Purging is a cleaning process of injection molding to remove residual resins, colors, contaminants, and degraded materials from the machine barrels, screws, nozzles, and molds. Purging is essential for maintaining the quality and efficiency of the injection molding process, especially when changing colors or resins. In this article, we will explain the benefits, methods, and best practices of purging an injection molding machine.
Table of Contents
- Benefits of Purging
- Purging Methods
- Purging Compounds
- Purging Procedures
- Purging Tips
Benefits of Purging
- Reducing material waste and cost: Purging reduces the amount of resin or colorant needed to clean the machine and mold. It also minimizes the production of scrap parts that are contaminated or discolored by the previous material.
- Saving time and labor: Purging speeds up the color or resin changeover process by eliminating the need for multiple purges or manual cleaning. It also reduces the downtime and labor required for maintenance and troubleshooting.
- Improving product quality and consistency: Purging prevents defects such as streaks, specks, splay, bubbles, burn marks, and dimensional variations that are caused by residual material or degradation. It also ensures that the product meets the specifications and expectations of the customer.
- Mechanical purging: This method uses a purging compound that has a higher viscosity, melting point, or abrasiveness than the resin to mechanically scrub and push out the residual material from the machine and mold. Mechanical purging is suitable for most resins and colors, but it may require higher temperatures, pressures, and screw speeds to be effective.
- Chemical purging: This method uses a purging compound that reacts chemically with the resin to break down its molecular structure and make it easier to flush out. Chemical purging is effective for removing resins that have a high tendency to degrade or cross-link, such as PVC, POM, PBT, and PC. However, chemical purging may require longer soaking time and careful handling to avoid safety hazards.
- Ultrasonic purging: This method uses a purging compound that is activated by ultrasonic waves to create micro-bubbles that burst and dislodge the residual material from the machine and mold. Ultrasonic purging is efficient for removing stubborn contaminants and deposits that are difficult to reach by mechanical or chemical purging. However, ultrasonic purging requires special equipment and expertise to perform.
Purging compounds are resin-based products designed to clean up different parts of a plastic injection molding machine. Some of these parts include molds and extruders. These compounds are able to remove carbon, resin, as well as any colors left on machines and molds. There are many types and brands of purging compounds available in the market, each with its own advantages and disadvantages. Some of the factors to consider when choosing a purging compound are:
- Compatibility: The purging compound should be compatible with the resin type, colorant type, processing temperature, and machine type. It should not cause any adverse effects such as corrosion, degradation, or contamination on the machine or mold components.
- Performance: The purging compound should be able to remove the residual material effectively and efficiently. It should have a good flowability, wetting ability, dispersion ability, and cleaning ability. It should also have a low tendency to stick or leave behind any residue on the machine or mold components.
- Cost: The purging compound should be economical and provide a good return on investment. The cost of the purging compound should be weighed against the benefits it provides in terms of material savings, time savings, labor savings, and quality improvements.
- Prepare the machine: Before starting the purging process, make sure that the machine is in good condition and ready for purging. Check the hopper for proper angle and cleanliness—no fines. The feed throat is not as easy to get at and rarely gets the attention it deserves. Take time to inspect it and remove any buildup of oils, waxes, resin, rust (yes rust, there are times when you can actually see condensation) or other contamination. Adjust the shot size to about 25% of injection capacity. Set the machine to manual mode and purge the barrel and screw until the previous material is no longer visible.
- Load the purging compound: Load the purging compound into the hopper or feeder according to the manufacturer’s instructions. Make sure that the purging compound is compatible with the machine and the resin. If using a chemical purging compound, follow the safety precautions and wear protective equipment.
- Purge the machine: Purge the machine according to the type of purging compound and method used. For mechanical purging, increase the backpressure and screw speed to the maximum safe level and purge until the compound is almost clean. For chemical purging, soak the compound in the barrel for 3 to 5 minutes or as recommended by the manufacturer and purge until the compound is almost clean. For ultrasonic purging, activate the ultrasonic device and purge until the compound is almost clean. During the purging process, make sure to clean the nozzle and sprue bushing as well.
- Rinse the machine: After purging, replace the purging compound with the next resin or colorant to be run. Feed 1 to 1-1/2 barrel capacities of the next resin (with colorant on) to rinse out residual purging compound. Set the machine to normal mode and resume production.
Here are some tips to optimize the purging process and achieve better results:
- Purge regularly: Purging should be done regularly as part of a preventive maintenance program. Purging can prevent problems such as carbon buildup, color contamination, resin degradation, and mold damage that can affect product quality and machine performance.
- Purge before shutdown: Purging should be done before shutting down the machine for a long period of time. Purging can prevent problems such as moisture absorption, oxidation, cross-linking, and crystallization that can occur when resins are left in the machine for too long.
- Purge after startup: Purging should be done after starting up the machine from a cold state. Purging can prevent problems such as cold slugs, air pockets, bubbles, and splay that can occur when resins are not melted properly.
- Use proper settings: Purging should be done with proper settings for temperature, pressure, speed, and time. Purging with too high or too low settings can cause damage to the machine or mold components or reduce the effectiveness of the purging compound.
- Use proper amount: Purging should be done with proper amount of purging compound. Purging with too much or too little purging compound can cause waste of material or incomplete cleaning of the machine or mold components.
Purging an injection molding machine is a crucial maintenance process that involves removing any residue or contaminants from the barrel and screw assembly. This is essential to ensure the integrity of the plastic material used in the injection molding process and prevent any potential defects or inconsistencies in the final product. Purging helps to optimize production efficiency, reduce downtime, and maintain the quality of the molded parts.
The first step in purging an injection molding machine is to select the appropriate purging compound or material. Purging compounds are specifically designed to clean the barrel, screw, and nozzle effectively. They typically consist of a blend of resins, additives, and cleaning agents that facilitate the removal of any old or degraded plastic material from the machine.
Next, the operator must follow the manufacturer’s guidelines for purging. This may involve setting the machine to a specific temperature, screw speed, and injection pressure suitable for purging. It is crucial to ensure that the purging compound flows smoothly through the machine to effectively remove any remaining plastic material.
The purging process usually begins by purging out the remaining resin or material in the machine with the new purging compound. This step helps in preventing any undesirable mixing of materials during the purging process. Once the old material is cleared, the purging compound is fed into the machine, and the screw is run at a specified speed to allow the compound to thoroughly clean the barrel and screw assembly.
During the purging process, the operator should pay close attention to the color and consistency of the purging compound as it exits the machine. The purging process is considered complete when the purging compound appears clean and free from any residual color or contaminants. The machine can then be shut down, and the barrel and screw assembly can be visually inspected to ensure that the purging process was successful.
It is essential to perform regular purging maintenance to prevent material degradation, carbon buildup, and color contamination, which can lead to product defects and production inefficiencies. The frequency of purging depends on the type of material being used and the production volume. As a general rule, it is recommended to purge the machine at the end of each production run and before starting a new production run with a different material.
In conclusion, purging an injection molding machine is a critical maintenance procedure that ensures the cleanliness and integrity of the plastic material used in the injection molding process. By following the manufacturer’s guidelines and using appropriate purging compounds, manufacturers can optimize production efficiency, reduce downtime, and maintain the quality of the molded parts. Regular purging is an essential part of maintaining a well-functioning injection molding machine and ensuring the production of high-quality plastic products.