How to Reduce the Burden of Plastic Scrap and Maintain Product Quality

A certain level of scrap or unacceptable parts is inevitable in any process. Whether it is the 3.4 occurrences per million units envisioned by Six Sigma quality adherents or a much higher percentage, it represents a drain on productivity and profitability, and keeping it to the absolute minimum possible should be the highest priority.

Many plastics processors accept scrap as a necessary evil and focus on handling it efficiently, running it through granulators and returning it to the process in a form that ensures they can reuse as much as possible while continuing to make good parts.

To achieve this objective, it’s important to keep process scrap and rejected parts clean and segregated by material and color, and then to make sure granulator blades are sharp and properly adjusted, and that screens are not heavily worn.

Monitoring these conditions for consistency – and making repairs as needed – will ensure uniform granulate, largely free of dust and oversized particles, so that the regrind processes, to the degree possible, like the virgin pellets with which it is blended.

Consistency is Key to Quality

Note the word “consistency” in the previous paragraph. While consistency and quality are not the same, a consistently good process is necessary to minimize or prevent scrap and to make good, quality parts.

Processors go to great lengths to determine the material and process conditions that result in consistently good parts. Of course, they identify the resin, additives and color that have acceptable processing characteristics and that will yield products that meet application requirements.

However, they also must make certain that the material is in good condition – properly blended, free of excessive dust, contamination and moisture – when it is delivered to the extruder or molding machine. Finally, temperature needs to be controlled during the molding and extrusion process.

Auxiliaries are Primary

Obviously, auxiliary equipment is essential to each of these critical process-control tasks:

Conveying

Conveying equipment, consisting of pipes, pumps, receivers and associated controls, is essential to getting the right materials from storage to the next step in the manufacturing sequence.

To prevent conveying the wrong resin to the process – which would surely result in scrap – operators need to be well-trained and supervised. And equipment that simplifies or automates the proper connections between source and destination, including rail-car and truck unloading systems, and resin-selection system (RSS) tables with automatic proofing, can lessen the chance of human error.

At the same time, the maxim “quality in, quality out” applies to conveying as well. The conveying system must be able to preserve the quality of the incoming resin, minimizing the generation of dust, angel hair and other by-products of high-speed vacuum conveying.

Adding dust collectors and angel-hair traps can remove most this contamination, but a variable-speed conveying system, which tailors conveying speed to the characteristics of the resin and throughput requirements, can actually prevent its formation almost entirely.

Blending

Blenders are the next quality-critical components in the process stream. Often in tandem with minor ingredient feeders, blenders receive and meter the different elements — usually virgin pellets, regrind, colorants and additives — that make up a plastic material recipe.

That recipe is developed to ensure that the correct aesthetic and performance qualities make their way into the molded or extruded part. In today’s blenders, gravimetric metering has largely supplanted volumetric methods, resulting in higher levels of accuracy.

Newer control algorithms use a feed-forward dispensing approach to increase accuracy even further. Instead of making a single dispense of each ingredient and adjusting for discrepancies in subsequent batches, the control uses incremental dispenses in each batch, continuously weighing the material and adding a little bit more until the target weight is reached.

This approach ensures that all materials are accurate to 0.025% of the batch recipe. Without the benefit of today’s advanced blending technology, processors would be hard-pressed to maintain recipe consistency – and prevent production of off-spec and scrap parts — over protracted production runs.

Drying

Dryers are arguably the most critical systems when it comes to material consistency and are especially important when processing engineering resins – nylon, polycarbonate, ABS and the like. That’s because these materials are hygroscopic. They naturally adsorb moisture from the air and, if that moisture is not removed before the polymer is processed, it will flash to steam causing streaks, bubbles, burning, brittleness and other defects in critical molded or extruded parts.

Because water in a plastic pellet becomes bonded to the polymer at the molecular level, removing it is difficult, requiring careful attention to four fundamental parameters: heat, low dew point (dryness), air flow and time.

Pellets in the drying hopper must be heated to a temperature specified by the resin manufacturer and surrounded by air that is extremely dry, typically with a dew point around -40°F, and held there for up to eight hours (depending on the material) in order to break those molecular bonds.

In the drying process, air is passed through a desiccant to extract all but a tiny percentage of its moisture and then it is heated to lower its dew point even further. As the air rises through the material bed in the drying hopper, it heats the pellets and its low dew point creates a pressure differential that causes water to migrate out of the pellet and into the air so it can be carried away.

By monitoring the temperature and dew point of air as it enters and exits the drying hopper, processors can confirm that conditions for proper drying are maintained. Even better, a probe can be inserted into the hopper to actually measure the temperature at multiple points in the material bed. Along with dew-point monitoring, this technique is the ideal way to ensure material consistency and minimize scrap.

The other critical dryer component is the desiccant and there are various approaches to how drying air is exposed to it. However, for maximum dew-point stability – required for enabling a consistent, low-scrap process — proven desiccant-wheel technology is far superior to loose-desiccant dryers.

Heat Transfer

Heat-transfer equipment plays a key role in creating the final conditions for production of a quality product. In molding and extrusion, the polymer must be melted by heat and shear generated by the screw and barrel of the processing machine. That heat must be carefully controlled and then removed in such a way that the plastic solidifies properly to minimize production of off-spec parts.

There are three main types of heat-transfer equipment: cooling towers and chillers produce cool water (generally between 40-80°F) that can handle many of the heat-removal tasks in plastics processing.

Creating and maintaining ideal temperature conditions in the most challenging applications, however, requires a temperature-control unit (TCU) or Thermolator®.

Used machine-side, TCUs deliver heat-transfer fluids (water, ethylene glycol for low-temperature applications or oil when high-temperatures are required) capable of maintaining temperatures from freezing or slightly below to 300°F or even higher. They are used to preheat injection molds to allow for complete filling and then they draw out excess heat so that the plastic cools at a precise rate that prevents unplanned shrinkage or distortion but still allows part ejection in the shortest time.

In extrusion, TCUs function in much the same way, keeping dies and sizing tooling at the proper temperature and filling cooling tanks for pipes, profiles and tubing. For cooling thick sections in profiles or pipe, TCUs may be used to deliver warm or “tempered” water that provides controlled heat removal to prevent scrap caused by uneven shrinkage, bowing, warpage or other distortion.

“Smart” Process Control

Consistent operation of the equipment discussed above is critical to producing quality products rather than scrap.

Each machine is equipped with the latest controls required to achieve that consistency and to troubleshoot situations where things go awry. Some of these controls – notably those available with blenders and dryers – can deliver trending information so that processors can confirm that straight-line conditions are maintained over time.

These same read-outs provide an early warning system, showing even small disruptions to smooth processing before they become serious enough to result in unacceptable quality. In cases where problems are not corrected in time, stored trend data can be used to investigate what happened and institute practices that avoid it in the future.

But what about equipment that doesn’t have controls that support process trending? What if you have more than one manufacturer’s equipment in your plant? What if you want to view process data from all your auxiliary equipment on one platform without walking onto the plant floor?

Well, Industry 4.0 has solutions that simplify and improve the way processors interact with all of their digital auxiliary-equipment data and helping them to use it to provide actionable information that can maximize productivity, improve quality and reduce scrap.

Thanks to such cloud-based platforms, anyone with an internet connection and the right security credentials can log in and see digital data about individual units or groups of machines, regardless of their location.  It’s no longer necessary to have expert personnel standing near the equipment in order to monitor and control auxiliaries in all those locations.

Instead, data is presented on user dashboards in the form of trend lines and charts so it is possible to know what is happening at any time.

Users can set up to ten customizable Key Performance Indicators (KPIs) that are most critical to the producing quality parts and then view them as trend lines at any time.

For instance, in dryers the most often used KPIs are setpoint and actual readings for dew point and drying temperature. In blenders, it’s setpoint and actual ingredient percentage for critical ingredients, and in TCUs, it’s setpoint and actual readings for to- and from-process temperatures. Processors can check these critical parameters in real time or view them historically. Or, they can set alarms to alert responsible people via email or text message if ever things begin to trend in the wrong direction.

What it takes to minimize scrap

With a solid understanding the features and capabilities of auxiliary equipment, and the latest Industry 4.0 tools that make the most of the information that those machines generate, processors can make certain that all their equipment operates together to maximize process consistency, therefore maintaining product quality and reducing the productivity/cost burden of scrap.