| TWINSHOT® Co-injection Technology
It enables a conventional machine with a single barrel and single screw to inject two materials in a single shot with one material totally enclosing the other. The process uses a screw with two independent melting zones to produce layers of different materials within a single barrel. During injection, the well known principle of fountain flow causes one material to cool against the mold walls and the other material to form the encapsulated core. A three-layer ?sandwich? construction is produced. Spirex, a global leader in plasticating systems solutions, and Twinshot Technologies, the developer of this process have entered into an agreement that makes the retrofit technology available to markets worldwide. The exclusive, international licensing agreement signed by the two ISO 9001-certified companies allows Spirex to manufacture, market and install Twinshot co-injection technology to existing standard injection molding machines in markets around the world. Unlike a conventional co-injection machine with two barrels, the technology is remarkably easy to use. Other than the screw recovery cycle the machine works exactly the same as a standard injection molder. In most cases TWINSHOT® can be controlled by the same software that comes with the standard machine. The process can be used with barrels down to 25mm (about 1.5oz). The ratio of materials is controlled by an external auger feeder mounted to the secondary hopper. Because this design requires no changes to the screw drive mechanism, retrofitting to any model of existing press is as simple as a routine barrel change. The technology leads to a faster recovery as both materials plasticize simultaneously. The technology uses a single screw as against other conventional co-injection machines thereby bringing down the cost. It also offers a flexibility to switch to single material molding on the fly. Standard co-injection molding machines typically require 5 to 7 years to pay back - it is usually profitable in the first year. With a suggested machine cost only 25% higher than a single-material machine, it is the lowest cost multi-material solution. Additionally, the savings in additives and reduced scrap alone can frequently justify the expense of the system. With TWINSHOT® 25-35% of a part's weight can typically be replaced by a lower-cost material. This can be helpful in the following instances:
? UV-protected outside/standard inside
? Virgin outside/regrind-recycled inside
? Colored outside/uncolored inside
? Soft-touch outside/structural inside
Twinshot will work with any cold-runner mold. The ratio of core to skin is dependent on the relative viscosity of the two materials as well as the geometry of the parts. Low cavity count and a balanced fill are the two most important characteristics. Ideally, the melt front should reach all edges of the cavity simultaneously. Portions of the cavity that fill early may not contain a core layer. As a result, parts with simpler geometry work better than complex parts. Walls as thin as .05" (1.3mm) have been successfully molded with Twinshot. Moreover, the two materials must be chemically compatible (non-reacting), and must be moldable at similar temperatures. Typically a difference of 50°C between the melt temperatures can be accommodated. Materials do not have to bond, although de-lamination may occur unless there is chemical bonding between the layers.
Nevertheless, Twinshot does not work with hot-runner molds. Multi-cavity molds are only possible if the fill rate to each cavity is balanced. One way to achieve balance on a multi-cavity tool is the "Melt Flipper" technology available from Beaumont Runner Technologies, Inc. Family molds with dissimilar parts may be problematic. Also, a conventional coinjection machine may be converted into a two-color machine for selective overmolding with the installation of a different manifold and a rotating platen. A Twinshot machine cannot undergo the same conversion. A conventional coinjection machine can be programmed to switch between materials multiple times. A Twinshot machine produces only a single skin and core layer. On the other hand, as the system is operated just like a conventional machine, operators need minimal if any retraining. The technology's durability, long wear life, and maximum uptime mean significantly reduced costs from maintenance and lost production. Lastly, the presence of only one barrel and screw to heat this method achieves extremely fast cycle and recovery times. Among many applications, foam core molding with the technology reduces pressure, sinking and warping. Parts with even a small amount of foaming can be molded with much lower cavity pressure thereby saving costs by running parts on a smaller machine at a lower hourly cost. It is claimed to achieve a Class A gloss surface without common challenges such as sinking. Moreover, with a TWINSHOT® foam core, thick ribs are no longer a problem. Because of the void-free surface, Twinshot parts not only look better, but are stronger than conventional structural foam parts. Another noted plus point of the foamed core is an improvement in flatness. Parts moulded using conventional co-injection techniques tend to warp in the direction of heavy ribs. Once internal stresses are reduced by foaming the core, parts exhibit up to 80% less warp.
Enhanced applications include soft-touch parts, where a tactile surface is needed for skin contact on a part that must otherwise be rigid. Handles, grips and housings are common applications. The technology eliminates the need for complex over-molding by simply combining the soft and hard layers in one simple cavity. Multi-layer and single-layer areas can also be combined. The process is also suited for the encapsulation of reinforced materials. Molders of glass-filled materials know that a glossy cosmetic surface can't be obtained with conventional molding. Its use can hide highly filled core material underneath a cosmetic skin of unfilled plastic. Further, the surface of the mold can also be protected from abrasion caused by the filler. Other enhanced-property applications of the process include inner oxygen-barrier layers in packaging applications; combining a layer of conductive polymer with a low-cost structural layer in electronic enclosure applications and backing up UV-modified skin materials in outdoor applications.
It can also be installed on the existing molding systems. Retrofitting involves replacement of barrel and screw, and mounting an auger feeder to control the ratio of materials. In most cases a retrofit can be accomplished in a single shift, once the pre-engineered kit is received. It is easily adapted using a slightly modified standard injection molding machine. As the process is a single-shot process, machine operation requires no modification. Set-ups are also simple with no exotic mold requirements. On a traditional co-injection machine, the timing of injection between the two barrels is very important. Particularly during the transition from skin to core, injection rates need to be carefully tapered off so the melt experiences a constant velocity failing which hesitation lines will be left on the part. The process makes use of only one injection stroke, so a simple velocity profile used for standard solid molding is required. The process also scores well as far as the maintenance costs are concerned. Except for an auger feeder on the second feed throat, there are no additional parts?extra heater bands, thermocouples, joints in the barrel, or nozzle valves. The process also needs no additional hydraulic power plants, accumulators, computers or any external equipment to the machine. |
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