Runner System
| In this chapter, you will learn about the functions, characteristics,
and shapes of the sprue, runner, and gate. |
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The molten plastics injected from the injector nozzle will go through a sprue
(sprue bush), a runner,
and a gate and fill up in the cavity.
As the temperature of molten plastics is lowered while going through the sprue
and runner, the viscosity will rise; therefore,
the viscosity is lowered by shear heat generated
when going through the gate to fill the cavity.
Sprue
A sprue is a channel through which to transfer molten plastics injected from
the injector nozzle into the mold. It is a part of sprue bush, which
is a separate part from the mold.
Runner
A runner is a channel that guides molten plastics into the cavity of a mold.
Gate
A gate is an entrance through which molten plastics enters the cavity.
The sprue, the runner, and the gate will be discarded after a part is complete.
However, the runner and the gate are important items that affect the quality
or the cost of parts.
Let's take a look at the detailed descriptions of runner and gate.
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Runner |
You need to determine the shapes,
dimensions, and positions of the runner
taking into consideration the required quality and material of parts, the number
of the cavity, so that injection pressure
and temperature of molten plastics will not be lowered.
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Cross Section of Runner |
There are three types of runner cross
section - round, trapezoidal, and U-shape.
Select a runner cross section that minimizes liquid resistance and temperature
reduction when molten plastics flows into the cavity.
In the figure above, a round shape is the ideal. Concerning the mold making
process, the round shape requires making half circles to both cavity
plate and core plate, which complicates
this task.
For this reason, it is considered better to select a runner in a trapezoid cross
section which is closer to square, if either the cavity plate or the core plate
is shaped.
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Size of Runner
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The size of runner is determined based
on the following factors.
- (1) Thickness of Runner
- The thickness of runner is determined based on the material or shape of
parts.
If a runner is too big, cooling solidification
will take too much time and molding cycle
will be longer. Also, a large runner wastes the molding
material , thus raising the cost.
On the other hand, too small of a runner would cause molding defects such
as short shot or sink
mark, adversely affecting the quality of parts.
- (2) Types of Plastics
- As some plastics are superior in
flow and the others inferior, determine the shape or the size of a runner
according to these factors.
- (3) Length of Runner
- If a runner is long, enlarge the size of the runner to reduce liquid resistance.
- However, this will lengthen the cooling time and waste the molding material,
causing the cost to rise.
- It is ideal to shorten a runner as much as possible.
To Mold Thin Parts
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Rapidly fills up by accelerating injection
speed and raising the mold temperature.
Make the runner small so that the flow speed will be accelerated to generate
shear heat (the heat generated when
molten plastics goes through the gate),
which decreases viscosity.
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To Mold Thick Parts
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Gradually fills up by decelerating injection
speed to avoid defective surface of parts such as sink
mark or air bubbles by degassing the molding material.
Apply the holding pressure and fill up molten plastics in the cavity
well. Make a runner larger so that sink marks or short shot will not occur.
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Also be extremely careful in determining the runner size of a three-plate
mold.
As the three-plate mold has a certain order for mold opening and tends to
have a longer runner, if the mold opening stroke necessary for removal is
miscalculated, then removal of parts can not be done.
For the runner of three-plate mold, careful study during the mold design phase
is critical.
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Layout of Runner |
Keep the same distance from a runner to
each cavity when molding multiple parts
. By doing so, injection pressure will
be evenly distributed and molten plastics will fill up evenly in each cavity.
Therefore, the ideal layout for a well balanced runner is a regular polygon
(right triangle or a right hexagon) to keep the same distances to all cavities
even and distribute injection pressure evenly. The figure below illustrates
the example.
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Gate |
The gate is categorized into restrictive
gate, which narrows the entrance, and nonrestrictive gate, which does not narrow
the entrance.
The gate has the following functions:
- Restricts the flow and the direction of molten plastics.
- Simplifies cutting of a runner and
moldings to simplify finishing of parts.
- Quickly cools and solidifies to
avoid backflow after molten plastics has filled up in the cavity.
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Restrictive Gate |
A restrictive gate has a narrow entrance to the cavity
to restrict the amount of molten plastics in order to improve filling in the
cavity.
The restrictive gate has the following characteristics.
- Generates shear heat by going through
the narrow gate, raising the temperature
of molten plastics and improving the filling in the cavity.
- Reduces residual stress, and thus reduces part defect such as warpage.
- As the cooling solidification time
is shortened, molding cycle is also
shortened.
- As the gate trace is less, it is possible to complete finishing process
in a short time.
The restrictive gate has the following types.
| Side Gate
|
Submarine Gate
(Tunnel Gate) |
The most common gate.
Put to the side of parts.
The gate trace will be left.
Often used for the structure with more than two cavities. |
The gate will be automatically cut off during mold opening.
The position is flexible (front, side, or back of parts).
The gate needs to be thought about not to be left inside the cavity.
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| Pin Point
Gate |
Fan Gate
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Suitable for molding multiple parts.
The position is relatively flexible.
The structure is complicated due to three-plate method of mold.
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Suitable for large and flat plate parts.
Finishing is difficult and cost is high due to the wide gate.
The gate trace will be left. |
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| Film Gate
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Valve Gate
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Suitable for thin plate parts.
Finishing is difficult and cost is high due to the wide gate.
The gate trace will be left.
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The valve of the gate opens the gate according to injection timing.
Sprue and runner
will not be discharged due to hot runner method.
Put to either the front or the back surface of parts. |
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Nonrestrictive Gate |
A nonrestrictive gate involves a method in which molten plastics fills up in
the cavity directly from the sprue.
The nonrestrictive gate has the following characteristics.
- Reduces the loss of injecting pressure
due to direct cavity filling from the sprue.
- Less molding material because there
is no runner .
- The simple mold structure reduces the cost and produces a mold with less
trouble.
- It is likely that residual stress is generated and parts have crack.
The nonrestrictive gate has the following type.
Direct Gate
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The sprue plays the role as gate.
Put on the front or back side of parts.
The gate trace will be left. |
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Determining Gate Position |
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- Point 1
Set a gate position where
molten plastics finished filling up in each cavity simultaneously.
Same as multiple points gate.
- Point 2
Basically set a gate position to the thickest area of a part. This
can avoid sink marks due to
molding (part) shrinkage .
- Point 3
Set a gate position to an unremarkable area of part or where finishing
process can be easily done.
- Point 4
Avoid injecting from the direction where the air in the cavity or
the gas generated from molten plastics is inclined to accumulate.
- Point 5
Fill up molten plastics using the wall surface in order not to generate
jetting.
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Now, let's look at the details of sprue parts.
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Parts of Sprue
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The parts related to a sprue are locating ring and sprue bush
The locating ring functions to position a mold when installing the mold to the
molding machine.
The sprue bush is the channel to direct molten plastics inject from the injector
of the molding machine into the mold.
Be cautious of the setting for the following items when installing each part.
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Locating Ring |
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Check the diameter of locating hole of the molding
machine.
Generally, the diameter of locating ring
should be (locating ring diameter) = (locating hole diameter) - (0.1mm to
0.3mm).
(Click a name in the figure to display the
description.)
The locate hole diameter is regulated to be
60, 100, 120, and 150, and the thickness of the locating ring is regulated
to be 15 mm by JIS.
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Sprue Bush |
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Set the correlation of the nozzle hole diameter
and the sprue bush as follows:
D1 < D2
D2 = D1 + (0.5 - 1.0)[mm]
R1 < R2 R2 = R1
+ 1.0[mm]
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D1: Nozzle diameter
D2: Sprue bush hole diameter
R1: Nozzle tip R
R2: Sprue bush R |
(Click a name in the figure to display the description.)
[Correlation of Nozzle and Sprue Bush]
The tip of the nozzle is spherical, and the curvature radius is regulated
to be 10 mm, 15 mm, 20 mm, or 30 mm by JIS.
The nozzle diameter is to be selected from f2.5,
f3, f4, f5. f6, f8, and f10
The high pressured molten plastics injected from the nozzle of the molding
machine is injected into the cavity
through the sprue bush.
At this point, the center bores of the sprue bush and the nozzle must be
aligned so that the high pressured molten plastics will not leak.
A locating ring is attached to the mold
to make positioning easier.
