Tài liệu thiết kế khuôn mẫu

Tài liệu thiết kế khuôn mẫu

C ORE T ECH S YSTEM

Mold Design Fundamentals

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Basic Tasks of a Mold

qCooling/Heating and Solidification of the Melt

The mold is probably the most important element of a molding machine It is a

arrangement, in one assembly, of one (or a number of) hollow cavity spaces built

to the shape of the desired product, with the purpose of producing large numbers

of plastic parts Thus the primary purpose of the injection mold is to determine

the final shape of the molded part (shaping function)

In addition to give the final shape of the molding, the mold performs several

other tasks It conducts the hot melt from the heating cylinder in the injection

molding machine and distributes the melt to the cavity (or cavities), vents the

entrapped air or gas, cools the part until it is ejectable, and ejects the part without

leaving marks or causing damage

The secondary tasks of a mold derived from these primary tasks include several

mechanical functions such as accommodation of forces, transmission of motion,

guidance and alignment of the mold components

The mold design, construction, the craftsmanship largely determine the quality

of the part and it manufacturing cost

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Functional Systems of the Injection

Molds

qMelt Delivery System: Sprue/Runner/Gate

qCavity (with Venting)

An injection mold is composed of several functional units Each unit performs

one or several task of the mold

The melt delivery system or runner system performs the task of receiving and

distribution of the melt The runner system is in fact a set of flow channels that

lead the melt into the cavities

Forming/shaping the molten material into the final shape of the part is the job of

the cavity During the filling and packing/holding stages, melt is forced by

injection/holding pressure to completely fill the cavity (or cavities)

Mold tempering or heat exchange system is used to control the mold

temperature, cool down the molten melt (or,if thermosets or elastomer are used,

heat the melt and cross-link the material) uniformly, solidify the molding to an

ejectable state Mold tempering system design has direct impact to the production

cycle time and the quality of the molded part

Ejector system is utilized to open the mold and remove the molded part from the

cavity Mold mounting, alignment, and guiding are accomplished by the

guidance/ locating system and machine platen mounts Other auxiliary units such

as force supplier and movement transmission unit are essential to accomplish the

functions of an injection mold

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Structure of A Mold Unit

Sprue Sprue

Primary Runner

Secondary Runner /Sub-runner Gate

Part

Cold-Slug Well

Cold-Slug Well Sprue Ejector Pin Sprue Bushing

Above figure shows the layout af a typical simple injection mold, which has

four identical cavities Melt from the nozzle enters the mold via the spure, which

has a divergent taper to facilitate removal when demolding

Opposite the sprue is a cold slug well, which serves both to accept the first

relatively cold portion of the injected material, and to allow a re-entrant shape on

the end of an ejector pin to grip the sprue when the mold opens

The melt flows along a system of runners leading to the mold cavities In

general, for a single cavity mold, only the sprue or primary runner appears in the

mold; whereas for a multicavity mold, secondary runners or subrunners are

needed to distribute the melt into each cavity

The gates at the entries to the cavities are very narrow passages in at least one

directions, so that the molded part can be readily detachable from the runners

after removal from the mold

Sometimes additional cold slug wells are added in the end of primary runners to

trap the cold slug during the filling stage

The mold is aligned with the nozzle on the injection cylinder by means of the

locating ring surrounds the sprue bushing.

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Mold Design Issues

mold base cooling channel/lines

runner (mainfold) system gate

q Cooling System Design

) Cooling Channel Layout

) Special Design

The primary tasks of an injection mold include the accomodation and

distribution of the melt, the shaping and cooling/heating of the molding,

solidification of the melt, as well as ejection of the molded part Besides, a mold

has to provide mechaincal functions such as accomodation of forces,

transmission of motion, and guidance of mold components

Hence the primary functional systems of a injection mold include the melt

delivery system ( sprue/runner/gate ), cavity (single-cavity or multicavity),

ejection system, guiding and locating system, as well as mold temperature

control unit (cooling system)

From the view point of mold design, we have to evaluate the suitable size and

layout of runner system and cavity, number of cavity, cooling system, etc

We will propose a few examples to illustrate how these design parameters

influence the productivity and quality of the moldings

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Determine Number of Cavities

q Single Cavity vs Multicavity Mold

) Productivity and complecity consideration

q Determination of Number of Mold Cavities

) Number of moldings required and period of delivery

) Quality control requirements (dimensional tolerance,etc.)

) Cost of the moldings

) Shape, dimensions, and complexity of the molding (position of

parting line and mold release)

) Size and type of the injection molding machine machine (shot

capacity, plasticizing capacity, mold release )

) Plastics used (gating scheme and gate location)

) Cycle time (increase in recovery time of plasticating unit,

injection time, pressure drop, and mold opening time)

The multiple mold cavities can produce several article at the same time and

hence has a higher output speeds and improved productivity However, the

greater complexity of the mold also increases significantly the manufacturing

cost The problems arising from a multicavity mold includes cavity layout, flow

balance, balanced cooling channels layout, etc

Theoretically, for the same product, cycle time do not increase prorate with the

number of cavities because th cooling time does not change However, one often

find that cycle time will increase as the number of cavities increases, for the

following reasons:

-Increase in recovery time of plasticating unit for the next shot and injection

time because the total shot volume is increased These increases in time are

significant for large shots

-Increase in pressure drop becaused of the increased flow length from sprue,

through runner system, to each cavity The pressure drop can be a determining

factor in the evaluation of numbers of cavity

-Increase in mold opening time because of the increased complexity

Both the technical and economic criteria have to be considered in determining

the number of mold cavity, such as the numbers of moldings required, the cost

and time of mold construction, the complexity of the molding, cycle time, quality

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When the number of parts produced in each cycle exceeds one, a multicavity

mold have to be used Many cavity layouts can be adopted in the production

For example, layout in series has the advantage that there is no space restriction

for each cavity; however, the unequal flow lengths to individual cavities may

lead to unbalanced flow and differential part weights in each cavity

Circular layout has the advantage of equal flow length and uniform part

quality; however, only limited number of cavities can be accomodated by this

layout

H-style layout and X-style layout belongs to the so-called symmetrical layout

They are good in flow balance Their disadvantage is that more larger runner

volume and much scrap will be generated Hot runner system can be adopted to

conquer this drawback

Layout of cavities not only influence the filling pattern and extent of pressure

packing, but also determines the equilibrium of injection force and clamp force

during the molding cycle

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Design of Runner System

Piston or Screw

Screw Chamber (Reservoir)

Heating Element Nozzle

Runner

Gate Sprue Cavity

) Accommodates the molten plastics material coming from the screw

chamber and guides/distributes it into the mold cavity

) Raises the melt temperature to the proper processing range by viscous

(frictional) heating while the melt is flowing through the runner

q Design Consideration

) Quality (filling pattern ) & Economics (cycle time )

A runner system is composed of the sprue, the runner(s), and the gate(s) that

connecting the runner with the cavity

The primary task of a runner is the delivery and distribution of melt from the

screw chamber into the mold cavity The runner system must be designed in such

a way that the melt fills all cavities simultaneously and uniformly under uniform

pressure and temperature This design criterion is referred to as the flow balance

of the runner system

Melt temperature may be significantly increased as it passes througn the narrow

runner passage or gate due to friction effect This viscous heating is important in

raising the melt temperature and reducing the flow resistance because of the

shear-thinning character of plastic material

The runner system has significant impact on the part quality and the economics

of manufacture Problems such as weld lines, pressure drop, material waste,

removability of moldings, etc.,are related to the design of runner system

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Common Runner Cross Sections

There are several types of cross section can be adopted for a runner The

selection of the runner cross section depends on its efficiency and ease or

difficulty of tooling

Circular or full round cross section provides a maximum volume-to-surface

ratio and hence offers the least resistance to flow and least heat loss from the

runner However, it requires a duplicate machining operation in the mold, since

two semi-circular sections have to be cut for both mold halves and aligned as the

mold is closed

Parabolic or U-type runner represents a best approximation of circular runner,

although more heat losses and scrab produced (mass is 35% greater), it needs

simpler machining in one (movable) mold half only

Trepezoidal runner is an alternative modification of circular runner, its

performance is similar to that of the parabolic runner Trapezoidal runner is

often used in three-plate molds since sliding movements are required across the

parting-line runner face

Half round and rectangular cross section may lead to larger flow resistance and

are unfavorable in the runner cross section

Normally, full round or trapesoidal runners are adopted in most practical cases

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Considerations in Runner Design

q Part Consideration

) Geometry, Volume, Wall Thickness

) Quality (Dimensional,Optical, Mechanical )

q Material Consideration

) Viscosity, Composition, Fillers,Softening Range, Softening

Temperature,Thermal Sensivity, Shrinkage, Freezing Time

q Machine Consideration

) Type of Clamping, Injection Pressure, Injection Rate

q Mold Consideration

) Way of Demolding, Temperature Control

Key factors affecting the design of a runner are summarized here

In the aspect of part consideration, the geometric dimensions of the runner

should be such that flow restriction is at a minimum, that is, the runner should

convey melt rapidly and unrestricitly into the cavity in the shortest way and with

a minimum heat and pressure losses The runner system should allow cavity

filling with a minimum numbers of weld line so that the mechanical and surface

properties of moldings can be improved The runner should permit the

transmission of holding pressure during the packing/holding stage so that the

dimensional accuracy can be ensured

In the aspect of material consideration, the flow character and the thermal

properties of material are related to the sizing of runner diameter and the runner

length Long or small runner should be avoided for material with short flow

length (high viscosity) Runner should be properly sized to minimize material

waste while not cause significant pressure loss

In the aspect of machine consideration, we should note the allowable injection

pressure, injection rate, type of clamping, etc

The runner should be design so that demolding and removal from the molded is

easy Location and number of runner ejectors should be considered in the mold

design phase

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Flow Balance in the Runner Design

) Increase in recovery time of plasticating unit, injection time,

pressure drop, and mold opening time

PLAY412

Consider the runner system design in the multicavity mold case

In a symmetric, naturally balanced cavity layout, all flow lengths from the

sprue to each cavity are of the same length In this ideal case the plastic melt will

fill all cavities simultaneously under the same pressure and temperature

conditions The molded part in each cavity has the same weight and final

properties

Unfortunately not all runners can be naturally balanced, especially for large

parts where multiple gating may be needed to produce a proper part Moreover,

the natural flow balance is difficult for molds with a large number of cavities

and is even impossible for the so-called family mold (combination mold) where

each of the cavities is of different size and forms one component part of the

assembled finished product

In these cases we have to balance the flow artifically Balancing ensures

virtually equal flow of plastic through each gate of a multicavity mold, and/or

through each gate (if there is more than one) into each cavity The melt should

arrive at all gates/cavities at the same time and with the same properties so that

all molded parts have uniform characteristics This type of runner system is

called the artifically balanced runner systems

On the other hand, even though the cavity layout is virtually balanced, the

desired balanced flow may not be achieved since the flow depends on the plastic

material used, the process condition setting, the accuracy of machining and the

finish inside the channel, temperature difference due to unbalanced

cooling/heating, , uneven venting, mold surface quality, etc

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Runner Design and Part Shrinkage

Runner cross-sectional Area

Part Shrinkage

Runner Length

Part Shrinkage

The runner system design has a significant impact on the quality of moldings

For example, the part shrinkage increases as the runner length is increased since

more pressure drop in the runner system and the melt is less packed within the

mold In general, the runner length should be as short as possible in order to

reduce the pressure drop and amount of scrap However, the runners must be of

adequate length to satisfy the other conditions such as flow balance

consideration, accommodation of cooling lines and ejector pins, etc

The part shrinkage reduces as the runner cross section is increased since the

filling process is promoted and the effective holding pressure is higher However,

increase the runner size also produces more scrap and material waste

The size of the runner depends on the size of the part and its wall thickness, the

design of the mold and the type of plastic being processed Plastics with low

viscosity (high melt flow index or long flow length) permit a longer or thinner

runner

The runner cross section should be as small as possible but still compatible with

the melt flow requirement such as pressure drop consideration

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For most thermoplastics, minimum recommended runner size=1.5mm (0.06”)

This table lists the recommended runner diameters for different thermo-plastics

in injection molding industry For most thermoplastics, the minimum

recommended dimension of runner is 1.5mm (0.06”), too small the dimension

may lead to excessive presure drop and filling difficulty

The recommended runner size also reveals the flow ability (processability) of

the plastic material Plastics with low viscosity (high melt flow index or long

flow length) such as polyethylene (PE) permit a smaller runner Larger runner

should be adopted for plastics that have shorter flow lengths (higher viscosity

values), such as polycarbonate (PC)

This table serves as an initial guess for runner sizing

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Both the number and location of ejectors depend on the plastic being processed

The stiffer the plastic is (at the moment of ejection), the fewer ejectors are

needed; also, the designer has higher degree of freedom to determine the ejector

locations For example, the ejectors can be placed under the connecting runners

(bridge runners)

For soft, flexible, or sticky plastics, more ejectors have to be adopted Care must

be taken in the ejector location so that the part can be ejected without leaving

marks or causing damage In general, more ejectors lead to an increase in the

comlexicity of mold and the cost of the hardware and of machining

In the design phase of the runner system, one should consider the ease of

demolding and removal from the molded part The runner system should provide

sufficient spacing for cavity in order to accommodate cooling lines and ejector

pins and leave adequate cross section to withstand the injection pressure force

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Runnerless Molding Technology

Moldings

Runner System:

•Scrap and material waste

•Pressure drop

q Runnerless Molding Technology:

) runners and sprues are kept a molten state during the processing

) runner systems are never actually ejected with the molded parts.

q Types of Runnerless Molding Technology:

) Insulated Runner System

) Heated/Hot Runner System

The conventional runner systemare referred to as cold runner systems since the

runners solidifies during the cooling phase of the injection molding cycle and is

ejected with the part During the molding cycle the pressure drop increas as the

runner is cooled down gradually Degating is required during mold opening (for

three-plate molds) or separately afterwards (for two-plate molds) and the runner

system is regarded as scrap The runner material may be reground and recycled

again, but it may have some physical properties degraded from the original,

virgin material For small products the mass of cold runners may be as much as

80% of the mass of the total shot

On the other hand, the so-called runnerless molding technology has been

developed to circumvent the drawbacks encountered in the cold runner systems

In these special mold designs the runners and sprues are kept a molten state

during the processing and are never actually ejected with the molded part There

are no runners to be reground and recycled, thus, savings in material, labor,

and/or overhead are realized

Typical examples of runnerless molding methods include insulated runners,

heated/hot runner systems

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Insulated Runner System

Molten state melt

Solidified resin shell Cooling Lines

Emergency

parting line

Parting line

q Oversized the runner diameter (15~30mm)

q Insulation effect of frozen skin shell

q Works for most olefinic resins(PE,PP ) and PS

In the insulated runner system, the runner diameter is oversized (say, 15~30mm)

in order to maintain the molten state of the material The large diameter runner

allows an inner molten melt to pass through during the molding cycle because of

the insulation effect of frozen skin shell surrounding the melt core

The insulation runner system has the advantage of extremely simple

construction, low cost tooling, and high efficiency, provided the system can be

left running undisturbed for long periods This design is suitable for most olefinic

plastics (such as polyethylene (PE), polypropylene (PP) ) and polystryene (PS)

The disadvantages of the insulated runner system includes:

- it requires fast cycle to maintain molten state within runner (at least 5

shots/min)

- it requires long start-up periods (15-25min) to stabilize the runner temperature

(up to 150 oC)

- it needs a long color change time

- it needs very accurate gate temperature control in order to have a satisfactory

production rate

- Additional emergency parting line is required to facilitate the removal of the

frozen runner in the case of prolonged delay in the cycle time

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Internally Heated Hot Runner

System

q Material is heated by the heating element in the center of the runner

q Annular gap for melt flow

Heater Cartridge

Heated Probe (Torpedoe)

Part

Melt

Tempertature Profile Vlocity Profile

In the internally heated hot runner system, the material is heated and kept at a

molten state by the heated probe (torpedoe) in the center of the runner The melt

is allowed to flow in the cross section of the annular gap of the runner

The advantages of the internally heated hot runner systems include:

-Less heat loss and lower heating power required since the thermal insulation of

polymer melt

-Less mold components mis-matching problem arising from thermal expansion

-Inexpensive (as compared with the external heated runner system)

-Little space required

The disadvantages of this design include:

-Higher shear rate and pressure drop since the restricted flow area

-Sophicated heat control required (temperature profile exists in the cross

section of the annular gap of the runner)

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Externally Heated Hot Runner

System

q Material is heated by the cartridge-heating manifold in

the housing of the runner

q Circular cross section for melt flow

Cooling Lines

Heater Cartridge Heated Manifold

Part Air gap insulation

constant temperature profile

In the externally heated hot runner system the material is heated by the

cartridge-heating manifold in the housing of the runner Thus a plug-like flow

profile and an approximately constant temperature profile across over the circular

flow area is developed Thus the flow resistance is smaller than that of the

internally heated system

The advantages of this design are:

-More uniform temperature distribution

-Better temperature control

-Lower melt stresses and pressure drop

-Color/material changes easily

The disadvantages of the externally heated hot runner system include:

-More complicated design

-More Expensive

-Significant thermal-expansion-induced mis-match problems for various mold

components

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•Flow length

Plastic Material

Then gate provides the connection between the runner and the mold cavity It

must permit enough material to flow into the mold to fill out the cavity, raises

melt temperature by viscous (frictional) heating, and freezes-off when the

holding stage is over It should be smaller in the cross section so that it can be

easily separated from the molded part (degated)

The type of the gate and its size and location in the mold strongly affect the

molding property and the quality of the molded part The factors which

determine the gate design is summarized here briefly

General speaking, the gate should be small, simple to demold and easily

separated from the part The gate should be connected to the molding in such a

manner that the latter is not distorted (the molding tends to deform concave to the

feed ) and does not exhibit blemishes Cost of tooling is also a consideration

factor The location of the gate must be such that weld lines are avoided or

shifted to a less critical position Molding defects such as jetting, burning,

thermal degradation, short shot, etc should be avoided in the production

Gating scheme and location of gates are crucial to the quality of the molding

Filling pattern and cavity pressure profile are closely related to the final

properties of molded parts, such an mechanical properties, cosmetics (surface

properties), dimensional accuracy A gate should provide appropriate filling

pattern and viscous heating effect, permit effective packing and holding of the

material within the mold These criteria depend on both part design as well as

physical properties of the plastic material

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There are several gate type can be adopted in the mold design, and each has its

own advantage for application

The direct gate or sprue gate feeds material directly into the cavity It is used

for temperature-sensitive or high viscosity materials, and is suitable for

producing part with heavy sections The direct gate can be applied in high quality

part because it allows effective holding (minimum pressure loss) and exact

dimensions can be obtained However, it is suitable only for single-cavity molds

Visible gate mark and the high stress concentration around the gate area are the

disadvantges

The side gate or edge gate is the standard gate for injection molding It is used

wherever the product can be or must be gated from the parting line and where

self-degating is not required or practical It is carried out at the side of the part

and is easy to construct and degate

The pin gate or pinpoint gate is a kind of restricted gates that are usually

circular in cross section and for most thermoplastics do not exceed 1.5mm (0.06

in.) in diameter It is generally used in three-plate molds (with automatic gate

removal) and hot runner construction It provides rapid freeze-off and easy

degating of the runner from the gate Flexibility in gate location is another

advantage of the pin gate It can easily provide multiple gating to a cavity for

thin-walled parts Viscous heating as the melt passing through the restricted

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Gating Scheme

The fan or fin gate is a fanned out variation of the edge gate It is used for large

flat parts (say,over 8cm x 8cm or 3 in x 3 in) or when there is a special reason

such as elimination of weld lines when the danger of part warpage and

dimensional change exists, the fan gate is often adopted

The film gate or flash gate involves extending the fan gate over the full length

of the part but keeping it very thin It is used for flat molded part in the situation

that the orientation of flow pattern in one direction is required, this is important

in the applications of optical parts It has the advantages that there is no weld

line, reduced warpage and improved part dimensional stability However,

postoperation for gate removal is required for this type of gate

The tab gate is used in cases where it is desirable to transfer the stress generated

in the gate to an auxiliary tab, which is removed in a postmolding operation The

tab gate is capable of preventing the jetting problem during the filling stage Flat

and thin parts require this type of gate

The disc gate or its variation, the diaphragm gate, has a conical manifold It is

used for rotationally symmetrical parts (hollow tubes) with core mounted at just

one half of the mold The advantage of using this gate system is that there are no

weld lines, and concentricity of the molded part is ensured This is a important

dimensional requirement for pipe fittings The cone or diagram region eliminates

stress concentration around the gate since the whole area is removed, but the

postoperation is necessary and more difficulty