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Mold venting
When molten plastic is injected into the mold, the air in the cavity has to be displaced. To accomplish this, vents are machined into the parting line to evacuate the air and are extremely important to the consistent production of high quality products. In many cases, this is an area in mold design and construction that is often overlooked.
Vents should be located at the extremities of the part and at locations where melt flow fronts come together. Venting is also easily achieved around ejector pins and core slides provided that there is sufficient clearance between the pin/slide and the mold. Typical mold vents are channels cut from the cavity or runner straight to the edge of the mold. Closest to the part, they are typically 0.0005-0.001 in.(0.013-0.025 mm) in depth and 0.063-0.5 in. (1.6-12.7 mm) in width. The initial vent thickness should be maintained for about 0.5 in. (12.7 mm) and then the depth can be increased to about 0.003 in. (0.076 mm) to the edge of the mold. The vents should be polished towards the edge of the mold to make them ‘self-cleaning.’ Build-up in the vents will eventually affect mold filling resulting in non-uniform fill and unbalanced cavities. For this reason, it is important that vents be inspected between production runs to ensure that they are clean and within specification. In some cases, reduction of the injection rate prior to final filling of the cavity will prevent burning and also prevent the mold from opening.
Figure 28. Schematic showing typical runner designs found in injection molds.
Figure 29. Insulated runner system
Figure 30. Hot-runner system
Continuous parting-line venting may be necessary in high speed molding operations. Even though burning is not evident, the lack of burn marks does not ensure that the molds are properly vented. Increasing vent areas may help reduce cycle time. Proper venting will also aid mold filling by decreasing the resistance due to air pressure on the flow front.
Mild sand blasting or vapor blasting of the mold cavity assists in venting and part release. However, for highgloss finishes, this blasting is not advisable. Vapor honing may help alleviate a venting problem area but care must be taken that honing is not too deep or wide to be noticeable on the finished part.
Gating Figure 31.Gating systems
The gate is the bridge between the runner and the cavity. Depending on the specific material and part design (wall thickness, geometry, etc.) there are many different types of gates which can be used (Figure 31).
The type and size of gate are very critical since they can affect many factors including mold-fill time, overall cycle, orientation, shrinkage, warpage, and part appearance.
Because it acts as a restriction to the polymer flow, a high shear rate is created at the gate often resulting in a temperature increase. There is also a high pressure drop across the gate which needs to be overcome by increased injection pressure or higher temperatures. The pressure drop can be reduced, to a certain degree, by using shorter gate land lengths.
A large gate provides easy filling with relatively low shear rates and pressure drops. However, if it is too large, it will require an excessive amount of time to cool, lengthening the cycle. It is also possible that insufficient packing and subsequent sinks or voids will occur if either sections of the part or the sprue and runner system freeze off before the gate.
A gate which is too small will require higher pressures to inject the material and may cause problems in part filling. If the gate freezes off before the part cools, it will not be possible to develop adequate packing, resulting in voids or sinks. With extremely small gates, jetting or melt fracture of the polymer flow will cause surface appearance defects, including delamination.