Plastic Troubleshooter
On-Line Solutions To Injection Molding Problems

What Happens During the Molding Process?

Drying Raw Material


Plastic materials are very susceptible to failure if they contain moisture in, or on, the molding pellets. To eliminate this moisture, the plastic material must be dried prior to molding.


Some materials, known as hygroscopics (hy-grow-SKOP-icks), actually absorb moisture directly from the atmosphere. The most common of these are nylon, ABS, polycarbonate, and polyurethane. These materials must be dried using a dryer unit that utilizes a desiccant. The desiccant absorbs the moisture from heated air that is passed over the plastic.


Other materials, known as non-hygroscopics, do not absorb moisture from the atmosphere, but are still susceptible to moisture from other sources such as condensation. Also, many of the fillers used in the production of these materials are hygroscopic themselves and will absorb moisture. Therefore, all materials must be dried, but some can be dried using only heat (the non-hygroscopics) while others (the hygroscopics) require heat plus a desiccant. The amount of drying depends on the specific material being dried. Normally the material must be dried for at least 2 hours, and sometimes up to 24 hours. Then, it must be molded within 2-3 hours because moisture will begin to form again immediately after drying.


Once dried, the material is loaded in the machine's hopper, and the process is ready to begin.


A - Heat is applied.


All materials are made up of extremely small, ping-pong ball shaped molecules. Molecules they take up the space available to them. These molecules must bond together to form a molded part. To get them to bond in the right manner and shape, we heat them up in the heating cylinder of the molding machine. Molecules do not like heat and try to get away from it. That's what causes steam when you boil water. The steam is actually millions of molecules jumping out of the pot to get away from the heat. In the heating cylinder of the molding machine, the plastic molecules are heated just enough to break them apart, but not enough to break apart the chains that are present. These chains are simply many molecules bonded together like a railroad train. In this state, the plastic softens (or melts) and is ready to inject.


B - Pressure is applied.


Pressure is created by the screw of the injection unit pushing forward. It contacts the melted plastic and causes it to be pushed forward too. Actually, the pressure caused by the screw moving forward lines up the molecular chains, like dragging a fork through a plate of hot spaghetti. Once the chains are all lined up they can flow easily through the heating cylinder and into the mold. At that point they continue to flow until they fill the runner, gate, and cavity images. Pressure is maintained during the entire filling process.


C - Cooling is applied.


The mold into which the plastic is injected is being cooled by the use of water flowing through channels that are drilled through various plates of the mold. These cooling channels bring the water close to the shape of the plastic part being molded without actually touching the plastic. The mold draws heat from the plastic and the cooling channels draw the heat away from the mold. As soon as the plastic enters the mold it will begin to cool down and solidify. The plastic must be injected quickly to keep it from solidifying (or "freezing") before it is finished filling the mold. The material is held under pressure while it freezes to allow the molecules to be forced together forming a tight bond. Once the plastic has solidified it can be removed (ejected) from the mold.


D - Ejection is applied.


After the part has cooled enough to withstand the force of ejection, the mold opens, and the parts are pushed out of the mold using the ejector system, located on the moving half of the mold. Once the parts have cleared the immediate are around the mold, the injection mold closes again and starts the next cycle.


On average, the entire injection process, including mold close, injection, cooling time, mold open, and parts ejecting, is between 20 and 30 seconds. Increased part complexity, thicker wall sections, and more exotic materials, will increase the overall cycle, mostly due to higher melt and mold requirements.


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