Gate: is a small channel or orifice connecting the runner with the impression. It has a small cross- sectional area when compared with the sprue, or runner. The small cross- sectional area is necessary because of following reasons: The gate freezes soon after the impression is filled. So that nozzle can with drawn without possibility of back flow of material, voids EST.
The gate is a channel or orifice connecting the runner with the impression.
The size and shape of the gate depends on the following factor:
1. The flow characteristics of the material to be moulded
2. The wall section of the moulding
3. The volume of the material to be injected into the impression
4. The temperature of the melt
5. The temperature of the mould
A small gate is desired so that: The gate freezes soon after the cavity filled. The injection plunger can be withdrawn immediately without forming voids due to suck-back. It is easy for degating. Small witness mark remains on the product. Better filling of cavities in multi-impression mould. More packing of material due to shrinkage effect is minimised.
Gate design: Gate is a small channel that connects the cavity with the runner.
Gate size: The size of the Gate should be designed to permit optimum flow of material.
Optimum Size depends on: The flow characteristics of the material, the wall section of the moulding, the volume or weight of the moulding, The moulding temperature, The mould temperature
Types of gate: To obtain the optimum filling conditions the type of gate must be carefully chosen. Direct sprue gate, Pin point gate, Rectangular Edge gate, Fan gate, Film gate, Tab gate, Spoke gate, Diaphragm gate, Ring gate, Sub-surface or submarine or Tunnel gate/ submarine gate, Overlap gate.
Gate Location:
The location of the gate is decided based on:
Appearance
Function of the part (end use)
The area near the gate is highly stressed due to:
Frictional heat generated at the gate
The high velocity of the melt.
Gate location affects the molecular orientation of the resin during injection process which produces:
Directional variation in structural properties
Gate area should be away from the load bearing surfaces.
Positioning of gate: it’s should be such that there is an even flow of melt in the impression, so that it fills uniformly and takes same time to fill each impression. The gate should be positioned in such that there is an even flow of melt in the impression. The gate should be positioned in such a way that no jetting can occur, causing troublesome marks.
GATE SIZE:
The size of the Gate should be designed to permit optimum flow of material.
Optimum Size depends on:
1. The flow characteristics of the material
2. The wall section of the moulding
3. The volume or weight of the moulding
4. The moulding temperature
5. The mould temperature
Gate Area = h x w
Where: h = depth of gate
w = width of gate
n . √A
Gate width W = ----------------------------
30
Where : n = material constant (o.6-0.9)
A = surface area of cavity (mm2)
Depth of gate h = n . t
Where: n = material constant ;
t = wall thickness of the moulding (mm)
Land length L = 0.5 – 1.5 mm.
Types of gate:
1. Sprue Gate: A direct (or sprue) gate is commonly used for single-cavity molds, where the sprue feeds material directly into the cavity rapidly with minimum pressure drop. The disadvantage of using this type of gate is the gate mark left on the part surface after the runner (or sprue) is trimmed off.
When the moulding is directly fed from a sprue or secondary sprue, the feed section is termed as a sprue gate. This type of gate is used for single impression two plate moulds.
2. Tab gate: The tab gate is used for feeding solid block type transparent moulding.
A projection or tab is moulded on to the side of the component and a conventional rectangular edge gate feeds this tab.
3. Edge (Standard) gate: An edge gate is located on the parting line of the mold and typically fills the part from the side, top, or bottom.
Rectangular edge gate: This gate feeds the material from one side into the mold cavity. A rectangular channel machined in one plate to connect the runner to impression. The gate dimensions can be easily & quickly modified.
The filling rate of the impression can be controlled relatively interpedently of gate seal time. All common moulding materials can be moulded through this type of gate. This is a general purpose gate and in its simplest form is merely a rectangular channel machined in one mould plate to connect the runner to the impression.
4. Overlap gate: The overlap gate is a variation of the basic rectangular type gate and used for block type moldings. This gate prevents formation of jetting by directing the melt flow against an opposite face of the impression; the gate is machined into plain mold plate in such a way that it bridges the gap b/w the end of the wall of impression.
The gate being attached with moulding surface. The cross-sectional form simple. It is cheap & easy to machine. Close accuracy in the gate dimension can be achieved. The gate dimensions can be easily & quickly modified. The filling rate of the impression can be controlled relatively interpedently of gate seal time. All common moulding materials can be moulded through this type of gate. This is a variation of rectangular type gate. The overlap gate is machined into the plain mould plate in such a way that it bridges the gap between the end of the end of the runner and the end wall of the impression.
5. Fan gate: Fan gate is used for thin walled large moulding. The dimension of width of fan gate is increases while depth decreases. The gate at the impression is relatively wide and because of this, a large volume of material is injected in a short time.
This is another edge gate which has variable width and depth. The width increases while the decreases so as to maintain a constant cross sectional area throughout the length of the gate. This type of gate is used for large area,thin-walled mouldings.
6. Disk (Diaphragm) gate: This gate is used for single impression tubular shaped moldings on two plate moulds. It may also be used in a similar manner for multi – impression tubular shaped moldings on underfeed and runner less moulds.
Determination of gate: If the internal bore is important then the gate should be cut in the cavity. If internal bore is not important then gate is cut on core.
7. Ring gate: Ring gate is used for tubular type moldings when more than one impression is required in a simple two plate mold. The gate provides for a feed all around the external periphery of the moldings.
It permits the use of a conventional runner system to connect the impression. The runner in the form of trapezoidal annulus is machined into the mold plate. This type of moldings would be ejected using a stripper plate. This gate is similar to the diaphragm gate .
8. Spoke (spider) gate:
This kind of gate is also called a four-point gate or cross gate. It is used for tube-shaped parts and offers easy de-gating and material savings.
Disadvantages are the possibility of weld lines and the fact that perfect roundness is unlikely.
9. Pin gate: This is a circular gate used for feeding in to the base of components and because it is relatively small in diameter it is prefer dot the sprue gate which necessitates a finishing operation.
10. Submarine (tunnel, chisel) gate:
A submarine gate is used in two-plate mold construction. An angled, tapered tunnel is machined from the end of the runner to the cavity, just below the parting line. As the parts and runners are ejected, the gate is sheared at the part.
If a large diameter pin is added to a non-functional area of the part, the submarine gate can be built into the pin, avoiding the need of a vertical surface for the gate. If the pin is on a surface that is hidden, it does not have to be removed.
Multiple submarine gates into the interior walls of cylindrical parts can replace a diaphragm gate and allow automatic de-gating. The out-of-round characteristics are not as good as those from a diaphragm gate, but are often acceptable.
11. Film gate: Film gate also called flash gate is very thin compare to another gate, this thin gate like film which have parallel runner before the gate, this type of gate used for straight edges. Generally this gate used in thin and flat requirement, like flat mobile phone cap, ipod cap and others. but sometimes this gate is not likely going to have a flat flow front, the make flow material more stable and flat, make temperature molds more higher than usual, but cycle time will also higher.Picture below shown film gate with main parts.
12. Hot-runner (hot-probe) gate:
A hot-runner gate is generally used to deliver hot material through Heated runners and electrically heated sprues directly into the cavity, producing runnerless moldings.
The packing cycle is controlled by the freeze-off of the part near the gate. The very hot material at the gate is torn from the part as the cavity is opened.
13. Valve gate:
The valve gate adds a valve rod to the hot runner gate. The valve can be activated to close the gate just before the material near the gate freezes. This allows a larger gate diameter and smooths over the gate scar. Since the packing cycle is controlled by the valve rod, better control of the packing cycle is maintained with more consistent quality.
The valve gate adds a valve rod to the hot runner gate. The valve can be activated to close the gate just before the material near the gate freezes. This allows a larger gate diameter and smooths over the gate scar. Since the packing cycle is controlled by the valve rod, better control of the packing cycle is maintained with more consistent quality.
Gate Balancing: When moulds incorporate a large number of different shaped impressions, it is not possible to have a balanced runner system. In these cases, balanced filling of impression can be achieved by varying the gate dimensions, which is termed as gate balancing.
GATE BALANCING IN MULTI-IMPRESSION MOULD (COMPOSITE MOULD):
For Round Gate:
d2 = d1(w2/w1)1/4
Where
d1 = The gate diameter of the first cavity (cm)
d2 = The gate diameter of the second cavity (cm)
w1 = The weight of the first cavity component (gm)
w2 = The weight of the second cavity component (cm)
For Rectangular Gate:
t2 = t1 (w2/w1) 1/3
Where
t1 = depth of gate in first cavity (cm)
t2 = depth of gate in second cavity (cm)
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