Reducing defects due to foreign materials and black dots is a very important issue at plastic molding production site.
The causes of its occurrence can be briefly divided into.
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Mixing when the material is put in the machine
Foreign materials are mixed in from dryer, hose, etc. when material is put into hopper. -
Material is carbonized at Screw
・ Resin touching inner wall of Cylinder is carbonized from thermal decomposition and or cross-linking reaction by continuous molding.
- Resin left at the surface of Cylinder and or Screw is oxidized and carbonized after exposing to air by vacating molding machine after completion of molding.
- Carbonized by the temperature getting higher than the working temperature from the resin easier to get carbonized like flame-retardant resin is staying, etc.. - Adhere to at the opening and closing of the mold
those are possibly considered.It is necessary to take countermeasures to all of causes.This time, let’s think about black dots.
Mechanism of occurrence
- Carbonization of resin gets started when stabilizing film is broken from abnormal heating of heating cylinder, etc..
- It is peeled off because it can't resist the flow of resin from getting larger by the progress of carbonization.
- New carbides are built at the peeled off parts. Since it gets in touch with the metal directly without stabilizing film.
- "Carbide is built and is peeled off after being a certain size and is causing black dots." Is repeated.
Countermeasures to reduce black dots defects
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Disassembly and cleaning
Disassembling Screw and getting rid of carbides.
Attention and care are required. Black dots defects might occur temporarily before it gets stabilized after restarting. Since carbide removal effect is very efficient, but stabilizing films are removed, too. -
Cleaning with purging pellet (cleaning agent)
Purging agent has foam type, surface active type, etc., but the type mixing glass (glass fiber) is used regularly for good cleaning effect. However, you need to pay attention to the concern on Screw wear and purging agent drainage
Arrival and Introducing a new purging pellet (cleaning agent)
- There are approx. 40 manufacturers confirmed in Japan. Some companies are launching new products, and purging agents different from conventional ones have been selling in the market.
- From all of voices at the molding production sites, they said they tried testing and evaluated many different cleaning agents and methods, however, results are almost same for each company and its product, etc. Those voiceds are not favorable and rather painful to accept. Could you please challenge by gathering new information for favorable and efficient manufacturing ?