Injection molding gates

Laminar flow Flow without turbulence, i.e., the movement of one layer of fluid past another layer with no eddying between them. Most melt flow, even at high velocities, is laminar. With sudden changes in melt velocity, such as may occur at some extrusion-die entries and at injection-mold gates, laminar flow may be disrupted by melt fracture. 

Splay Mark Also called silver streak. Fan-like or streaking on the surface of fabricated plastic parts. Causes include moisture in the plastic material, thermal degradation of plastic material, relatively cold plastic on mold or die surface due to fast an operation (called jetting), injection mold gate restriction, foam molding, etc. 

An alternative approach for injection molding is the use of a rotary table injection machine. This type of machine uses the multistation concept, each station having a single cavity mold generally using a point gating/ injection point as previously mentioned. Consider a rotary table machine with eight stations, station one would be the injection station and station seven would be the unload and load metal insert station. This type of process has been successfully used for different types of lip type rotary shaft seals where more expensive elastomers are used and waste needs to be kept to a minimum. 

The polymers studied and the experimental procedure were described in the last section. The samples were made of the same PC-TLCP composite and the pure TLCP Vectra A950, respectively. To study the influence of sample forms and geometries on mechanical constants, plate samples (80 x 80 x 2 mm-) were also injection molded with a film gate. Tensile samples were then cut from the plate in the longitudinal (//) and transverse... 

Many injection molded products will influence the final product s performance, dimensions, and other characteristics. The mold includes the cavity shape, gating, parting line, vents, undercuts, ribs, hinges, and so on (Table 3-17). The mold designer must take all these factors into account to eliminate problems. At times, to provide the best design... 

These cutting operations are usually used only for machined prototypes, or very low volume production of simple shapes. High speed routing is sometimes used for slotting or gate removal on injection molded products. Standard end mills (two-flute), cir-... 

Design Considerations for Injection Molded Parts (Parting lines, draft angles, wall thickness, fillets and radii, bosses, ribs, opening formations, shrinkage, gating, vents, potential knit lines)... 

In addition to the process parameters, there are large numbers of factors that decide the success of an injection molding operation. Mold design, product design, position, and size of gates, type and positioning of runners, etc., are extremely important factors to be taken care of. A detail consideration regarding these factors has been discussed below [213]. 

A typical injection pack/hold pressure during injection molding of polyamide 66 components is 1,000 bar and the injection temperature is 280°C. The gate freezes shut when the average temperature inside the mold reaches 225°C. 

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. 

For the flow of polymer melts, the Reynolds number, Re = pUch/r], is usually 10 5, with the exception of the reaction injection molding process, RIM, where Re —> 1 — 100 at the gate. The geometric and dynamic conditions of the lubrication approximation, applied... 

Inevitably, in multi-cavity injection molds, some of the mold cavity gates are located further than others from the sprue that delivers the melt from the plasticating unit. If the runner system that distributes the melt from the sprue to the individual cavities has a constant... 

Figure 6.80 Schematic diagram of an off-center gated slit-shaped injection mold.

---