In automotive manufacturing, polymeric composites are being used increasingly due to their favorable characteristics, including being lightweight, highly-conformable or shapeable, strong, and durable. Some composites are further colorable and can be finished to have most any desired texture.
The increased use in automobiles includes, for instance, in instrument and door panels, lamps, air ducts, steering wheels, upholstery, truck beds or other vehicle storage compartments, upholstery, external parts, and even engine components. Regarding engine components, and other under-the-hood (or, UTH) applications, for instance, polymers are configured, and being developed continuously, that can withstand a hot and/or chemically aggressive environment. Regarding external parts, such as fenders, polymers are being developed that are online paintability and have high heat and chemical resistance over longer periods of time. And many other potential usages in automotive applications are being considered continuously.
With this trend, finding ways to efficiently and effectively join polymer components is becoming progressively important. Compression molding and post-mold joining techniques—e.g., ultrasonic welding—are being used more commonly.
In ultrasonic welding, two workpieces are joined, wherein one or both includes a polymeric composite. With reference to the figures, and more particularly the first figure, FIG. 1 shows schematically a conventional ultrasonic welding arrangement including two workpieces 102, 104 to be welded together, and a welding tool 106, such as an ultrasonic welding horn.
For ultrasonic welding, the workpieces 102, 104 are held together, putting them under pressure, while ultrasonic vibrations are applied to pieces—e.g., to a top workpiece of the two.
To direct energy to an interface 108 between the workpieces 102, 104, energy directors 110 are sometimes positioned between the workpieces 102, 104, or formed in one or both workpieces so that they are positioned between the pieces during welding.
Generally, when ultrasonic energy is transmitted through the workpieces 102, 104, being under pressure, the energy over time may concentrate at an apex of the energy director(s) 110 resulting in a rapid buildup of heat. This causes the director to melt. The molten material flows across the joint interface forming a molecular bond with the mating surface.
Regarding director formation and placement, in a common case, protruding energy directors are formed when compression molding one of the workpieces using recesses in the mold. The directors extend between the workpieces 102, 104, forming a path for welding energy (e.g., ultrasonic vibrations), transmitted to the proximate workpiece 102 to propagate to the area of the interface 108 between the pieces and toward the distal piece 104.
The vibrations create frictional heat, initially at faying interfaces (i.e., tool-to-workpiece, workpiece-to-workpiece), and then intermocular friction in the composite material, causing the material to melt. When the melting occurs at the interface 108, such as due to the vibrations transmitting to the energy directors 110, the workpieces are joined there by molecular bonds (e.g., fusion or covalent bonds) of the molten material.
Sometimes, flawed, or discrepant welds are formed. A discrepant weld, generally, is any weld differing undesirably from a target weld configuration. A common flaw is that a weld is undersized, or otherwise less robust than desired. In one scenario, in which under a given sheet gage it is desired to have welds measuring about 7 mm, or more, in width, discrepant welds can have widths between about 0 mm and about 4 mm.
FIG. 2 shows the workpieces 102, 104 receiving welding energy (e.g., high-frequency (HF) acoustic vibrations) to be joined, but in an undesirable manner, forming a discrepant weld 202. The weld 202 is flawed, having an insufficient primary portion 204. The weld region or zone 202 in this example also includes insufficient ancillary welding 206. FIG. 3 shows the weld horn 106 being withdrawn from the newly, insufficiently, joined pieces 102, 104.
FIG. 4 shows an example scenario 400 in which a sufficient, desired, weld 402 is formed. Welds lacking desired quality can be referred to as discrepant, as described above.
The ancillary welding 206 can be referred to as squeeze out because it is likely cooled molten material pushed aside from the primary portion 204 under the pressure pushing the workpieces 102, 104 together during welding. Generally, the higher the amount of heat generated in forming the primary weld portion 204, the greater the amount of squeeze out.
An attempt to re-weld the existing discrepant weld 202, using the same tooling and process, would be ineffective at least because the workpiece 102 has likely been changed (joined to the workpiece 104) so that it will not generate sufficient frictional heat at the faying interfaces again for welding in response to the vibrations. More particularly, the workpiece structure, especially at a surface or perimeter of the previously-welded zone, would have been changed during the first welding to being too constraint to generate sufficient welding heat again.
Attempting to repair a weld by adding additional welds around the discrepant weld 202 is not practical due to limited space, material, time, and energy. It would be space, time, material, and energy consuming to form the extra new welds around the discrepant weld 202.
An undesirable alternative is to scrap the workpieces 102, 104 having the discrepant weld 202. Hypothetically, some of the workpieces improperly joined could also be recycled, somehow, but this process also takes resources and does not cure the challenge of avoiding undesired discrepant welds.
Yet another alternative is conventional mechanical fastening of the workpieces 102, 104 together instead of welding, or after a partial weld has been identified. The workpieces 102, 104 can be screwed together, or connected by nuts and bolts, for instance. These connections have shortcomings including unwanted added weight, unsightly exposed portions of the fasteners, more time than desired, and possibly less-robust joints.