types_of_moulds-split_mould.ppt

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Types of Moulds

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Injection Moulds
The molds consist of two main parts: the cavity and core. The core
forms the main internal surfaces of the part. The cavity forms the
major external surfaces of the part. Typically, the core and cavity
separate as the mold opens, so that the part can be removed. This mold
separation occurs along the interface known as the parting line.
An injection mold is usually made in two halves or sections and held
together in the closed position by the molding press. The mold is
generally made out of tool steel and is provided with channels for
cooling, heating and venting, Ejector pins and other devices may be
incorporated.

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Types of Moulds
1.Two Plate Mould
2.Three Plate Mould
3.Multi-daylight mould
4.Stack Mould
5.Runnerless Mould
6.Insulated Hot Runner Mould
7.Split Mould

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Two plate mould
It is the simplest of all the mould design configurations,being
constructed from two distinct half units, the core half and cavity half.
The core half of the mould is usually attached to the moving platen
of the molding machine since the mould ejection actuation system is
commonly positioned behind the moving platen.
The cavity half of the mould is therefore attached to the fixed platen
of molding machine directly in front of the machine injector unit for
material feeding of the mould.
Cooling channels are positioned in both core and cavity components
to control the mould temperature.

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Two plate mould-two impressions

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Advantage
1.Simplicity of design
2.Utilization of standard mould parts
3.The cheapest design
Disadvantage
1.Limitations in component gate positioning when conventionally
feeding
2.Lack of available space for balanced feeding of multiple cavities
3.High material waste level(Sprues and runners).

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Three plate mould
The three-plate mould differs from the more common two-plate design
format in terms of utilizing more than one split or parting line.
The tool construction is divided into three distinct plate build-ups which
separate from each other on opening.
One opening provides clearance for component ejection, while the other
allows for sprue ejection and clearance.

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Three plate mould
This type of mold is made up of three plates:
(1) the stationary or runner plate is attached to the stationary platen,
and usually contains the sprue and half of the runner,
(2) the middle plate or cavity plate, which contains half of the runner
and the gate, is allowed to float when the mold is open; and
(3) the movable plate or force plate contains the molded part and
ejector system for the removal of the molded part.
A linkage system between the three major mold plates controls the
mold-opening sequence. The mold first opens at the primary
parting line breaking the pinpoint gates and separating the parts
from the cavity side of the mold. When the press starts to open, the
middle plate and the movable plate move together, thus releasing
the sprue and runner system and degating the molded part. This
type of mold design makes it possible to segregate the runner
system and the part when the mold opens. The die design makes it
possible to use center-pin-point gating.

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Schematic of a two-cavity, three-plate mold
with cutaway view -first stage of opening
phase
Second phase of opening
Final opening phase

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Multi-daylight mould
The basic mould consist of two half, when these two half's are opened the
moldings can be extracted.The space between the mould plates is termed
as Daylight.
A mould in which more than one daylight occurs when it is opened.This
term refers to underfeed mould.
In stripper plate mould, it consists of three plates namely, fixed mould
plate, a moving mould plate and a stripper plate.When the mould is fully
opened there are two daylights.The molding and feed system are removed
from the first daylight, that is from between the striper plate and the fixed
mould plate.

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Stack Mould
A Multilevel injection mold in which two sets of cavities are filled
simultaneously.
The stack mold, reduces the clamp force required by multi-cavity molds.
Typically, multiple cavities are oriented on a single parting line and the
required clamp force is the sum of the clamp needed by each cavity plus
the runner system.
In stack molds, cavities lie on two or more stacked parting lines. The
injection forces exerted on the plate separating parting lines cancel, so the
resulting clamp force is the same as for just one parting line.
Stack molds produce more parts per cycle than would otherwise be
possible in a given size molding press. The stacked injection mold is just
what the name implies. A multiple two-plate mold is placed one on top of
the other. This construction can also be used with three-plate molds and
hot-runner molds.
A stacked two-mold construction doubles the output from a single press
and reduces the clamping pressure required to one half, as compared to a
mold of the same number of cavities in a two-plate mold. This method is
sometimes called "two-level molding“.

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Runnerless mould
In this process of injection molding, the runners are kept hot in order
to keep the molten plastic in a fluid state at all times. In effect this is a
"runnerless" molding process and is sometimes called the same. In
runnerless molds, the runner is contained in a plate of its own.
Hot runner molds are similar to three-plate injection molds, except
that the runner section of the mold is not opened during the molding
cycle.
The heated runner plate is insulated from the rest of the cooled mold.
Other than the heated plate for the runner, the remainder of the mold is
a standard two-plate die

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Advantage
1.Reduced cycle time as a result of having a component cooling
requirement only.The runner and feed system remain molten above the
quickly frozen gate.
2.Material savings results from having no sprue or runner systems to
granulate.
3.Labor and post molding finishing costs are significantly reduced without
the need for degating of the moldings.
4.The ability to gain greater control over the mould filling and flow
characteristics of the molten polymer during the filling phase of the
molding cycle.

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Insulated hot runner mould
The insulated hot runner mould is the simplest of all the hot-runner
designs.
In this type of molding, the outer surface of the material in the
runner acts like an insulator for the molten material to pass through.
In the insulated mold, the molding material remains molten by
retaining its own heat. Sometimes a torpedo and a hot probe are
added for more flexibility. This type of mold is ideal for multicavity
center-gated parts.

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Advantages
1.The feed system can easily be stripped and cleaned,resulting in very
little material or color contamination occurring.
2.Mould start-up times are faster when compared to other hot runner
mould systems.
3.Moulds are very much cheaper to manufacture than the other hot
runner mould designs
4.Thermally unstable polymers may be processed using such a system.

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Split Moulds
A mould in which the cavity is formed by two or more parts held
together by a chase bolster during the injection phase.
A two or more steel blocks containing the impression which can be
opened, normally at right angles to the mould’s axis, to facilitate the
molding of external undercut type comments.
In Split mould, i )guiding the splits in the desired direction ii)
Actuating the splits and iii ) securely locking the splits in position
prior to the material being injected into the mold.

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There are three main factors in the design of the guiding and
retention systems for a sliding splits type mould.
1.Side movement must be prevented to ensure that the split
halves always come together in the same place.
2.All parts of the guiding system must be of adequate strength to
support the weight of the splits and to withstand the force
applied to the splits by the operating mechanism.
3.Two split halves must have a smooth, unimpeded movement.

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Undercut Moulds
A molding which has a recess or projection is termed an Undercut
molding.
External Undercut
Any recess or projection on the outside surface of the component
which prevents its removal from the cavity is termed an external
undercut.

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There are two forms of undercut to be considered:
a) The undercut may be local, in that the recess or projection occurs in
one position only.For example-clip on a pen cap.
b) The undercut may be continuous recess or a projection on the
periphery of the components.For example-water connector has a
number of undercuts.
In the above case, it is necessary to split the cavity insert into parts and
open these, generally at right angles to the line of draw, to relieve
the undercut before the moulding is removed.
Since the cavity is in two pieces, a joint line will be visible on the
finished product.The joint line, on an undercut component, is
comparable to the parting line on an in line of draw component and
the same careful consideration must be exercised in deciding its
position before attempting to design the mould.

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Internal undercut
An internal undercut is any restriction which prevents molding from
being extracted from the core in line of draw.
The specific design adopted depends upon the shape and position of
the restriction.
A component which has a local undercut portion can be successfully
molded in the conventional mould by incorporating the undercut form
on a form pin.
A circular hardened steel pin which incorporates molding form and is
used for the molding of internal undercuts.It may have either a straight
or an angled action is termed as form pin.

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Finger Cam
A hardened steel rod, fitted at an angle to the mold plate for the purpose of
operating splits and side cores.
The splits mounted in guides on the moving mould plate, have
corresponding angled circular holes to accommodate these finger cams.

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The distance traversed by each split across the face of the mould plate is
determined by the length and angle of the finger cam.The movement can
be computer by the formula
M=(L sinø) – (c/cosø)
As the required movement is known from the amount of component
undercut, the following rearranged formula to determine the finger cam
length is of greater use, apart from checking purposes
L=(M/sinø) + (2c/sin2ø)
Where,
M= splits movement
Ø = angle of finger cam
L=working length of finger cam
c =clearance

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Dog-leg cam actuation
Here, method of actuation is used where a greater splits delay is required
than can be achieved by the finger cam method.
The dog-leg cam which is of a general rectangular section, is mounted in
the fixed mould plate.Each split incorporates a rectangular hole, the
operating face of which has a corresponding angle to that of the cam.

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Cam track actuation
A hardened steel member fitted to one mould plate for the purpose of
operating splits and side cores.
This method of actuation utilizes a cam track machined into a steel plate
attached to the fixed mould half.A boss fitted to both sides of the split, runs in
this track.The movement of the splits can thus be accurately controlled by
specific cam track design.
To ensure smooth operation a generous radius should be incorporated at each
point where the cam track form changes.A radius or taper should also be
included at the entrance to form a lead-in for the boss as it re-enters the track.

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Spring actuation
In this design, which obviates the use of cams altogether, incorporates
compression springs to force the splits apart and utilizes the angled faces
of the chase bolster to close them.The outward splits movement must
therefore be limited so that they will re-enter the chase bolster as the
mould is closed.
The splits are mounted on the mold plate and retained by guide strips.
Studs project from the base of the splits into a slot machined in the mould
plate.
A compression spring is fitted between the studs in a link-shaped pocket
situated in the lower mould plate.

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Hydraulic actuation
In this design the splits are actuated hydraulically and it is not
dependent on the opening movement of the mould.
The splits can be operated automatically at any specific time by
including this function in the operating programme of the machine.
On machines which do not programme for auxiliary cylinder control it
is necessary to add a separate hydraulic operating circuit to the existing
system.

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Split core
A steel core manufactured in two or more parts to facilitate the molding
of internal undercut components.
The split core design is used for components which have extensive
internal undercuts that cannot be incorporated on a form pin.
The split core may be moved forward either in a straight plane or an
angled plane.

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Mould for Internally Threaded components
The internal thread comes within the broad definition of an
internal undercut in that the thread forms a restriction which
prevents the straight draw removal of the molding from the
core.
i) Split core design
ii) Collapsible core design
iii) Unscrewing mold design

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Moulds for Externally threaded components
An external thread comes within the broad definition of an external
undercut, namely a restriction situated in the outside form of a
molding which prevents straight draw removal of the molding from
the cavity.
In undercut component, the threaded type of molding can be
unscrewed from the cavity and in certain cases this allows a
simplified design.
But in automatic operation is required some form of rotary motion
within the mold is necessary to perform the unscrewing operation
automatically.
Another method often used is stripping design, allowing for fast
production cycles, is limited to those components which incorporate
roll threads.

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Standard Mold bases
The importance of standardizing a mould base is to manufacture on a
rigidly controlled basis,all possible parts of the mould which permits
considerable pre-fabrication and eliminating a lot of work done by highly
skilled man power in the mould and die making shop.
It takes many hours to produce mould components by tool room methods
for which some times large and most expensive machines are required.
When components are standardized and put in the market in a ready to
assemble condition, the only major work on any mould to be done will be
core and cavity for which more skilled man hours would be made
available.This in turn will increase the rate of production, faster
delivering of the moulds thereby increasing the profitability of the mould
and die making industry.

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Advantage
The advantage of standard mould base can be broadly grouped into
three areas, for designers, manufacturers and customers.
Designers
•Risk due to drawing errors are minimized
•By adopting standardization, the draughtsman does only the
essential portion of the job i.e core and cavity profiles, ejection
location, gate & cooling details only.
•A wide variety of mould drawings could be drawn in minimum
time without fatigue or repetitive drafting.
•Mould parts could be easily numbered so that the batch production
in manufacturing will be easy.

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Manufacturer
*The time spent on the manufacturing mould base can be utilized for
manufacturing core and cavity details so that the efficiency of the output
can be easily increased with the availability of manpower.
•Machinist replace highly skilled mould makers
•Duplication will be easy if required and consumes less time.
•Better control on cost can be achieved on batch production techniques.
•Interchangeability of parts will be easy.
•Tool maintenance is reduced.
Customers
•The delivery time can be easily reduced
•The quality of the product can be assured
•Total cost of the tool will be reduced.

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Calculation of strength of cavities
a)Rectangular cavity
The maximum deflection commonly allowed is 0.13-0.25mm depending upon
the size of the tool.Of this 0.1-0.2mm may be due to clearances between the
blocks of the built-up mould and elongation of the bolster or register faces.For
stress design purposes, therefore, a maximum deflection of 0.025-0.05mm is
usually take.The approximate thickness of the side wall required may be
calculated from the following formula:
t = 3( c.p.d4 / E.γ )
γ - Maximum deflection of side walls(cm)
c- Constant , t – thickness of cavity wall (cm)
p- Maximum cavity pressure(say 630 Kg/cm2)
d- Total depth of cavity wall(cm)
E- Modulus of elasticity for steel(0.1 x106 kgf/cm2)

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b) Cylindrical cavities:
Increase in radius due to the internal pressure of injected material can
be determined from
γ = (rp / E) [{r2+R2)/(R2-r2) +m)]
γ – Increase of inside radius(cm)
r- Original inside radius(cm)
R- Original outside radius(cm)
m- Poisson’s ratio(0.25 for steel)
p- Cavity pressure, say630kg/cm2

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Calculation of Strength of Guide pillar
The size of the working diameter of guide pillar to use depends on the
size of the mould and whether or not a size force is likely to be exerted
on it. The moulds with deep and heavy cross-sectional cores exert side
thrusts, and the guide pillars should be strong enough to absorb them
without any damages.
Side thrust(Q) can be calculated from the equation,
Q = 2/3d.h.pf(K.g) for circular cores
d= Maximum diameter of core(cm)
h= Height of core(cm)
pf= Cavity pressure causing side thrust taking into consideration the
effect of clamping force(kg/cm2) say 300kg/cm2

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Q= a.h.pf
a= Maximum side of core(cm)
h= Height of core(cm)
pf= say 300 kg/cm2
Working diameter(d) of guide pillar
d(4Q/N fs )½
Q= Side thrust (kg)
N= Number of pillars
fs= shear stress, say 16 Tons/sq.inch=158 x 16 kg/cm2

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Determination of Support Pillar requirements:
W= 8ZS/M; Z= LB2/6; A=W/P
W=permissible load on support plate(kg)
S=permissible working stress(kg/cm2)[840kg/cm2]
M=Distance between supports(cm)
L=Length of support plate(cm)
Z=Section modulus(cm3)
B= Thickness of support plate(cm)
P=Maximum unit pressure on support plate(kg/cm2)[630 kg/cm2]
A= Permissible projected area of molding/impression(cm2)

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If the calculated permissible projected area(A) is found less than actual
projected area of molding then it denotes the additional support pillars
are required to withstand the stresses.
Placement of one row of additional support pillars, equally diving the
span(M) in the ejectors grid will quadruple the permissible projected
area.
The support pillars should be placed as close as possible to the points
of maximum stress, and can be made use for guiding the ejector place
assembly.

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Mould design check list
Machine:
1.Is the weight of the molding, runners and sprue within the shot capacity of
the machine?
2.Is the expected output well within the plasticizing capacity of the machine?
3.Is the clamping pressure of the machine sufficient for the projected area of
the moldings and runners?
4.Will the mould pass between the machine tie bars?
5.Do the clamping arrangement for the tool suit the platen bolt holes?
6.Is the closed thickness of the tool above the minimum and below the
maximum required for the machine?
7.Is the machine opening stroke sufficient for the ejection and extraction of the
molding?

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Molding:
1.Is the flash at the tool parting line position visually acceptable on the molding?
2.Are the positions of any print lines arising from tool blocks, cores, ejectors, etc.,
visually acceptable on the molding.?
3.Is the gate position visually acceptable on the molding?
4.Will the position of any flow or knit lines that may occur be acceptable in
appearance?
5.Will any knit lines cause weakness in a critical area?
6.Will any heavy section in the molding cause unacceptable sink marks?
7.Is the design free of any undercut areas that will prevent ejection?

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Mould
1.Is the mould parting line chosen the most efficient for mould operation and
tool construction?
2.Have the Core and cavity been designed in the easiest manner for
machining on the available equipment?
3.Will any slender blades or pins deform under cavity pressure or flow?
4.Is the cavity of adequate strength to resist internal cavity pressure?
5.Area all tool components exposed to side thrust from cavity pressure?
6.Is the tool construction free from any chance of horizontal flash?
7.Does the ejector packing provides sufficient support to the die plate to
prevent distortion under cavity pressure?
8.Can all parts of the tool be dismantled and separated in the event of tool
breakdown or modification?
9.Are all necessary parts hardened?
10.Have all allowances for molding shrinkage been provided?

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11.Has sufficient molding taper been allowed on all parts forming molding
surfaces?
12.Will the tool dimensions produce moldings within component tolerances?
13.Will the molding remain on the ejection side when the mold opens?
14.Is the ejection stroke sufficient to clear the molding?
15.Have sufficient ejectors been provided to prevent sticking, cracking or
distortion of the molding?
16.Are the ejector and ejector bars sufficiently strong?
17.Is the ejector mechanism suitable for the particular machine ejector systems?
18.Has the ejector return mechanism been provided?
19.Have adequate guide pins been provided between the tool halves?
20.On split tools or moving cores in the opening movement provided by cams,
cylinders, etc., sufficient to clear the undercuts on the component?
21.Are all inserts positively located for position and prevented from displacing
themselves during tool closing or under plastic flow in the cavity?

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22.On split tools or moving cores, is the mechanism sufficiently fool proof to
prevent damage by fault operation?
23.On all splits and moving cores, is the cavity pressure resisted by solid steel
locking faces and not by the split or core-operating cam?
24.Have adequate cooling channels been provided?
25.Is the cooling too close or too distant from the mould surfaces?
26.Are the runners of sufficient size?
27.Has a sprue puller hook and sprue cold well been provided?
28.Are runner hooks and runner cold wells necessary?
29.On multi-daylight tools has sufficient opening been provided between the
plates to allow extraction of moldings and runner system?
30.Is the mold sufficiently vented?
31.On offset designs is the out-of-balance force too great?
32.Has an injection side platen location spigot been provided?
33.Do the spherical nose and orifice of the cylinder nozzle mate with the
spherical seating and bore of the sprue bush?

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34.On split tools can be in the forward position without interfering with the
closing of the splits?If not-is mechanism provided to ensure ejectors are returned
before the splits close?
35.Is the sufficient clearance provided between ejector chains or links to
withdraw the component through them without difficulty?
36.Have tool lifting eye bolts been provided?

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END

types_of_moulds-split_mould.ppt

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