As well as the shape of the moulding, the information about material provided by the manufacturers of the raw materials can also be referred to for a first general assessment of the ultrasonic welding response of thermoplastics. Of this data the following items permit approximate conclusions to be drawn about the welding response.
Density
This shows in comparison with the basic types whether there are large quantities of additives, e.g. glass fibre (GF), glass globes (GG), asbestos, talcum, etc, which affect the welding response. In most cases the density is increased by these additives.
Shear Modulus G’ And Mechanical Loss Factor Tan δ According To The Temperature
Favourable welding properties can be expected from a high G’ or E shear modulus which is constant up to the glass transition temperature. At the same time the mechanical loss factor tan δ (attenuation) should be low up to the glass transition temperature and as constant as possible. Hard amorphous plastics have these favourable properties at room temperature. The sound waves are conveyed to the joining surface without much loss and converted into heat. Most reinforced materials increase the stiffness, as a result of which the shear modulus is also raised.
The shear modulus is also affected in the case of unfilled thermoplastics by the moisture content, the level of crystallinity and the crystal orientation and self-contained stress. These influences are also operative in the case of reinforced thermoplastics.
A marked fall in the shear modulus curve up to the glass transition temperature (Tg) or up to the melting zone (Tm) means an increase in the mechanical loss factor and causes a marked attenuation of the sound waves on the way to the joining surfaces. In general, the energy losses are greater in the case of semi-crystalline plastics than in the case of hard amorphous plastics. Mouldings of the same shape usually require a higher generator power output or longer welding period and a higher amplitude, in the case of semi-crystalline plastics, as compared with those made of amorphous plastics. In general, it is desirable to have a shorter welding period.
Melting Heat Or Heat Content And Specific Calorific Capacity Cp
The higher the value is in particular at the glass transition temperature or in the melting range, the greater the amount of energy which is required for plasticising the material in the joining zone. This means a longer welding period or where necessary a more powerful ultrasonic welding installation, the latter being preferable.
Melting Range Or The Thermoplastic Range
The heating of the joining zone beyond the melting range must be guaranteed by the choice of appropriate welding parameters.
Sound Velocity
The sound velocity in the synthetic material is temperature controlled and is important where the moulding acts as a sound conductor, e.g. in far-field welding.
Melt Viscosity
The viscosity of a plastic melt (expressed, for instance, by the MFI, Melt Flow Index) influences the welding response.
High-molecule, viscous plastics, characterised by a low MFI, generally require more energy to be melted. This means a longer welding period or that a higher power output of the ultrasonic welding apparatus is necessary.
Plastics with a low melt viscosity, characterised by a high MFI, melt more quickly. Molten material can suddenly leave the joining zone in such cases. To avoid this the welding pressure, the welding period, the amplitude, the triggering and the design of the joining zone should be coordinated with particular care.
Most reinforced and filling materials increase the melt viscosity, i.e. the molten material is more viscous. Small quantities of some fillers, e.g. mica and talcum, reduce the melt viscosity and the molten material is more mobile and flows more quickly.
Reinforced Materials, Filling Materials And Other Additives
Reinforced materials:
Glass fibres, glass globes, carbon fibres, talcum, asbestos, etc.
Filling materials:
Wood flour, chalk and other mineral and organic filling materials.
Other additives
Stabilisers, lubricants, dyes, softeners, flame-retarding additives, antistatic coatings, etc.
The nature and quantity of these additives can affect the welding response and the welding result. The construction of the mouldings and the welding conditions should be adjusted accordingly.
