Thermoplastic Injection Molding and Thermoset Injection Molding
While the two processes are similar, there are differences between thermoplastic and thermoset injection molding. Here is the thermoplastic injection molding cycle …
After ejecting the previous shot of parts, the cooled mold is closed. The next molding cycle begins
The reciprocating screw augers the raw resin and the color beads from the hoppers to the front of the heated barrel. The material is compressed together, mixed and heated as it moves forward
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 “metering 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 cooled mold with melt. Now the screw holds the melt under pressure in the mold (this is called “dwell,” or “dwell time”). The cooled mold solidifies the resin by reducing its temperature below the melt point.
After the resin solidifies, 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
Thermoset Injection Molding — a familiar high-volume process in thermoplastics, now equally applicable to many thermoset materials.
Thermoset molding has always been the mainstay of Standard Plastic. As a natural outgrowth of services to our thermoset molding customers, we’ve expanded into thermoplastic custom injection molding, but thermoset has always been our first love. Molding thermoset resins is a challenging and exacting craft with results that range from the utilitarian to the decorative, and both at once!
In many applications, thermoset moldings are economical, feature-rich alternatives to metal castings, even to the extent where some existing metal molds can be converted to thermoset molds, extending the useful life of a design while picking up some superior properties of thermoset materials in the process. For other applications, only thermoset’s unique properties, flexible design and economy of production will meet the requirements.
Most of our customers own their own plastic molds, but for reasons of economy and convenience, outsource the actual molding and secondary processes to Standard Plastic. It makes sense. The specialized equipment and the trained operators required for high quality thermoset molding are capital-intensive to say the least, and if you can’t keep an in-house thermoset line busy, costs can get out of control. Then of course, there’s machine maintenance, mold maintenance and storage, resin purchasing and stockpiling, support machinery (preformers and preheaters), secondary operations (deflashing, post-molding inserts, labeling, packaging, palletizing), warehousing, drop-shipping, and scheduling for Just-In-Time (JIT) production delivery. That’s what Standard Plastic brings to the party. With the capabilities to run lots of jobs at the same time, we keep our lines busy, and your costs down.
It’s the old “why keep a cow just to get milk?” theory; cut your costs and have it delivered by the bottle.
Whether you’re looking for an outsource thermoset molding facility to do entire high-volume production runs, post-production MRO replacement part job shop runs, specialized molding of complex or critical-tolerance pieces, or even design-test prototyping, Standard Plastic can fill the bill.
And, of course, if you need new plastic mold — or modifications to existing tooling — we can handle that for you, too.
Thermoset Molding Capabilities:
The names on our thermoset presses, preformers and preheaters read like a Who’s-Who of thermoset molding equipment: Adamson, Bipel, Dake, Hannifin, Lawton, Logan, Mytron, Racine, Stokes, Thermall, and Williams & White.
Thermoset Compression / Transfer Molding Presses:
Tonnages: 40 • 75 (4) • 100 • 200 (4) • 300 (3) • 350 • 380
Support Equipment: Thermall and Mytron Preheaters • Thermoset Resin Preformers • Molding Press Cooling Towers
Facilities for Oil or Electric Heated Molds
Thermoset Injection Molding Presses:
Tonnages: 75 • 200 • 260 (2) • 300
Support Equipment: Molding Press Cooling Towers
Thermoset Molded Product Examples:
To illustrate the wide variety of thermoset molded products, here are a few selected examples from Standard Plastic’s showcase. They demonstrate typical problem-solving features of various molding processes and thermoset resins.
Aircraft Main Power Terminal Housing (Polyester Compression Molding)
Milling Machine Adjustment Wheel (Phenolic Compression Molding)
Clothes Dryer Blower Fan Blade (Phenolic Compression Molding)
How the two processes differ:
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 crosslinking 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 color additives are rarely used in thermoset molding, the mixing action of a tapered screw is not needed.