injection mold,plastic injection mold--dongfeng mold

the injection mold that is mounted between the stationary and moving platen of the molding machine28. Therefore, the mold one or more hollow cavities shaped like the desired product. As shown in Figure 8, a typical mold is a series of plates. The cavity and core plates contain the geometry of the parts and runner system (if needed). The ejector pins and sprue puller are mounted between the ejector and ejector (or sprue) retainer plates. These plates are supported by top and bottom clamping plates, a support plate, and support pillars. The mold splits between the cavity and core plates to produce the two mold halves. The cavity or A side of the mold is mounted to the stationary platen while the core or B side is mounted to moving platen. These halves are aligned using four leader pins and bushings.

Figure 8. Cross-section of a typical two-plate mold[21].

Melt is delivered to the mold at the sprue bushing, which fits into the top retainer and cavity plates. A locating ring surrounding the sprue bushing aligns the mold with the stationary platen, thereby aligning the sprue bushing and nozzle. Melt flows from the sprue bushing into the runners, through the gates, and into the cavities. Figure 9 presents the layout of cavities in a multi-cavity mold. The sprue delivers melt from the nozzle to the runners, which split the melt stream for delivery to the four cavities. The core and cavity design control the shape, size and surface texture of the molded part. Cavities are located between the cavity and core plates, with placement and parting line locations dependent of part design. The term ¡°mold half¡± does not mean that the two mold halves are of equal width.

As illustrated in Figure 9, the sprue is tapered to facilitate the part release. Runner diameters typically have round (shown in Figure 9), trapezoidal, or modified trapezoidal cross-sections because these designs provide the best surface-to-volume characteristics[22],[23],[24]. The dimensions of these runners depend on the material and size of the part[25]. When melt flows from the runners to the cavities, the melt passes through a reduced cross-sectional area in the mold called a gate. Gates control the melt flow entering the cavity and ease separation of the molded part from the runner system[26]. The design, sizing, and location of the gate influence the 1) shear experienced in the gate, 2) direction melt flow (i.e., orientation) and level of balanced cavity filling, 3) presence of flow instabilities, such as jetting, 4) location of vent and parting lines, 5) number and strength of weld and meld lines, 6) the amount of runner scrap, and 7) the need for secondary operations.

Figure 9. Layout of cavities in a multi-cavity mold[27].

In general, gate size is determined by the part wall thickness[28], overall part size, and material properties. Thicker parts require larger gates to facilitate packing, but a gate depth less than the part thickness allows for proper e

jection without an ugly gate vestige (i.e., mark left on the part when the gate is removed). With thin-walled parts, the gate depth may be larger than the part thickness to decrease the fill pressure28. Parts with long flow lengths and large cavity surfaces need larger gates to reduce fill pressures and to prevent premature gate freeze off. Higher viscosity resins also require larger gates than easier flowing resins, larger gate cross-sections reduce the shear applied to the polymer melt, and short lands decrease the occurrence of jetting and other flow instabilities. Generous radii on the cavity side of the gate also create laminar flow and prevent jetting28. Some materials have wide processing windows while other materials can only be used with a narrow range of molding conditions. This behavior often occurs when small gates tend to cause thermal degradation of the material and excessive residual (molded-in) stresses in the part. Although too small a gate results in loss of strength of the steel in the land area and may cause the steel to break[29], long lands promote jetting. Therefore, the land length of the steel (i.e., gate) is usually 50% the gate depth. Common gate designs for cold runner molds are shown in Figure 10.

Injection molds have traditionally been machined from tool steel. Mold temperature is controlled from water lines that are drilled in the core and cavity plates of the mold. Water heated or cooled in a mold temperature controller and pumped into the mold. Since machining cannot produce smaller features, newer tooling has included electroformed nickel (used for digital versatile disks) and silicon inserts produced using conventional semiconductor fabrication (exposure and etching) processes.

Sprue gate			Edge gate				Tab gate
	Pin gate				Tunnel gate
		Disk gate		Spoke gate		Ring gate

Figure 10. Selected gate designs[30],[31],[32].