Traditional welding methods use an electrode, or arc, to heat metals and spark a fusion reaction. Processes like arc welding work well for metal-on-metal manufacturing, but not when you want to join metal to a material that has a different melting temperature — a material like plastic.

When furniture makers or automotive manufacturers need to bond plastic to metal, the construction often requires rivets and adhesives. Adhesives, however, take time to settle, and rivets can cause structural damage.

A team in Germany has a quicker, cleaner approach to welding metal to plastic: The HPCI® Joining Gun.

the HCPI joining gun from Fraunhofer
The HPCI Joining Gun (Image Credit: Fraunhofer)

The "HeatPressCool-Integrative" device takes seconds to fuse conventional steels, titanium, or aluminum alloys to thermoplastic materials.

The gun, built by Germany's Fraunhofer Institute for Material and Beam Technology IWS , uses inductive heat to target the metal at the bonding point. The residual heat causes the thermoplastic material to partially melt. The fusion begins as the plastic solidifies.

The bond is especially strong because of a pre-treatment of the metal. The process, known as laser ablation, removes material as deep as 100 micrometers, creating a pattern of laser-etched anchoring structures.

an image of metal after laser ablation, so that plastic more easily bonds to it
The metal, after laser ablation. On the bottom of the image (in white), the laser structured metal (mild steel) is visible. On the top of the image, the glass-fiber-reinforced PA6 with certain fibers and plastic penetrates the metal structure. Laser ablation is done by cw-laser-radiation.

A surface layer of the plastic penetrates the array, cools down, and contracts to form a secure fit in the structure.

An induction coil serves as the heat source, and magnetic flux concentrators offer an efficiency boost. The mandatory joining pressure is applied via high performance ceramic ram geometries coupled with an actuator.

The HPCI device resembles the spot resistance welding gun widely used in car body engineering for metal to metal joining.

Although there are no restrictions to the choice of metal, the synthetic joining partner must have thermoplastic properties. Conventional steels as well as several aluminum and titanium alloys have already been tested.

The device, which can be mounted on a robot arm in place of a spot welding gun, is valuable for applications requiring multi-material bonding, from car-body manufacturing to the joining of stainless steel panels to dishwashers or refrigerators.

“Generally the joining gun was designed for applications in car body manufacturing but we have recently found partners that are highly interested in the technology from other industry sectors as well,” Philipp Götze, a Fraunhofer researcher and engineer of the joining gun, told Tech Briefs.

Götze and his team were the Manufacturing/Robotics/Automation category winners in this year's Create the Future Design Contest. (See the rest of the 2019 'Create the Future' standouts.)

In an edited interview with Tech Briefs below, Götze reveals how a HPCI joining gun stacks up against traditional welding methods.

Tech Briefs: What inspired this idea? What need did you see in manufacturing?

Philipp Götze: We see an increasing use of plastics in combination with metals in all kinds of industries and especially in the automotive industry. So, we looked into how conventional metal to metal constructions are manufactured now and searched for a corresponding solution that enables equally efficient metal-plastic connections.

Tech Briefs: Can you help our readers visualize how the gun works? Take us through the steps of how the joining gun is used during a sample car-body application, for example.

Philipp Götze: One example for an application in the automotive industry could be local reinforcements of metal structural parts, such as the B-pillar with carbon fiber reinforced plastics. Both parts would be aligned by robots, and then the joining gun would press them together and heat the metal locally up to the melting point of the plastic, while maintaining the joining pressure. The now fluid plastic penetrates the pre-treated (laser-etched) metal surface.

After cooling, the plastic re-solidifies and achieves a strong bond to the metal surface. Depending on the geometry of the part, one, two, or multiple spot joints can be created by a single or a number of joining guns. We have illustrated that example here:

Tech Briefs: How does the pre-treatment impact the efficiency of the manufacturing process? Do manufacturers need to spend a lot of their time pre-treating the metal?

Philipp Götze: Pre-treatment can also be done locally (only on the metal surface) and takes about one second per joining spot. It doesn’t impact the efficiency much, especially compared to adhesive bonding where extensive pre-treatment has to be done on both surfaces. The whole pre-treatment process can also be done at external suppliers of the automotive industry, so it doesn't affect OEMs at all.

Tech Briefs: Beyond automotive, what other applications are possible?

Philipp Götze: This goes all the way from consumer goods to aerospace applications. It's most useful in any scenario where plastic parts have to be fixed to metal parts or vice versa.

Tech Briefs: Why has it always been so challenging to join multiple materials?

Philipp Götze: Plastics and metals especially have entirely different material properties, which on the one hand can provide great potential in a hybrid material configuration, but on the other hand also inherit massive challenges. Conventional welding technologies can not be applied, simply due to the huge differences in melting temperatures and electrical conductivity as well as the entirely different molecular or macromolecular structure of both materials.

Glues have to adhere both on the metal, as well as on the plastic surface and therefore require careful selection and surface pretreatment on both sides.

The biggest challenge for our process is the difference in thermal expansion ratio. Without our special pretreatment process on the metal surface, the joint could break just because of the induced stress during heating and quenching. We have come to the point where we can use that to our advantage by carefully adjusting the metal surface, to make the plastic claw into the metal surface and provide a form fit.

Tech Briefs: What is the joining gun such an important invention?

Philipp Götze: Compared to conventional joining technologies like adhesive bonding or riveting it's much faster and more easily automated. It provides great potential in various industries and can enable lightweight constructions for the mobility of the future. Absolutely no additional and potentially harmfully material like glue is needed, and the parts can be de-mounted after use which allows efficient recycling.

What do you think? Share your questions and comments below.