There are many different variations of blow molding, but all can benefit from the use of commercial purging compounds.
In order to fight against color and carbon contamination, blow molding machines require the appropriate type and grade of purging compound and processes. Following specific purging tips and recommendations will help maintain high-quality products and minimize total purging costs.
Commercial purging compounds contain a base resin and other additives engineered to clean your screw, barrel and die. The two main types of compounds used are mechanical and chemical.
The Basics Of Mechanical Purging Compounds
Most mechanical purging compounds rely on shear force in the barrel with simple physical adjustments of the machine and differences in material viscosities to clean machines. Some mechanical purges also rely on material affinity - they are attracted to carbon and other resins but not to metal so that they don’t stick to the screw and inner wall of the barrel. Unlike other types of compounds, mechanical purging compounds do not require time to soak because they do not clean by chemical reactions.
Unlike other types of purging compounds used specifically in certain environments, mechanical purges are effective across a wide variety of applications.
The Basics Of Chemical Purging Compounds
Unlike mechanical types, these purging compounds clean your machine by the chemical reaction that occurs in the barrel and/or die when the purge is exposed to heat in your barrel. This kind of purge breaks the chemical bonds of the contaminants to help get rid of them.
Chemical purges require soak times of between five and 30 minutes, and often are aided by the inclusion of foaming agents.
Chemical purges are often more effective than mechanical purges in low-pressure and low flow environments.
How To Choose The Right Blow Molding Purging Compound
To properly clean your blow molding machines, you must use the proper purging compound. First and foremost, the compound should have a similarly low melt index, which is better for resin compatibility.
The following are typical melt indices (MI or MFI) for blow molding resins:
- LDPE: <2g/10 min
- HDPE: <1g/10 min
- PP: <2g/10 min
If you’re using different resins, you may not necessarily use the same purging compound. You must purge between resin changes with the appropriate compound to completely clean your machine and prevent contamination.
How To Make The Most Of Purging
To prevent layering of resins and contaminants in your barrel and head, reduce defects such as black specks, off-color or thin spots, and minimize costly screw pulls, purging cannot be a reactive measure.
Proactive maintenance is critical to maximizing your efficiency and profitability.
Sealing your screw and barrel eliminates the risk of degradation, which creates havoc (and kills your profitability) upon startups. Even throughout your production periods, you must periodically purge to protect against contamination.
Deposits that are just beginning to build are removable, but if they’re not purged soon enough, carbon forms and adheres to the metal. Without preventative purging, the carbon is encased by subsequent resin and will break off and contaminate your product.
With the proper purging compound – used as part of a proactive, preventative maintenance program – your blow molding machines are equipped to continuously produce high-quality products and help you maximize your bottom line.
Talk to an expert to help you determine the proper purging compounds and processes for your business.
Comments