Apparatus for radio-frequency bonding of thermoplastic members

In a bonding apparatus using radio frequency energy for bonding a first thermoplastic member having a predetermined peripheral shape to a second thermoplastic member thereby to form an embossed pattern having a peripheral border region (B) surrounding a plurality of raised areas (R) with flat regions (F) therebetween,

• the thermoplastic members being fabricated of materials which absorb radio frequency energy, the material of at least one member being susceptible to thermal runaway,
• the bonding apparatus including a first, die, electrode (14) and a second, backing, electrode (22),
• the die electrode (14) comprising a planar mounting portion (16) and a die member (18),
• the die member (18) having a peripheral shape with edges and relief features each with an operative face (18F) thereon,
• the die member (18) having a height dimension (18H), the operative face (18F) of the relief feature having a first depth dimension (18E),
• the peripheral shape of the die member (18) corresponding to the peripheral shape of the first thermoplastic member, while the relief features correspond to the peripheral border region (B) and the flat regions (F) of the embossed pattern,
• the backing electrode (22) having a planar surface (22S) thereon,
• the improvement comprising:
• a conjugate bonding member (24) mounted to the backing electrode (22), a portion of the surface (22S) of the backing electrode (22) defining a margin (22M) surrounding the conjugate bonding member (24), the margin (22M) having a predetermined width dimension (22W),
• the conjugate bonding member (24) having a peripheral shape that corresponds to the peripheral shape of the die member (18),
• the conjugate bonding member (24) having relief features (24F) sized and located to correspond to the size and location of each relief feature (18F) on the die member (18),
• the conjugate bonding member (24) having a height dimension (24H),
• the relief feature (24F) on the conjugate bonding member (24) having a second depth dimension (24E), wherein
• the sum of the height dimension (18H) of the die member (18) and the height dimension (24H) of the conjugate bonding member (24) being at least three (3) centimeters, and
• the ratio of the second depth dimension (24E) to the first depth dimension (18E) is at least eight to one,
• whereby, the concentration of the electric field in the vicinity of an edge of the die member (18) is minimized and the concentration of the electric field in the vicinity of confronting edges of respective relief features on the die electrode and the conjugate bonding member is minimized,
• so that a first thermoplastic member is fully bonded to a second thermoplastic member in the peripheral border region (B) and the flat regions (F) without the occurrence of overheating of either thermoplastic member.