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Melt strength is a measurement of how strong a plastic is when it is in a molten state. For common plastics, melt strengths are available in the form of charts which also outline other important physical properties of the plastic which may be important for people to know about. Companies which sell materials like nurdles also usually list the melt strength of the products they sell so that people can make an informed decision about the best product for their needs.
Plastics often spend some time in a molten state as they are worked with. For plastics which are extruded, melt strength can be especially important. If a plastic has a poor melt strength, it may not be able to hold its shape after extrusion, resulting in a product which will deform before it has a chance to cool and harden. This characteristic can also be relevant to injection molding, thermal molding, and a wide variety of other processes, depending on the details of the specific process.
Melt strength is not necessarily fixed. There are additives which can be blended with a plastic to alter the melt strength and other properties, allowing companies to tailor plastics to their individual needs. Some of the additives used in plastics processing have attracted controversy; chemicals such as phthalates, which soften plastics, for example, have been accused of causing health problems in people who have been exposed to them by using products which contain phthalates.
Companies which utilize extrusion molding may purchase raw plastic and melt it down in their extrusion machines, or may develop their own plastics, depending on the size of the company and the products it makes. Melt strength can be determined during materials testing, in which samples of the plastic are analyzed in a laboratory to gather data about their properties. Materials testing can also be used to uncover safety issues which may be of concern to the manufacturer.
For extrusion molding, plastics with a high melt strength are favored. A wide array of plastics are available for extrusion molding, including products with an assortment of properties which may be valuable for different kinds of products. Petrochemical companies are also always interested in developing more products, especially products which address consumer concerns about durability and safety. Concerns about plastic as a pollutant have also lead companies to increase efficiency, and to utilize recycling and reclamation technology to reduce plastic waste.
@Calvin77 - Melting is better than burning and it might prevent more house fires that way. Plastic is in practically everything around the house. Garbage cans, travel mugs, phones, socks, power cords -- it'd be great if they all could withstand heat better.
The plastic in car interiors is another type of plastic that needs a very high melt strength. Think of places like California and Texas, where is gets above 100 degrees outside. Imagine how the dash of your car has to hold up when all the windows are increasing the interior temperature. It's amazing how the inside of your car doesn't melt into a pile of goo.
Melt strength is very important for toys that will be out in the sun all day -- like sandbox or beach toys. Especially if they are dark colored or black. Plastic piping used in greenhouses also have to have high melt strengths -- the greenhouses are made to gather heat. The last thing you want is a melting greenhouse – especially on your organic garden.
The same goes for polystyrene, which is one of the most common plastics. It's in CD cases, plastic toys and lots of other stuff around your house. I think that the higher the melting strength, the better – then things won't burn if you leave them by the heater.
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