Black Specks Injection Molding: Why Do They Occur?

by | Injection

black specks

Black specks in plastic are one of the most stressful defects for injection molding or extrusion technicians, as they may or may not have a pattern of behaviour.

We know that these black dots indicate a certain level of degraded material within the system.

It is crucial to thoroughly analyze the causes that generate this degradation process to be able to evaluate the best procedure for its correction.

We can narrow down the origin of black specks to the following causes.

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Black Specks Injection Molding: Polymer Overheating

Under normal conditions, with the parameters of temperature, injection speed, pressure, etc., we should not expect an overheating of the polymer if this is well set up.

Usually, the overheating process occurs on start-ups after a long stop.

The issue worsens when working with sensitive materials or plastics with very different process temperatures in the same machine.

Plastic is a poor conductor of heat. To reach the process temperature during preheating, at start-up, the layer of plastic close to the cylinder walls is heated well above the inner layers, favoring material degradation.

Black Specks Injection Molding: High Shear Rates

High shear rates are another cause of black specks in plastic.

Due to significant variations in back pressure and screw speed, a considerable increase in shear can occur, with consequent degradation of the polymer chains.

Black Specks Injection Molding: 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 specks in plastic.

Black Specks Injection Molding: 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.

Black Specks Injection Molding: 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.

Related article: The Two Main Causes of Black Specks during Production

Why does the presence of black specks in plastic increase by 60% on Mondays?

It is not uncommon for black dots or spots to appear after equipment has been off for the weekend.

The Cause

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.

The solution:

There are two ways to do it, let’s see how.

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

If we know the origin of the problem, the solution is quite simple: just clean it before the machine stops and leave a virgin high resistance material 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 eradicate the problem?

The most significant gain is achieved by integrating the use of a Purge compound in the production processes within 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.

Avoiding the appearance of black specks in plastic and assuming it as a strategic and competitive advantage translates into less stress for them and an increase in the productivity of the Company.

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