Extrusion blow molding is one of the most demanding plastic processing environments. From large industrial drums and garbage cans to automotive fuel tanks and bulk containers, these parts require thick walls, consistent parison control, and clean melt flow. Most of these applications run on accumulator head machines from manufacturers such as Davis-Standard, Uniloy, Kautex, Jackson Machinery, or R&B Plastics.
When contamination appears in extrusion blow molding, it can quickly become expensive. Die lines, burn streaks, color contamination, and surface defects often originate from a single source: the accumulator head.
If you run HDPE, PP, or PPO in an intermittent extrusion process, proper purge management can significantly reduce scrap and downtime.
Why the Accumulator Head Is the Biggest Challenge
Unlike injection molding or continuous extrusion, accumulator head blow molding is a low-pressure environment during melt storage. Resin collects in the head before being pushed out in one large shot. That low-pressure zone makes it difficult to generate the shear needed to scrub residue from internal surfaces.
Polyethylene compounds, especially HDPE, tend to layer inside the accumulator. Over time, degraded resin builds up in dead spots. When the machine cycles, that contamination breaks loose, causing:
- Burn streaks
- Black specks
- Color streaking
- Die lines in finished bottles or tanks
Because most accumulator dies are not polished, glass-filled mechanical purging grades are typically safe to use and often necessary for proper cleaning.
Step One: Clean the Extruder First
Before focusing on the accumulator head, always clean the extruder barrel and screw. If the screw is still contaminated, you are simply feeding new residue into the head.
Using an aggressive mechanical grade such as Asaclean® EX, or a chemical grade like NCR with strong foaming action, ensures proper barrel cleaning before moving downstream.
In accumulator blow molding, skipping this step almost guarantees recurring contamination.
The Correct Way to Purge an Accumulator Head
Purging an accumulator head requires a controlled, staged approach to generate proper scrubbing action.
Once the extruder is clean:
- Fill the accumulator to 100 percent and push out 25 percent.
- Fill to 100 percent again and push out 50 percent.
- Fill to 100 percent again and push out 75 percent.
- Finally, fill to 100 percent and purge the full 100 percent.
This progressive cycling allows the purge compound to contact internal surfaces multiple times, dislodging layered polyethylene buildup and carbon deposits.
Rushing this process often leaves residue behind, which reappears during production.
Use the “Disco Purge” Technique
Accumulator heads respond well to what operators call a “disco purge.” This involves varying screw speeds, stopping and restarting rotation, and adjusting back pressure when possible.
Changing screw speed increases shear and improves the mechanical scrubbing effect of the purge compound. In low-pressure accumulator zones, this technique can make the difference between partial cleaning and complete cleaning.
When dealing with stubborn contamination or color transitions, combining aggressive mechanical scrubbing with screw speed variation significantly improves results.
Selecting the Right Purging Compound
Accumulator heads require purging compounds specifically designed for difficult environments.
The key is matching the purge grade to the contamination challenge.
Do Not Forget Shutdown Sealing
Extended downtime events, including maintenance shutdowns or seasonal plant closures, create another contamination risk. Resin left inside an extruder or accumulator head can oxidize and degrade, creating severe streaks at startup.
To prevent this, seal both the extruder and accumulator head with a shutdown-approved grade of Asaclean® before turning off the machine. Proper sealing protects internal metal surfaces and dramatically reduces startup scrap.
Cleaner Machines, Better Bottles
Accumulator head extrusion blow molding machines require a deliberate purging strategy. Insufficient cleaning leads directly to die lines, burn streaks, and costly production delays.
By cleaning the extruder first, using staged accumulator purging cycles, applying the disco purge technique, and sealing properly during shutdowns, processors can:
- Reduce startup scrap
- Minimize color change downtime
- Prevent carbon buildup
- Improve overall bottle quality
For large drum and fuel tank producers running HDPE or PP, a structured purge program is not optional. It is essential to maintaining profitability and product consistency.
If your team is dealing with tough resin transitions or temperature swings, we can support you with a customized purge protocol. Request a free sample or schedule a consultation with one of our purging experts today.
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