10Ultrasonic_Welding

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Design guide

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Picking up good vibrations

Semi-crystalline

In the last two design guides, we discussed hot air staking. A similar effect can be produced by using ultrasonic energy as the heat source. This technique is also used for other assembly tasks with thermoplastics, including full or spot welding, inserting and swaging. It is usually referred to as ultrasonic welding. Ultrasonic means beyond sound, and by that we mean beyond the range of the human ear. Ultrasonic welding is usually carried out at frequencies ranging from 15kHz to 40kHz. This is well within the range of cats (65kHz) and dogs (50kHz), not to mention dolphins (150kHz), but just beyond the threshold of audibility for humans (50Hz to about 15kHz). When ultrasonic vibrations are applied to the interface between two parts to be joined, the vibrational hammering and friction generates heat which locally melts the plastics parts to form a fusion weld. The amplitude of the vibration is an important factor in producing the weld; to understand this we need to look briefly at the way an ultrasonic welder works. The key component is a transducer or converter where electrical energy is transformed into mechanical vibrational energy with the same frequency. For this reason, the first step is to transform the standard low frequency electrical power supply to a high frequency running at 20kHz to 40kHz. The transformation into mechanical energy is usually performed by a piezoelectric transducer, typically a polycrystalline ceramic. A lead zirconate titanate ceramic is commonly used. This

Amorphous

The Design Guide moves on to discuss ultrasonic welding of plastics Ease of welding Far-field

Material

Near-field

ABS ABS/PC alloy Butadiene-styrene Cellulosics PMMA Polyamide-imide Polycarbonate Polystyrene GP Polystyrene toughened Polysulphone PPO PVC rigid

excellent excellent to good good fair to poor good good good excellent good good good fair to poor

good good fair poor good to fair fair good excellent good to fair fair good poor

Acetal Fluoropolymers Polyamide Polyester thermoplastic Polyethylene Polymethylpentene Polypropylene PPS

good poor good good fair to poor fair fair good

fair ineffective fair fair poor fair to poor poor fair

Ultrasonic weldability of some common thermoplastics Source: After Lucas-Dawe Ultrasonics

type is referred to as a PZT converter. The piezoelectric material has the property of expanding and contracting when an alternating voltage is applied. In other words, it vibrates. However, the extent – the amplitude – of the vibration is small, something like 0.01mm to 0.02mm. This is not enough to produce a good weld so the ultrasonic welder includes two devices to increase the amplitude. The first is known as a booster and is an integral component of the ultrasonic welder. The

booster is a tuned resonator device that typically doubles the amplitude. The second is the horn, also known as a sonotrode. This is not a part of the machine and must be designed specifically to suit the individual welding task. The horn is in contact with the parts to be joined, and is contoured to fit the joint. Its function is to transfer both vibration and pressure to the joint while further increasing the amplitude of vibration at the tip, typically to a figure of 0.05mm to 0.12mm.

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Designers’ notes ● Ultrasonics can be used to perform a variety of welding, staking, inserting and cutting operations on thermoplastics ● Horn design is critical, and is a job for a specialist ● Rigid thermoplastics weld better than flexible ones ● Amorphous thermoplastics weld better than semi-crystalline ones ● Try to avoid far-field welds in semicrystalline materials

Horn design is a complex affair and is best left in the hands of the machine supplier. The suitability of a thermoplastic for ultrasonic welding depends on its ability to transmit high frequency vibration. This means that rigid materials are better than flexible ones. Melting behaviour is also important. Materials that melt over a broad temperature range and resolidify gradually work best. This is why amorphous plastics are better for ultrasonic welding than semi-crystalline plastics. The table shows the quality of weld to be expected when joining a particular thermoplastic to itself under optimum conditions. The table distinguishes between near and far-field welds. A near-field weld is one where the joint interface is within about 6mm of a horn tip contact point; anything else is a far-field weld. Far-field welding is much less effective because of energy losses within the material.

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10Ultrasonic_Welding by Jayesh Rambhia - Issuu