Black dots are one of the most stressful defects for injection molding or extrusion technicians, as they may or may not have a pattern of behavior.
We know, without a doubt, that these points indicate a certain level of degraded material within the system
It is important to thoroughly analyze the causes that generate this degradation process in order to be able to evaluate the best procedure for its correction
Basically the black dots that appear on the pieces injected have the following origin:
overheating Under normal conditions, with the parameters of temperature, injection speed, pressure, etc., well defined, an overheating of the polymer should not be expected.
Usually this happens on starts, after a long stop.
The issue becomes more complicated when working with sensitive materials or when plastics with very different process temperatures in the same machine
Plastic is a poor conductor of heat and to reach the process temperature during preheating, at start-up, the layer plastic close to the cylinder walls is heated well above the inner layers, favoring material degradation
- High shear rates
Due to large variations in back pressure and screw speed, a considerable increase in shear can be generated with consequent degradation of the polymer chains.
- Residence time
An increase in the residence time of the polymer within the system is likely to generate, among other problems, the presence of black spots.
- Material degradation by negative flow inside the barrel.
A faulty behaviour of the check valve can cause, among other things, negative flow zones with the consequent increase in residence time and subsequent degradation of the polymer
- Degradation of additives
Some additives are very sensitive to temperature; Unlike the blackheads produced by the aforementioned causes, these occur spontaneously and can be controlled relatively easily by reducing the temperature curve or sometimes by changing the provider.
- Why does the presence of blackheads increase by 60% on Mondays?
It is not uncommon for black dots or spots to appear after equipment has been off for the weekend.
Overheating of the polymer during start-up
When the machine stops, despite emptying the screw, the material remains attached to the screw and to the barrel jacket.
During preheating, at start-up, the degradation process of the material begins, which will subsequently generate black spots
There are two ways to do it
- Subsequent cleaning
There are good purging products on the market whose formulation incorporates abrasive charges to deal with the problem of the black points once generated.
Depending on the severity of the problem, they will be more effective or not.
- Preventive cleaning
Knowing the origin of the problem, the solution is quite simple: just clean it before the machine stops and leave a virgin material with high resistance in the spindle to degradation, such as HDPE or PP.
Why wait until you have defective parts to use a purging compound when we can eliminate this situation from the root?
The most significant gain is achieved by integrating the use of a Purge compound in the production processes within a routine preventive maintenance of the machine.
Today, most of the technicians involved in the production of plastics are increasingly aware of the importance of using a good purging compound for changes of colour or material and to avoid the appearance of black spots and assume it as a strategic and competitive advantage that translates into less stress for them and an increase in the productivity of the Company.