Definition: Melt: A material, solid at room temperature that has been heated to a molten condition. Uniformity: 1: the state or quality of being uniform: overall sameness or homogeneity; absence of variation. 2: something that is uniform
Melt uniformity is a termed used to describe the melt of a process whether it is injection molding, extrusion, blow molding and or any process which tends to melt or soften a plastic material prior to shaping it.
In melting and or softening the material (semi-crystalline or amorphous) the correct melt temperature needs to be known. Most manufactures list their recommendation as to barrel settings and also their melt temperature suggestions. Usually there is a range; some may be 100 degrees and others only 10 to 20 degrees. It is up to the processors to realize where they should be. A good starting point for melt temperature is to start at the mid point of recommendations, it can than be adjusted, for process conditions.
In establishing the melt temperature and or range, set the barrel heats at manufactures recommendations and or to temperatures you have established so as to achieve the desired melt temperature. Does this now establish a uniform melt? That is the question that must be considered. In processing or purging out the material how does it look? Lumpy, cold what is the actual melt temperature as taken with a pyrometer? Understand that it takes a uniform cycle and time to establish a melt temperature that is consistent.
Uniformity is a mixture, meaning that all the material being processed is treated and or uniform in regards to its history and uniformity of mix and shear. But what causes this to be true? How can one achieve this uniformity?
That comes from understanding the screw, both the L:D ratio, compression ratio, wear factors and if molding or extruding any other design features to the screw configuration. It also comes from understanding any additives being added into our mixture, such as but not limited to color concentrates, UV additives, regrind, and or any other components.
In extrusion for example, it being a continuous/constant process of placing material at the throat and pumping it through and out the end of a die. This process is simple yet not, does the material have enough time in the barrel to melt, and or mix? If I do not work it or shear it enough well it all melt or soften uniformly? What screen pack is being used, and in what sequence? What are the screw configurations and RPM?
An example; At an extrusion trail for a TPO which was based on polypropylene, a semi-crystalline material the results were non uniform melt, resulting in unmelted pellets and poor performance. The extruder was originally used to process rigid PVC and was designed for this. It had a low compression ratio, short L:D ratio and used no screens in the breaker plate region. Though the material melt temperatures were correct the melt uniformity was not there.
In injection molding which uses an interrupted process where the screw moves back and forth and the shot size (barrel usage) can vary. Typically the L:D ratio is shorter than extrusion, but compression ratios are similar. But uniformity is dependant on where screw usage is set in comparison to the shot capacity of the barrel. Screw configuration and back pressure also influences the uniformity of the melt.
An example would be color mixing. If the screw is only rotating for 2 or 3 revolutions per cycle to create the shot size than a uniform color within the material is not really achieved with a standard color concentrate. (Note using a powder, micro pellets etc. can over come these issues). The screw is not rotating enough to mix and shear the material into a uniform mix, most machine / material manufactures typically recommend utilization of 25 to 75% of the shot size rating of a press. When falling out of this range the chance of a non uniform melt exists even with everything within manufactures specifications.
Uniformity of the melt is critical to achieve a consistent product produced and in most processes is not thought about, until finding breakage, and or defects that it than becomes an issue.
SL Silvey
360-882-3183
