edge bonding is not as strong as varnish impregnation and is not applicable for cutting or gapping. Edge bonding adds less than 0.002" to the core surface. This process can not be performed before annealing.


Cutting a core can serve two purposes: one is to ease assembly and the other is to allow the introduction of an "air" gap into the flux path. Cutting is a common option for rectangular wound cores where the core is cut completely into two pieces. These cores are usually referred to as cut cores or C-Cores and must be varnish impregnated to prevent de-lamination. After cutting, the cut faces may be lapped and etched as needed to reduce audible noise and losses. Normally, for a rectangular core, a single cut is made across the "G" dimension in parallel with the "F" dimension; although cuts can be made on any dimension. Multiple cuts may be made where necessary but increase the likelihood of delamination. Multiple cuts allow the customer to place smaller gaps throughout the core which helps to reduce heat buildup caused by fringing effects. An alternate method to multiple cutting is to use a single cut core and then add bonded lamination stacks. Toroids may also be cut. Cutting is standard for all three phase wound cores. Partial cutting can be done where the cut does not go completely through any one lamination of the stack. This has been done where reducing residual flux is important and losses can be sacrificed.


Lapping is a grinding process used to obtain adequate flatness of the core mating surfaces in order to reduce vibration, audible noise and excitation (VA) losses. Lapping may be performed at different levels, if at all, depending on the needs of the application versus cost requirements.


When a core is cut or ground, burrs are formed across the lamination stack, in effect “shorting” the laminations, which increases losses in the core. The etching process is performed to remove these burrs. This process is not always necessary and may be a consideration when weighing cost versus efficiency.


Electro Core uses the term gapped to refer to cut cores that are shipped as one piece. These cores may have only been cut across one leg or, if cut into two or more pieces, reassembled with some sort of banding. Two types of gaps are available with these cores: open and closed. Open gaps are only on one leg and are cut to size. Tolerances are difficult to hold and may range from 0.005" to 0.031" or more depending on the core configuration. Closed gaps are filled with a spacer, usually Mylar or Nomex, and the perimeter of the core is often wrapped with wire or banding to maintain the closed gap. Gap tolerances for closed gaps are usually within 0.0005" to 0.002".

Heat Seasoning

If a cut core will encounter temperatures of above 200o F in service, the bonding material may begin to soften and any induced stress in the core may cause laminations to shift, in turn enlarging the gap between the mating faces and therefore increasing losses and possibly contributing to noise. This can be compensated for somewhat by reheating the core after cutting to relieve the induced stress and then finishing processes would be applied such as lapping and etching where needed.

Epoxy Coating

An epoxy coating may be applied to the outer surface of cores for mechanical and electrical protection. The standard material available is the 3M #260 electrical resin.

Technical information as follows:

U.L. Recognition #: E34-947M

U.L. recognized for 130o C operating temperature

Standard nominal flat surface coating thickness and tolerances:

0.020" +0.007", -0.003"

Other restrictions:

1. Cores which weigh more than 30 lbs will require special handling.

2. Maximum practical coating thickness of .040"

Core Caps and Cases

Core caps and cases are used in a similar fashion to epoxy coating; to protect the windings from the sharp edges of the core steel and also to provide electrical insulation. Caps tend to be expensive and not practical for small quantities. They can, however, provide greater protection from external stresses and offer more consistent physical dimensions than epoxy coating. Epoxy coating can also be applied over a core case to provide even greater protection.


Electro-Core has years of experience with machining laminated silicon steel. Operations performed include slotting, machining special gaps, drilling/tapping holes, milling and grinding. Machining laminated silicon steel can be problematic, so consult with us early in your project.