The key to optimum profitability in extrusion is pretty simple: Run a quality product at the minimum material specification while maintaining a maximum line rate. But achieving this balance—and the payoff it provides—is tricky because there are so many factors to manage simultaneously, including:

- Material mix
- Extruder output
- Extrudate fit in sizing tooling
- Vacuum level in cooling tank
- Secondary cooling
- “Hot” and “Cold” dimensional measurements
- Puller speed

Ultimately, your goal is to get the extruder output aligned with the puller speed downstream. But getting line speed “right enough” to produce an acceptable product is different from getting it optimized for quality and output with minimum material consumption. Optimizing output using typical downstream feedback pose challenges because the feedback loop – from the puller back to the extruder outlet—is so long.

Monitoring this feedback loop is complicated because the extruder output fluctuates constantly, most commonly because of the changing condition of the extruder’s screen pack. As this screen filters out impurities or solids in the melt, it gradually becomes blocked, resulting in steady reductions in extruder output. Other reasons for diminished extruder output include screw and barrel wear or occasional fluctuations in temperature, material viscosity, and backpressure.

Regardless of what causes the extruder output change, the operator or control system has to respond. Yet, there are often not enough experienced operators to make all of the right decisions all of the time. For example, the typical operator response to reduced extruder output is to decrease the speed of the puller to compensate for the reduced flow. However, slowing the line costs added production time and labor expense. And then, when the extruder screen pack is changed to allow full flow, extruder output rises again and requires the operator to boost puller speed.

If you’re reliant on operator skill – or an aging control system – this can be a constant problem, making you wish you could shorten or sharpen this costly control loop.

#### The Basics of Gravimetric Control

One new solution is to move control closer to the extruder, using gravimetric material control. Instead of relying on a downstream control system to continually respond to small extruder output changes, this method eliminates line-speed guesswork by directly monitoring material flow through the extruder and making downstream line speed adjustments automatically.

Gravimetric control of extrusion begins by replacing the traditional material hopper at the top of the extruder with a more compact, lightweight gravimetric hopper – essentially a blender/hopper atop a load cell. In operation, this hopper works together with its controller to track extruder input by measuring the “loss-in weight” as the material enters the extruder for processing. This data creates one “anchor point” for the gravimetric control process. The other “anchor point” is at the end of the process – a calculation of the per-foot weight of product with optimum characteristics. Together, these two data points capture the essentials for precise, quantitative extrusion control: Managing critical product dimensions (ID/OD/Wall thickness) and product weight-per-foot.

#### Comparing Control Methods

Let’s look at a before-and-after to clarify the differences between conventional extrusion control and the newer, gravimetric method. Start with an ordinary extrusion line that is set up with proper equipment, tooling, cooling systems and the like.

**Without gravimetric material control.** The extruder is set at a given speed, estimated to produce a steady flow of extrudate through the die, through the sizing plate, and into the vacuum tank. As noted, extruder speeds and output typically fluctuate, causing the pounds-per-hour delivered out of the extruder to change constantly and gradually drop.

With a conventional control system, we know that any drop in extruder output immediately affects:

- Gauge – ID/OD/Wall thickness
- Yield – Lower line speed
- Layer ratios – Multi-layer dies

However, the ability to detect these faults—at the die—is limited because our quality-assurance mechanism – the end of our laser measuring loop – is located downstream. So, it takes some length of downstream product to even recognize the upstream problem. Then, to compensate, an operator (or a control system) may decide to ramp up extruder speeds and pressures in an attempt to maintain throughput or, more commonly, will decide to ramp down line speed to maintain quality at the expense of production time and yield.

The cost and impact of extruder performance fluctuations only multiply when production involves multi-layer pipe products. If output from any extruder feeding a multi-layer die falls off, then the adjustment becomes even more complex. The other extruder must respond with a comparable output reduction to maintain correct material ratios through the die, while the puller speed must also slow proportionately. If both adjustments aren’t prompt and correct, scrap product and a line shutdown are the result.

**After gravimetric material control. ** Gravimetric material control is all about ensuring a steady flow of material through the extruder. Periodically, the blend hopper is precisely refilled and weighed, and then ongoing loss-of-weight data is used to track the material throughput of the extruder in pounds per hour.

Once the computerized control knows the precise, real-time output of the extruder, as well as how much material equals an optimal foot of product, it is easy to calculate and perform real-time puller speed adjustments with precision. The control immediately knows when and how extruder output is changing and can instantly boost extruder speed or pressure as needed to maintain the proper rate of flow (lbs/hr) to the die. The result: optimized quality and output.

Once line-speed variations are stabilized using gravimetric material control, other extrusion controls – for vacuum, for dimensions, for cooling – are free to self-adjust within relatively narrow quality parameters.

#### Calculating Product Weight per Foot

This math can look a little daunting at first, but if you get confused, ask Conair for help. Let me walk you through an example, calculating the weight of one foot (12 inches/30.38 cm) of 1-inch OD (2.54 cm) polyethylene pipe, with a 0.75 inch ID and a 0.25 inch wall thickness.

Start by calculating the pipe volume. Do this by calculating the rod volume (volume of a solid pipe) minus the volume of the ID, which is hollow, as detailed in the illustration below.

Since we are calculating in metric, the result will be in cubic centimeters.

Now that you have the volume of one foot of pipe, you can calculate its weight per foot based on the weight of the material (grams/cc), then convert to pounds/foot. In our example, we’re using polyethylene (0.95g/cc).

Now, convert grams/foot to pounds/foot (214.63g/ft = 0.473 lbs/ft).

Finally, convert pounds/foot to pounds/hour, based on the line, or puller, speed. Note that puller speed is determined based on the cooling capacity of the tank system used in the application. This speed is entered as a constant speed by the operator, based on quality control specifications for the line.

#### Summary of Calculations

**Gravimetric system sets the extruder LB/hour automatically – How does the TrueWeigh do this?**

Given: Pipe Outside Diameter 1 inch, Wal thickness 0.25 inch, Inside diameter 0.75 inch and length is one foot. Puller speed desired is **25 feet per minute**.

Proof:

Rod Volume = O.D. X Pi r squared X Length

Pipe Volume = (O.D. X Pi X r squared) – (I.D. X Pi r squared) X Length

Pv = (2.54 x 3.14 x 1.6129) – (1.905 x 3.14 x 0.9072) x 30.38 cm

Pv = (12.863cm – 5.426cm) x 30.38cm

Pv = 225.93 cubic centimeters

Weight/Foot – Pv x grams/cc (where grams per cc of PE is 0.95)

Weight = 214.63 grams per foot = **0.473 lbs/foot
**LBS/Hour = 0.473 Lbs/Foot X 25 Feet/Minute X 60 Min/Hour

**LBS/Hour = 709.5**

#### Conclusion

Optimizing extruded product output using a typical feedback loop poses challenges because the feedback loop – from the puller back to the extruder outlet—is so long. Gravimetric extrusion control eliminates guesswork and improves precision by determining the material flow per foot required to produce optimal output, monitoring actual material flow through the extruder, then adjusting puller speed accordingly. This control method virtually eliminates the huge swings in material flow, dimensionality, vacuum, or line speed that can lead to over-adjustment, excessive scrap, or line shutdowns, resulting in improved control, higher product quality, and greater profitability.

Are you interested in learning more? Check out Alan Landers’ recent webinar on Gravimetric extrusion control for pipe and tubing production.