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Shrinkage is inherent in the injection molding process. Shrinkage occurs because the
density of polymer varies from the processing temperature to the ambient temperature
(see Specific volume (pvT diagram)). During injection molding, the variation in shrinkage both globally and through the cross section of a part creates internal stresses.

Variable residual stresses arise and the part deforms as layers of different frozen-in specific volume interact with each other Process-induced residual stress data are much more useful than in-cavity residual stress data for molding simulation. Following are definitions of the two terms, along with an example that illustrates the difference between them.

Furthermore, asymmetrical thermal-induced residual stress can occur if the cooling rate of the two surfaces is unbalanced. Such unbalanced cooling will result in an asymmetric tension-compression pattern across the part, causing a bending moment that tends to cause part warpage. This is illustrated in Figure 3 below.

Residual stress is a process-induced stress, frozen in a molded part. It can be either
flow-induced or thermal-induced. Residual stresses affect a part similarly to externally applied stresses. If they are strong enough to overcome the structural integrity of the part, the part will warp upon ejection, or later crack, when external service load is applied. Residual stresses are the main cause of part shrinkage and warpage.

Theoretically, cooling time is proportional to the square of the heaviest part wall thickness or the power of 1.6 for the largest runner diameter.

Baffles and bubblers are sections of cooling lines that divert the coolant flow into areas
that would normally lack cooling. Cooling channels are typically drilled through the mold
cavity and core. The mold, however, may consist of areas too far away to accommodate
regular cooling channels. Alternate methods for cooling these areas uniformly with the
rest of the part involve the use of Baffles, Bubblers, or Thermal pins, as shown below.

Parallel cooling channels are drilled straight through from a supply manifold to a collection manifold. Due to the flow characteristics of the parallel design, the flow rate along various cooling channels may be different, depending on the flow resistance of each individual cooling channel.

Mold cooling accounts for more than two-thirds of the total cycle time in the production of injection molded thermoplastic parts. Figure 1 illustrates this point. An efficient cooling circuit design reduces the cooling time, which, in turn, increases overall productivity. Moreover, uniform cooling improves part quality by reducing residual stresses and maintaining dimensional accuracy and stability (see Figure 2).

This example illustrates various aspects of the power of CAE software for the plastics industry, and how such software can be strategically applied. It shows how to put into practice many of the concepts that are discussed in detail in the Design and Processing > Physics topics. These topics include injection pressure, filling pattern, melt-front area and melt-front velocity, runner design and balancing, and gate design.

The gate location should be at the thickest area of the part, preferably at a spot where the function and appearance of the part are not impaired. This leads the material to flow from the thickest areas to thinner areas to the thinnest areas, and helps maintain the flow and packing paths. Gate location should be central so that flow lengths are equal to each extremity of the part. Numerical simulation of the molding process is an effective tool that can be used to compare the effects of various gate designs.

Gates can have a variety of configurations. They are classified into two categories- manually trimmed and automatically trimmed-based on the method of de-gating.

china injection mold company, china injection mold manufacturer. A gate is a small opening (or orifice) through which the polymer melt enters the cavity. Gate design for a particular application includes selection of the gate type, dimensions,and location. It is dictated by the part and mold design, the part specifications (e.g.,appearance, tolerance, concentricity), the type of material being molded, the fillers, the type of mold plates, and economic factors (e.g., tooling cost, cycle time, allowable scrap volume). Gate design is of great importance to part quality and productivity.