BLOW MOLDING: The Role of Screw Design in Industrial Blow Molding
Know How: Blow Molding
Screw design is seldom discussed when buying a new machine. In fact, it should be near the top of the list.
The feed screw is the heart of the blow molding system. All of your processing preparation is developed in the extruder. If the temperature is not at the proper setpoint, or if the material is not properly mixed by the time it is discharged, you won’t be able to do much about it in the accumulator head. (That said, a good heating system on the accumulator head is also important to maintain a constant temperature.)
Many blow molding machines have feed screws improperly designed for the specific material they are running. In fact, screw design is seldom discussed when buying a new machine. That’s unfortunate, as it should be one of the top items on the agenda. The right screw imparts less shear on the material, which results in lower melt temperatures.
Screw design is the first step in good parison formation, and a major move toward achieving lower overall cycle times. Screw speed should be variable to allow for different resins, parts, and operating needs. Variable speed control will also give fine output adjustment.
Industrial blow molding machines tend to be very large, which makes it difficult to pull the screw from the barrel for maintenance and changes. In most cases the accumulator head—or melt pipe in the case of dual heads—leaves very little or no room to remove the feedscrew.
Because the main frame is very high, it’s difficult to get the proper moving equipment in position to pull the screw out of the barrel. An overhead hoist would be ideal for moving head(s) over or off the machine. Once done, the hoist can be used to remove the feedscrew from the extruder barrel. The reverse procedure will also use the hoist for reassembly.
All this time, effort, and money is well worth it. Too often, processors that make material switches instead ask their machine builders to supply so-called general-purpose screws that allow for processing of several different types of materials without needing to change the screw. While this sounds like an ideal solution, it really isn’t.
So how does an industrial blow molder go about choosing a screw design? First, you’ll need to research the material(s) you’ll actually be using on the machine. For the sake of discussion, let’s say HDPE with 0.3 melt index will be the material you’ll be running most frequently on your new industrial blow molding machine.
You also must consider the amount of flash regrind you’ll be running. On average, you’ll be running a mixture of 40% to 60% regrind, with the rest virgin. I have seen up to 100% regrind being used for some applications. In any case, you’ll want to keep the ratio of regrind to virgin as consistent as possible. Processing becomes very touchy if the percentage of regrind keeps changing, as this will have a direct effect on maintaining a steady process and constant melt temperature.
In our scenario, the extruder will have a 4.5-in. screw diameter and 30:1 L/D and is rated at 1100 lb/hr for our 0.3-MI HDPE. You should be able to reach this output at 80% of the screw’s maximum speed. This unit will have a smooth-bore feed-throat. When you are running a lot of scrap, this is a better option than grooved feed sections. The scrap should be ground to 8 mm.
In the picture I have painted above, you’d be wise to have the screw designed for the HDPE specified. But in many cases, blow molders also want to run PP in the same machine, so they’ll opt for a general-purpose screw “optimized” to run both materials. In reality, though, this will lead to compromises and process variations.
Specifically, you’ll experience lower outputs and higher melt temperatures. The net result will be an increase in cycle time for both materials.While both materials can be run on barrier screws—with perhaps a spiral mixer at the discharge—on average you’ll get 35% less output when running PP than with HDPE if you use the same screw. This gap in output would be closer if you had two different screws, each designed for one material only.
I have run into quite a few processors that also want to run engineering materials such as ABS or glass-filled PC using a general-purpose screw. This will adversely affect the outputs and cycle times of each material. In fact, running engineering-grade materials on general-purpose screws will cause even more issues than you’ll experience running HDPE and PP on one screw.
Once you got the right screw it’s a good idea to get a footprint of its dimensions. You can refer to this footprint as a starting point to determine screw wear. If you notice a decrease in output, the most logical cause would be a worn screw or feed-throat section. Check them both at the same time. It would be wise to harden the inside of the barrel and the outside of the feed screw. Your material supplier should be able to provide some advice on what type of hardening would be best.
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