Thermoplastic Polyurethane (TPU) Injection Molding at Mythentec

Thermoplastic polyurethanes belong to the broader material group of thermoplastic elastomers. TPU is therefore a TPE, but exhibits a distinct set of properties within this group. The material consists of elastic soft segments and stiffer hard segments.

Unlike chemically cross-linked elastomers, TPU becomes plastically deformable at elevated temperatures. The material can therefore be processed on conventional injection molding machines and solidifies again upon cooling. This reversible process enables cost-effective production of complex molded parts with short to medium cycle times.

The characteristic properties of suitable TPU grades include, among others, high abrasion resistance, good tear resistance, and excellent elastic recovery. Many grades also offer high impact resistance, good damping properties, and flexibility at low temperatures.

The specific properties always depend on the type of TPU selected. Therefore, statements regarding resistance to specific media, UV stability, temperature resistance, or biocompatibility cannot be generalized to the entire material group. When selecting a material, the component’s intended operating conditions and functional requirements must be fully taken into account.

A Comparison of Polyester-TPU and Polyether-TPU

A key distinguishing feature is the chemical composition of the soft segments. In technical applications, polyester- and polyether-based TPU grades are used in particular.

Polyester-TPU is typically characterized by high mechanical strength, good abrasion resistance, and comparatively good resistance to mineral oils and many industrial greases. These materials are therefore often suitable for molded parts subjected to mechanical and tribological stress. However, their suitability must be carefully evaluated in cases of prolonged contact with moisture, warm water, or environments contaminated with microorganisms.

Polyether TPU generally offers advantages in terms of hydrolytic stability, microbial resistance, and low-temperature flexibility. It may therefore be suitable for components exposed to moisture, fluctuating temperatures, or lower operating temperatures. Here, too, the available material grades differ in terms of hardness, strength, and chemical resistance.

In addition to the classification into polyester and polyether TPUs, a distinction is made between aromatic and aliphatic types. Aromatic TPU materials cover a large portion of standard technical applications. For transparent, light-colored, or components subject to prolonged UV exposure, special aliphatic grades may be required, as conventional aromatic TPU grades can be prone to discoloration when exposed to light and UV radiation.

Excessive residual moisture can lead to the degradation of polymer chains during plastification. Possible consequences include streaks, bubbles, dull surfaces, inconsistent flow behavior, and a reduction in mechanical properties.

Therefore, proper pre-drying is generally required before processing. The drying temperature and duration depend on the specific type of material and the manufacturer’s specifications. Storage and material handling between drying and processing must also be organized in such a way as to prevent the material from absorbing moisture again.

TPU is also sensitive to unnecessarily high thermal stresses. The melt temperature, dwell time, and screw speed must be adjusted to suit the material and the shot volume. An excessively long residence time or excessive shear stress can lead to material degradation, discoloration, or a change in the component’s properties. This must be taken into account especially when shot weights are small relative to the plasticizing unit.

The flow and solidification behavior also differs between soft and hard TPU grades. Soft grades can be more easily stretched or deformed during demolding and, depending on the geometry, require longer cooling times. Harder grades, on the other hand, place different demands on temperature control, flow paths, and pressure transfer.

TPU can be processed using both single-component and two-component injection molding. In two-component injection molding, for example, soft functional areas can be directly bonded to a hard thermoplastic base material. Typical functions include sealing, damping, grip, protection, and vibration isolation.

The elastic properties of TPU open up a wide range of design possibilities, but require a design tailored to the material. Wall thicknesses, transitions, ribs, openings, and demolding directions affect not only how the mold is filled, but also warpage, shrinkage, and the part’s behavior during demolding.

Wall thicknesses that are as uniform as possible promote homogeneous cooling and reduce the risk of sink marks, air pockets, and internal stresses. Significant changes in wall thickness should be avoided or implemented with smooth transitions. Radii on inner and outer edges improve material flow and reduce local stress peaks when the part is subjected to mechanical stress later on.

Draft angles are particularly important for TPU. Although flexible components can stretch to a limited extent during demolding, this elasticity should not be considered a substitute for a design optimized for demolding. Insufficient draft angles can lead to excessive ejector force, surface damage, or permanent deformation. Textured or grain-patterned surfaces generally require larger draft angles than smooth surfaces.

Ribs, sealing lips, locking areas, and film hinges must be designed to match the specific TPU hardness. Very thin or delicate components can place high demands on flowability, venting, and mold precision. For elastic areas subjected to continuous loading, creep behavior, stress relaxation, and potential compression set must also be taken into account.

For hard-soft components, the interface between TPU and the base plastic must be planned during the design phase. The appropriate design depends on whether the composite is created primarily through chemical bonding, form-fit, or a combination of both principles. As part of its plastic-compatible development services, Mythentec assists with the coordination of materials, geometry, and manufacturing processes.

The mold design is adapted to the flow properties, hardness, and demolding sensitivity of the selected TPU type. Injection, venting, temperature control, and the ejection system are of particular importance.

The gate location affects the flow path, weld lines, pressure requirements, and orientation of the material. Gates and runners must be sufficiently sized and designed for optimal flow to prevent excessive shear and unnecessary pressure losses. At the same time, consideration must be given to where the gate will be cut off after demolding and what visual or functional requirements apply to this area.

Effective mold venting ensures complete mold filling and reduces the risk of burn marks, visible flow defects, or incompletely formed details. Venting areas should be provided, in particular, at the ends of longer flow paths, at parting lines, and in thin-walled functional areas.

Depending on the material type and mold surface, TPU can produce component surfaces that are smooth and glossy as well as matte, textured, or non-slip. The final surface appearance is not determined solely by the mold structure. Material hardness, pigmentation, mold temperature, flow behavior, and demolding conditions also influence the visible result.

Textured surfaces can improve the tactile feel, reduce reflections, or make minor signs of wear less noticeable. However, during the design phase, it is important to keep in mind that deep texturing can make demolding more difficult and may require correspondingly larger demolding angles.

TPU can be colored in a wide range of shades. Color batches and additives must be compatible with the base material and the intended conditions of use. For light-colored, transparent, or color-stable components, special attention must be paid to UV exposure, the material base, and potential color changes during processing.

Subsequent printing, marking, coating, or bonding is possible, depending on the material and process. However, the comparatively low surface energy of certain TPU grades may require pretreatment or specially formulated ink and adhesive systems. Suitability should therefore be tested under realistic production conditions.

Depending on the project, Mythentec handles not only injection molding but also downstream processes and the further processing of plastic components. This may include, for example, assembly, testing, labeling, or the integration of components into subassemblies.