Runner: The runner is a channel machined into the mould plate to connect the sprue with gate to the impression. They are supposed to the distribute the material so that it fills all cavities at the same time under equal pressure. The wall of the runner channel must be smooth to prevent any restriction of flow. The runner should be polished in the line of draw direction In order to eject the runner from the mould plate.
Size of runner: The wall section and volume of moulding. The overall length of runner. Runner cooling consideration. Standard cutter size and plastic material to be used. The runner size should not be below 2 mm dia or above 10 or 13mm. The cross-sectional area of branch runner should be equal to the main runner. The Runner size will be decided based on: The Wall section and Volume of the moulding, the overall length of the runner, Runner cooling consideration, Standard cutter size, and the plastic material to be used. Runner diameter D = √ W x 4 √ L / 3.7
1. Wall section and volume of the moulding
2. The distance of the impression from the main runner or sprue
3. Runner cooling consideration
4. Plastic material to be used
Runner Design: The designer should keep in mind the following points during design of runner. Shape and Cross section of the runner, Size of the runner, Layout of the runner
Runeer efficiency: is defining as the ratio of cross-section area to the periphery of runner. 1) Cross-sectional area. 2) Periphery of runner.
Cross Sectional Area
Runner Efficiency = ------------------------------------------
Periphery of the runner
1. Round runner: This type of runner has two semi circular cross – section which are machined in both mould halves (core & cavity). Smallest circumference with a given cross- sectional area, Smallest cooling, Smallest lost of heat & friction, Good effect of holding pressure.
2. Half round runner: A name of half round runner is semi circular runner which is machined in one plate of mold half. Trapezoidal runner: More plastics melt is required (approximately 25%) compare to round runner. A 10 degree taper angle is provided on the side of the wall of a square runner to get cross-section of trapezoidal runner.
3. Modified trapezoidal runner: This type of runner is specified for molds, which have complex parting surface where use of round runner causes ejection difficulties.
4. Square runner: Square runner is not use in general practice because of its ejection difficulties. It is also difficult to machine in mold plate.
5. Rectangular runner: Rectangular runner is not use in general practice because of its ejection difficulties. It is also difficult to machine in mold plate.
6. Hexagonal runner: is basically a double trapezoidal runner, where the two halves of the trapezium meet at the parting surface. The cross- sectional area of this runner is approximately 82% of that of the corresponding round runner.
Runner layout: The layout of runner system will depend upon the following factors: The number of impression, the shape of component, the type of mold, the type of gate.
Runner balancing: The runner balancing means that the distance the molten plastics material travels from the sprue to the gate should be the same for each moulding. This system ensures that all the impression will fill uniformly and without interruption providing the gate lands and the gate area are identical.
Important consideration in runner layout:
1. Runner length should be reduced to pressure losses
2. Runner system should be balanced
Runner balancing means that the distance the plastic material travels from the sprue to the gate should be the same for each impression.
Runner size:
The Runner size will be decided based on:
The Wall section and Volume of the moulding The overall length of the runner
Runner cooling consideration Standard cutter size
The plastic material to be used
√ W x 4 √ L
Runner diameter D = -------------------------------
3.7
Where:
D= runner diameter (mm)
W = Moulding weight (gm)
L = runner length (mm)
The empherical formula is used when:
1. The weight of moulding is up to 200 gms.
2. The wall thickness of the moulding is less than 3 mm.
Other Points to be Considered for deciding the runner size.
1. For rigid PVC and Acrylics, increase the calculated value by 25%
2. The runner size should not be below 2 mm dia, nor above 10 mm or 13 mm dia.
3. The calculated value should be increased to the next cutter size.
4. The cross sectional area of the main runner should be equal to the area of the branch runners.
Layout of the Runner depends on:
1. Number of Impression
2. Type of Mould
3. Type of Gate
4. Layout of the Cavities
Points to be kept in mind when designing runner layout:
1. Minimum Runner length
2. Balanced Layout
Radial arrangement of Runners with Multiple Cavity Moulds:
Radial arrangement of Runners with Multiple Cavity Moulds:
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