Mold and Die_ppt

Mold and DieMechanical Engineering DepartmentKasetsart University

Standard 2-Plate Mold Components

Function of Mold Base Components• Top Clamping Plate

– Supports the “A” cavity plate, locating ring, and sprue bushing.

• Locating Ring– Its intended purpose is to properly

situate the mold in relation to the injection nozzle of the machine.

Function of Mold Base Components• “A” Cavity Plate

– Contains and supports the cavity or cavities or the core insert, sprue bushing, and the runners for the parts to be molded.

– In some cases, the cavity may be cut directly into the solid steel plate, while in others the cavities can be constructed separately and inserted into pockets within the cavity plate.

Function of Mold Base Components• “B” Cavity Plate (Core Plate)

– Contains and supports a core section of the molded part and also contains the leader pin bushings.

– The plane between these two plates is the normal parting line (P/L) of the mold, which separates the two halves of the tool.

Function of Mold Base Components• Support Plate

– Is used to provide strength to the cavities to avoid deflection during melt injection inside the cavities.

• Ejector Housing– The ejector housing parallel blocks are

added to provide the height required for the movement of the ejector system.

Function of Mold Base Components• Bottom Clamping Plate

– Secures the movable half of the mold to the movable platen of the injection molding machine.

• Ejector Retainer Plate– Mounted on top of the ejector plate, this

plate retains the ejector head pins, ejector return pins, and sprue puller pin through counter bored holes.

Function of Mold Base Components• Ejector Plate

– Is bolted together with the ejector retainer plate to form a unit. It acts as a back support plate for the ejector pins, return pins, and the knockout bar.

• Ejector Pins– Enter the cavity to make contact with

the molded part.

Function of Mold Base Components• Sprue Puller Pin

– It is used to pull the solid sprue out of the bushing automatically when the mold opens and the molded parts and the runner system are ejected.

• Return Pins– They contact the stationary cavity plate

and prompt the movement of the ejector plates back to normal position prior to the next shot.

Function of Mold Base Components• Leader Pins

– Used to align the plates on the closing of the mold, are hardened and ground steel pins mounted into one of the mold halves.

• Shoulder Bushings– Hardened and ground steel bushings

are mounted into the other half of the mold, in-line with the leader pins.

Injection Molding M/C Nozzle

•The nozzle is a tabular shaped component of various lengths and inside diameters, long enough to reach the mold.

Injection Molding M/C Nozzle

•It commonly consists of a one-piece unit or removable separate nozzle tip screwing into the main body of the nozzle.

Mold Cold Runner System•The cold runner system transfers the

thermoplastic melt from the injection nozzle to the cavities.

•It consists of sprue, sprue puller, runner, cold slug pockets, and gates.

•It is desirable to keep the travelling distance of the material to a minimum to reduce pressure and heat losses.

Mold Cold Runner System

Figure 1 Illustration of molded components in a complete shot

Mold Cold Runner System•The purpose of the runner cold slug

pockets is to catch the melt that has chilled at the front of the nozzle.

•The runner is a channel machined into the mold cavity plate surface (P/L) to connect the sprue with the entrance (gate) to the cavity.

Cold Runner Sprue

•The sprue transfers polymer melt and heat from the molding M/C nozzle to either a runner system or directly to a cavity.

•The contact radius surface between the injection nozzle and the sprue bushing helps with alignment of the flow channels between these components.

Cold Runner Sprue

•Fig 10-67

Figure 2 Contact radius surfaces between injection nozzle tip and sprue bushing

Cold Runner Sprue

•The radius of the sprue bushing should be slightly larger than the injection nozzle tip to ensure sufficient sealing w/o flashing.

•If there were any misalignment, an undercut would be created and inhibit the sprue from being pulled from the bushing.

Cold Runner Sprue

•Fig 10-68

Figure 3 Interface problems between injection nozzle and standard sprue bushing

Cold Runner Sprue

Guidelines when designing a sprue:

•The sprue must not freeze before the runner system and cavities.

•The sprue must be ejected easily and reliably.

•The sprue interfaces with the injection M/C nozzle must not have any flash to avoid sprue ejection problems.

Cold Runner Sprue

Guidelines when designing a sprue:

•At the base of the main runner, in line with the sprue, a sprue puller pocket should be provided to act as a cold slug well.

•The sprue bushing “O” diameter should be a minimum 0.031 in larger than the injection nozzle tip orifice diameter.

Cold Runner Sprue

Standard Sprue Bushings

Figure 4 Standard commercially available sprue bushing

Cold Runner SpruePerformance Alloy Sprue Bushings

Figure 5 Performance alloy sprue bushing to improve temperature control

Cold Runner SprueTranziSprueTM

Figure 6 TranziSprue, a sprue bushing with temperature control

Cold Runner SprueExtension Nozzle Sprue Bushings• They are used to reduce length of the sprue,

reduce pressure drop, and lower injection pressure required for the molding process.

Figure 7 Extension nozzle sprue bushing and short sprue mold

Guidelines for dimensioning sprues

Sprue Puller• During mold opening, the cold sprue is pulled

from the sprue bushing by an undercut in the sprue puller pocket.

Figure 8 Ejection sequence of the cold sprue and runner system

Sprue Puller

Figure 9 Recommended sprue puller designs

Sprue Puller

Reverse taper:

It is the most common design used for molding both high and low melt temperature materials, unreinforced, impact modified, fiber glass, and mineral reinforced resins.

Sprue Puller

“Z” type sprue puller:

It is used in similar applications as the reverse taper. It is not recommended with brittle materials.

Sprue PullerAnnual ring:

This design is recommended only for unreinforced resins.

Designing the Cold Runner System

Important considerations for designing the cold runner system:

▫Cold runner system layout

▫Cold runner cross section geometry

▫Cold runner dimensions

Cold Runner System Layout

The layout of the runner system will depend on the following factors:

▫The number of cavities

▫The geometry of the molded parts

▫The type of mold

▫The type, geometry, and size of the gates.

Cold Runner Cross Section Geometries

nn

Figure 9 Typical cold runner cross section geometries

Cold Runner Cross Section Geometries•The half round runner design is the most

insufficient in delivering a melt, because its ratio of pressure loss to runner unit volume is very high.

•The parabolic runner best approximation of circular

cross-section, simpler machining in one mold

half only.

Cold Runner Cross Section Geometries•The trapezoidal runner

alternative to parabolic cross-section,

more heat loses and scrap thanparabolic cross-section.


•A full round runner

slowest cooling rate,low heat and frictional losses,center of channel freezes last therefore

effective holding pressure,machining into both mold

halves is difficult and expensive.


The runner should provide •a maximum cross section area from the

standpoint of pressure transfer and •minimum contact on the periphery from

the standpoint of heat transfer.

Cold Runner Dimensions

Considerations required for

specifying the cold runner dimensions:

• The wall thickness and volume of the molded part

• The distance of the cavity from the main runner or sprue

• The mold cooling system for the runner and gates


Considerations required for

specifying the cold runner dimensions: (cont.)

• Type of cold runner cross section design

• The thermoplastic melt flow rate

• The thermoplastic viscosity and shear rate characteristics


• The runner length should always be kept

to a minimum to reduce pressure losses and the cold runner system should be balanced.

• Runner balancing means that the distance, the volume, and heat transfer characteristics should be identical for each channel.

Cold Runner Layout

Figure 10 Difference between balanced and unbalanced runners

Cold Runner Layout

Figure 11 Three different balanced runner configurations

Cold Runner Layout

Circular Layout• Equal flow length to all cavities,• Easy demolding especially of parts

requiring unscrewing devices,• Only limited number of cavities can be

accommodated.

Cold Runner Layout

Series Layout• Space for more cavities than with

circular layout,• Unequal flow lengths to individual cavities,• Uniform filling possible only with corrected

channel diameters.

Cold Runner Layout

Symmetrical Layout• Equal flow length to all cavities

without gate correction,• Large runner volume,• Much scrap,• Rapid cooling of melt (Remedy: hot

manifold or insulated runner).

Mold Cavity Gating

The gate is always the narrowest point in the gating system (except the sprue gate) which encounters a resistance to flow. This is a desirable effect because• The melt entering the cavity becomes more

fluid and reproduces the cavity better,• The surrounding metal is heated up and the

gate remains open longer for the holding pressure.

Mold Cavity Gating

The gate size is important for the following reasons:

• The correct type of gate allows for simple gate separation from the molded product and automatic molding process.

• After de-gating, only a small witness mark remains on the molded part.

• Uniform quality control of multi-cavity molds can be achieved.

Mold Cavity Gating

The optimum type, geometry, dimension, and location of the gate are determined by the following factors:

• The viscosity and shear rate characteristics of the resin to be molded.

• The shot size or volume of material to be injected.

• The melt and mold processing temperatures.

Mold Cavity Gating• The crystallinity rate or time required to

freeze the melt in the mold cavity

• The size, complexity, and wall thickness of the molded part.

• Molded product requirements (flatness, roundness, tolerances, strength, toughness)

• Type of injection molding process (runnerless, 2- or 3- plate)

Types of Mold Cavity GatesSprue Gate

• The molded part is injected directly from a sprue, the feed section is called a sprue gate.

• The main disadvantage is that it leaves a large gate mark on the molded part, requires manual sprue removal, and single cavity molds.

Types of Mold Cavity GatesRectangular Edge Gate

• The cross section geometry is simple and cheap to machine (only in one mold plate).

• Close accuracy in the gate dimensions can be achieved.

• The gate dimensions can be easily and quickly modified.

Types of Mold Cavity GatesRectangular Edge Gate

• One disadvantage is that after gate removal, a witness mark is left on a visible surface of the molded part.

“W” should be between D and H

Types of Mold Cavity GatesOverlap Edge Gate

• This gate, being attached to the molding surface, does require more careful removal and finishing than for edge gate.

Types of Mold Cavity GatesFan Edge Gate

Types of Mold Cavity GatesFan Edge Gate

• A width of the gate at the cavity is relatively wide and, because of this, a large volume of material can be injected in a short cycle time.

• This gate can be used for large area, thin-walled molded part.

• The fan shape appears to spread the flow of the melt uniformly.

Types of Mold Cavity GatesTab Edge Gate

• This gate prevents the undesirable “jetting”, leaves large witness marks, and is developed for high viscosity amorphous resins.

Types of Mold Cavity GatesSprue Diaphragm Gate

• This gate is used for single-cavity tubular shaped injection molded part.

Types of Mold Cavity GatesExternal Ring Gate

• This gate is used for tubular type molded parts when more than one cavity is required in a simple 2-plate mold.

Types of Mold Cavity GatesInternal Ring Gate

• This gate is used for molding small tubular molded parts.

Types of Mold Cavity GatesSpider Gate

Types of Mold Cavity GatesFilm Edge Gate

Types of Mold Cavity GatesFilm Edge Gate

• This gate is used for injection molding large, thin-walled, good surface finishing thermoplastic component to help in the production of warp-free products.

• The gate normally extends across the complete width of the molded part.

Types of Mold Cavity GatesPin Point Gate

Types of Mold Cavity GatesStandard Tunnel Gate

Figure 14 Tunnel gate, sequence of ejection


• It is a circular or oval gate that submerges and feeds into the cavity below the P/L of the mold.

• The gate is sheared off from the cavity automatically during ejection.


• The standard tunnel gates are found in two varieties: short tunnel and long tunnel gates.


• When a long tunnel is used, the angle between the part and the tunnel should not exceed 30 to ensure gate shearing.

• The steel safety margin must be at least 0.078 in or greater.

Types of Mold Cavity GatesBanana or Cashew Gate

• It is a variation of a tunnel gate and can provide gating into the lower base of the molded component.

Gate Molding Effects• Ideally, the gate should be positioned to

allow an even flow of the melt into the cavity, so that fills uniformly and the advancing melt front spreads out and reaches the various cavity extremities simultaneously.

• The location and the type of gates used in the molds affect the geometry and dimensional size of the molded part.

Gate Molding Effects

Figure 15 Molding effects caused by a single external

edge gate

Figure 16 Effects caused by two external runners and

edge gates


Figure 17 Effects caused by two internal spider runners

and gates

Figure 18 Effects caused by four internal spiders and

gates


Figure 19 Molding effects caused by the type of gate and location






Figure 22 Effects caused by type, number, and location of gates

Mold Venting Systems

• The openings through which gases trapped inside the cavity escape are known as “vents”.

• The ideal vent would be one that will allow gases to expel freely from the cavity while completely blocking the escape of molten polymer, which would cause flashing.


Consequences of inadequate mold venting:

For the molded part For the mold For injection molding

Burn marks due to diesel effect

Abrasion through combustion residues in the combustion gas diesel effect

Irregular processes through blockage of venting channels

Structural defects/surface defects through detachment of the polymer from a structured mold wall

Corrosion by aggressive gases diesel effect

Longer cycle times due to increased back pressure in the cavity

Overpacking due to injection pressure set too high when vents clogged

Mold coated by combustion residues in the combustion gas diesel effect

Short service life of machine due to higher loading


Consequences of inadequate mold venting:

For the molded part For the mold For injection molding

Displacement if weld lines due to changes in vents

Mold exposed to direct heat due to strong air heating during compression hardening of outer layer

Escaping gases during the process may be harmful to health, depending on material

Entrapped air (voids) Increased cleaning of venting elements

Longer setup time through higher scrap rate

Incomplete mold filling

Higher repair and maintenance costs

Greater need for pressure due to increased back pressure in the cavity

Reduction in strength especially at weld lines

Injection molding machine has higher energy requirements


• Lack of proper venting will cause excessive use of injection pressure for the molding process, which will cause a high degree of internal stresses.

• Relief edge vents have to be small enough to prevent the polymer melt from entering the venting channels.

Product Design for Venting

Several product design geometries can lead to venting problems:

• A thermoplastic injection molded product designed with thin-walled sections connected and surrounded by thick sections should be avoided.


Figure 23 Venting problems, outer heavy/middle thin wall section


• Blind, deep hole in the cavity should be avoided.

Figure 24 Venting problems with a cavity/center deep blind

hole

Product Design for Venting• Avoid any geometry that could cause

a preferential filling along the P/L or around core pins, where the unvented section of the cavity is last to fill.

Figure 25 Venting problems of thick upper/thin base taper core

Venting Characteristics of Thermoplastic Polymers

There are two characteristics of thermoplastics that lead to venting problems:

• Thermoplastic that release large quantities of vapor when heated require more mold venting than materials that do not.

• Thermoplastic that have sharp melting points, fast melt flow rates; for a fast injection, the mold must be well vented.

Mold Venting Process Problems

Inadequate venting causes many injection molding problems:

• Flashing problems

Figure 26 Poor mold venting causes flashing problems


• Short shot

Figure 27 Poor mold venting causes incomplete molded parts


• Internal gas voids

Figure 28 Micro structural analysis showing internal gas voids


• Poor weld lines

Figure 29 Micro structural analyses showing the weld lines

Mold Venting Process Problems• Burn marks

Figure 30 Burn mark problem of a thermoplastic molded part

Mold Venting Process Problems• Core insert corrosion

Figure 31 Core insert corrosion caused by poor mold venting

Mold Venting Process Problems• Poor surface finishing:

caused by the inability to fill the mold cavity quickly.

Figure 32 Two molded bushings showing poor surface finishing

Mold Venting Process Problems• Mold deposit spots

Figure 33 Molded cup showing mold deposit problems

Mold Venting Design• P/L Cavity Venting

Figure 34 Mold base plates and cavity insert mount clearances


Figure 35 In-line P/L cavity venting (in)


Figure 36 P/L cavity venting to the ring groove vent (in)

Mold Venting DesignTable 2 Cavity relief edge vent depth for common types of

resins


Figure 37 P/L cavity venting systems

Mold Venting Design• Mold Cavity Insert Parting Surface

Venting

Figure 37 Different mold cavity insert parting face venting systems


Venting

Figure 37 Different mold cavity insert parting face venting systems


Venting

Figure 38 Horizontal parting face insert cavity venting (in)


Venting

Figure 39 Vertical parting face insert cavity venting (in)

Mold Venting Design• Ejector Ring Venting System

Figure 40 Ejector ring and P/L cavity venting system

Mold Venting Design• Core or Ejector Pin Ring Groove Venting

Figure 41 Core or ejector pins ring groove venting details (in)


Figure 42 Core or ejector pins ring groove vent projection view

Figure 43 Two screw bosses using cores with ring groove vents


Figure 44 Additional cold runner system locations for venting

Mold Venting Design• Ring groove vents for sprue puller pins,

runner/gate ejector pip vents, and 3-plate mold runner venting systems.


Mold Venting Design


Mold Venting Design

Figure 45 Sintered porous insert plug mold

venting

Mold Venting Design• Using Sintered Porous Insert Plugs

Figure 46 Sintered vent plug insert mold venting

application

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