Polyoxymethylene (POM) Injection Molding at Mythentec

POM belongs to the group of engineering thermoplastics and has a semi-crystalline structure. This results in a combination of properties that is particularly appealing for mechanical functional components. POM exhibits high strength and stiffness along with good toughness and, due to its low moisture absorption, maintains relatively stable dimensions even under varying environmental conditions.

Other characteristic features include low coefficients of friction and good wear resistance. As a result, moving components can operate with low friction against each other or against other materials in suitable applications. These properties make POM an attractive choice for applications such as slide guides, gears, bearings, and mechanical transmission elements, among others.

Good fatigue strength and favorable springback behavior are also important for components that are subjected to repeated mechanical stress. Depending on the type of material, latching, spring-loaded, or snap-fit functions can therefore be integrated directly into an injection-molded part.

POM also exhibits good resistance to numerous fuels, oils, greases, solvents, and other media. However, specific resistance to media must always be evaluated based on the type of POM used, the concentration, temperature, and duration of exposure. There are limitations, particularly with regard to strong acids and highly oxidizing media.

In addition to standard grades with unchanged properties, modified POM compounds are available. Depending on the application, grades such as reinforced, tribologically optimized, electrically modified, UV-stabilized, or those designed to meet specific regulatory requirements can be used. This allows the property profile to be specifically tailored to the component’s intended function.

A Comparison of POM-C and POM-H

A basic distinction is made between POM-C, a copolymer, and POM-H, a homopolymer. Both material groups possess the combination of high strength, good dimensional stability, and favorable sliding properties characteristic of POM, but they differ in certain technical properties.

• POM-H typically offers slightly higher strength, stiffness, and hardness. Depending on the application, this can be advantageous for precision components subjected to high mechanical stress.
• POM-C, on the other hand, often offers advantages in terms of chemical and thermal resistance as well as processing stability. This can be particularly relevant for applications involving contact with media or those with heightened requirements for long-term performance.

Therefore, it does not make sense to choose between POM-C and POM-H based on a blanket rule. Key factors include mechanical stress, temperature range, contact with media, friction pairing, component geometry, and the required dimensional accuracy. In addition, the properties of the specific material type and any additives must be taken into account.

POM can be cost-effectively processed into complex, technical production parts using plastic injection molding. However, this semi-crystalline material places specific demands on process control. To ensure reproducible parts, temperature control, filling behavior, holding pressure, and cooling must be tailored to the material type, geometry, and mold.

Unlike highly hygroscopic plastics, POM absorbs very little moisture. When stored properly, intensive pre-drying is therefore not necessary for many types of POM. However, the processing specifications provided by the respective material manufacturer and the actual storage conditions are always decisive.

Special attention must be paid to thermal process control. POM has a defined processing window and should not be exposed to unnecessarily high material temperatures or long dwell times. The mold, plasticizing unit, and process parameters must therefore be coordinated so that the material is melted in a controlled manner and processed without unnecessary thermal stress.

Due to its semi-crystalline structure, POM exhibits a comparatively pronounced volume contraction during cooling. The resulting processing shrinkage must be taken into account as early as the mold design stage. Its actual magnitude depends, among other factors, on the material type, wall thicknesses, flow paths, mold temperature, and holding pressure conditions.

The properties of POM can be utilized most effectively when the component geometry is tailored to the material and manufacturing process as early as the development phase. In addition to mechanical performance, flow behavior, shrinkage, demolding, and cooling must also be taken into account.

Wall thicknesses that are as uniform as possible promote uniform filling and cooling. Significant material buildup should be avoided whenever possible, as it can lead to longer cooling times, uneven shrinkage, and localized sink marks. If varying wall thicknesses are required for design reasons, transitions should be made as smooth as possible.

Radii reduce stress peaks while improving the flow behavior of the molten plastic. Adequate draft angles facilitate the safe removal of the part from the mold and protect sensitive surfaces or structures.

Ribs can be used to increase the stiffness of a component without having to increase the wall thickness significantly. However, their dimensions and connections must be designed in such a way that local material buildup and the resulting surface or dimensional deviations are avoided as much as possible.

POM is also suitable for integrating mechanical functions. Depending on the load and geometry, locking tabs, snap-fit connections, guides, bearing seats, or gear teeth can be incorporated directly into the injection-molded part. Particularly for such functional areas, permissible elongation, continuous load, notch effect, and manufacturing tolerances must be taken into account as early as the design phase.

When it comes to high-precision assemblies, one should not focus solely on the tolerance of individual components. What is crucial is how all components interact under the conditions in which they will ultimately be used.

When working with POM, particular attention must be paid to the significant shrinkage characteristics of this semi-crystalline material and the requirements for controlled temperature management.

The location and sizing of the injection points influence flow paths, pressure distribution, orientation, and the effect of holding pressure. The gate design is therefore determined based on the part geometry, material type, and quality requirements. For more complex geometries, it is also necessary to consider where flow fronts meet and what effects this may have on mechanical or optical functional areas.

Adequate mold venting promotes controlled mold filling and reduces the risk of trapped air. At the same time, the temperature control must be designed so that the part cools as uniformly as possible. A reproducible thermal balance of the mold is crucial, especially for parts with tight tolerances.

The expected processing shrinkage is also taken into account in the design of the cavity. Since it is not simply a constant material parameter but is influenced by material type, geometry, and process conditions, the mold design is tailored to the specific application.

Depending on the quantity, component size, and project requirements, different mold and gate designs can be used. The goal is always to develop a design that is both technically reliable and cost-effective for the planned mass production.

POM injection-molded parts can be produced with different surface textures directly in the mold. The resulting surface is influenced by factors such as the mold surface, material type, and process conditions. Depending on the part, smooth engineering surfaces can be produced, as well as defined textures.

However, POM exhibits a unique characteristic when it comes to subsequent surface treatments: Its low surface energy makes painting, printing, and—in particular—bonding more difficult compared to many other plastics. If such further processing is to be carried out, the material type, pretreatment, and process used must be coordinated at an early stage.

Mechanical post-processing operations such as drilling, milling, or other machining steps are generally possible and can be used, for example, for functional surfaces or geometries to be added later. In many cases, it is more cost-effective to integrate the required functions directly into the injection-molded part.

Mechanical joining methods, snap-fit connections, or suitable welding methods can also be used, depending on the component, POM type, and joint location. If there are specific requirements for marking or labeling, appropriately suited or modified material types can also be considered.

Mythentec can integrate injection-molded parts into downstream processing and assembly steps as needed. The technical feasibility of specific processes is evaluated based on the component's function and the specific material used.