Screw cleaning is a problem that every injection molding factory must face. Whether it is shutdown cleaning before holidays or material change, color change cleaning, if cleaning is not thorough, it will have a negative impact on normal production.
Commercial Purging Compounds (CPC) have been shown to be the best way to remove resin from plastic processing machinery. However, due to limitation of price and other factors, many injection molding factories are still using traditional methods to clean screw. This processing method is not only time-consuming (because nozzles, screws and other components need to be removed), but also cannot clean internal parts of machine (such as check rings, etc.), leaving cleaning dead spots. What's more, cleaning agent is very good at removing carbonization (black spots) and masterbatch deposits.
Traditionally, there are three main ways of screw cleaning, namely:
• Mechanical cleaning: cleaning with particles such as glass or minerals.
• Chemical cleaning: Use chemical reagents for cleaning.
• Mixed cleaning: Use a combination of mechanical and chemical cleaning.
In general, although mechanical cleaning methods can be very effective, they can contain particles that can trap or clog important parts of machine, causing blockages such as hot runner valve needles. Chemical cleaners, or foaming or expanding cleaners, may be more effective at cleaning "dead spots" that are common in some screws and components.

 

To maximize cleaning effectiveness, following basic guidelines need to be followed, namely:
• Make sure barrel is completely filled with cleaning agent.
• Once wash begins to flow from machine, make sure screw speed is between 70 and 120 rpm. Use a short blast cleaning method to clean check ring and nozzle throughout process.
• During cleaning process, stop rotation of screw in time, so that cleaning agent can enter any dead angle of screw.
• Make sure barrel temperature is set at temperature of material currently being removed for cleaning.
By varying rotational speed of screw and material flow rate, a highly fluctuating environment of pressure and agitation can be provided for cleaning process, so that cleaning compound can work optimally until cleaned material is free of obvious contaminants.
Periodic cleaning procedures or preventive cleaning procedures should be established to avoid accumulation of contaminants.
As a best practice, a routine or preventive cleaning process should be established for regular cleaning to avoid build-up of contaminants. Benefits of preventive cleaning for injection molding plants are mainly reflected in following aspects:
• Reduce waste.
• Improve machine utilization.
• Reduce downtime.
• Reduce resin consumption.
• Increase productivity.

 

Leaving material in barrel during extended downtime (such as a holiday) is not recommended due to use of normal resins which will oxidize over time, resulting in carbide contamination. It is recommended to use a screw cleaner for pre-shutdown cleaning.
After thorough cleaning of screw using mechanical or hybrid cleaning, it is strongly recommended that machine be subjected to a barrel sealing procedure, i.e. shutting down machine after filling barrel with cleaning compound to minimize material oxidation or carbon build-up, especially with non-glass filled cleaning compounds.
Implementing a preventative screw cleaning program is critical to improving efficiency and profitability of a plastics processing facility. Whether it’s black spots due to resin degradation, or color contamination left over from previous production runs, most cosmetic issues are related to a lack of good screw cleaning procedures.

Incorporating appropriate screw cleaner into preventative maintenance cleaning process can greatly reduce carbon deposits, eliminate possibility of resin and color cross-contamination.
Following preventive screw cleaning procedure is recommended for injection molding and consists of following four steps:
1. Clean screw and barrel with a mechanical or mixed cleaning agent. The first step is to remove contamination and buildup from screw and barrel.
2. Intermittent cleaning before, during and after production runs. This step varies by resin and color. Cleaning with a chemical or mixed cleaning agent every two to three days is very necessary.
3. Seal barrel with a non-glass cleaner for extended downtime, this step is great for removing oxygen from barrel (eliminating any possibility of oxidation) and preventing any carbon buildup that may have occurred in initial stages.
4. Clean hot runner system and mold. After screw and barrel have been systematically cleaned, it may be necessary to increase cleaning temperature to help clean dead areas of these components.

Following are key cleaning procedures to follow during downtime:
Usually, there is no need to modify RPM or temperature settings after a thorough cleaning.
• Make sure all auxiliary equipment is disconnected.
• Clean hopper and feed port.
• Empty barrel so that no resin remains in barrel.
• Add one barrel volume of cleaning agent. Typically, only capacity of one barrel is required for a normal cleaning process; older equipment and more difficult-to-clean resins may require more cleaning agent.
• Continue cleaning until clean and free of any contamination.
• During this process, cleaning agent will undergo a thermal mixing reaction, decomposing color and degrading polymers in barrel and on screw.
• For some difficult-to-clean materials, a cleaning agent soaking procedure may be required. In some cases, slightly lowering barrel temperature and allowing cleaning agent to stay in barrel for a while will result in better cleaning results.
• After barrel is completely cleaned, fill the entire barrel with cleaning agent, stop screw rotation and turn off all heaters on machine.

Following are the key procedures to be followed when starting up injection molding machine:
• Turn on machine and set operating temperature to specified temperature.
• Do not rotate screw until heated to operating temperature and proper soaking has been achieved.
•Follow other start-up procedures required by process.
• Once operating temperature is reached, rotate screw slowly.
• If material coming out of machine contains any contaminants, rinse machine with more cleaning agent until it is thoroughly clean.
• Add resin required for normal production operation and rinse remaining cleaning agent.
• Get machine ready and start production.
By following these procedures, turning machine off and on is easy and can start normal production as quickly as possible after turning it on.

Considering that no two cleaning agents are the same, here are key pieces of information you should get from your cleaning agent supplier:
• Make sure to select appropriate cleaning agent for material and machine used.
• Train operators on correct methods to ensure desired cleaning results are achieved.