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5 Causes of Black Specks in Your Injection Molded Parts

Views:0     Author:Mike Shi     Publish Time: 2021-02-26      Origin:Site

The causes of impurities and black spots during the moulding process can be divided into internal factors and external factors.

External factors mean that there are already black spots and impurities in the cavity before the injection is started. The poor surrounding environment and irregularly maintained moulds are the reasons for this situation.

Internal factors mean that black spots and impurities will be generated in the cavity after the injection starts.

The reasons are bad maintenance of the injection moulding machine screw, material contamination and improper adjustment of process parameters.


1.Surrounding environment

* Impurities and black spots have a great relationship with the surrounding production environment. Nowadays, injection moulding factories are developing toward a dust-free workshop. Like the factory, I used to produces milky white home appliances. The surface quality requirements are strict and no black spots are allowed. After eliminating many reasons, it still can’t solve the problem of impurities. Trace the root cause and keep the surrounding environment clean. Related, the factory itself is a metal house with ventilated areas and an empty environment. In order to solve the problem, an iron frame was temporarily erected, and a plastic sheet was propped up outside to form a confined space. The impurity rate was greatly reduced. To

* There is a lot of dust in the sky side of the mould and injection moulding machine. When the manipulator is running, the gas blows the dust into the mould cavity.


2.The Mould

* The mould is used over its‘ service life, or the mould maintenance is not in place, the iron filings rubbed by the moving and fixed moulds, the thimble rubs off the iron filings (foam), and the slider rubs off.

* During the moulding production, the mechanic did not regularly clean the parting surface of the mould, resulting in excessive oil stains and material debris, which would contaminate the product


3.The Injection moulding machine

* When the injection moulding machine is processed and produced, the crushed material used contains metal substances and high-hardness materials for injection moulding. After a long period of operation, the surface coating of the screw, the non-return ring and the rubber head are bound to be corroded and damaged, and the material remains to Wear on the crevice. 

* If there are steps, unevenness and damage between the nozzle and the sprue sleeve, the nozzle and the screw thread, and the nozzle flange, the material will be stagnated and the material will be decomposed and carbonized. 

* During production, the nozzle and the sprue bushing are not evenly matched and the deviation is too large. If the paper shell, aluminium sheet and other conductive media are used to delay the material, but after the nozzle is heated at high temperature, the medium may melt and decompose, Impurities produced by injection into the product (remaining in the hot runner). 

* The heating ring is loose and the actual temperature far exceeds the set temperature. 

* The relay is damaged, it is always closed, and the barrel is overheated.


4. The process parameters


*The process parameters are set improperly, and the set temperature of the barrel is higher than the processing temperature required by the actual material.

*The heating ring is loose and the actual temperature far exceeds the set temperature.

*The set backpressure exceeds the backpressure required for processing, and the material is excessively squeezed.

*The height of the plasticized material in the screw is much higher than the amount of plasticization required for injection.


5. The Materials
* The gaps and dead corners were not cleaned up when the feeding barrel, feeding box, suction device, and mixing machine were changed. To
* When starting up, the screw is not flushed sufficiently, leaving behind the last processed material. 
* The heat-sensitive material produced last time. The material in the screw is not cleaned or replaced during the shutdown. 
* Sundries such as bag rope, masterbatch bag, etc. fall into the material box when feeding, and the product is driven into the product during moulding.


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