Plasma-SealTight®: Strong plastic-to-metal bonds in the injection molding process

Plastic-metal hybrid components are firmly established in industrial manufacturing and are becoming increasingly relevant in an era of lightweight construction. This material combination comes into its own for components whose property profile unites the individual strengths of both materials: The metal provides high strength and stiffness, while the plastic offers greater design flexibility combined with low density.

A reliable covalent bond is the key to joining two dissimilar materials to create a high quality, durable and reproducible compound.  If not properly sealed, the joint interface between the plastic and metal materials constitutes a risk factor.

Together with plastic compound manufacturer Akro-Plastic GmbH, Plasmatreat has developed a process for hybrid injection-molded components which significantly improves bond adhesion and can be integrated into mass production lines. The atmospheric PlasmaPlus® coating Plasma-SealTight® lies at the heart of this development. This coating facilitates the formation of a covalently bonded joint between the two materials to create a very strong, media-tight hybrid compound.

 

 

Plasma-SealTight® process: Reliable, reproducible and environmentally friendly

The Plasma-SealTight® process is designed as an automated inline solution for continuous production systems which can be fully integrated into the injection molding line. All process components within the overall concept (plasma polymer layer, plastic recipe, process parameters) are precisely matched to ensure that the process is reliable and reproducible even at high production capacities.

The first step involves cleaning the surface of the metal inlay to a microfine level with Openair-Plasma®. An adhesion-promoting, corrosion-proof, plasma polymer coating is then applied to the metal surface immediately after cleaning. Both the plasma cleaning and plasma coating process are completed in a matter of seconds and are area-selective, i.e. applied only to targeted areas. The thermoplastic is then injected onto the coated metal in the subsequent injection molding process.

The pretreatment steps used in conventional processes such as solvent-based primers, which are often harmful to the environment, can be completely replaced by the Plasma-SealTight® coating while maintaining quality assurance.

 

 

Improved bond strength compared with adhesion promoters

Plasma-SealTight® bonds to the metal (stainless steel, copper, aluminum, steel) at molecular level and at the same time generates organic compounds (functional groups) which ensure a strong adhesive bond between the metal and the plastic.

Tensile test of a test specimen treated with Plasma-SealTight® before injection molding
Plasma-SealTight® technology ensures that the plastic-to-metal bond holds. At over 22 MPa the plastic fails, but not the adhesive bond.

 

Tensile shear strengths of over 40 MPA can be achieved between plastic and metal with Plasma-SealTight®.  Plasma coating achieves significantly higher bond strength than other processes such as adhesion promoters, laser micro-structuring or etching.

 

Media-tight hybrid bond

Water can penetrate the boundary layer and spread across the surface of materials that do not have a chemically bonded joint. In combination with oxygen, water corrodes the metal insert in the hybrid compound, leading to failure of the component function or even delamination.

 

If water penetrates the boundary layer of the plastic-to-metal bond, it can lead to corrosion, failure of the component or even delamination (left). The plasma polymer layer fills every cavity and so prevents the penetration of water (right).

The plasma coating prevents the penetration of aggressive media by filling in even the tiniest of cavities and bonding covalently with the metal surface. This protective seal provides a long-time stable barrier to water, salt solutions, oils and gases and prevents these media migrating below the surface.

Benefits of Plasma-SealTight® technology

  • increased component quality
  • media-tight interface
  • can be fully integrated into injection molding line
  • high process reliability and reproducibility
  • complete elimination of solvents, primers and acids
  • very low use of chemicals (precursors)
  • no VOC emissions
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