6.0 FEED SYSTEM
6.1 Introduction
Before entering into the impression, the plasticized material passes through the sprue, main runner, sub runners and gate. This is caned the feed system. In other words, the feed system is the channel for the plastic material to flow from the nozzle tip to the entrance of the impression.
6.2 Runner
The runner is a channel machined into the mould plate to connect the sprue with the entrance (gate) to the impression. In the basic two - plated mould, the runner is positioned on the parting surface. The wall of the runner channel must be smooth to prevail any restriction to flow. As the runner has to be removed with the moulding, there must be no machined marks left which would tend to retain the runner in the mould plate. The runner channel must be polished "in line of draw".
Other factors that are to be considered are:
1. The shape of the cross - section of the runner
2. The size of the runner and
3. The runner layout.
6.2.1 The shape and cross-section of runner
The following cross - sectional shapes are usually used
- Fully round
- Rectangular
- Hexagonal
- Trapezoidal
- Modified Trapezoidal
The reasons why these shapes are prepared are as follows:
The runner should provide a maximum cross - sectional area from the stand point of pressure transfer and a minimum contact area from the stand point of heat transfer. The ratio of cross - sectional area to periphery will give a direct indication of the efficiency of the runner design. Higher the valve, the greater the efficiency. A square section is modified to a trapezoidal section by providing a 10 degrees angle on the runner wall. As the plastic melt progress through the runner and the mould system, the melt adjacent to the cold mould surface will rapidly decrease in temperature and solidify. The material which follows will pass through the center of the solidify material and the solidified material acts as an insulation and maintains the temperature of the central region. Therefore the gate should be positioned in line with the center of the runner to receive the material from central flow stream. This condition is achieved with the fully round runner also.
The trapezoidal design is not as satisfactory since the gate cannot be positioned in -line with the central flow stream. The cost for a mould contained round runner is higher because the channels in two mould plates are to be accurately machined and matched to have an efficient runner system. The choice of runner section also depends on the positive ejection of the runner system. The Increased mould cost being relatively small, a fully round runner is preferred for simple two-plate moulds with flat parting surface. For moulds with complex parting surface, the semicircular channels of the round runner should be used. For multi plate moulds the trapezoidal or modified trapezoidal section should be used.
6.2.2 Runner size
The following factors are to be considered when deciding on the runner size.
1) The wall section and volume of the moulding
The cross - sectional area of the runner must be sufficient to permit the melt to pass through and fill the impression before the runner freezes.
2) The distance of the impression from the main runner
Longer the runner length, the greater is the resistance to flow. Hence care should be taken while designing a mould. It is important to note that the sub runner is smaller in size and length than the main runner. We also should take care to note that the total length of runner is as short as possible.
3) Runner cooling considerations
The larger the cross - sectional area of the runner, the higher the period material taken to cool sufficiently to be ejected. So it is undesirable to make the runner diameter more than 10 mm. However for melts with higher viscosity the runner can be up to diameter 12.5 mm ex: rigid PVC and acrylics.
Balanced runner
6.3 Gates
The gate is channel or orifice connection the runner with the impression. It has a small cross - sectional area when compared with rest of the feed system. This small cross - sectional area is necessary so that:
- The gate freezes soon after impression is filled so that the injection plunger can be withdrawn without the likelihood of void being created in the moulding suck back.
- It allows for simple degating and in some moulds the degating can be automatic.
- After degating, only a small degating mark remains.
- Better control of the filling of multi-impressions can be achieved.
- Packing the impression with material in excess of that required to compensate fir the shrinkage is minimized.
The size of the gate can be considered in terms of the gate cross-sectional area and the gate length, the later being known as gate land. The optimum size for a gate will depend on a number of factors including
- The flow characteristics of the material being moulded
- The wall section of the mouldings
- The volume of the material to be injected into the impression
- The temperature of the melt
- The temperature of the mould
6.3.1 Position of the gate
The position of the gate should be such that there should be an even flow of melt in the impression, so that it fills uniformly and the advancing melt front spread& out and reaches the various impression extremities at the same time. In this way two or more advancing front can meet to form a weld line with consequent mechanical weakness and surface roughness in the moulding. Such an ideal position for the gate is possible in certain shaped mouldings such as those with circular cross section. For example, a cup or a cone in which material is fed through the centre of the base or apex. The direction of melt flow of a centre feed compared with the side feed is shown in figure.
a) Edge gating of cup-shaped moulding
b) Same moulding with a sprue gating
A central gating at the edge is done for slender core like components such as pen caps. A side gating in this case can cause deflection of the core. This is because side gating gives rise to a more rapid flow of material down one side of impression, resulting in a differential pressure which can move the core out of position. This also results in a thinner wall section on one side, thus adding another weakness to that of weld line.
Position of gate for a pen cap
a) Edge gate – note the deflection of core
b) Underfeed pin gate – even flow holds core central
When the edge gate is used, the gate should be positioned so that the melt flow immediately meets a restriction. An example is shown in the figure given below. The impression is fed in the centre at one end and the material, on entering at high velocity “jets” and quickly sets on reaching the cool mould walls. More material then enters and flows around the original jetted material. The resulting flow lines are often visible on the finished component.
Jetting is prominent inhomogeneous snake like strands on the surface of molding.
Position and choice of gate for solid block type of moulding
a) If edge is used, jetting of material occurs
b) Overlap gating avoids this undesirable feature
Jetting originating at the gate, spreading over entire part. This trouble can be overcome by overlap feeding or tab feeding.
6.3.2 Balancing of the gate
It is often necessary to balance the gates of a multi impression mould to ensure that the impressions fill simultaneously. This method is adopted when a balanced runner system can not be used. By adopting the method of balanced gating, there are two ways or varying the restriction.- By varying the land length.
- By varying the cross sectional area of the gate.
In practice, balanced gating is a matter of trial and error. The land length normally is kept constant; starting with a small gate width, the mould is tried out with a short injection stroke so that a short moulding is obtained. On inspection, it will be obvious which impressions are filled first. The gate width can be progressively enlarged and adjusted until balanced filling is achieved.
6.3.3 Type of gate
The types of gate must be carefully chosen to obtain the optimum filling conditions. The types of gate commonly used are, sprue gate, edge gate, overlap gate (or tab ), fan gate, diaphragm gate, ring gate, film gate, pin gate, winkle and submerged gate..
1) Sprue gate
When the moulding is directly fed from a sprue or secondary sprue, the feed section is term as sprue gate. The main disadvantage with this type of gate is that it leaves a large gate mark on the moulding. The size of this mark depends on : (1) the diameter at the small end of the sprue, (2) the sprue angle and (3) the sprue length. Thus the gate marks can be minimized by keeping the dimensions of the above factors to a minimum. Note that as the sprue entry is controlled by the nozzle exit diameter and , as it is undesirable to reduce the sprue angle below two degree inclusive for withdrawing purpose, the sprue length is the logical dimension for the designer to attempt to reduce. On a basic two-plate mould, the sprue gate is used only for single-impression moulds. In this case, the impression is positioned in the centre of the mould and the sprue is a direct feed into it.
2) Rectangular gate
This is the general purpose gate and it is a rectangular channel machined in on mould plate to connect the runner to the impression. The advantages are
1. The cross sectional form is simple and it is cheap to machine.
2. Close accuracy in the gate dimensions can be achieved.
3. The gate dimensions can be easily and quickly modified.
4. All common moulding materials can be moulded through these types of gate.
The disadvantage of this type of gate is that after gate removal a witness mark is left on a visible surface of the moulding.
The following empirical relationship for gate depth has been found useful.
h = nt
h = depth of gate
n = material constant
3) Fan gate
This is other types of edge gate but does not have a constant width and depth. The fan shape spreads the flow of the melt as it enters the impression and a more uniform filling is obtained with less flow marks and surface finish. The width increases and the
depth decreases so as to maintain a constant cross-sectional area throughout the length of the gate. The effective length of the gate land between the runner and the impression progressively increases from a minimum at the centre line to a maximum at the outer gate wall. This type of gate can be used with all conventional moulding materials apart from certain grades of rigid PVC.
4) Tab gate
This is particular gating technique for feeding solid block types mouldings. A projection or tab is moulded on to the side of the component. That is mainly used to avoid undesirable jetting on the moulded part. The melt is thereby caused to advance in a smooth steady flow and providing the shape of the impression allows it, the
impression will fill uniformly. Thus the tab gate is an alternative to the overlap gate. The choice of the gate will depend mainly upon whether the witness mark left by the gate is best, from the appearance point of view, on the top or the side. This gate , while being developed particularly for the acrylics, may be used for any of the common moulding materials.
5) Overlap gate
It can be considered as a variation of basic rectangular edge gate and is used to feed certain type of moulding. In this type of gate we have a better control over the flow rate. This has the disadvantage of big remnant mark after degating. For block type moulding, the rectangular gate is replaced by the overlap gate which, by virtue of its position, directs the melt flow against an opposite impression face. The overlap gate ,
a) View of cavity plate b) moulding with gate attached
c) Overlap gate machined d) cross-section through mould.
which is of general rectangular form, is machined into the plain mould plate in such a way that it bridges the gap between the end of the runner and the end wall of the impression. This gate can be used for all general moulding materials except PVC.
6) Diaphragm gate
This gate is used for single impression tubular shaped mouldings on two plate moulds. The sprue leads into a circular recess slightly smaller than the inside diameter. This recess forms a disc of material and acts as a runner which allows
material to flow rapidly from the sprue to the gate. The gate may be cut either on the core or in the cavity. Thus by a simple machining operation on the face of the moulding the bore diameter is not disturbed.
7) Ring gate
The function of this gate is identical to that of a diaphragm gate. This type of gate is used for tubular type mouldings when more than one impression is required in a simple two plate mould. The gate provides for a feed all round the external periphery of the moulding and permits the use of a conventional runner system.
The runner in the form of trapezoidal annulus is machined into the mould plate. The trapezoidal runner is normally used since this type of moulding would be ejected using a stripper plate.
8) Film gate
This is a long rectangular type edge gate and it is used for large thin - walled components to assist in the production of warpage free products.
The gate normally extends across the complete width of component although a smaller width may be used initially. The gate is similar in principle to the diaphragm and ring gates in that it provides for a large flow area and results in a quick fill time. The gate is used for all common moulding materials. It is particularly useful for those materials which exhibit differential shrinkage for which central feeding is not possible.
9) Pin gate
This is a circular gate used for feeding into the base of the components and it is preferred to the sprue gate because it is small in diameter. It is often to be preferred to the sprue gate which necessitates a finishing operation. However, the pin gate may only be used in certain types of moulds and these are generally more complex in design than the moulds in which sprue gating or side gating techniques are used.
A section through a typical pin gate is shown in figure. This shows a three-plate underfeed type mould. The gate is essentially a circular orifice which connects the secondary sprue to the impression.
To permit he use of a pin gate , one of the following mould design must be adopted.
- Three-plate underfeed type mould
- Hot runner moulds
- Two plate mould with special nozzles.
The gate dimensions which must be considered are the land length(L)., and the gate diameter(d). To minimize the pressure losses, as for all other gates, the land length is kept to a minimum consistent with the strength of the steel used. A lsnd lrngth of between 0.5mm and 0.75mm is suitable.
10) Round edge gate
This gate is formed by machining a matching semi circular channel in both mould plates between the runner and impression. The slight radius incorporated on the
Entry into the impression is an advantage in preventing damage to the moulding when the gate is removed. However, because of its form, the round edge gate suffers many disadvantages as compared with the corresponding rectangular edge gate.
- The matching form is more difficult to machine
- Precise dimensions are more difficult to achieve.
- The filling rate can not be controlled independently of the gate seal time.
11) Submerge gate
The submerge gate is circular or oval gate which submerges and feeds into the impression below the parting surface of the mould. This gate is even called subsurface gate or submarine gate.
The advantages of this gate are:
- The form being on one mould plate, no matching problem and precise dimensions can be achieved.
- If the more oval form is used, the filling rate of the impression can be controlled independently of the gate seal line.
- The gate is sheared from the moulding during its ejection (degating is automatic).
- Leaves very small degating marks.
- More uniform filling is obtained with less flow marks and surface finish.]
The moulding and feed systems are removed separately from the mould and this means that a separate runner ejection is advantageous, particularly as a certain amount of deformation of the runner is necessary to remove the secondary runner from the mould.
12) Winkle gate:
This is a curved variation of the subsurface gate and for that reason it is sometimes called a “curved subsurface gate” or “curved tunnel gate”. The basic design of a winkle gate is shown below.
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