Effective Weld Area: Do Not Measure the Outline

When a customer asks what power a part needs, the figure usually offered first is the outline perimeter. Selecting from that number gets it wrong most of the time.

What counts is fused area, not perimeter

Strength comes from the area at the interface where chains actually diffuse and entangle. Several discounts sit between that area and the perimeter measured on the drawing.

Discount one: joint form. The fused width of an energy director is set roughly by its base width and how far the melt flows; a shear joint's area is engagement depth times perimeter, and the depth can be several millimetres. At the same perimeter the two can differ by a factor of several.

Discount two: corners. Melt flows poorly into sharp corners, and the inside of a corner is often incompletely fused. A part with many corners has noticeably less real area than its perimeter suggests.

Discount three: interrupted sections. Joints are frequently broken to make way for snap fits, bosses and cable routing. A break contributes no area and is a stress concentration besides.

Discount four: filler. A 30% glass loading means roughly that share of the resin at the interface is missing. This discount multiplies rather than adds.

Why selection goes wrong

Power has to cover the energy needed to open the whole fused area at once. Working from the perimeter over- or under-estimates, and both directions cause trouble.

Over-estimating buys a machine whose weld area is never used, wasting cost. Under-estimating is more common: the machine reaches the line and the whole joint will not open at once, the middle fuses while the ends do not, the cycle gets extended, and the surface marks and flash grow.

Getting the figure closer

Fix the joint form first, then compute the area. The order matters, because the joint form determines how area is calculated.

Mark the interrupted and corner sections separately. Do not run one perimeter figure end to end. List effective sections, corner sections and breaks, and discount the corners.

Multiply by the filler discount. At 30% glass, count roughly 70% of the interface as effective resin.

Consider zoning on large parts. Beyond a certain joint length, one horn cannot hold amplitude uniform and the middle welds while the ends do not. That calls for several horns driving separate zones rather than a larger single horn.

What to send for selection

These make a selection possible: part drawings showing joint position and form, the resin grade and filler content, the effective joint length and any interruptions, the maximum part envelope, and the daily output and cycle requirement.

The last two decide more than the model: whether automation is needed, and whether a second station is. Often the model is right and the cycle time is wrong, and the line still will not run.

Joint forms and the dimensional system are in the joint design guide on the downloads page. Power ratings, maximum tooling size and amplitude ranges by model are on the ultrasonic welding machine pages.

Send drawings and the cycle requirement to 1427498429@qq.com, or call +86 769 8202 9510 / +86 137 6001 0932.