A comprehensive understanding of the PMMA injection molding process, manufacturing parameters, and precautions
- A comprehensive understanding of the PMMA injection molding process, manufacturing parameters, and precautions
- Briefly describe the basic information about PMMA plastic materials and their applications.
- What are the four stages of PMMA injection molding?
- Understanding the properties of PMMA materials in injection molding processes
- PMMA material injection molding process parameters
- Drying treatment of PMMA materials
- Injection temperature parameter settings
- Injection pressure parameter settings
- Injection speed parameter settings
- Pressure holding pressure and time parameter settings
- Cooldown time reference
- PMMA injection mold runner and gate design
- PMMA injection mold temperature control
- Product demolding and post-processing
- What should be noted in the PMMA injection molding process?
- Benefits of using Elimold’s professional PMMA injection molding services
- in conclusion
Injection molding is a very popular process in the manufacture of plastic products . The processes and materials may vary, but the ultimate goal is always to produce high-quality products. These processes are used in many industries. Top companies use these processes, which help achieve optimal results. When combined with high-quality materials, they produce outstanding results.
PMMA (Physical Polymer Materials) is commonly used in the manufacture of transparent plastic products. We frequently encounter the molding and production of transparent PMMA . PMMA, also known as plexiglass, is currently the best quality and most affordable of all raw materials for producing fully transparent products. This article will introduce PMMA, explaining the injection molding process, advantages, applications, and points to note regarding PMMA materials.
Briefly describe the basic information about PMMA plastic materials and their applications.
Polymethyl methacrylate (PMMA) resin has good transparency and optical properties.Its weather resistance, chemical resistance, hardness, and appearance are among the characteristics that have earned it the nickname “Queen of Plastics.”Advanced material. This material is mainly prepared through free radical polymerization of methyl methacrylate (PMMA) in bulk (sheet) or suspension form. Compared with traditional glass (mainly composed of silicate complex salts), PMMA, in addition to having the same excellent transparency, also has the characteristics of being lightweight and tough, and its impact strength is more than 10 times that of traditional inorganic glass. However, compared with other transparent plastics, PMMA has low impact strength and poor heat resistance, abrasion resistance, and scratch resistance, which limits its applications.
PMMA, as a high-performance transparent material, is widely used in various lamps, lighting equipment, optical glass, instrument dials, housings, scales, optical fibers, commercial advertising windows, billboards, aircraft cockpit glass, bulletproof glass for aircraft and automobiles, and various other applications (medical, military, and architectural). In particular, with the rapid growth of the liquid crystal display (LCD) market, the demand for optical-grade PMMA molding compounds has increased significantly. With the continuous development of new application areas, PMMA molding compounds have enormous development potential and broad application prospects. Global demand for PMMA is constantly growing, and the annual growth rate is projected to reach 7% in the future.
What are the four stages of PMMA injection molding?
The injection molding process for PMMA is no different from that for other plastic materials; it mainly involves using injection molding machines and molds to manufacture customized plastic products. The main process typically consists of the following four stages.
- Stage 1 Clamping: In this stage, both sides of the mold are placed into the clamping device, and then the machine pushes the two halves of the mold together to ensure that the mold is tightly closed, preparing for the subsequent injection molding process.
- The second stage, injection molding, is the most critical step in the injection molding process. The injection molding machine delivers PMMA plastic raw material into the injection mold cavity. The PMMA raw material is heated and melted within the cavity, and then injected into the closed mold shape through a nozzle at high pressure and high speed.
- Stage 3 Cooling: After injection, the mold needs to be cooled. During this stage, the PMMA plastic melt inside the mold gradually cools and solidifies into a solid form.
- Stage 4 Ejection: As the mold opens, ejector pins slowly push the solidified plastic product out of the open cavity. The manufacturer should then use a tool to remove any waste and refine the final product for customer use.
Understanding the properties of PMMA materials in injection molding processes
1. At processing temperatures, the rheological properties of PMMA melt are essentially non-Newtonian, meaning its melt viscosity is primarily governed by pressure (shear rate). Therefore, reducing viscosity and increasing fluidity mainly relies on increasing pressure. However, its melt viscosity is much higher than that of PE and PS, requiring higher pressure for injection molding. Furthermore, its melt viscosity decreases with increasing temperature, making it more temperature-sensitive than other thermoplastics. Therefore, careful temperature control is crucial for successful molding; otherwise, PMMA products are prone to developing silver streaks during storage and use.
2. PMMA granules are highly hygroscopic, absorbing up to 3% moisture from the air. They must be dried before molding; otherwise, bubbles and air streaks will form, reducing transparency. Therefore, drying is necessary: drying temperature 80-90℃, for at least 3 hours. This is especially important when using recycled materials.
3. Since PMMA is an amorphous polymer, its molding shrinkage is small, ranging from 0.5% to 1.8%.
4. Due to the high melt viscosity, poor fluidity, and fast cooling rate of PMMA, the products are prone to internal stress. Therefore, the PMMA molding process has strict requirements, the mold design has high requirements, and the products generally require post-processing.
PMMA material injection molding process parameters
The following are basic parameters for the injection molding process of PMMA materials, summarized by the Elimold engineering team based on years of experience.
Drying treatment of PMMA materials
PMMA has a certain degree of hygroscopicity and needs to be dried at 80-90°C for 3-4 hours before injection molding to remove moisture and prevent defects such as silver streaks and bubbles from appearing in the product. If drying is not thorough, the surface of the injection molded part may appear hazy, affecting its transparency.
Injection temperature parameter settings
The injection molding temperature for PMMA is generally between 230-260°C. It’s important to note that excessively high temperatures can cause material decomposition, affecting product quality. For example, PMMA is prone to discoloration and degradation when the temperature exceeds 270°C.
Injection pressure parameter settings
When manufacturing PMMA plastic products, the injection molding machine pressure is typically in the range of 80-130 MPa. Higher injection pressure helps fill the cavity, but excessive pressure may cause flash and increased internal stress in the product.
Injection speed parameter settings
When manufacturing PMMA plastic products, injection molding machines should use medium or high speed injection to ensure that the melt fills the cavity quickly, reduce the residence time of the melt in the cavity, and avoid degradation.
Pressure holding pressure and time parameter settings
When manufacturing PMMA plastic products, the holding pressure of the injection molding machine is generally 50%-70% of the injection pressure. The holding time should be determined according to the thickness and size of the product to prevent shrinkage.
Cooldown time reference
The cooling time is relatively long, typically requiring 30-60 seconds, because PMMA has poor thermal conductivity. Insufficient cooling may cause the product to deform.
PMMA injection mold runner and gate design
The runner should be as short and wide as possible, and the gate should be large, such as a fan-shaped gate or annular gate, to facilitate the flow and filling of the melt.
PMMA injection mold temperature control
Mold temperature is usually controlled at 60-90°C, which helps to improve the surface quality and dimensional stability of the product.
Product demolding and post-processing
Demolding: Since PMMA products have high requirements for surface quality, a suitable release agent should be selected to avoid damaging the surface of the product.
Post-processing: For some high-precision products, annealing may be necessary to eliminate internal stress.
What should be noted in the PMMA injection molding process?
Materials play a crucial role in any molding process. The properties of the material and the components even determine the machine used. This contributes to achieving optimal molding results. PMMA also has some properties that need to be considered.
Nozzles and grooves
Although PMMA does not require a special injection molding machine, it does place some additional demands on the machine to achieve optimal molding results. For example, due to the material’s high viscosity, deeper spiral grooves and larger nozzle orifices are needed.
Strength and Absorption
In addition, the required product must be considered. If the product requires higher strength, a special screw will be needed. A screw with a longer diameter is required for low-temperature plasticizing. This helps with better mixing and improves strength. Remember the water absorption rate of PMMA we mentioned earlier. It absorbs a large amount of moisture. Therefore, it is essential to store it dry to prevent water absorption and damage.
Temperature and diameter
Temperature is crucial in injection molding. It determines the success or failure of the molding process. Therefore, for PMMA, set the temperature to 60-80°C. This will facilitate good melting and mixing. Additionally, the runner diameter should be considered. The runner is a critical system in this process. Set the main runner to 8-10 mm. A gate diameter of less than 50 mm ensures thorough mixing and prevents overflow.
thickness
However, the diameter depends on the characteristics of the product. The thickness of the product determines the diameter. Your flow channel diameter is 6-8 mm.
gate
Regarding gates, you have several options. Using the appropriate gate size is crucial. For high-quality products, you need to use depth gates and fan gates. The size of a depth gate is 0.7-0.9t, and the size of a fan gate is the same. There is also a TAB gate in the mold, which is also 0.7-0.9t in size. “t” represents the wall thickness of the product. Additionally, there is a pin gate, whose diameter should be 0.8-2mm, larger than all other gates. These are the gate size requirements for this process.
Injection speed
Next, consider the injection speed. Injection speed refers to the rate at which material is injected into the mold; it affects the quality and appearance of the product, and even the quality of the mixture. Therefore, injection speed is crucial and should never be ignored unless you are not aiming for ideal results. For PMMA materials, you can use a fast injection method. However, remember to avoid generating internal stress, as internal stress can compromise the material’s quality. You can also adjust the injection speed. That is, adjust it according to each stage of the process. You can use a multi-stage injection process. This allows for fast and slow injections at different stages of the injection process. For objects with thicker walls, use slow injection. For other objects, you can use a combination of slow-fast-slow injections for better results.
Duration
Next is the holding time. Holding time refers to the time between the first and second resin injections. Temperature is crucial for this step. At 260°C, the holding time is 10 minutes. Or more precisely, it should not exceed 10 minutes. However, at around 270°C, the holding time should not exceed 8 minutes. Therefore, the higher the temperature, the shorter the holding time; the lower the temperature, the longer the holding time. Additionally, you should measure the air injection amount based on the melt temperature. For PMMA, the melt temperature is approximately 210-270°C.
Benefits of using Elimold’s professional PMMA injection molding services
Using Elimold’s professional PMMA injection molding services offers numerous benefits. Firstly, the highly automated and efficient process saves time and money. Our professional PMMA injection molding services typically utilize state-of-the-art equipment and materials, ensuring the highest quality parts are produced. Finally, our team of engineers provides expert advice and guidance to ensure that part designs achieve maximum efficiency and performance. Contact us now to collaborate on your next PMMA injection molding project.
in conclusion
This concludes the brief introduction to PMMA, its injection molding process, manufacturing parameters, and precautions. After reading this article, you will have a completely new understanding of PMMA injection molding and can confidently discuss or perform injection molding processes. Be sure to keep all the details in mind, as they will be helpful in your production process.
Many companies use it to produce a variety of products, such as automotive lights, instrument parts, transparent pipes, and even optical lenses. When high-volume production is required, these companies use injection molding, a process that is well-suited for PMMA.