In a great example of Yankee ingenuity, Xaloy of New Castle, Pa., is commercializing a single-screw, single-barrel package that permits coinjection without investing in a separate (and costly) injection unit.
Called “Twinshot”, the system includes a screw with two independent melting zones and a barrel with two feed ports. Skin material is fed into the first port from a standard hopper or volumetric feeder. Core material enters through a second port and is metered into the system by an auger feeder with speed control. Other components include a dual-purpose non-return valve and a special 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 required because the melt front needs to reach all surfaces of the cavity simultaneously.
It doesn’t work for everything.
The 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.
Examples of how it can be used 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; or 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.
As an example of savings, Anton Hagen, global product manager for screws and multi-material molding for Nordson Xaloy pointed to an automotive mirror housing that has resin costs of $5 and energy costs of $1. “With coinjection, a 15 percent savings in material costs is easily obtainable, reducing the resin cost per part by 75 cents and cutting the total cost per part by 12.5 percent.”
A video illustrating the Twinshot process is posted at http://www.xaloy.com/Product-Twinshot-Multi-Material-Molding.
Xaloy is a longtime player in developing screw and barrel technology. Founded in Los Angeles in 1929 as Industrial Research Laboratories, Xaloy invented the world’s first bimetallic casting alloy in 1931 and invented a new barrel heating system in 2007. Founded in 1954 and headquartered in Westlake, Ohio, Nordson has operations and support offices in more than 30 countries.