Have you ever heard of zone zero on an extruder? Do you know where it is? It is the feed throat. This is an area that most extruder manufacturers do not consider a barrel zone. Yet all material entering the extruder must first pass through the feed throat. The design and temperature control of the feed throat have a great deal to do with extruder output and energy efficiency. In a previous column, I discussed feed throat design and how it affects extruder performance. Now let's focus on the thermal aspects. The same principles of solids feeding that occur in the barrel immediately adjacent to the feed throat also apply to the feed throat itself. That is, the polymer must adhere more to the barrel than to the screw in order to move forward. The most effective way to increase the polymer-to-barrel friction is to transfer heat into the barrel wall. As I said, all material passing through the extruder passes first through the feed throat, so this can become a control point in the process.
As a processor, the only way to maximize feed throat performance is to adjust its temperature. But it is not as easy as it sounds, because the only control device usually supplied by the machine builder is a coolant flow valve, which is often not designed for precise flow control. Additionally, coolant temperature and flow rate often change throughout the day as plant demands or even outside temperatures change. Consequently, the feed throat temperature can change without any operator adjustment. You can solve this problem by adding a modulating valve as a flow-control device, controlled by a thermocouple attached to the feed throat. This does not necessarily optimize feed throat performance, but it does keep it at a fixed set point.
Have you ever seen the situation where, during a long screw stoppage, certain polymers melt in the feed throat while the barrel heaters are still at operating temperature? To avoid this, most people run full coolant flow to the feed throat. They do not think of the feed throat as part of the process, but as an independent device separate from it. That is simply not correct. Far from it. Not only does the feed throat determine the feed rate, but overcooling the feed throat continuously draws heat out of zone 1, consuming as much as 20% of the total drive power. The feed throat has a metal-to-metal connection, and this type of connection is very effective in heat transfer. Because the first zone thermocouple is located in the barrel some distance from the feed throat, a temperature gradient is set up between the zone 1 setting and the feed throat. That gradient further affects the feed rate because it cools the barrel at a location critical for developing solids feeding, solid-bed compaction, and initiating melting.
Figure 1 Effect of overcooling on feed rate
The performance of extruders that do not use a separate feed throat — that is, the barrel extends through the feed throat jacket — further illustrates this. Because the thermal conductivity of the barrel is much higher than that of the barrel-to-jacket-to-coolant path, the barrel inner wall temperature is maintained at a higher level. As a result, the barrel inner wall temperature at the feed throat is substantially higher. In tests using the same screw, this design increased output by as much as 23%. The main disadvantage of the jacket design is that polymer melts faster in the feed throat and on the screw, so it requires more operator attention during screw stops. Nevertheless, it shows the positive effect of a hotter feed throat on output. I have also found that extruders with this design generally have more stable output.
A hotter feed throat improves output and stability
For most polymers, I like to start with a feed throat that is warm to the touch (110 to 120°F) and then gradually reduce coolant flow, thereby increasing feed throat temperature, until output no longer increases. This takes a while because the feed throat changes temperature slowly due to its mass and heat transfer to the gearbox, hopper, and resin. Each polymer, extruder, and operating condition requires a slightly different feed throat temperature to optimize output — but generally, the hotter the better.
