Co-injection molding

In contrast to multi-color and multi-component injection molding, co-injection molding uses the same gate and runner system. Here, the component that forms the outer skin of the part is injected first, followed by the core component. The core component displaces the first and a combination of the no-slip condition between polymer and mold and the freezing of the melt creates a sandwiched structure as depicted in Fig. 3.51. 

In principle, the gas-assisted injection molding process is similar to co-injection molding. Here, the second or core component is nitrogen, which is injected through a needle into the polymer melt, blowing the melt out of the way and depositing it against the mold surfaces. 

CO-INJECTION MOLDING OF PYC WITH OTHER THERMOPLASTICS PROCESSING, PROPERTIES, AND APPLICATIONS... 

FIGURE 14 53 Schematic diagram illustrating the sequence of events in co-injection molding. 

Co-injection molding involves injecting two or more different materials into a mold, generally in sequence. To accomplish this, molding machines are fitted with multiple injection units (Figure 14-53). Two-shot molding, the most common version, often utilizes an indexing mold. In this process, the mold is rotated 180° after the first shot fills a portion of the cavity, then the second shot finishes the part... 

The middle seventies saw the advent of high-speed injection molding. For thin wall parts, the production rate was limited by the hydraulic response of the machine. Nitrogen assist accumulators help inject the melt to reduce the overall cycle time. The early seventies also saw the introduction of co-injection molding. In this process a first melt is injected and cooled. The cavity is rotated and the second melt is injected to form the finished part. Usually both injections use the same material but a different color. The process is extensively used for calculator keys where the different colors eliminate a printing step. 

Co-injection molding is used to produce parts that have a laminated structure with the core material embedded between the layers of the skin material. As shown in Figure 2.14, the process involves sequential injection of two different but compatible polymer melts into a cavity where the materials... 

Structural foam molding, gas counterpressure structural foam molding, and co-injection molding. The high cost of tooling is the factor which governs access... 

Gas counterpressure structural foam molding and co-injection structural foam molding generally follow the same design rules as structural foam molding, although co-injection molding differs somewhat and... 

The guidelines for inside radii (IR), Sec. 8.2.3, apply to structural foam-molded parts. The ideal inside radius is 0.5 to 0.6 times the wall thickness (IR = 0.5 to 0.6 W). The minimum inside radius should be 0.093 in for most parts, 0.060 for thin wall (0.156-in) parts. The same recommendations apply for co-injection molded parts. The outside radius should be equal to the inside radius plus a wall thickness (OR = IR + HO. 

Structural foam is also more generous in freedom of rib design. The thickness of the base of the rib (B) can follow the specification indicated in Table 8.18. However, the recommendations in Sec. 8.2.4 should be followed for co-injection-molded parts. The height of the rib (C) should not exceed 3 times the nominal wall (HO (C < 3H0 since the law of diminishing returns sets in at that point and taller ribs, while feasible up to 8 times the wall thickness, tend to become too thin to foam due to the reduction in thickness resulting from draft. Besides, the... 

Generally, two resins are used in the co-injection molding process. However, three resins may be used especially if the outer skin resin is not compatible with the inner foam core. In this respect the third resin is an intermediate layer for bridging the foam core and the skin. 

Co-injection molding this process, unlike multicomponent and multicolor molding, uses the same runner system. The component which forms the outer skin of the part is injected hrst, followed by the core component. 

Solomon S, Abu Bakar A, Ishak Z A, Leong Y W, Ishiaku U S and Hamada H (2007) Drop weight impact properties of (CO) injection molded short glass fiber/short carbon fiber/polyamide 6 hybrid composites, J Reinf Plast Comps 26 405-418. 

Typical processing methods - mono- and multi-layer film extrusion, blow molding, pipe coextrusion, extrusion coating, co-injection molding, lamination ... 

Fuel filler neck hose is co-injection molded with a 0.6 mm layer of fluoroelastomer Vi-ton, which reduces hydrocarbon emission to less than 0.01 g/day. Viton is used for fuel sender seals and the inner layer of turbo hose for transmitting hot fuel vapors. Fluo-roelastomers have good barrier resistance to aromatic hydrocarbons and oxygenated hydrocarbons, which is a reason for their enduring use for fuel delivery applications. 

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