PC polycarbonate injection molding process and plastic combustion performance
PC polycarbonate injection molding process and plastic combustion performance
PC is commonly known as polycarbonate. Because of its excellent mechanical properties, it is commonly known as bulletproof glue. PC has the characteristics of high mechanical strength, wide operating temperature range, good electrical insulation performance (but the arc-proof performance remains unchanged), good dimensional stability, and transparency. It is widely used in electrical products, electrical instrument housings, and electronic product structural parts. There are many modified PC products, usually glass fiber, mineral fillers, chemical flame retardants, and other plastics are added. PC has poor fluidity and high processing temperature, so the processing of many grades of modified materials requires a special plasticized injection structure.
1. Plastic processing
PC has a high water absorption rate. It must be preheated and dried before processing. Pure PC is dried at 120°C, and modified PC is generally dried at 110°C for more than 4 hours. The drying time cannot exceed 10 hours. Generally, the air extrusion method can be used to judge whether the drying is sufficient.
The use ratio of recycled materials can reach 20%. In some cases, 100% recycled materials can be used, and the actual amount depends on the quality requirements of the product. Recycled materials can not be mixed with different masterbatch at the same time, otherwise it will seriously damage the properties of the finished product.
2. Mold and gate design
The common mold temperature is 80-100℃, and the glass fiber is 100-130℃. Small products can use needle gates, and the gate depth should be 70% of the thickest part. Other gates have rings and rectangles.
The larger the gate, the better, to reduce defects caused by excessive shearing of the plastic.
The depth of the vent hole should be less than 0.03-0.06mm, and the runner should be as short and round as possible.
The draft angle is generally about 30'-1°.
Third, the selection of injection molding machine
Due to cost and other reasons, current PC products use modified materials, especially electrical products, and must increase fire resistance. When flame-retardant PC and other plastic alloy products are molded, the requirements for the plasticization system of the injection molding machine are Good mixing and corrosion resistance are difficult to achieve with conventional plasticizing screws, so please specify in advance when purchasing. Ramada has a dedicated PC screw for customers to choose.
4. Melting temperature
The air injection method can be used to determine the processing temperature. Generally PC processing temperature is 270-320℃, some modified or low molecular weight PC is 230-270℃.
Five, back pressure
The back pressure of about 10bar can be appropriately reduced without air lines and color mixing.
Six, injection speed
It is more common to use faster injection speed for molding, such as electrical switch parts. Commonly it is slow→rapid prototyping.
7. Detention time
If the residence time is too long at high temperature, the material will degrade, release CO2, and turn yellow. Do not use LDPE, POM, ABS or PA to clean the barrel. Apply PS cleanup.
8. Matters needing attention
Some modified PCs tend to produce dark brown liquid bubbles due to too many recycling times (decreased molecular weight) or uneven mixing of various components.
Combustion products are related to the type of plastic
Generally speaking, plastic combustion products are related to the type of plastic and the nature of the groups contained in the main carbon chain. In a fire, plastic combustion products mainly include smoke, toxic gases, combustible gases and burning droplets.
Smoke such as polystyrene, polyurethane foam, burning in a fire will quickly produce a large amount of smoke. The smoking law of plastics is: most types of plastics will be thermally decomposed and produce very thick smoke. The use of ventilation can drive away some of the smoke, but can not greatly reduce the smoke concentration; some flammable polymers will be heated under hot conditions. There will be smoke in the flame, but the concentration is low; urea-formaldehyde foam, whether it has flame or not, can produce dense smoke as long as it is heated, and the spread speed is very fast.
Toxic gas Under fire conditions, any plastic burning will produce toxic gas, and its toxic component is mainly carbon monoxide. In addition to carbon monoxide, the combustion of some types of plastics also produces other gases, which are more toxic when mixed with carbon monoxide. Different types of toxic gases are produced when plastics with different chemical compositions burn: carbon, hydrogen, or carbon, hydrogen, and oxygen are the main components of plastics. The toxic gas produced by the combustion of plastics is carbon monoxide, which is the most dangerous when the fire is fierce. ; Nitrogen-containing plastics, such as melamine formaldehyde and polyurethane, can produce carbon monoxide, nitrogen oxide and hydrogen cyanide when burned. This mixed gas is extremely toxic; chlorine-containing plastics, such as polyvinyl chloride, are heated in flames or overheated It will produce hydrogen chloride, which can cause death when it reaches a certain concentration; fluorine-containing plastics, such as polytetrafluoroethylene, will produce hydrogen fluoride gas in a fire or overheated, which is corrosive and toxic.
Burning droplets Generally, only thermoplastic products melt and flow when heated to produce combustible droplets, while thermosetting plastic products do not produce droplets when burned, except for polyurethane foam. The appearance of burning droplets will accelerate the spread of fire and affect the safety of evacuation and fire fighting. Burning droplets may bring about two results: one is that the plastic melts out of the flame zone and prevents re-combustion, and the other is that the droplets burn and produce tar-like droplets. The latter result is more likely.
Combustible gas The combustible gas in the combustion or thermal decomposition products of plastics, such as carbon monoxide, hydrogen cyanide, etc., and air mixture will burn or explode under appropriate conditions. When the oxygen concentration in the fire field changes, it may cause more violent combustion or explosion occur. PC is commonly known as polycarbonate. It is not flammable. When an external fire is forced to burn, black smoke will be emitted, small particles will be deposited at the burning place, and the flowers and fruits will have a rotten smell. The quality of this material depends on the toughness. The general board material is polymer PC (good material), but pay attention to the coating (scrape the surface with a knife, if the scraped powder is very long, it is coated). There are users who remove the coating in China, but the labor cost is calculated, that is, the price is lower. The general product materials are medium-molecular or high or low, depending on the needs of the product. This type of polycarbonate is the most common, such as the transparent parts in the lampshade (coated) of game consoles, optical lenses and electrical enclosures, etc., transparent and There is a certain gap in the price of other colors or black. Low-molecular-weight PC-generally found in record materials (VCD, CD, etc.), the current price of this material is low.
Almost all polycarbonates used as plastics contain bisphenol. When they were identified, their color reaction with p-methylaminobenzaldehyde or the Gibss indophenol test gave positive results.
Polycarbonate is completely saponified by heating it in a 10% potassium hydroxide ethanol solution for a few minutes. During the reaction, potassium and potassium carbonate precipitated out. After filtering and acidifying the precipitate with dilute sulfuric acid, carbon dioxide gas will be released. When barium hydroxide solution is added, barium carbonate precipitates.