DESIGN FOR ASSEMBLY (PART 3)
In this month’s article in the series of assembly we shall look at welding.
There are a number of different welding methods:
Ultrasonic welding
is the most popular method and uses ultrasonic energy to create surface and
intermolecular friction so generating heat and in turn forming a weld.
Ultrasonic welding is suitable for most thermoplastics. Amorphous polymers
are generally ideally suited, semi-crystalline polymers, on the other hand,
are generally more difficult to weld ultrasonically.
Vibration welding
involves rubbing together under pressure the two parts to be welded until
sufficient heat is developed to cause the materials to melt at which point
the motion is stopped and the parts allowed to cool forming the weld. As
with ultrasonic welding, most thermoplastics can be vibration welded
although amorphous materials tend to be easier to vibration weld than
semi-crystalline polymers.
Spin welding
is again a friction process and as the name suggests, is used to join parts
with rotationally symmetrical joining surfaces. One part is rotated while
the other is held firmly, pressure is applied and the motion stops once the
material is in a molten state in the weld zone. This is the most efficient
joining process for parts with circular cross sections. Cylindrical parts
up to 600mm diameter with a joint area as large as 58cm2 have
been successfully joined using this method.
Induction (Electromagnetic) welding
uses inductive energy to achieve fusion temperature. Again, this method may
be used with most thermoplastics. Before welding an electromagnetic welding
material, such as a gasket, is placed between the two parts to be joined
usually in a groove or other recess. During welding an electromagnetic
field is formed which heats the electromagnetic sensitive material, the heat
is transferred to the surfaces of the parts to be joined and the parts
soften. Welding is completed with the application of pressure.
Resistance welding
is similar to induction welding except that an electrically conductive wire
or braid is used in the place of electromagnetic material. An electric
current is applied to the wire or braid which then heats up and softens the
thermoplastic material on the two areas to be joined which can then be
welded with the application of pressure. It is a quick and simple method
ideally suited to very large parts. The wire remains in place which may
weaken the finished weld.
Hot tool welding
uses electrically heated tooling to soften the plastic part surfaces to be
joined. The tool is then removed and the parts pressed together before the
surfaces cool. This method can be used for all sizes of parts and almost
any thermoplastic material. It is particularly suited to semi-crystalline
polymers such as polypropylene and polyethylene. Unfortunately, the cycle
times for this type of welding can be long, especially for larger parts.
Hot gas welding
is used for very large parts although not usually injection moulded
components as there are better alternative methods.
Extrusion welding
evolved from hot gas welding. A V-groove on the part to be joined is heated
using hot air or gas to soften it. It is then filled under pressure with an
identical material which has been plasticised via extrusion. Once deposited
pressure is applied. Usually used for larger sheet type structures.
Contact us for a copy
of our free Design Guide
Can't find what you are looking for? Use the Site Map for a quick search