Induction sealing of paperboard

A method of sealing packaging material having a layer of thermoplastic material and a layer of electrically conductive material comprising:

• (a) compressing the packaging material together between first and second rigid members so that a first thermoplastic material portion is pressed against a second thermoplastic material portion with no relative motion between the first and second rigid members and the first and second thermoplastic material portions, the first and second rigid members being electrically isolated from ground and the first rigid member having an electrically conductive material on its surface which contacts the packaging material and which is in the form of a first loop and also having a second electrically conductive material in the form of a second loop, said first and second loops being separated by a rigid, nonconductive material;
• (b) generating a primary electromagnetic field having an effective range for inducing a current in an electrically conductive material brought within the effective range;
• (c) inducing from said primary electromagnetic field a first current in the second electrically conductive material loop of the first rigid member by moving the first rigid member into the effective range of said primary electromagnetic field thereby generating a second electromagnetic field from the first current flowing in the second electrically conductive loop of the first rigid member,
• (d) inducing from the second electromagnetic field of the second loop a second current in the first loop thereby generating a third electromagnetic field;
• (e) fusing the first and second thermoplastic material portions together by inducing from the third electromagnetic field of the first loop a third current in said electrically conductive material layer of the packaging material for a period of time sufficient to generate enough heat as a result of the induced current flow to soften and fuse together the first and second thermoplastic material portions without adversely affecting the packaging material and;
• (f) solidifying the heated thermoplastic material into an effective seal after it fused together by removing the first rigid member from the effective range of the first electromagnetic field so that no significant induced currents flow, no heat is generated, and the thermoplastic material is allowed to cool and harden.