Polyoxymethylene (POM), as a high-performance engineering plastic, is widely used in automotive, electronics, medical, and industrial manufacturing fields due to its excellent mechanical properties and chemical stability.
The POM injection molding process involves injecting molten POM plastic into molds to produce plastic parts with high precision and durability.
This article will provide an in-depth exploration of the process characteristics, key parameters of POM injection molding, and its practical applications in manufacturing, offering valuable insights for readers to fully understand and master this plastic injection molding process.
What Is POM ?
POM (Polyoxymethylene), also known as polyacetal or acetal resin, is a highly crystalline engineering thermoplastic. It is renowned for its excellent mechanical properties, low coefficient of friction, and good dimensional stability,can be easily processed on injection molding machines.
Process POM is widely used in injection molding for high-performance products such as precision gears, bearings, automotive components, electronics, and medical devices.
POM is mainly divided into two categories based on chemical structure: homopolymer POM (POM-H) and copolymer POM (POM-C):
Homopolymer POM (POM-H): Polymerized from formaldehyde monomers, its molecular chain consists purely of repeating -[CHâ‚‚-O]- units. It has a higher crystallinity (typically 75-85%) and a more regular molecular chain arrangement. Therefore, it offers better mechanical properties and fatigue resistance. A typical example is DuPont’s Delrin series.
Copolymer POM (POM-C): Contains a small amount of comonomers (such as dioxolane) introduced into formaldehyde (or trioxane), forming repeating units of -[CHâ‚‚-O]- along with minor units like -[CHâ‚‚-CHâ‚‚-O]-. The introduction of comonomers disrupts the regularity of the molecular chain, reducing crystallinity but significantly improving thermal stability and chemical resistance.
When selecting materials for injection molding, POM-H is preferred for applications requiring high strength and wear resistance, while POM-C is recommended for better processing stability and chemical resistance.
Properties of POM Plastic
Polyoxymethylene (POM) plastic possesses a variety of excellent properties, making it a standout among engineering plastics.
High Strength And Stiffness
POM has excellent tensile and flexural strength, making it suitable for high-strength applications, although it has a high shrink rate during processing.
Its high tensile strength typically ranges from 60 to 90 MPa, with a flexural modulus of about 2.8 to 3.0 GPa, maintaining good toughness even at low temperatures around -40°°ä.
Low Friction
POM exhibits a low coefficient of friction and excellent self-lubricating properties, making it ideal for sliding components such as gears and bearings.
Low Moisture Absorption
POM has a low moisture absorption rate (about 0.2% to 0.5%), which helps maintain the dimensional accuracy and mechanical properties of parts.
Dimensional Stability And Creep Resistance
POM also offers excellent dimensional stability and creep resistance, maintaining shape and size even under long-term stress, which is particularly important for precision part manufacturing.
Chemical Resistance
POM plastic is highly resistant to solvents, fuels, and mild inorganic acids, widely used in automotive fuel systems and industrial pipelines.
Heat Resistance
It has good heat resistance, able to endure continuous use temperatures up to approximately 105°°ä; homopolymer POM can withstand about 175-184°°ä, while copolymer POM ranges from about 160-170°°ä. Compared to copolymer POM, homopolymer POM has higher instantaneous heat resistance.
Additionally, POM’s mechanical and processing properties can be further enhanced by adding fillers and additives, such as improving toughness and wear resistance.
Due to its exceptional properties, POM plastic is widely used in the automotive industry, electronics, medical devices, and industrial equipment to manufacture various high-performance POM parts, such as gears, valves, slides, and connectors.
Property | Unit | Typical Value |
|---|---|---|
Density | ²µ/³¦³¾Â³ | 1.41 – 1.42 |
Melting Point | °°ä | 165 – 175 |
Tensile Strength | MPa | 60 – 90 |
Flexural Modulus | GPa | 2.8 – 3.0 |
Elongation at Break | % | 20 – 40 |
Coefficient of Friction | – | 0.2 – 0.3 |
Moisture Absorption | % | 0.2 – 0.5 |
Heat Deflection Temperature | °°ä | ~105 |
Shrinkage Rate | % | 2.0 – 3.5 |
POM Injection Molding Processing Parameters
The injection molding process of POM requires careful control and optimization of various parameters to achieve high-quality parts with excellent mechanical properties and dimensional accuracy. Understanding these processing parameters is crucial for manufacturers to maximize the advantages of POM plastic.
Drying Temperature
Although POM has a low moisture absorption rate (0.2% to 0.5%), it is sensitive to moisture during processing. Moisture can cause defects such as bubbles and degradation. Therefore, drying the raw material before molding is crucial.
The drying temperature typically ranges from 80°°ä to 100°°ä, and drying time is usually 2 to 4 hours, depending on the grade and moisture content of the material.
Melt Temperature
Maintaining the appropriate melt temperature is essential to avoid thermal degradation of POM. The melting temperature generally lies between 165°°ä and 175°°ä.
High temperatures beyond this limit can cause degradation, discoloration, and poor mechanical properties.To prevent degradation, the temperature of molten POM should be kept below 210°°ä during the injection molding process.
Injection Pressure And Speed
POM injection molding typically requires maintaining a medium to high injection speed and an injection pressure between 70 and 120 MPa , to ensure the molten material fills the mold cavity completely and achieves the desired part density and surface finish.
High pressure is also necessary to compensate for POM’s relatively high shrinkage rate (2.0% to 3.5%) and to minimize deformation during cooling.
Viscosity and Material Flow
POM has a relatively low melt viscosity, which allows it to flow easily into molds with complex geometries. However, its viscosity is sensitive to temperature and shear rate, so process parameters must be carefully controlled to achieve consistent flow behavior and part quality.
Shrinkage and Dimensional Accuracy
POM has a shrinkage rate of approximately 2 to 3.5% during injection molding, which is important to consider for part design and tolerances.
In summary, mastering the processing parameters for POM injection molding is crucial to achieving high-quality parts that fully leverage POM’s excellent mechanical properties and chemical stability.
What Are The Challenges Of POM Injection Molding?
POM injection molding faces several challenges that require careful consideration.
Firstly, POM is difficult to bond; if painting or gluing is needed, advanced surface treatment techniques are often necessary to ensure proper adhesion and finish.
Secondly, POM is prone to rapid degradation if overheated, releasing toxic and corrosive formaldehyde gas, which poses safety risks and can damage molds.
Thirdly, POM exhibits a high shrinkage rate of 2% to 3.5%, which complicates the molding process and necessitates precise mold design and accurate modeling to minimize dimensional deviations.
Additionally, POM’s tendency to react under certain processing conditions can lead to defects if parameters are not carefully controlled. Moisture control is also critical, as even its low moisture absorption can cause issues during molding.
Addressing these challenges requires thorough feedback and solutions throughout the injection molding process to ensure high-quality, stable parts.
Advantages Of POM Plastic Injection Molding
POM injection molding offers numerous advantages that make it a preferred choice for manufacturing a wide range of high-performance plastic parts.
High Precision and Dimensional Stability
Due to its low shrinkage rate and excellent dimensional stability, POM injection molding produces parts with tight tolerances and consistent quality. This precision is critical for components such as gears, rods, and sheets used in mechanical assemblies where exact dimensions are essential to proper function.
Low Friction and Wear Resistance
POM’s low coefficient of friction and self-lubricating nature reduce wear and tear on moving parts, enhancing the lifespan of molded components such as bearings, slides, and valves. This characteristic also helps minimize energy loss in mechanical systems.
Efficient Manufacturing Process
The injection molding process for POM allows for high-volume production with rapid cycle times and minimal waste. Optimized processing parameters help prevent defects such as warping or sink marks, ensuring cost-effective and consistent part quality.
Enhanced Surface Finish and Aesthetic Options
POM parts can achieve smooth surfaces with good gloss, and while bonding can be challenging, advanced surface treatments enable painting or coating for improved aesthetics and functionality.
Overall, POM injection molding combines the material’s superior properties with efficient manufacturing to deliver high-quality, durable, and precise plastic components suitable for a broad spectrum of industrial applications.
Applications Of Injection Molded POM Parts
Injection molded POM parts are widely utilized across various industries due to their excellent mechanical properties, chemical resistance, and dimensional stability. These applications leverage POM’s unique combination of strength, low friction, and durability.
Automotive Industry
In automotive manufacturing, POM is extensively used for components that require resistance to fuels, oils, and high temperatures. Typical applications include fuel caps, fuel filler necks, valves, gears, and electrical connectors.
Its ability to withstand aggressive automotive fluids and maintain dimensional stability under thermal stress makes it ideal for modern fuel systems and interior parts that demand both performance and aesthetic quality.
Medical and Healthcare Equipment
In the medical and healthcare industry, POM is favored for its high strength, stiffness, and toughness, making it suitable for equipment that requires high purity and FDA compliance.
It is used in precision parts such as surgical instruments, drug delivery components, and diagnostic equipment.
Industrial and Fluid Handling Applications
POM is commonly used in industrial applications for fluid handling, including plumbing and irrigation systems, due to its low moisture absorption and dimensional stability when in contact with water.
Its resistance to wear and corrosion ensures long service life in demanding industrial environments where contact with water and various chemicals occurs frequently.
Electrical and Electronics
POM is also favored in electrical and electronic applications for insulating components, connectors, and housings. Its good electrical insulation properties, combined with mechanical strength and resistance to solvents, make it suitable for parts that require durability and safety in electrical systems.
Consumer Goods and General Engineering
Beyond specialized industries, POM is used in consumer goods such as zippers, fasteners, and household appliance parts. Its low friction and wear resistance enhance the performance and longevity of moving parts in everyday products.
Overall, the versatility of injection molded POM parts, supported by their excellent mechanical and chemical properties, enables their widespread use in diverse applications that demand high precision, durability, and reliability.
Conclusion
In summary, POM injection molding is a highly effective manufacturing process that leverages the exceptional mechanical properties, chemical resistance, and dimensional stability of polyoxymethylene plastic.
By carefully controlling processing parameters such as drying temperature, melt temperature, injection pressure, and speed, pom injection molding manufacturers can produce precise, durable, and high-performance parts suitable for a wide range of industries including automotive, medical, industrial, and electronics.
Despite some challenges like shrinkage and sensitivity to overheating, the advantages of POM injection molding—such as low friction, excellent fatigue resistance, and chemical stability—make it a preferred choice for producing complex components that demand reliability and longevity.