Injection molding of thermosets: is basically identical to that of thermoplastics, with the exception that the thermoset material is injected cool into a heated mold. The heated mold causes a cross-linking reaction in the material and a hard part is removed from the tool.
Process Characteristics
1. Part Cost - low
2. Tool Cost - high
3. Production Rate - high
4. Thermoset Materials are very stable
Thermoset Injection Molding:
How thermoset injection molding works:
While the two processes are similar, there are differences between thermoplastic and thermoset injection molding. Here is the thermoset injection molding cycle ...
After ejecting the previous shot of cured parts, the heated mold is closed. The next molding cycle begins...
The reciprocating screw moves the raw resin from the hopper to the front of the heated barrel. The resin is compressed and heated as it moves forward. (See Resin Heating, below)
By the time the resin reaches the front of the barrel, it has been heated to the point where it plasticizes; it a hot fluid and starts to flow. To make room for this build-up of "melt," the rotating screw slowly retracts. The melt builds up in the heated "front zone" of the barrel.
To account for molds of different sizes, the maximum distance the screw will retract is controlled so as to build up a precise amount of "melt" in front of the screw. This is similar to and takes the place of metering a preform pill to the proper volume to fill the mold.
Once a "full shot" of melted resin has built up, the rotating screw starts to move forward again. Now it acts as a ram to force the measured melt into the mold.
In the full forward position, the screw has completely filled the heated mold with melt. Now the screw holds the melt under pressure in the mold. Once under pressure in the heated mold, the resin finally reaches its cure temperature, cross-linking occurs, and the finished parts are now locked into their final, rigid form.
After curing, the mold is opened and the entire "tree" is ejected the finished parts, the runners and the sprue. Then the mold is closed and clamped and the next molding cycle begins...
When comparing thermoset injection molding to thermoset transfer molding, there are obvious similarities. Both are examples of closed mold processing. Also, an injection mold is essentially a transfer mold turned on its side. There are obvious differences, too. The compression chamber (the well, or pot) and transfer plunger used in transfer molding are replaced by a heated barrel and a screw/ram in injection molding. A constant supply of plastic thermoset resin, usually in the form of pellets or granules, is fed into the barrel of the injection unit through a large hopper. As the screw augers the plastic resin through the barrel from the hopper to the nozzle, the resin is heated in two ways...
First, and primarily, the resin is heated by the injection unit's barrel, which is itself heated in three temperatures "zones:"
The FEED Zone not directly heated. In this zone, the plastic resin pellets are packing into the screw chamber, forming a long thin "ribbon" of resin material that wraps around the screw.
The REAR Zone Heater bands around the barrel heat its middle part, which in turn starts heating the ribbon of resin pellets. In this zone, the resin begins to melt and changes from cold pellets into a warm slush.
The FRONT Zone more heater bands bring the barrel and the resin fully up to melt temperature. In this zone, the resin becomes a hot, flowing fluid that is ready for injection into the mold. This "melt" accumulates in the front zone and the nozzle, where it is held at melt temperature until the reciprocating screw has metered the appropriate amount of melt for a full shot, at which point the screw rams the melt through the nozzle into the mold.
A secondary source of heating comes from "shearing" as the resin is augured through the barrel by the screw. Shearing occurs as the resin is scraped from the inner walls of the barrel by the flights of the screw, and as the resin drags along the inner surfaces of both barrel and screw. All this scraping and dragging creates friction, which in turn creates heat.
The illustration below shows the main differences between thermoplastic injection molding and thermoset injection molding:
The differences are quite logical when you consider the differing objectives of the two processes:
In thermoplastic injection molding, the molded parts retain their shape after they cool below their melting point. Therefore, the primary objective of the thermoplastic injection molding process is to get the resin up to its melt point as quickly as possible (which is one reason for the tapered screw; see Resin Heating), and then to get the resin down below its melt point once it's in the mold (which is why the mold is cooled). A secondary objective is the thorough mixing of the resin pellets with the colorant beads. This is another reason for the tapered screw; it does a superb job of mixing the resin and colorant as they are compressed together more and more moving up the taper.
In thermoset injection molding, the molded parts retain their shape after they are heated above their cure point the point at which cross linking occurs. Therefore, the primary objective of the thermoset injection molding process is to get the resin up to its melt point (so it can be injected into the mold), but not to get it up to its cure point until after it is in the mold (which is why the mold is heated). Since the resin can be brought up to melt temperature more accurately by controlling the barrel heat, no additional compression heating is needed, and so the screw root is straight. Also, since colour additives are rarely used in thermoset molding, the mixing action of a tapered screw is not needed.
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