Polyethylene Terephthalate (PET) Injection Molding at Mythentec

Polyethylene terephthalate is a thermoplastic polyester whose property profile is largely determined by its material composition, processing, and degree of crystallization. Various PET grades and compounds are available for engineering injection-molded parts, allowing the material to be tailored to meet mechanical, thermal, and geometric requirements.

PET is essentially a semi-crystalline thermoplastic. During processing, ordered crystalline regions can form within the material. The rate and extent of this crystallization are influenced, among other factors, by the material formulation and the temperature control within the mold. This is particularly relevant for injection molding, as crystallization and cooling affect shrinkage, dimensional stability, and mechanical properties.

PET should not be confused with PETG or PBT. PETG is a glycol-modified polyester with altered crystallization behavior and a different set of properties. PBT is also a thermoplastic polyester, but differs from PET in terms of crystallization rate and processing, among other factors. The choice between these materials is therefore always based on the specific component requirements.

Properties of Polyethylene Terephthalate

Engineering-grade PET grades combine good mechanical strength with high dimensional stability. Depending on the material grade, properties such as stiffness, strength, dimensional stability, and wear resistance may be particularly relevant for use in functional plastic components.

PET also exhibits good resistance to numerous oils, fats, and other media. However, its specific chemical resistance depends on the medium, concentration, temperature, duration of exposure, and the type of material used, and should be verified on a project-by-project basis for critical applications.

The comparatively favorable sliding and wear characteristics of certain PET grades also allow for their use in functional components subject to mechanical stress. At the same time, tight geometric requirements can be met if the material, component design, mold, and process control are appropriately engineered.

Unreinforced and reinforced PET

In addition to unreinforced PET grades, a variety of modified compounds are available for technical applications. The addition of reinforcing or functional additives allows the property profile to be specifically tailored.

For example, glass-fiber-reinforced PET grades can achieve significantly higher stiffness and dimensional stability than unreinforced variants. They are therefore particularly suitable for structural engineering parts subjected to mechanical loads. At the same time, reinforcing fibers alter the material’s flow, shrinkage, and warpage behavior and must be taken into account during component and mold design.

In addition, depending on the manufacturer and application, other modified PET compounds are available. The selection is not based solely on individual properties, but takes into account the overall stress conditions the component will be subjected to in its final application.

PET is sensitive to moisture in the melt and can suffer thermal or hydrolytic damage at inappropriate temperatures or during prolonged residence times. At the same time, mold temperature and cooling conditions have a significant influence on the material’s crystallization.

To ensure reproducible production, the material condition, melt conditioning, injection phase, holding pressure, mold temperature control, and cooling must therefore be coordinated. There are no universally applicable process parameters. The appropriate settings depend on the specific type of PET or compound, the part geometry, and the mold design.

At Mythentec, the injection molding process is therefore designed based on the specific material data and project requirements. Relevant process parameters can be digitally recorded and documented in a traceable manner during series production.

Drying and Moisture Management

PET can absorb moisture during storage and handling. If moist material enters the plasticizing cylinder, the water it contains comes into contact with the hot polymer melt.

Under these conditions , hydrolysis may occur , leading to a shortening of the polymer chains. The consequences may include, among other things, reduced mechanical properties, changes in melt viscosity, or quality deviations on the component surface.

PET should therefore be dried in accordance with the specifications of the respective material manufacturer and then handled under controlled conditions as much as possible until it is processed. What matters is not a general drying time, but a sufficiently low and reproducible moisture content of the material.

Even during production interruptions or extended downtime, it is important to keep in mind that dried material can reabsorb moisture from the environment. Moisture management is therefore an integral part of the overall material logistics process and not merely a single preparatory step immediately prior to injection molding.

Temperature Control, Crystallization, and Process Stability

PET is processed at relatively high melt temperatures. At the same time , unnecessary thermal stress should be avoided. The melt temperature and residence time must therefore be selected so that the material is reliably plasticized without being subjected to excessive thermal stress.

Another distinctive feature is the crystallization behavior of PET. The mold temperature and the cooling rate influence the extent to which crystalline structures form. As a result, these process parameters affect, among other things, mechanical properties, shrinkage, dimensional stability, and cycle time.

In addition to temperature control, injection speed, holding pressure, and holding time play an important role in mold filling and compensating for volumetric shrinkage. Maintaining as uniform a mold temperature as possible also promotes reproducible cooling conditions and can help reduce variations within the part.

Flow behavior, shrinkage, crystallization, and demolding must be taken into account as early as the component development stage. Making changes to a component after it has already been designed is often more time-consuming than coordinating product development and injection molding technology early on.

As a general rule, wall thicknesses should be as uniform as possible, and transitions should be designed to facilitate flow. Larger accumulations of material can cause localized differences in cooling conditions, which may lead to sink marks, variations in shrinkage, or warping. Transitions between different wall thicknesses should therefore be as smooth as possible.

Ribs and other reinforcing elements make it possible to increase the stiffness of a part without creating thick wall sections. Radii at edges and transitions reduce local stress peaks while also facilitating material flow during mold filling. Adequate draft angles facilitate the subsequent removal of the molded part from the mold.

Particularly with reinforced PET compounds, the orientation of the fibers must also be taken into account. This can lead to direction-dependent mechanical properties and varying shrinkage behavior. The gate location, part geometry, and flow direction should therefore be considered together as early as the design phase.

As part of our development services, we assist with the design of technical components tailored to plastics and manufacturing processes, taking into account the material, tooling, and subsequent mass production as an integrated system.

The mold design has a significant impact on the reproducibility of a PET part’s manufacturing process. In addition to the actual part geometry, the gate, venting, and mold temperature control must be tailored to the material used and the part’s geometry.

The location and dimensions of the gate influence the flow path of the melt, the orientation of reinforcing fibers, and the position of potential weld lines. For more complex components, it is therefore essential to consider, as early as the mold design phase, how the cavity will be filled and where the melt fronts will meet.

Proper venting ensures complete mold filling by allowing displaced air and process gases to escape from the cavity. Inadequate venting can lead to quality issues, particularly at the end of longer flow paths.

Uniform and controlled mold temperature control is also particularly important for PET. It affects both cooling and crystallization behavior and, ultimately, cycle time, shrinkage, and part properties. The cooling system and temperature control should therefore ensure conditions that are as uniform as possible throughout the entire molded part.

The tool design and process parameters cannot be optimized independently of one another. Rather, a properly designed tool creates the necessary process window for stable and cost-effective mass production.

The surface quality of a PET injection-molded part is determined as early as the molding process. The mold surface, material condition, venting, filling behavior, and temperature control all combine to influence the visual and functional characteristics of the final part.

For example, moisture or thermally damaged material can contribute to surface defects. Similarly, weld lines or variations in cooling conditions may become visible. A reproducible surface, therefore, does not begin with downstream finishing, but rather with material preparation, mold design, and process development.

Depending on the application, additional steps may be required after injection molding. These include, for example, assembly, labeling, or other finishing processes. Which processes are suitable for a PET component depends on the material type, component geometry, and the functional or aesthetic requirements.

If necessary, Mythentec can machine and assemble injection-molded parts beyond the actual molding process. This allows for the appropriate manufacturing and post-processing steps to be taken into account as early as the project planning phase.