PEEK injection molding at Mythentec

PEEK is a semi-crystalline high-performance thermoplastic that is characterized by a combination of thermal stability, mechanical strength, and chemical resistance. These properties are largely retained even at elevated operating temperatures, making PEEK an established material for technically demanding injection-molded components. Typical operating temperatures are significantly higher than those of conventional engineering plastics, although the actual continuous operating temperature always depends on the component design, the type of load, and the selected material type.

A key feature of PEEK is its crystallinity. As a semi-crystalline plastic, PEEK forms an ordered crystal structure when cooled from the melt. The degree of crystallinity influences mechanical properties, dimensional stability, temperature resistance, and chemical resistance. In injection molding, it is mainly controlled by the mold temperature and cooling conditions. This means that PEEK differs significantly in its processing from amorphous high-performance plastics, for which this relationship is irrelevant.

PEEK is used in various forms. In addition to unreinforced types, glass fiber or carbon fiber reinforced compounds are also used when increased stiffness, reduced shrinkage, or improved dimensional stability are required. These reinforcements influence the flow behavior, surface quality, and anisotropic component characteristics and must be taken into account in the design of the component and tool. The selection of the suitable PEEK type is therefore always application-specific and in accordance with the functional requirements.

The injection molding of PEEK differs significantly from the processing of conventional engineering plastics in several respects. The reasons for this are the high melting temperature, the pronounced crystallization, and the comparatively low process tolerance. Stable series production processes therefore require a narrowly controlled and reproducible process window that consistently coordinates the material, tool, and plant technology.

PEEK is processed in the injection molding process at significantly increased cylinder temperatures. At the same time, a high and uniform mold temperature is required to achieve a defined crystallinity in the component. Too low a mold temperature can lead to incomplete crystallization, increased internal stress, or limited mechanical properties. Conversely, high mold temperatures affect cycle time and cost-effectiveness, which must be taken into account in series design.

At Mythentec, PEEK is processed on suitable injection molding machines with controlled temperature control and continuous process monitoring. Relevant process parameters are recorded and documented in order to detect fluctuations at an early stage and keep series processes stable in the long term. This is particularly important for applications with increased quality requirements.

Process window, drying, and crystallinity

Although PEEK is not considered to be highly hygroscopic, defined material drying prior to injection molding is state of the art. Even low levels of moisture can lead to material degradation, surface defects, or fluctuating component properties at high processing temperatures. The drying parameters are based on the material type and delivery condition and are integrated into the overall process.

The crystallinity of PEEK develops during the cooling phase in the mold. Among other things, it influences the stiffness, heat resistance, and chemical resistance of the molded part. This property can be controlled within defined limits by carefully selecting the mold temperature and cooling conditions. If necessary, a downstream tempering process can also be used to achieve a uniform crystal structure throughout the entire cross-section of the component.

These relationships make it clear that PEEK injection molding is less tolerant of deviations than many other injection molding materials. Clean process design and validation are therefore crucial for reproducible results.

Stabilizing serial processes and traceability

For series applications, long-term process stability is crucial, in addition to pure processability. Mythentec relies on digitally networked injection molding machines with real-time recording of key process data. Batch-related traceability back to the raw material used is part of the manufacturing concept.

This structure makes it possible to systematically analyze deviations and document process changes in a traceable manner. Especially for PEEK components used in safety-relevant or regulated applications, this transparency forms an important basis for approvals, requalifications, and series monitoring.

In PEEK injection molding, a functional and production-oriented component design is a key factor for process reliability, component quality, and cost-effectiveness. Due to the high processing temperatures and pronounced crystallization, PEEK is more sensitive to design weaknesses than many other plastics. Design decisions therefore have a direct impact on flow behavior, shrinkage, dimensional stability, and internal stresses.

A wall thickness distribution that is as uniform as possible supports homogeneous cooling and reduces the risk of warping or local stresses. Large variations in wall thickness or massive cross-sections make controlled crystallization more difficult and extend the cycle time. Where functionally necessary, transitions should be rounded and designed in a stepped manner to avoid material accumulation.

The relatively high melt viscosity of PEEK requires short, well-defined flow paths to ensure complete mold filling and reproducible component properties. This influences both the component geometry and the subsequent mold design.

Typical design aspects are:

• Uniform wall thicknesses to support stable crystallization

• Generous radii at transitions to reduce notch stresses

• Sufficient draft angles, especially for fiber-reinforced types

• Design functional areas so that they are not located in critical flow or seam areas.

• Consideration of directional shrinkage in glass or carbon fiber reinforced compounds

The insertion of metal inserts also requires a coordinated design. Different thermal expansion coefficients of metal and PEEK can lead to stresses in the component. These effects can be controlled by using suitable geometries, preheating the inserts, and adjusting the process parameters.

The high processing and tool temperatures place special demands on tool materials, temperature control, and design. These aspects must be taken into account early on in the project phase in order to avoid subsequent adjustments or restrictions in series production.

Consistent and efficient mold temperature control is crucial for achieving controlled crystallization in the component. Temperature gradients in the mold can lead to inhomogeneous material properties, warping, or dimensional deviations. At the same time, the mold temperature directly influences the cycle time, which is why a balance between component requirements and cost-effectiveness is necessary.

Tool design, temperature control, and injection concept

The design of PEEK injection molding tools differs from standard tools in several respects. In addition to temperature-resistant steels and suitable coatings, temperature control is of central importance. Tools must be able to maintain consistently high surface temperatures without creating local overheating or underheating.

The injection concept is selected to ensure short flow paths, uniform filling, and stable seam zones. For fiber-reinforced PEEK types, the flow direction is also relevant, as it influences the mechanical properties and shrinkage of the component. Venting concepts must be adapted to the high melting temperature in order to avoid burns or air pockets.

Tool procurement, industrialization, and series transition

The tools are procured through collaboration with qualified external partner companies. Mythentec takes care of the technical coordination, defines the requirements together with the customer, and accompanies the entire process from tool approval to series production.

As part of industrialization, process parameters are defined, initial sampling is carried out, and series processes are gradually validated. The goal is a stable transition to series production, in which tools, materials, and processes interact reliably under real production conditions.

After injection molding, PEEK components can undergo further processing or finishing steps. Due to their high temperature resistance and chemical resistance, many common processes are possible, but they must be tailored to the material in terms of the process. Mythentec offers various in-house processes for this purpose as well as the organizational integration of external services.

Typical post-processing and further processing steps are:

• Laser marking or pad printing for permanent marking of components and serial numbers

• Laser and ultrasonic welding for joining PEEK components or hybrid assemblies

• Assembly of individual parts into functional assemblies

• Conditioning of plastic parts to stabilize component properties

• Packaging and handling under defined conditions, especially for sensitive applications

The interface between injection molding and assembly plays an important role in assemblies. Tolerances, joint edges, and contact surfaces must be designed to function reliably even under the specific properties of PEEK. This applies to purely mechanical connections as well as joining processes such as welding.