6. Runner Design

The fig shown below commonly used runner cross section shaped. The full-round runner is the best in terms of maximum volume-to-surface ratio, which minimizes pressure drop and heat loss. However, the tooling cost is generally higher because both halves of the mould must be machined so that the two semi-circular sections are aligned when the mould is closed.

Fig 4: Commonly used Runner  Cross-Section

The trapezoidal runner also works well and permits the runner to be designed cut on one side of the mould. It is commonly used in three plate moulds. Where the full-round runner may not be released properly, and at the parting line in moulds, where the full interferes with mould sliding action. To compare runners of different shapes, the flow efficiency (L) of the melt through a runner, which is an index of flow resistance, is employed. The higher the flow efficiency of the melt through the runner, the lower the flow resistance is. Flow efficiency can be defined as L = A/P, Where L = flow efficiency of the melt through a runner. A = Cross section area. P = perimeter. Once molten plastic has been injected into the mould cavity, it takes time before the moulding has cooled and become sufficiently rigid to allow it to be demoulded. This period is called the cooling time and often forms a significant part of the moulding cycle. To allow a molded part to cool and solidify, heat must be removed from the mould. The cooling water enters from one side of the plates, circulates and leaves. This normally refers to a fill analysis or a flow analysis. A fill analysis stops when the part volume is just filled to 100%. The flow analysis is a fill analysis but continues through the packing and even cooling phases of the moulding cycle. We can identify and resolve a number of moulding issues using fill analysis before running a flow analysis. Solving filling problems is typically an iterative process requiring several analyses. Once the first fill analysis is done the results are reviewed, and a problem is identified and fixed. This may require much iteration of previous steps. There is one major assumption in the sequence described here, namely, that the problem being addressed is a filling related problem. And not packing, cooling or warpage related. The other problem, too are solved by an iterative process, but only after the filling is optimized. Once the runner system is sized, packing of the part can be investigated. Although a flow analysis can be one without a gate or runner, it is not very much recommended. The freeze time of the gate and runners significantly affects the packing of the part. Without a runner and gate, the packing analysis will be less accurate.

Fig 5: Filling pattern of material into cavity and Air trap in  component

Entrapped air in the mould can escape through air vents that are ground into the parting line of the mould. If the trapped air is not allowed to escape, it is compressed by the pressure of the incoming material and is squeezed into the corners of the cavity, where it prevents filling and causes other defects as well. The air can become so compressed that it ignites and burns the surrounding plastic material. We provide vent holes to escape the compressed air after filling.We can produce a uniform volumetric shrinkage with the use of a packing profile. The volumetric shrinkage distribution should be within 2%. When there is a uniform volumetric shrinkage across the part, the likelihood of warpage is reduced. Warpage is simply caused by a variation in shrinkage, therefore, when the shrinkage variation is reduced, so is the warpage.The fill time result shows the position of the flow front at regular intervals as the cavity fills. Each colors contour represents the parts of the mould which were being filled at the same time.

Fig 6: Fill Time and Confidence of  fill

The pressure at V/P switchover results shows the pressure distribution through the flow path inside the mould at the switch over point from velocity to pressure control. Pressure should be zero at extremities of each flow path at the end of filling. The pressure drop occurs as the cavity completing the filling and. The maximum pressure 0.8010Mpa is noted.