Twinshot Technology for A-B-A Resin Combinations
Technology without Costly Investment in Dual-Injection-Unit Systems
A patented single-screw, single-barrel equipment package from Nordson Xaloy provides the cost-savings and versatility of two-component molding while requiring substantially less investment than more complex coinjection molding systems that use separate injection units.
Called Twinshot, the Nordson Xaloy system replaces a standard screw and barrel with counterparts designed to produce a two-component part in a single molding cycle with a single screw, single shot and single recovery. It is available as an option on new injection molding machines and as a retrofit that can be installed in one or two days. The Twinshot system is limited to simple “A-B-A” sandwich structures and cannot be used for hot-runner molding or on equipment with L/D ratios less than 20:1, but within these limitations it can process a wide range of resin and color combinations and can readily be switched back to single-material molding.
The advantages cited for Twinshot two-component molding include: confining high-cost specialty material to the skin layer with general-purpose material in the core; using recycled or wide-spec material in the core; combining a structural core with a cosmetic exterior; adding new product value by, for example, combining a “soft-touch” TPE skin with a rigid core; and obtaining the benefits of a foam core (reduction in weight and molded-in stress and elimination of warpage and sink marks) while obtaining a cosmetic surface finish.
The Twinshot system includes a screw with two independent melting zones and a barrel with two feed ports. The skin material is fed into the first port from a standard hopper or volumetric feeder. The core material enters via the second port and is metered into the system by an auger feeder with speed control. Other components are a dual-purpose non-return valve and an Eliminator nozzle tip. Layer thicknesses are controlled by the speeds of the primary screw and the auger screw.
The mold cavity fills in a single shot. There is generally sequential injection of the two materials through the same gate (or gates), with some amount of simultaneous injection. The skin material cools against the mold walls as the molten second material fills out the core. The result is a three-layer, or A-B-A, sandwich structure with the B, or core, layer completely encapsulated. The ratio of core to skin depends on the relative viscosity of the materials as well as on part geometry. When using multi-cavity molds, a balanced fill is a necessary condition for optimal use of the Twinshot system. Ideally, the melt front should reach all surfaces of the cavity simultaneously.
Nordson XALOY Europe GmbH
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