23.0 FLOW SYSTEM
23.1 Introduction:
The design of a flow system to inject molten alloy in to a die cavity greatly influences the success of a die. The failure of many dies can be traced to faulty design of the flow system. By applying the basic principles of hydraulics a proper molten metal flow system can be developed. The following explains the recent trend in the die - casting die design, which lays much emphasis on the molten alloy flow system.
The system aims at an efficient controlled delivery of molten alloy through the flow system to the die casting with minimum mixing of molten alloy and air.
23.2 Machine suitability
Choice of suitable machine is an important aspect of die designing. Two main considerations determine suitability:
1. Clamping force.
2. Injection capability.
Clamping force can be calculated by multiplying the projected area of the impression including runners, gate overflow by the injection pressure. The result should be less than the clamping force of the machine.
The injection capability or metal pumping capacity of the machine must be determined from the PQ2 diagram. In a typical PQ2 diagram, metal pressure “P” is shown on the vertical axis and flow rate “Q” on the horizontal. Metal pressure is approximately proportional to the square of flow rate. Pressure P is plotted on linear scale and Q on a squared scale.
The flow system comprises of a series of passage of varying lengths and diameters which carry molten alloy at varying high velocities from the under side of the injection plunger to the gate. In addition to variation in length and diameter a number of changes in directions occur. At each change energy is consumed to overcome the resistance to the flow.
The flow system should be properly designed and manufactured to minimize the energy losses. The flow system should converge from gooseneck to gate, sharp corners should be avoided and the surface of the flow path should be smooth and well finished.
The use of controlled fully converging flow paths through nozzle, sprue and runners and gate contracts with traditional designs which typically includes large random changes in cross sectional area and shape.
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23.2.1 The gooseneck:
The bore of the gooseneck should be sufficiently large to ensure a low velocity through the major bend, minimizing pressure losses.
23.2.2 The nozzle
The nozzle is basically a heated pipe connecting gooseneck to the sprue bush. The bore of the nozzle should be only slightly smaller than the bore of the gooseneck so that the velocity of the molten metal through the nozzle is kept down to the recommended value.
Low goose neck and nozzle velocity ensures that the pressure losses are low when the molten alloy reaches the sprue.
If calculation indicate the need for a smaller nozzle exit diameter, the step down should be at the exit and the change in diameter should be smooth to minimize pressure losses. The metal velocity through the nozzle should not exceed 30m/s. Higher velocities are acceptable if the distance from nozzle to gate is short and the gate velocity is same or only slightly higher the nozzle velocity.
Velocity can be decreased by:
- Use of a large diameter nozzle (provided it does not exceed the gooseneck bore).
- Use of a small diameter injection sleeve and plunger.
23.2.3 The sprue:
The sprue has two basic functions:
· To provide a smooth flow path between nozzle and runners.
· To provide a means of rapidly solidifying molten metal alloy at the entrance of the die after The cavity has filled and the flow has stopped. (this requires a large temperature difference Between sprue and nozzle.)
Conventional sprue designs do not satisfy the converging flow system. There are massive variations in cross sectional area of flow paths at various points of the sprue. This causes pressure losses and the flow velocity of molten metal is not uniform.
Runner type sprues have been developed to achieve a smooth flow path with uniform cross section, minimum change of direction and smooth transition from nozzle to sprue and sprue to runner
Standardized runner sprue is shown in fig. The shape of flow path is good, metal flow characteristics are excellent and the transitional curve into the runner is a relatively large radius.
23.3The runner system from sprue to gate:
23.3.1 main runner:
The main runners join the sprue to the final tapered tangential runners feeding the gate. They should be as short as possible and must have a constant cross sectional area.
23.3.2 Tapered tangential runners:
Tapered tangential runners provide the final flow path from main runners to gate. They provide an efficient flow path based on simple geometry and permit the use of very thin gates (from 0.15 - 0.3mm). The velocity of molten metal in a 10 mm long parallel section 2 x 2 mm running into the simple disc shaped shock absorbers.
23.3.3 Runner cross section:
Circular sections give best flow efficiency and minimize thermal losses, but are the least practical cross section from the tool - maker's view point.
For tapered tangential runners modified square or trapezoidal shapes are used. Area calculations are based on square sections and are converted into trapezoidal shapes.
Pairs of tapered tangential runners can be used to feed a long gate into a single cavity. A thin delta shaped region between the paired runners is found. Depending on the size of the cavity and flow requirements varying delta configurations are used.
The angle of exit of molten alloy from gates fed by tapered tangential runners varies with the velocity of runners. In low velocity system the angle is near 90° to the direction of runner flow and reduces as the Velocity increases. .
Paired tapered tangential runner for a high velocity system requires a. Huge turning delta from the main runner to minimize pressure losses and to reduce the possibility of separation of molten alloy from the wall of the flow channel.
For a low velocity paired runner system the bend is sharp and the delta is small, improving flow across it. The angle of metal exit from the gate is near perpendicular to the runner.
A further development of the above system is to make the bend approximate to an elliptical curve. Theoretically the static pressure of the molten metal in the bend area is high and chances for cavitation are reduced. The delta region becomes short compared to its width. The flow across it is improved.
The delta region is initially cut to gate depth. After trials if the flow path is found unfavorable (pre mature freezing may occur), the delta geometry is changed to encourage flow.
General rules for runner design :
- Ratio of width to depth is 2 : 1 to 3 : 1
- Ratio of gating thickness to runner thickness is 1 : 6 to 1 : 10
- Ratio of gating area to runner area is 1 : 3 to 1 : 4
- Side walls should be machined at an angle of 2 deg. To 5 deg
- Runner should be designed short & straight into the cavity wherever possible
- Long runners will result in undesirable loss of metal temperature & over heating of the die on the gating area.
- Never design a runner having abrupt changes in direction, which may tend to develop turbulence
- Side of the runner away from the gate is slightly angled to provide draft for ejection
- The side next to the gate has a definite angle (approach angle) which directs the metal in to the gate.
- Bottom is flat but corners is a radius
- The runner as approaches the sprue / biscuit down the stream will become progressively larger.
23.3.4 Shock absorbers:
The small end of tapered tangential runners terminates in simple disc shaped shock absorbers. They control the very high transient velocities which may occur at the small ends of the runners. High energy release through the extremities of the gate, causing severe damage to the die. Shock absorbers also protect the die when the gate does not flow completely as in the case of a cold die during set up, when very high velocities can be reached.
Simple shock absorbers are in the form of small circular cavities cut tangentially to the standard 10 mm long 2 x 2 mm parallel section at the small end of the runner. Metal flow into and around the shock absorber cavity traps air that is compressed, causing the metal to decelerate.
The volume of the shock absorber is arrived by making it 2 mm deep (for machining
convenience) and calculating its top area to match that to the entry to the tapered runner.
Gate Design
With thin gates as soon as the hot injected metal comes in contact with the relatively cold die, the outer skin freezes. This introduces a chocking effect. Hence the metal has to flow through the core portion, which is still in the molten stage.
During filling time, the gate progressively solidifies from the outer skin to the inner core
This increases the resistance to flow. This would demand higher injection pressure when very thin gate is used
- The proportion of the area of the runner to the area of gate must be1.25:1to1.6 : 1
- Thin gates are preferable for trimming operations.
- It is very unlikely to achieve solid front fill with thin gates
- With very thick gate there is a possibility of pinhole porosity
- at the gate area coupled with trimming problems
- However with thick gates it is possible to achieve solid front fill
Gate Thickness:
Component weight | |||
Upto 100 grams | 100 – 1000grams | 1kg – 5 kgs | |
Zinc alloy | (0.3 – 0.6) mm | (0.5 – 1.2)mm | (0.8 – 1.8)mm |
Aluminium alloy | (0.5 – 1.0)mm | (0.8 – 1.8)mm | (1.5 – 3.5)mm |
General rules for Gate Design:
- Preferably only one gating should be provided. In case more gating, care should be taken that individual metal stream entering the cavity does not interfere.
- The cavity should be filled from one direction to another; care should be taken to avoid the formation of jets in the incoming stream.
- It is preferably for large castings to provide the gating point somewhere in the middle, this will shorten the distance the metal has to travel through the cavity.
- Care should be taken while deciding the place & direction of gating that no air pockets can be developed during the filling period.
- On a correct gating, the metal entering the cavity should push the air to the air vents
- On thin walled castings, the best surface finish can be obtained generally with thin gating & high injection pressure.
- On thick walled castings, sound & pressure tight castings can be obtained only with thick gating, slow injection speed but high pressure
- The metal stream should fill the cavity with the least possible obstruction. i.e.. Direct hitting on cores should be avoided as far as possible.
- The gating point should be so arranged that easy breaking of gating be ensured with out spoiling the corners of the casting.
- It is always advisable to start a new die with a thinner gating since it is easy to increase as & when the condition so require
- The gating section depends on the volume of the casting; hence the thin gating are to be made longer than thick gating for castings of the same weight.
- The travel of small individual streams should be as short as possible. On long travel, the alloy may cool on the die wall to an extend that fusing at the meeting points is hardly possible
- Small passages should be fed directly from the runner with hot metal, otherwise deep flow marks may occur
- The meeting points should be preferably on thick sections where the die temperature can be maintained hot.
23.3.5 Fan gate:
The fan gate has been largely superseded by the tapered tangential runner. But fan gate is useful in some applications. To increase its efficiency the inlet should be larger than the gate.
Correct design and dimensions of gates is essential to achieve high casting quality and efficiency. To obtain desired metal flow velocities gate thick ness from 0.15 - 0.3 mm and gate lands 1.0 - 1.5 mm are used for castings up to 1.5 kg. Thin gates simplify the trimming of castings.
23.3.6 Gate positioning:
General guidelines for gate positioning are given below:
a) Center gating:
Center gating offers the best feeding configuration. It permits the use of short direct feed system. It achieves an ideal flow path across the die cavity.
b) Edge gating using tapered tangential runners:
The following are the recommendations:
· The longest uninterrupted length of casting available for casting should be chosen but it must satisfy the shortest and smoothest flow path across the cavity.
· The direction of metal flow in the cavity should be chosen such that at least one side or comer be open during fill time to allow for venting.
c) Gate area:
The total gate area must always be smaller than nozzle exit area. But it should not be less than 40% of nozzle exit area.
d) Gate velocity:
The velocity of the molten alloy through the gate depends on:
- The energy of the machine.
- The pressure losses in the total flow system including the gate.
- The type of runner.
True velocity through gates fed by tapered tangential runners is higher than the figure arrived at using the formula. .
Velocity (v) = volume flow rate(q)
Gate area (a)
In the schematic diagram of a typical tangential runner, the component in the direction of arrow “A” is the one calculated by the formula. It is at 90° to the direction of flow in the runner. The component of gate velocity along the runner is shown as vector AB. Depending on the efficiency of the runner, between 80 and 100% of the runner velocity can be expected as a component of gate velocity which will alter the angle of flow from the gate and its velocity this is also true for constant area from gates.
True gate velocity can be calculated using the formula:
True gate velocity = √ ((runner velocity) ² + (gate velocity)²)
Where runner and gate velocity are the figures calculated using the formula V = Q/A
Adequate results can be achieved using the formula provided.
1. An upper limit of 35m/s placed on molten alloy velocity at the runner inlet (except in very short feed systems).
2. An upper limit of 45m/s is placed on gate velocity normal to the runner.
3. The gate is not less than 40% of nozzle exit area.
In the above case true gate velocity = √(352+ 452)
= 57 m/s
The approximate true gate velocity should not exceed 60m/s.
If it is necessary to exceed the upper limits, the die must be protected by providing adequate shock absorbers, avoiding sharp bends or recessed ejector pins in the flow path.
If the flow system is designed to operate at reduced injection pressure the true gate velocities for both reduced and full injection pressure should be calculated using the above formula and recorded in the step by step design procedure.
23.4 Air vents:
The use of converging flow system with correctly designed runners ensures that air in the feed system is pushed out ahead of the molten alloy flow and expelled through the cavity to the air vents.
Air vents with total entrance areas up to 20% of gate areas are recommended (depth 0.075 mm). Depths greater than 0.075 mm may lead to flashing problems. Corrugated vents up to 1 mm may be used. Vents are positioned where the flow on the cavity is expected at a late stage of cavity fill.
For casting metals that melts at high temperature such as aluminium, magnesium and brass.
23.5 Overflows:
No overflows need be cut prior to the first casting trial unless their use is specified for ejection purposes.
On trial if the castings exhibit poor finish or blister the following conditions must be checked first:
1. Machine operations and performance.
2. Feed system design and performance.
This will establish that both machine and die are operating at the design parameters.
The actual purpose of an over flow
• To maintain an even die temperature
• To venting of the die cavity
• To receive the cold metal ( first flow)
• Reception of lubricants remains.
If overflows are found necessary, its positions are determined by examination of flow patterns in the casting.
The design considerations:
- Depth of overflow must not be great, as the solidification will take time leading increase in the cycle time.
- The depth of the overflow should be about 3 times the section of the casting and width about double the depth
- Overflows should be matched close to the cavity approximately 3 to 6 mm and their thickness between 0.5 to 1.5 mm depending upon casting section.
- Overflow should be so provided that heat will enter the appropriate area of the die
- Shape should be such that it should be easy machining and ejection
- Generous radii should be provided at bottoms/corners of the die
- Each overflow should have one ejector pin
- Better to have series of overflows than one longer overflow
- If the metal flows a long distance, the overflow size should be enlarged.
- Overflow should be sized according to the volume of the cavity,from which the overflow receives molten metals.
23.6 Determination of gate area:
Gate area is determined using pQ2diagram. The pQ2 diagram shows the machine characteristic line A -B (maximum pressure against maximum flow rate) for a typical 160 ton (1.6MN) hot chamber machine fitted with a 60 mm injection sleeve. The line intersecting the machine characteristic line represents a range of nozzle diameters. At each point of intersection with the machine characteristic line the flow rate (Q) area (A) and molten alloy velocity (V) for each nozzle are stated. Even though the values on the diagram refer to nozzle velocities, it is assumed that the molten alloy velocity through a gate of a given area will be similar to alloy velocity through a nozzle of the same area at the same flow rate. (Inertia forces during fill time increases pressure, compensating for friction and other losses).
When less injection pressure is desirable a reduced pressure line CD should be drawn parallel to the original machine characteristic line and new values for flow rate and velocity are to be considered.
If the die is designed to operate at reduced injection pressure or reduced speed or both strict control of operating conditions must be applied otherwise the die may be damaged.
23.7 Procedure to calculate runner and gating dimensions:
The method given below is based on the angle / Velocity charge, which determines metal flow angle and velocity through the gate:
Step | Description | Example |
Step one | Choose exit angle θ appropriate to the casting shape to be made | 300 |
Step two | The vertical lines within the shaded band are the recommended runner entry velocities. Select appropriately | 25 m/s |
Step three | From the point of intersection of the chosen runner entry velocity line and chosen angle of exit line trace the curve back to the base line of the chart | 50 m/s |
Step four | Gate velocity normal to the runner (arrow A) is determined by measurement from the point of intersection to the base line or by calculation. | 25xcot30 = 43m/s |
Runner entry velocity (arrow) times Cot θ or by √side C2 side B2
The calculated value can be used to develop the total runner system using the PQ² diagram for the chosen machine.
E.g.: From the available PQ2diagram in order to achieve a gate velocity of 43 m/s at full power the gate area will be 154 mm2
(1.6 MN machine with 60 mm plunger
23.8 Procedures used to calculate runner and gate dimensions:
This procedure uses PQ2diagram for the design. The casting is illustrated in the fig.
Step | Description | Example |
Step one (calculate projected area and mass) | 1. Calculated projected area(AP) | 51200 mm2 |
2. Mass(m) of the casting from drawing or model | 1 kg | |
Step two (Determine machine locking suitability) | 1. Select plunger size | 80mm dia |
5027 mm2 | ||
2. Pressure on metal | 16 MN/m2 (160 kg/m2) | |
3. Area of runners & overflows (AT) | 10200 mm2 | |
4. Total projected area (APT) = AP+AT | 61400 mm2 | |
5. Force (pressure x area) | 61400 x 10-6 x 16 | |
6. Locking force (2.8 mm) | 0.9824 MN | |
7. Compare Force & Locking force | 28 MN | |
Step three (Calculate liquid volume casting) | 8. Liquid volume of Zinc alloy at 410o = 6.12 kg/dms3 | 1/6.12 = 0.164 dm3 = 164 cm3 |
Step four (Confirm displacement capacity of selected machine | 1. Shot volume | 244 cm3 |
2. Maximum volume | 817 cm3 | |
Step five (Calculate the tie of displace liquid volume of casting through the largest diameter nozzle at full machine power) | 1. Displacement time | Liquid volume / max. flow rate (from PQ2 diagram) |
2. Largest nozzle diameter | 25 mm | |
3. Area | 491 mm2 | |
4. 164 / 12750 ( for reference only) | 0.013 s | |
Step six
Establish gate velocity, flow rate, metal pressure and velocities for various areas are available at PQ2 diagram
Gate area | Velocity | Flow rate | Time | |
Possible combination 1 | 380 mm2 | 31.5 m/s | 1200 cm3/s | 0.014 s |
Possible combination 2 | 314 mm2 | 36 m/s | 11250 cm3/s | 0.015 s |
Possible combination 3 | 254 mm2 | 41 m/s | 10400 cm3/s | 0.16 s |
Possible combination 4 | 201 mm2 | 45.5 m/s | 9200 cm3/s | 0.18 s |
Step seven
From the drawing, determine the maximum practical length available for gating - say 600 mm
Step Eight
Establish depth of gate for possible combination 1, 2, 3 & 4
Depth = Area / length
Combination 1 - 380 / 600 = 0.63 mm
Combination 2 - 314 / 600 = 0.52 mm
Combination 3 - 254 / 600 = 0.42 mm
Combination 4 - 201 / 600 = 0.34 mm
Step nine Establish appropriate runner dimensions based on the smaller gate area from the combinations by apportioning the difference between nozzle and gate area over the rest of the flow system | Nozzle area | 380 mm2 |
Smaller gate area | 201 mm2 | |
Total area of runner entries (estimate) | 250 mm2 | |
Main runner area | 300 mm2 | |
Sprue runner area at entry (estimate) | 360 mm2 | |
Step ten | Total area of runner entry | 250 mm2 |
Velocity | 41 m/s | |
From the PQ2 diagram, check flow rate and velocities through the same area at reduced pressure | Reduce power to flow rate and velocity | 14 MP2 |
Nozzle velocity = Q/A | 10400/380 =27.4 m/s | |
Gate velocity v = Q/A | Combination 1 | 10400/380 = 27.4 m/s |
Combination 2 | 9700/314 = 30.9 m/s | |
Combination 3 | 8900/254 = 35 m/s | |
Combination 4 | 7800/201 = 38.8 m/s | |
Runner | Entry Velocity = Q/A | 8900/250 = 35.6 m/s |
Repeat step nine and ten for larger gate area if considered necessary.
Step Eleven
Design total flow system apportioning the difference in area between the nozzle and the entry to the tapered runners over the rest of the system. Draw an area chart, which shows graphically the convergence of the total flow system. The inlet to a bend should be larger than the exit. The ratio of the inlet to exit will depend on the sharpness of the bend. A sharp bend will require a large inlet to exit ratio (e.g. 1.3: 1).
Step twelve
Approximate true gate velocity = √ (runner velocity)2+ (gate velocity) 2
= |
= √ 352 + 392
= 52 m/s
Approximate true gate velocity should not exceed 60 m/s.
23.9 Procedure when PQ² diagram is not available:
The following procedure helps to calculate runner and gate dimensions in the absence of PQ2diagram. The procedure is less formal and less satisfactory. But excellent results are obtained practically.
The gooseneck / nozzle relationship of many die casting machines are not satisfactory. The bore of the nozzle is many times smaller than the bore of the gooseneck. This result in a high-pressure loss and increase in velocity at the wrong end of the flow system. By increasing the bore of the nozzle so that its area is approximately 15% less than the area of the goose neck, lower nozzle velocity can be achieved. The acceleration of the molten alloy through the sprue and runner takes place under controlled conditions.
Step one
Calculate the total projected area and the mass of the casting from the component or drawing.
Step two
Determine the machine locking suitability
(Repeat the same procedure when PQ² diagram was used)
Step three
Plot the known areas of gooseneck and nozzle on an area chart.
Step four
Establish length of gate i.e. the maximum practicable length for gating the casting.
Step five
Establish area of gate and plot on the area chart. In general the gate area should not be less than 40 % of the nozzle area and not greater than 100%. The most satisfactory results are obtained with gate areas in the range of 45 - 55% of nozzle area.
Step six
Design a preliminary flow system layout, noting bends and divisions. It is at these points that major
Losses occur.
Step seven
Apportion the difference in area between nozzle and gate over sprue, runners and tangential runners and Plot on chart. Provide allowances for bends and divisions.. Sprue runners, main runners and tapered runners are designed in accordance with the procedure explained in step eleven of the previous case.
23.10 Die casting die design calculations:
The following information is supplied with a die casting die:
1. Shot weight.
2. Shots per hour.
3. Die temperature.
4. Ejection temperature.
5. Water flow rate.
6. Locking force.
7. Ejector force.
8. Metal temperature.
9. Plunger velocity.
10. Plunger diameter.
11. Injection pressure.
12. Model and year of manufacture.
Important points to be taken into consideration while designing:
· Cavity fill time should not be more than 40 milliseconds for painting grade finish and 20 milliseconds for plating grade finish.
· Gate velocity should be between 30 - 60 m/s.
· Flow rate for a machine is fixed by the maximum plunger velocity and the plunger
diameter.
For HMT BUHLER machines the maximum plunger velocity is 3 m/s.
The flow rate and the gate area are determined from the graphs (PQ² graph). The graph is explained in the chapter ‘die casting die designs’. The same is used in the step by step procedure to determine the gate area.
23.10.1 Determination of gate area:
Step one:
machine specification:
Example:
HMT BUHLER | H250D |
Locking force | 2.5 MN(250t) |
Max. inj.force(with intensifier) | 0.35 MN (35t) |
Max. inj. Force (without intensifier | 0.22 MN (22t) |
Plunger diameter | 50,60,70,80,85 |
Max. plunger velocity | 3 m/s |
For all HMT BUHLER die casting machines | |
Step two:
Determine maximum flow rate:
Flow rate = plunger velocity x plunger area
From the machine specification chart,
Plunger velocity = 3 m/s
Plunger diameter = 50mm = 50 x 10-3 (with diameter 50mm)
Flow rate Q = 3 x π/4 (50 x 10-3)
= 5.888 x 10-3 m3/s
Step three:
Pressure = force / area
Maximum injection pressure =0.22 divided by π/4 (50 x 10-3)2
Step four:
Injection pressure should be less than locking pressure.
(Provide adequate safety factor)
Injection force = locking force
= Projected area of component (including runners and gate)
Let injection force = Fl
Step five :
Maximum injection force = 0.22 MN
Maximum injection pressure = 112 Mpa.
Injection force = Fl
There fore injection pressure = 112 x F/22
Back pressure on HMT BUHLER machines is 20 Mpa.
Therefore effective injection pressure = (112 x F/22) – 20
Step six:
Force = P1 x A1 = P2 x A2
P1x (d1)2 = P2 x (d2)2
Plunger diameter d1 = 50 mm
Hydraulic cylinder diameter d2 = 130mm
Injection pressure P1 = 40 Mpa
Hydraulic pressure P2 = 7.4 Mpa
Step seven:
Gate area mm2 | flow rate | V= velocity m/s | fill time s |
From graph | From graph | flow rate x 1000 divided by Gate area | volume(dm3) divided by Flow rate |
Step eight:
Select the appropriate gate thickness and gate length to satisfy the selected area. The gate from 00.3 mm to 1 mm. The limitation on gate length is influenced by the component.
Example for step seven:
Assume
Injection pressure = 40 Mpa
Flow rate = 5.9 m/s
Liquid volume of component = 104 cm3
Material = aluminium alloy
Draw a straight line connecting metal pressure 40 Mpa and flow rate 5.9 m/s in the die casting die design aid for aluminium (scale 2). Read the flow rate from the graph.
Serial no: | Gate area(mm2 ) | Flow rate(1/ s) | Velocity(m/s) | Fill time (millisecond) |
!. | 40 | 3.4 | 85 | 30.6 |
2. | 50 | 4 | 80 | 26 |
3. | 60 | 4.3 | 71.67 | 24 |
4. | 80 | 4.8 | 60 | 21.7 |
5. | 100 | 5.2 | 52 | 20 |
6. | 120 | 5.4 | 45 | 19.26 |
7. | 140 | 5.5 | 39.29 | 18.91 |
8. | 160 | 5.6 | 35 | 18.57 |
9. | 180 | 5.7 | 3 1. 66 | 18.24 |
Gate area 120 mm² satisfies the gate velocity and fill time requirements.
Tapered runners are preferred. If runner is straight the velocity will decrease and the metal may freeze at the gate. Around bends the runner area can be reduced by 10 - 30% depending on the severity.Go to next chapter
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