company news about Factors that affect the success of ultrasonic plastic welding

We all know the basic rules of ultrasonic welding，high-frequency electric energy turned into high-frequency mechanical energy，this repeated vibration transfers to the thermal plastic or metal，generates friction and energy between the plastic to plastic，plastic to metal or metal to metal.

In the process of ultrasonic welding，the heat that generated by friction melts the two plastic surface into each other. In the process of ultrasonic riveting welding, the tip controls the flow direction of the melted plastic，forms, and then presses. in the process of ultrasonic nut imbedding，the welding tip injects the nut into the plastic.

Various ultrasonic system configurations could be chosen with different frequencies(15kHz-50kHz) and power (600W-4800W)，or different control methods ，such as pneumatic ultrasonic welder，servo ultrasonic welder or different applications like handheld welder，non-standard ultrasonic welder，metal welder etc.

Many factors affect the result of the ultrasonic welding: Mold (upper horn plus lower fixture)，frequency，material，welding line design，welding parameter and spare parts injection etc.

Welding system frequency

Typical welding frequency includes 15kHz，20kHz，30kHz，35kHz，and 40kHz. Appropriate welding frequencies could be chosen per the product sizes，inner electric components types，welding strength，welding result etc. Basically, the following principles could be referred:

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• For small and high-precision electronic products (containing PCB board and microelectronic component inside) housing welding， a 40kHz ultrasonic welder is often used. It has a smaller amplitude with smaller welding pressure to avoid damaging the inner electronic components.
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• For the small products with demanding requirements on the look, a 40kHz ultrasonic welder could be applied. Smaller amplitude and pressure help to weld a better looking.
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• For the plastic welding with medium or large size, 15kHz or 20kHz are usually applied.
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• For the relatively soft material such as PP，or the material with thin poor stiffness，low frequency and big amplitude, a 15kHz welder is normally applied.
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• For far-field welding-the welding horn is far away from the welding line, for instance, ，if this distance is bigger than 12mm， on this occasion a 15kHz ultrasonic welder could be used.
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• 20kHz welder is applicable to most small or medium-sized plastic welding，which is also the most widely used frequency in ultrasonic applications.

Material

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• For ultrasonic plastic welding，it only applies to thermoplastic welding for the thermoplastic could melt in a specific range of temperature. For the thermosetting plastics，it degrades while heating so it could not be welded by the ultrasonic.
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• The weldability of the thermoplastic depends on the stiffness，elasticity modulus，density，friction coefficient，heat conductivity coefficient，specific heat capacity，glass transition temperature(TG) or melting temperature(TM).
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• Generally,  a material with good stiffness has excellent far-field welding ability for it easily transmits the vibration energy，soft material with low elasticity modulus would be hard to weld for it decreases the ultrasonic vibration. On the contrary，for rivet or spot welding，the softer the material，the easier the welding.
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• Normally，plastics are divided into two types-amorphous and crystalline. The ultrasonic energy can transmit easily in the amorphous material so it is easy to weld. However，it is not very easy for the ultrasonic energy to transmit in the crystalline material so it takes a larger amplitude and energy for the ultrasonic welder to weld crystalline material，the welding line should also be designed carefully.
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• The weldability could further be affected by the factors such as water content，releasing agent，lubricant，plasticizer，filler reinforcers, pigments, flame retardants, and other additives, and actual resin grades. In addition, attention should also be paid to the degree of compatibility between different materials. There is a certain degree of compatibility between certain grades of some materials and not others.
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• It should be considered whether the welding is near-field or far-field welding. The distance between the welding tip contacting the plastic part and the welding line is less than 6mm is called near-field welding, if the distance is larger than 6mm, it is then called far-field welding. The longer the distance, the greater vibration attenuation，the harder the welding. 

Tooling & welding head

Generally, customers will choose the same brand tooling and welding head. In fact, as long as the frequency is the same as the equipment, other brands could also be chosen.

The welding horn material could be aluminum, titanium, or hard alloy steel. The tooling material could be aluminum, stainless steel, and resin mold. The material chosen normally depends on the material type, the material fiberglass content, connector structure and size, welding strength, and service life. For instance, to lengthen the service life of the welding horn, hard alloy steel could be a choice.

The ultrasonic welding horn could be designed and optimized by FEA (finite element analysis), which allows the engineer to evaluate the welding head vibration and pressure before actual manufacturing. An optimal welding horn design is one with uniform output amplitude and minimum stress. In the picture above, the left picture is the welding horn design before optimization, the right one is the horn design after optimization.

In the process of welding horn design and manufacturing, the horn symmetry is of vital importance, asymmetric horn leads to non-axial vibration which increases the horn stress and causes the horn invalidity.

A good tooling design is also important, the tolling has two main functions: align the parts below the welding horn and rigid support welding area. The rigid support help reflects the ultrasonic energy to the welding line, which is why the tooling is usually made of metal working pieces.

In order to increase the durability of the welding horn and prolong its service life, the welding horn surface could be dealt  with tungsten carbide treatment or chromium plating treatment. The tooling could be designed in sections to better match the products.

Welding parameters

Welding parameters affect the welding result in the welding process. These parameters include amplitude, welding pressure, trigger pressure, welding distance, and energy.

Different plastic material requires different amplitude, the amplitude is usually adjusted in the software percentage setting, or adjusted by changing the booster with different ratio. Welding pressure could be adjusted by the rotary button or the software setting. The pressure trigger is when the pressure reaches the setting value after a certain amount of pressure on the plastic parts, the ultrasound starts, the value is adjusted by the rotary button or the software setting.

Several control modes in the welding process:

Times mode

Energy mode

Distance/position welding mode

Different plastic uses different welding modes. For instance, the thin plastic material usually adopts energy mode, products with large dimensional tolerances are welded in distance mode, Products with high tolerance requirements are welded in position mode.

In addition, you could monitor all the parameters in the welding process, also you could set the parameter qualification range to avoid malfunctioning workpieces.

Plastic welding is special processing, in the early stage of product development, one should work closely with the ultrasonic equipment manufacturer, using their experience, to evaluate the product structure and welding line design and test the sample product to greatly improve the qualification rate in the later mass production process.

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