These processes considerably extend the use of ultrasonic. They offer the possibility of formlocked combining of thermoplastic synthetics with other materials – metals, glass or dissimilar plastics. Unlike welding, in the case of fusion forming only one plastic part is locally plasticized and shaped in its viscous state.
Forming by ultrasonic has important advantages over other techniques. Because the forming takes place in the melting phase, only negligible stresses arise in the shaped parts – provided the machinery is correctly adjusted. The problem of stress relaxation is practically non-existent. Fixed connections with no play in them are achieved, coming up to very exacting demands, even in their longterm behaviour.
All corners and edges should be radiused if possible. Ultrasonics can cause stress concentrations in sharp inside edges which can cause stress cracks or melting of the material.
Extended part areas such as ribs, brackets, and studs may break during welding because of vibration and overheating. Inserted components such as springs or wires on electric components are also at risk. lt is best to use generously rounded edges and corners and short welding times at low amplitudes. lf necessary, potting compound such as silicon can be used to dampen the oscillation of electronic components, springs, etc.
ldeally, one part should mate with the other, and the parts should not be free to slide with respect to each other during the welding process. The fit should be close but not so tight that force must be used to put the parts together before welding. The ideal clearance will fall between 0.05 and 0.1 mm, depending on the size of the parts. The distance that one part should mate into the other should be at least one mm if possible. lf it is not possible to design the parts so that they mate in the weld area, it is also possible to hold the parts in alignment by the fixture or horn.
Recommended gap a=0.025~0.05mm; b= min. 1.0mm
Part design can influence the uniformity of energy delivery to the joint area. For instance, bends, sloping faces and openings in the energy path can reduce the strength of ultrasonic vibrations in the weld area.
Poor or no welding at all zone X
lt is best if the joint area is all in the same plane and the plane of the joint area is parallel to the horn surface. lf the joint area has a step and is not in one plane, the unequal distances from the horn surface may produce uneven welding.
Unequal distances L1 and L2; zone "a" not at right-angle to horn surface.
Preferably, the contact surface between the horn and the part should be as large as possible and be flat. Some contouring of the horn surface is often possible. lf the surface of the horn is smaller than the contact area between the parts to be welded, welding may still be accomplished, but some of the energy may not get to the weld area. Also, higher force may be necessary to get a good weld. The higher force may lead to marking of the part surface under the horn.
Horn surface area of contact should be as large as possible; for instance, "a" should be larger than "s" if possible.
The ideal part construction and design would place the contact surface of the horn as close to the welding area as possible. The term "Near field welding" applies if the distance from the horn to the weld area is 6 mm or less. Applications which permit nearfield welding have the least problems although far field welding can also be very satisfactory if care is used.
In far field welding, the contact surface of the horn is greater than 6 mm from the welding area. The walls of the upper plastic part transmit the ultrasonic vibrations to the weld area, similar to a transmission line. Rigid, amorphous thermoplastic parts are excellent, low loss ultrasound conductors and best for far field welding. Rigid, semi-crystalline thermoplastics will absorb some energy, making far field welding somewhat more difficult. Soft, semi-crystalline thermoplastics have a high mechanical loss rate. Ultrasonic vibrations are therefore absorbed if the distance between horn and weld area is too great. Welding is difficult and melting of the contact area of the horn is likely to occur.
The Energy Director is a V-shaped elevation on one of the two contact surfaces. It causes a line contact as the ultrasonic vibrations begin to melt the plastic. The line contact focuses the ultrasonic energy. As the melt progresses, the energy director turns fluid and flows to fill the space between the two parts. The downward force of the horn causes the melted material to spread over the entire contact surface. At this moment, the ultrasonic energy supply is shut off and the joint area, still under pressure, cools down in a short period of time, completing the weld.
Amorphous thermoplastics do not have a well defined melting point but have a relatively wide softening range. Therefore, welding parameters are less critical and there is reduced chance for thermal damage if the ultrasonic energy is on longer than necessary to obtain a good weld.
Semi-crystalline thermoplastics, on the other hand, have a clearly defined melting point. Most of these materials are sensitive to heat at temperatures above their melting point. Even short times at higher temperatures may cause thermal damage. There is air contact with molten plastic when the energy director is melting and spreading sideways; therefore, the material can crystallize before there is enough heat to weld the entire surface of the joint. Crystallized zones can crack and flake off from the welding area. The air contact may also cause oxidation of the plastic resin. Due to these possible problems, energy directors are not suggested for semi-crystalline materials.
Shear joints are good for achieving a hermetic seal and for semi-crystalline thermoplastics in general. A shear joint is obtained with a step and little contact surface. The small surface and the resulting high energy flow cause rapid melting. The two parts slide into each other, forming a vertical weld joint. The sliding of the two melting surfaces prevents bubbles and limits air contact. The weld is homogenous and usually free of leaks. The regular and even welding procedure is easy to control. Since there is little air contact, cooling is slower and crystallization and flaking of the material is impossible.
The integrity of a shear weld is influenced by the amount of overlap of the two parts. The walls of the lower part must be supported in the welding zone to prevent bulging due to welding pressure, especially if the walls are thin.
Stud Welding
Stud welding is a special type of shear welding. lt is an inexpensive solution for weldings where a hermetic seal is not required. A plastic stud molded on one part slides into a hole in the second part with bottom interference. The weld joint also provides positioning.
These processes considerably extend the use of ultrasonic. They offer the possibility of formlocked combining of thermoplastic synthetics with other materials – metals, glass or dissimilar plastics. Unlike welding, in the case of fusion forming only one plastic part is locally plasticized and shaped in its viscous state. In this way effective use is made of the heat energy between the horn surface and the surface of the plastic part.
The horn transfers the mechanical oscillation energy to the rivet spigot. It is the riveting tool while at the same time being worked on the face side to the desired rivet headshape. This recess corresponds to the volume of the shaped plastic. Particular attention must be paid to the wear on horn tips, especially when working with abrasive materials. Plastics with mineral fillers or glass fibres require the use of suitable horn materials. Hardened tool steels of hardnesses above 60 HRc, or a suitable coating are recommended.
Thin metal parts can be excited by ultrasonic vibrations and there is a tendency for the parts to climb up against the horn. A clean bond is not guaranteed. Clamping down devices will help. The vibrations can also lead to the break up of exposed parts. Such problems are solved by using sound-compensating materials, possibly combined with clamps designed for the purpose.
lf metal parts are fixed with several rivet heads, all rivet heads should be shaped in one working cycle. lf rivet joints are made individually, the sound energy is conducted through the metal part to the already shaped rivet heads and can lead to breakage.
The horn must not touch the part to be attached. The plasticized material must solidify under pressure during the cooling time. This procedure can be compared with the stress and cooling time for injection moulding. lf the horn lies on the upper part, the pressure on the rivet head is reduced. The result is a non-homogeneous structure with resultant loss of strength.
When metal parts are being riveted, this problem is solved very neatly in the form of a contact breaker. A suitably equipped absorption tool, connected electrically to the controls, causes cut off of the ultrasonic energy if the horn touches the metal part. A welcome secondary phenomenon with this system is that component tolerances are automatically compensated for.
The general shape of a rivet joint is known from the machine construction. The fixing of the rivet pin should in all circumstances be provided with a ringshaped undercut, with a radius or at least with a bevel. In either case the part to be riveted on must of course be recessed.
Head shapes F and G are not defined, these applications are limited to places which are not visible on the finished product.
For partially crystalline thermoplastics and larger spigots, steps must be taken to assist with the melting. A rhombic shaping (Kourl pattern) of the horns has proved very successful. Quite understandably, these two variants do not meet any special requirements for strength. They are used in preference for the fixing of metal parts in electrical engineering.
For the larger spigot diameters, from about 6 mm upwards, the use of hollow spigots as in illustration H is recommended. Accumulations of material and therefore sink marks on injection moulded parts can thus be avoided. The quantity of material to be shaped is reduced, which is beneficial in terms of the welding time and the energy requirement.
The suggested standardization represents approximate values. These can of course be varied and adapted to individualrequirements.
The flanging technique is known from metal-working. The most important characteristic in the case of ultrasonic flanging is that the material is plasticized by the ultrasonic energy and shaped in the viscous melting phase.
A typical application is shown as picture. The designer has a relatively free choice in shaping jointed flange connections, though the parts being shaped must exceed the volume calculation.
Even if such joints meet very high specifications, they can never be airtight because of the unequal thermal expansion of both parts. lf airtightness is essential, a separate sealing element must be inserted. The other pictures shows an airtight flanging joint where an 0-ring is used.
When soft materials are being welded, unacceptable welding ridges often occur. Here flanging offers an alternative to traditional welding.
This always results in very high tangential stresses, which often lead to formation of cracks. As a rule one tries to absorb these stresses with excessive wall strengths surrounding the embedded metal part. Such accumulations of material are unhelpful for achieving reasonable cooling times for injection moulding.
Plastics with a high stress-strain ratio, such as for example Standard Polystyrene, are particularly susceptible to stress fractures. All other thermoplastics too, though, can fail in their longterm behaviour under the influence of weathering or chemicals which trigger off stress fractures. One reason for using ultrasonic embedding, which should not be ignored, is the considerable saving in energy.
Embedding procedure
This requirement can be adhered to very easily by using inserts with a flange, and taking appropriate measurements. Also the tendency to protruding flash is significantly less because the flange forms a barrier against the rising molten material.
lf shafts, axles or other unfavourably shaped parts have to be embedded, it is advisable to locate the metal part inside the fixture, and allow the ultrasonic energy to act upon the plastic part. The points in the Design Principles for Ultrasonic Plastic Welding described under near and far field welding must also be taken into account. Marking must be expected on the coupling surface. By using a protective foil between the sonotrode and the plastic part, this can be avoided.
The amplitude is the movement on the horn surface. The amplitude is depending on the output power of the power supply, Generator setting, Gain of the Booster and Gain of the horn. The amplitude is chosen depending on the material to weld.
The amplitude is measured in micro meter. The value is peak. Typical amplitude values are 10 – 60 micro meter.
STRONG ULTRASONIC MACHINERY CO., LTD.
No. 288-21, Xinshu Road,
Xinzhuang District,
New Taipei City 24262,
Taiwan.
TEL: +886-2-2203-6999 FAX: +886-2-2206-5588 Email: strong.usonic@msa.hinet.net
