New potential for particle foams with additive manufacturing

Thanks to the combination of the particle foam EPP and the free design possibilities of 3D printing in toolmaking, an innovative lifestyle product with a new design was created from a conventional walking stick. This innovation was selected for the Swiss AM Guide 18.

New potential for particle foams with the help of additive manufacturing

Particle foams such as expanded polypropylene (EPP) have long been used in the logistics and packaging industry. In addition to a comparatively low density with good mechanical properties, expanded particle foams are characterized by high thermal insulation and damping properties. For this reason, they are also employed in the automotive industry, in insulation technology and other areas. The company Robert Hofmann GmbH recognized the potential of this material and combined it with the advantages of additive manufacturing in the area of tool making.


The production of particle foams
The fabrication of products and components from particle foam is carried out as follows: expanded polypropylene (EPP) is supplied in the form of beads. These beads are filled into the cavity between molds. Through several smaller openings in the molds, water vapor is injected under pressure and at elevated temperature of up to 150 °C. The introduction of heat energy due to the water vapor leads to the expansion and sintering of the beads, which bond together at the surface. After the forming process and removal from the molds, the water content of the component due to the water vapor is reduced with a tempering oven. Besides expanded polypropylene (EPP) other available materials include expanded polystyrene (EPS), expanded polyethylene (EPE) and expanded thermoplastic polyurethane (ETPU).

For serial production, the used molds are typically made of metal. In the design of the molds, the challenge is to introduce and distribute water vapor into the cavity as evenly and efficiently as possible. In this respect, the number and arrangement of the integrated openings in the form of slots or nozzles play a decisive role. They have a significant influence on the distribution of water vapor and thus on surface quality and component properties. Especially in the gaps between the slots there may occur air entrapment and incorrect sintering of the beads.


Difference to conventional production 

In conventionally manufactured molds, slots or nozzles are usually drilled into the tool. At a distance of about 10 – 15 mm, a tool can have several hundred slots. Additive manufacturing processes such as laser melting make it possible to decrease the distance between the slots to less than 1 mm. In this way, slots can be arranged much closer to each other, thereby integrating several tens of thousands of slots into the mold. In addition, the width of each slot can be reduced to about 0.1 mm. This leads to a more vapor-permeable mold, which introduces vapor much more homogeneously. As a result, a significantly better surface quality and structural integrity are achieved. As there are no larger distances between the slots, the sintering of the particles is improved and potential fractures are avoided. Further advantages of the additive process are the increased freedom in complex geometries, possible material savings as well as the flexibility and time savings in the fabrication of the molds.


New application example: The walking stick

To demonstrate the potential of this process chain and the advantageous material properties of a particle foam such as EPP, Robert Hofmann GmbH developed a novel application example in the form of a walking stick. A walking stick as a product from the healthcare sector shows that the properties of EPP such as energy absorption, reduced density and high structural integrity can also be employed for consumer products. The handle of the walking stick is made of EPP using the process chain described above. The user experiences the haptic advantages of EPP, as the pressure on the palms is distributed evenly due to the “bounce effect” of foam material. In addition, the reduced mass, low liquid absorption and thermal insulation properties of EPP ensure a pleasant handling. The required molds are laser melted of steel using machines from Concept Laser GmbH.

During the forming process of the particle foam, an insert is positioned in the mold. The expanding EPP material surrounds and fixates it. The insert is made of plastic employing the additive manufacturing process fused deposition modeling (FDM). It possesses a frame for the integration of an Apple Watch and contains a screw insert to which a fiber-reinforced carbon shaft is attached. With this lightweight structure, the walking stick can support a load of up to 120 kg.

The use of additively manufactured molds for the fabrication of the EPP handle leads to improved product properties


Increaed efficiency 

From an economic point of view, there are further advantages. The more homogenous and finely distributed injection of vapor improves the energy efficiency of the installation. In addition, cycle times can be reduced. As a result, the targeted use of additive manufacturing not only improves the product itself but also makes the overall manufacturing process more efficient.

In summary, the showcase demonstrates impressively how several different materials and processes are combined to create an innovative product with a new design from a conventional walking stick. The advantages of particle foams such as expanded polypropylene (EPP) are linked to the freedom of additive manufacturing, thereby enabling innovative process routes with novel applications.

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