In both extrusion and injection molding applications, melt quality largely dictates the success of an operation. Total output or cycle time is critical, however, if part quality is sacrificed in the pursuit of high throughput, there is little opportunity for successful molding – especially in a competitive global marketplace. There are a number of attributes that dictate melt quality: melt temperature, target viscosity, and adequate dispersion. Poor dispersion is a very common quality issue that resin processors encounter, and is very easy to diagnose – even with an inexperienced processor. The most classic example of poor dispersion is color swirling in the final molded part.

Color Swirls

In the case of color swirling, the colorant is simply a visual marker for poor melt quality. If the colorant is removed from the feedstock, the same poor melt quality exists, just without an obvious visual marker that invalidates the final product. The non-isothermal melt is the problem, color swirling is merely a symptom. The non-uniformity can stem from many sources, for example, the lack of proper mixing elements in the screw design, dead spots, or areas of low flow for material hang up, screw and barrel wear, and more. As important as uniform color dispersion is, this is more of a side-effect of a high quality, isothermal melt. Again, if the color concentrate is removed from the feedstock, the swirling defects still occur – just without an obvious visual marker.

General Purpose Screws

With poor dispersion comes variance in melt temperature, and naturally variance in viscosity and pressure. The commonly used “general purpose screw” is a main source of concern with these types of dispersion issues. With all general purpose screws, there is little ability to remedy the effects of solid-bed-break-up. Due to solid-bed-break-up and the insulating characteristic of polymers, and the natural helical flow pattern within the screw channels, at best there is a non-isothermal melt, leading to in-molded stress in the final parts, etc. At worst there can be un-melted solids found at the discharge end or conversely over-heated degraded resin. A screw’s ability to accurately establish and control a target viscosity and melt temperature is generally more indicative of good design than high throughput. Many molders unfortunately disregard the physical properties of molded parts. In the case of many injection molding operations, the primary concern is to fill the mold cavity, and the easiest way to do this is to increase melt temperature – higher rpm, higher backpressure, higher barrel temps, etc. This makes fill easier and fast. Increasing melt temperature has a myriad of consequences that are easily and often overlooked, or rarely considered.

• • Material degradation
  • Part specification violation
  • Mechanical or performance failure

It is abundantly clear that color swirling is an unacceptable part defect that must be remedied. However, in the pursuit of an overall successful molding operation, proper color dispersion is a given. To highlight this concern, we can look to a very common occurrence – many molders opt for poor quality general purpose screws when processing resins with no colorant, because after all, there is no opportunity for swirling when there is no colorant, so why bother with mixing? To reiterate, proper mixing is critical – regardless of colorants or additives or lack thereof. Adequate dispersion and distribution dictates the quality of the extrudate, naturally dictating the success of the operation.

A good starting point for any resin processor is to investigate whether the proper screw is being used for the application in question. The screw is the workhorse of the entire manufacturing process. It controls output and quality. When we look to process around the wrong equipment, at best we waste valuable time and assets, at worse we simply fail to complete the project as quoted. One cannot solve an upstream problem by inspecting a downstream result. To avoid the expensive headache of color swirling, or more importantly overall poor melt quality, every processor should seek out the very best screw assembly for every application. Although specialty screws can be expensive, the gains in output, quality, stability, repeatability, processing window, and wear life generate a very healthy return on investment. In many cases, what is accepted as a “good process” can be transformed into a great process by investigating the choice of screw being used.