Plastic Assembly History – Heat Staking vs. Ultrasonic Welding of Threaded Inserts

Historically, plastic assembly projects have always relied on the most reliable, repeatable, and consistent method of joining plastic parts available to them. Half a century ago, this meant using ultrasonic welding tools which at the time seemed like a sci-fi welding solution of unlimited potential.

Heat staking machines, on the other hand, were largely seen as inconsistent machines best used for installing metal inserts in to plastic bosses or housings. However, the forward march of technology continues unabated, and heat staking has taken strides in new directions beyond what ultrasonic welding machines are capable of.

One of the most obvious reasons for this is that heat and plastic technology has advanced a great deal since the 1970s. New polymers respond more precisely to heat, which is more accurately controlled by modern temperature controls and more advanced thermal head systems – our entire industry’s understanding of plastic thermodynamics has vastly increased.

So why isn’t heat staking a more popular plastic assembly method today? The truth is, it is, but only for companies who share a demonstrated commitment to adopting modern approaches and – importantly – phasing out obsolete production methods.

The Problem with Old Ultrasonic Welding Machines Still in Use Today

Any molding facility with more than a decade in business is not only likely to have ultrasonic welding machines on the factory floor – they’re probably using them too. Beyond the fact that most of these machines are only capable of installing one insert at a time or only being able of heat staking posts on a single plane, there is a more serious discrepancy occurring with these older machines.

Namely, improperly calibrated ultrasonic welding machines may only be cold pressing the insert using pneumatic pressure. The loud squeal of ultrasonic energy can dissipate upon contact with the insert, giving operators a sense of false confidence as to whether the insert is properly welded or not. A cross section would show that the plastic element has melted inconsistently and flowed around inserts’ knurls and undercuts – making it stick for the moment, but only superficially.

You can actually test this without cutting a cross section into a component to see the assembly process results firsthand. This half-effective cold press method offers almost no resistance to rotational torque, so you should be able to twist off most inserts with a screw and your thumb and forefinger – not the kind of build quality most manufacturing brands are looking for.

Complicating factors is the fact that even new ultrasonic welding machines are susceptible to this type of malfunction. In most cases, it goes unnoticed until consumer feedback and review paints a grim public picture of the finished product’s durability.

Heat staking, on the other hand, is far a more precise and reliable method with the correct equipment thanks to modern technology.

Heat Staking of Inserts Builds Quality Components

A properly aligned and calibrated heat staking system outperforms its ultrasonic counterpart in this key respect. Since the heat insertion system completely fills insert knurls and undercuts – the plastic will melt with precise temperature and pressure to flow precisely to achieve optimal fill – it resists rotational torque very effectively.

Heat staking specialists can set up the system so that there isn’t enough pneumatic pressure to install an insert until the proper temperature is reached. This lets technicians know for certain that the finished procedure was a success.

Heat staking also produces considerably less stress on component substrates and will not cause cold solder stress cracks in SMT components in electronic assemblies. Ultrasonic welders, by their nature, must generate stress on the material surrounding the insert. The only stress that a thermal press puts on component substrates is at the specific contact points with precise temperature and pressure – which can be controlled with a level of precision far superior to that of ultrasonic vibration.

Perhaps the most critical factor in favor of heat staking is the fact that heat imitates the molding process in a significant way. It precisely re-forms the plastic held under pressure, much in the same way a molding machine does. It also resets the plastic’ memory through post-cooling, leaving a tight stake without stringing on the tip cavity.

Thermal Press manufactures the highest-quality heat staking and thermal assembly equipment using cutting edge technology. Contact us to find out how we can make your plastic assembly processes more efficient.