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Gates and Runners Chapter 10






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Gates and Runners Chapter 10. Professor Joseph Greene All rights reserved Copyright 2000. Overview. Gate Location and Number per Cavity Hot Runner Gate Types and Configurations Cold Runner Gate Types and Configurations Cold Runner Ejection and Pullers Cold Runner Molds.
Gates and Runners Chapter 10

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Gates and runners chapter 10 l.jpgSlide 1

Gates and RunnersChapter 10

Professor Joseph Greene

All rights reserved

Copyright 2000

Overview l.jpgSlide 2

Overview

  • Gate Location and Number per Cavity

  • Hot Runner Gate Types and Configurations

  • Cold Runner Gate Types and Configurations

  • Cold Runner Ejection and Pullers

  • Cold Runner Molds

Gate location and number per cavity l.jpgSlide 3

Gate Location and Number per Cavity

  • Rule 1: One gate per cavity should be sufficient

    • Avoids undesirable weld lines in the product

  • One Gate per cavity

    • Outside Center Gating (OSCG)

      • Used with hot runner or three-plate mold, OSCG should be located so that an approximately equal volume of plastic will flow about the same distance toward the outside rim of the product

      • Venting is generally not a problem since plastic is flowing toward the parting line

      • Main problem is with core shift or core deflection

        • C/L of cavity may not coincide with the C/L of the core

          • Result is that one side of the cavity space is filled faster than the other, exerting a side pressure on the core and bending it.

        • C/Ls coincide, but the gate is offset just enough to produce a result similar to the above.

Gate location and number per cavity4 l.jpgSlide 4

Gate Location and Number per Cavity

Gate

Gate

Gate

Vent

P/L

Vent

P/L

P/L

  • Core shift is more serious with

    • Thin-walled products, where higher injection pressures are needed

    • Products with very little draft angle, where products get jammed between cavity and bent core. This prevents pulling out of the cavity, causing scratches or damages to the cavity walls

    • Visible and measurable effect of core shift is uneven walls in the product.

      • Causes warpage due to differential shrinkage.

    • Thinner side may not fill

  • Solutions

    • Proper selection of practical tolerances,

    • Well designed, long, and correctly preloaded tapers for alignment

    • Selection of stiffer (expensive) materials, e.g., tugnsten-carbide cores)

    • Gate selection (type and location) has the most effect on core shift

Fig 10.1

Gate

C/L

Vent

P/L

Vent

Gate location and number per cavity5 l.jpgSlide 5

Gate Location and Number per Cavity

C/L

Ejection

Cooling

P/L

Vent

Vent

Gate

ISCG

Gate

Vent

P/L

  • One Gate per cavity

    • Inside Center Gating (ISCG)

      • Used in hot runner or three-plate molds

      • Same gate location considerations apply as with OSCG

      • Venting problems are the same

      • Added complication is the ejection of the gate

      • Should be used ONLY in special cases when it is necessary to avoid the gate vestige (mark) on the visible side of the product (plates, bowls, etc.)

    • Side Gating Near Top

      • Hot runner edge gating (HREG) or Tunnel gate (TG)

      • Product design my not allow for vestige on either side of part (lenses)

      • Side location is better than gating near the rim for thin walled parts

      • Plastic will usually fill the bottom first & then flow toward the rim (OSCG)

Gate location and number per cavity6 l.jpgSlide 6

Gate Location and Number per Cavity

Gate

Vent

P/L

  • One Gate per cavity

    • Side Gating Near Top

      • Recommended that the side gate be located so that the plasic stream will not flow freely into the top surface

      • It should be directed to hit the core, at least partially.

      • The stream could be directed against a core pin in the top surface near the gate. This creates turbulence in the plastic flow and avoids streak marks.

      • Venting is critical because may travel so that it encircles a portion of the cavity space and traps the air in the bottom.

        • Ejector pins are natural vents

        • Otherwise use vent pins or vent inserts (Chap 11)

Gate location and number per cavity7 l.jpgSlide 7

Gate Location and Number per Cavity

HR or 3-plate

Edge Gate, TG, or HREG

Tunnel Gate

Vent

Vent

Edge Gate

P/L

P/L

Vent through pin

  • One Gate per cavity

    • Outside Gating Near Rim (Fig 10.6)

      • Used in HREG, two-plate and TG

      • Used in basic, general purpose molding, usually flat products

      • Not recommended for containers, particularly thin-walled products due to existence of weld lines and air entrapment.

      • Venting is very important, especially through the ejector pins

    • Gating of an Elongate Product

      • May have HR, HREG, three plate, two plate, or TG (Fig 10.7)

      • Rule is to locate the gate so that the plastic will flow the whole length of the product to avoid formation of weak area across from gate. (Fig 10.8)

      • Weak area is similar to weld line as flow splits.

Flow Direction

Vent through pin

Fig 10.6

Fig 10.7

Gate location and number per cavity8 l.jpgSlide 8

Gate Location and Number per Cavity

HR or 3-plate

Gate

Live Hinge

P/L

P/L

Weak spot

Fig 10.8

  • One Gate per cavity

    • Gating with a Live Hinge

      • Used in HR, three-plate, and two-plate molds

      • Live hinge is usually with PP is a very thin passage from one protion (ox) to the matching other portion (cover), producing a hinge.

      • Usually gate is located in the large portion, and the smaller portion is filled through the hinge.

      • Gating in both parts could produce a weal weld line at the hinge which could break after a few cycles of use.

Gate

Fig 10.7

Gate location and number per cavity9 l.jpgSlide 9

Gate Location and Number per Cavity

  • Two or More Gates per Cavity_ Large Products

    • Sometimes two or more gates are required for large parts where flow distance from single gate would be too long

      • automotive products, bottle crates, etc.

    • Problems with multiple gating

      • Freeze-up of gates

        • Not a problem with cold runners because gate is ejected with part

        • Hot runners that use open gates have the plastic pressure to open gate, which is partially frozen at the end of the previous cycle.

        • If pressure at the gates is uneven, or if one cavity is cooler than the other, the cavity will be filled from the gate that is easier to fill and not from the other

      • Minimum distance between gates

        • Hot runners requires a necessary distance because of physical dimensions of hot runner components

      • Weld lines and venting

        • Weld lines occur when two flow fronts meet creating a weak point.

        • Strength of weld line can be improved by venting trapped air between fronts

        • Trapped air can cause burning of plastic

Gate location and number per cavity10 l.jpgSlide 10

Gate Location and Number per Cavity

Vent

P/L

Gate

Gate

Weld line

Gate

Gate

  • Two or More Gates per Cavity_ Slender Products (Fig 10.11)

    • Conditions are similar to other slender (cylindrical) products explained earlier, except that the cavity is filled from open end of the product from two or more gates.

    • Restrictive to two-plate molds using edge or tunnel gates

      • Plastic enters on opposite sides of the cavity space and rises toward the end.

      • Forces on core are balanced and the rising plastic holds the core steady, even if it is very slender.

      • Two gates are placed 180° and three gates are placed 120°

      • A Continuous gate could be used to reduce weld lines effects or a solid ring which must be removed later.

      • Venting is very important, especially on the end farthest from the gate.

Gate location and number per cavity11 l.jpgSlide 11

Gate Location and Number per Cavity

C/L

Ejection

Cooling

P/L

Gate

Vent

Vent

ISSG

  • Two or More Gates per Cavity_ Slender Products (Fig 10.11)

    • Inside Side Gating at Rim

      • Method used with hot runner edge gates (HRED) for thin-walled containers (fig 10.12).

      • Problems are

        • venting the closed end

        • weld lines

        • limited space for suitable cooling channels

        • limited space for ejection from this side

      • Advantages are

        • Faster cycling

        • Thinner walls

        • Core shift is less of a problem

Gate location and number per cavity12 l.jpgSlide 12

Gate Location and Number per Cavity

  • Gate Vestige

    • Vestige is the visual appearance (on the product) of the point of separation of the plastic between the runner and the product.

    • Gate is usually round and the break-off point may be

      • shiny when separated from hot plastic

      • dull when cold plastic is broken

    • Protrusion: Height by which the vestige extends above part

    • Want to minimize protrusion

    • Hiding the gates

      • Textured or matte surface will hide the gate in cold or hot runners

      • Inside raised surfaces, e.g., lettering

    • If gate hiding is a requirement

      • For three-plate molds (rarely HR) use inside gating

      • For two-plate molds use side, submarine, or undergating

Gate location and number per cavity13 l.jpgSlide 13

Gate Location and Number per Cavity

  • Dimple

    • Dimple is a thickening of the product, usually in the shape of a spherical radius (Fig 10.13B)

      • Aids in providing an unrestricted flow of resin from the gate

      • If no dimple the plastic must flow around the obstruction and stretch

      • With the dimple, the effects of obstruction is reduced and flow is improved

      • With the dimple, the added mass of plastic at the spot remains hot after ejection, as it shrinks the vestige is sucked in and eliminated

    • Dimples should always be used, unless product forbids it

      • Design suggestions

        • Diameter d= 4 - 10 mm

        • Height h= 0.75t (for t<0.75mm), or h=1.0t for 0.75mm < t < 1.5mm

Gate location and number per cavity14 l.jpgSlide 14

Gate Location and Number per Cavity

  • Recessed Gate

    • Small area can be recessed below the level of the product (Fig 10.15) even if protrusion is left standing.

      • If gate is recessed, the depth, h, should be at least equal to t.

      • With wall thickness t<1.5mm ,the recess should be combined with a dimple

  • Hot runner Edge Gate with a Dimple

    • Vestige of the this gate is similar to that made by a cold runner tunnel gate.

    • Created when mold opens and the plastic is sheared off from the partially frozen gate by the motion between the cavity and the product.

    • Vestige is flat and somewhat shinier than if sheared with cold plastic

    • Impossible to hide this gate. Best to make small as possible

  • Valved gates

    • Vestige of valve gate is always circular and looks similar to ejector pin mark. Length of protrusion can be controlled by design of valve pin.

    • Flow of plastic should be broken into swirls to reduce jetting. Gate opposite core

  • Hot runner gate types and configurations l.jpgSlide 15

    Hot Runner Gate Types and Configurations

    • Open Gate

      • Gate is open for the flow of plastic under pressure

        • At the end of injection (or hold), the plastic in the gate freezes sufficiently to act as a plug.

        • This reduces drool into the cavity wile the mold is open for ejection.

        • A unsightly vestige is created and is usually conical

        • On the next cycle the plug is pushed into the cavity where it usually melts.

        • Goal in designing open gates is to find the geometric balance such that the plug freezes readily in the land but can be easily pushed into the cavity.

        • Valve gates are less sensitive to drool, but are more expensive

      • Types

        • Circular gate

        • Annular gate

        • Edge gate

    Hot runner gate types and configurations16 l.jpgSlide 16

    Hot Runner Gate Types and Configurations

    Hot plastic

    Gate

    Land

    Product

    • Circular Gate (Fig 10.17)

      • Disadvantage of long cylindrical land

        • Break point is not determined

        • Plastic can break anywhere along the length of land

        • Usually breaks near hot plastic and leaves long projection on product

      • Short land is better

        • Provides a well-defined breakpoint above conical extension of the product

        • Will still have a small conical projection of plastic

        • With heavy wall, or with using a dimple, the projection may disappear if product is still warm at ejection.

      • Problems with short land

        • Tooling strength of the gate area where plastic tends to push the steel outward into the cavity space.

        • Control of the heat flow away from the gate causing warmer area near gate if not enough cooling is provided.

        • Adding two radii (R1and R2) to the edge of the hot runner has a much shorter land resulting a a higher strength and better heat conduction

    Hot runner gate types and configurations17 l.jpgSlide 17

    Hot Runner Gate Types and Configurations

    Hot plastic

    Cooled

    Steel

    Heated area

    contact with

    product

    Air gap

    Heat Probe

    Gate

    Product

    Hot

    plastic

    Insulating

    plastic

    Heat

    probe

    Gate

    Product

    • Another Open Gate (Fig 10.19)

      • Simple design

        • Exposes product to hot nozzles

        • Provides poor temperature control in the gate area

      • Improved Design (Fig 10.20)

        • Nozzle does not contact the cooled cavity, but is insulated from it by plastic layer.

        • Temperature in both nozzle and cavity can controlled better and easier to create freeze-off

      • Circular Gate Advantage

        • They can be small

        • Well suited for heat-sensitive materials

        • Less expensive to produce

        • Easier to operate than other open gates

    Cooled

    Steel

    Hot runner gate types and configurations18 l.jpgSlide 18

    Hot Runner Gate Types and Configurations

    Hot

    plastic

    Insulating

    plastic

    Heat

    probe

    Gate

    Product

    • Annular Gates

      • Is an open gate with a heated probe at its center to prevent premature freeze-off (fig 10.21)

      • Heated probe or nozzle or nozzle tip is centrally located withing a well in the cavity block. At end of well is the gate and the pointed nozzle tip

    Cooled

    Steel

    • The tip will gradually wear (replaced); Material is usually BeCu

    • Due to tip creating an annular opening, the plastic enters cavity like tubing.

    • For heat sensitive materials, the space between the nozzle and the cavity with a molded or machined high heat resistant material, Vespel.

    • This prevents degradation of material in gap.

    Hot runner gate types and configurations19 l.jpgSlide 19

    Hot Runner Gate Types and Configurations

    • Annular Gates

      • Advantages

        • Wall thickness T of the tubing can be much smaller than the diameter of a circular gate of a cross section equal to that of the tubing.

          • Larger cross section (flow passage can be created by only small increase in the dimension of the the gate diameter.

          • Equivalent circular gate would be too big and have problems with drool.

        • Frozen gates rarely occur

          • Thin layer of plastic close to the nozzle will remain hot (and viscous) after the rest of the gate has frozen .

          • During next shot, plastic will easily enter cavity because hot plastic stream will quickly melt the surrounding layer of plastic

        • Less stringing occurs

          • Annular gates are used for plastics that string easily and not heat sensitive (PS)

          • Used in molds requiring very high filling speeds, e.g., disposable cups

        • Smaller vestiges occur in annular gates than circular gates

        • Wider operating window is possible since better control of temperature

    Hot runner gate types and configurations20 l.jpgSlide 20

    Hot Runner Gate Types and Configurations

    • Annular Gates

      • Disadvantages

        • More diversion of plastic from center to the outside of nozzle through two or more branch passages.

        • If too close to the gate and/or if temperature is too low, the diverted streams may not have enough time to melt in homogeneous tubular stream and may cause visible flow lines.

        • Annular gating is not applicable to all resins because high pressures are required to overcome high flow resistance in narrow gap.

        • Narrow gap between tip and the gate can easily plug up with contamination in the plastic. Caution if use dirty plastic.

        • Plastic can degrade after long exposure to heat n nozzle and may wash out into plastic part.

      • Three common annular gate Designs (Fig 10.23)

        • Heat conducted from a hot runner distributor

        • Torpedo heated by an inside cartridge heater.

        • Nozzle tip heated by an outside band heater.

    Hot runner gate types and configurations21 l.jpgSlide 21

    Hot Runner Gate Types and Configurations

    Heat Nozzle

    Insulation

    Plastic Supply

    Product

    Cooled Core

    • Annular Gates

      • Circular and annular open gates can be used with any hot runner mold but can be also used in insulated runner molds, which distribute the plastic without heated manifolds.

      • Hot Runner Edge Gates (HREG) (Fig 10.24)

        • Principle is same for circular open gates

          • At end of injection cycle, the material in the gate freezes.

          • As the mold opens, the product moves with the core out of the cavity, shearing the gate and leaving a plug (or slug) between the open cavity and the hot plastic in the runner system.

          • Next injection, the plug is pushed into the cavity space, melts, and disappears

        • Land of HREG can be cylindrical, but better if tapered at 5° per side.

        • Gate can be circular, rectangular, or any other shape cut by EDM.

    Bubble

    Hot runner gate types and configurations22 l.jpgSlide 22

    Hot Runner Gate Types and Configurations

    • Annular Gates

      • Design recommendations for HREG

        • Bubble should be as large as possible to create a plastic pool that will not easily freeze.

        • Land should be small in the order of 0.5 to 1.0 mm. The smaller the better but limits are set by the strength of the steel.

        • Land must be smaller than the wall thickness of the product opposite the gate so that the slug can be easily pushed out into the cavity on the next shot.

        • Gap between the land and the gate should be as small as possible

          • (0.03 - 0.05mm) to bring the heat conducting nozzle (and heat) to the plastic close to the gate area.

          • Nozzle should not contact the cooled cavity.

        • Reaction force from the plastic injected through the gate must be well supported to prevent deflection of the nozzle away from the gate.

          • Two gates can be located in the well at 180° to feed two cavities, or

          • Three small or four small cavities can be located at 120° or 90°

          • For one cavity, there must be mechanical support opposite to the gate, or

          • Nozzle design must be stiff enough to withstand the deflecting force.

    Hot runner gate types and configurations23 l.jpgSlide 23

    Hot Runner Gate Types and Configurations

    • Valved Gates

      • Principle of valved gates is that

        the gate opening and/or closing is achieved

        independent of the injection pressure.

        • It opens under injection pressure at the beginning of injection

        • It does not depend on the injection pressure for removal of the frozen plastic from gate.

        • Gates can be opened and closed independently, mechanically using a pin or thermally using a special heater.

    • Mechanically controlled gates: single or double acting operators

      • Single acting: Gates are opened by the plastic pressure acting on a step in the valve pin. They may be closed by

        • A spring which acts as soon as the pressure drops enough.

          • For low injection pressure, the spring must be weak.

          • For high pressure it must be stronger. Springs can anneal.

        • An in-line air cylinder or a wedge acting anytime after pressure drops

      • Double acting: Gates are opened and closed by in-line air cylinders

        • Opened usually at the start of the injection cycle. No step in valve pin

    Hot runner gate types and configurations24 l.jpgSlide 24

    Hot Runner Gate Types and Configurations

    • Valved Gates

      • Basic valve gate (Fig 10.26)

        • Early design of gate- cylindrical pin enters a cylindrical gate

          • Problems include poor alignment, deflection of pin, wear of plate, nd valve pin breakage.

          • Stoke, S, of pin must be sufficient to clear the gate and to ensure that the end of the pin which was cooled while inside the gate is heated again while immersed in hot plastic

        • Improved design included a tapered point of the valve gate pin, with a matching seat as gate (Fig 10.27) avoids some of the problems of alignment

          • But creates the problem of the closing forces acting on the gate. The gate must be strong enough to resist this force.

        • Some designs of valve actuator (Pneumatic piston), the length of thevalve pin is calculated so that with

          • Best conditions: The pin will just touch the seat without pressing down on gate

          • Worst conditions: There will be a slight gap at the seat

          • A stop inside the valve bushing should limit the valve pin stroke to prevent excessive loading at the gate and limits the pin travel when gate is not tapered.

          • Additional length of valve pin is important in gate design.

    Hot runner gate types and configurations25 l.jpgSlide 25

    Hot Runner Gate Types and Configurations

    • Factors Affecting Gate Size and Shape

      • Definitions and terms

        • Rheology: Science of deformation of plastics in response to an applied pressure or stress. OR... Rheology is the the Science of Plastic Flow

        • Melt Index (MI): Indication of the viscosity of the plastic material.

          • It is defined as the amount of plastic that flows out of a cylinder in 10 minutes

          • The cylinder has a weight pushing a rod through the plastic and is at a specified temperature.

          • Higher Melt index = lower viscosity

        • Plastics are Non-Newtonian fluids. The viscosity is NOT constant. Water is a Newtonian fluid and has one constant viscosity

          • Plastics are shear thinning and thin out with increased shear rate

          • Plastics thin out with increased temperature

        • Viscosity is the materials resistance to flow.

          • Low viscosity fluids, like water with a viscosity of 1 centipoise, flow easily and do not have much pressure drop. Units are cenitpoise or Pa-sec

          • High viscosity fluids, like sludge, tar, or melted plastics, flow with a lot of resistance and require a large pressure drop to flow.

    Hot runner gate types and configurations26 l.jpgSlide 26

    Hot Runner Gate Types and Configurations

    • Factors Affecting Gate Size and Shape

      • Definitions and terms

        • Shear rate: The rate of change of velocity of a moving plastic divided by the distance from the center of the channel or tube.

          • Shear rate:maximum at the mold wall and minimum at the center of flow channel.

          • The higher the shear rate = the lower viscosity.

          • Shear rate = volumetric flow rate (cm3/sec) divided by volume of part (cm3)

            • Q = volumetric flow rate (cm3/sec) = shot volume/shot time

            • r = radius of runner or tube

            • L, W, t are Length, Width, and thickness of part

          • Example,

            • Product has a mass of 106 g PS, which corresponds to a volume of 100cm3. (Note: density of PS = 1.06 g/cc)

            • The part is injected in 1 sec, so Q = 100 cm3/sec.

            • The gate size has a cross section of 1 mm3,corresponding to a diameter (2r) of 1.27mm = 0.127 cm, therefore r = 0.0635 cm

            • The speed of injection is 100,000 mm3 / 1 mm2 = 100,000 mm/sec or 100 m/sec.

            • Shear rate = 100 cm3 /sec / (3.14)(0.0635) 3 =527,000 sec-1. Very high

    Hot runner gate types and configurations27 l.jpgSlide 27

    Hot Runner Gate Types and Configurations

    • Factors Affecting Gate Size and Shape

      • Definitions and terms

        • Shear stress- unit pressure on the fluid that is subjected to shearing action

        • Shear sensitivity: Various plastics respond differently to the amount of shearing in the mold. Some plastics degrade if shear too much (PVC)

        • Shear insensitive materials: Some plastics are insensitive to the amount of shearing (PP, ABS)

    • Factors Affecting Gate and Land Size in Open Gates

      • Part weight and size: Longer flow length and the larger the cavity surface, the larger gate is required to reduce fill pressure

      • Product wall thickness: Large wall thickness requires large gate to provide material during packing phase.

      • Resin: Viscous resins require larger gates and shorter lands to reduce restriction at gate.

      • Location of cooling lines for mold: Too close to gate will cause premature gate freeze-off. Too far from gate, will cause drooling.

      • Injection time: Very fast injection requires a larger gate to reduce local pressure drop and prevent excessive shearing

    Hot runner gate types and configurations28 l.jpgSlide 28

    Hot Runner Gate Types and Configurations

    • Factors Affecting Gate and Land Size in Open Gates

      • Melt temperature: Gate can be reduced to increase shear rate and heat the plastic more and fill the part easier.

      • Entrance effects: Sharp corners or restrictions impede the flow of resin and can cause shear-induced degradation. Use a generous radius on the cavity side of the gate to help provide smooth laminar flow and prevent jetting.

      • Nozzle tip position: If tip is too close to the gate, the gate is less likely to freeze off prematurely, and as a result, a smaller gate can be used.

      • Dimple: If dimple is too large, the cycle will slow down.

  • Requirements for a Correctly Designed Gate

    • Permit unrestricted flow- to the greatest possible extent- to prevent degradation of the plastic.

    • Prevent drooling or stringing

    • Provide correct shearing to condition the resin and to reduce its viscosity to achieve the greatest flow length possible.

    • Hide or disperse the cold slug without impeding flow. This also is helped by a dimple.

  • Hot runner gate types and configurations29 l.jpgSlide 29

    Hot Runner Gate Types and Configurations

    • Consequences of improperly Designed Gate

      • Jetting (visible flow lines away from the gate)

      • Blushing (a concentric, cloud-like blemish around the gate)

      • Stringing (threads of resin sticking to the product)

      • Warping (deformation of the product)

      • Degradation of the resin

      • Improper filling (short shots)

      • Premature freeze-off of gates, and

      • Bad vestiges (gate marks)

    Hot runner gate types and configurations30 l.jpgSlide 30

    Hot Runner Gate Types and Configurations

    • Gate Shape and Size

      • Land Length (Gate height)

        • Land should be short as possible to achieve lower pressure for filling and to improve degating by reducing the height of vestige.Lengths= 0.13 to 0.25mm

      • Gate diameter

        • Too small a gate can be recognized by blemishes at the gate and surface imperfections.

        • Too high an injection pressure and short shots

        • Premature gate freeze-off

      • Role of Shearing in Gate Diameter Sizing

        • High shear rates can raise local melt temperature of the melt and reducing viscosity making the plastic flow easier in the cavity

        • High shear rates can improve the gloss of the plastic

        • If gate is too large, then very little shearing may occur and could cause gate freeze-off

        • Fig 10.29

    Hot runner gate types and configurations31 l.jpgSlide 31

    Hot Runner Gate Types and Configurations

    • Gate Shape and Size

      • Role of Shearing in Gate Diameter Sizing

        • Fig 10.29

          • Fig 10.29 creates four times as much shearing as in B and none in C.

          • Shear rate in gate should be greater than 1,000 sec-1

          • In thin walled molding (<1mm), the shear rate should be between 100,000 and 1,000,000 sec

          • All materials have a maximum shear rate at which they will degrade.

          • Table 10.1

    Hot runner gate types and configurations32 l.jpgSlide 32

    Hot Runner Gate Types and Configurations

    • Gate Shape and Size

      • Time of Exposure to Shear

        • Longest time exposure to shear has an effect on the resin

        • Long exposures can degrade some plastics

    • Establish a Proper Gate Size

      • Use past experience, Use computer analysis, Use empirical approximation

    • Computer analysis

      • Gate is part of the hot runner and should be designed with the rst of the hot runner

        • Pressure drop: less than 6,000 psi or less than 5,000 psi for General Purpose

        • Melt Temp Rise: Less than 15C but less for shear sensitive plastics

        • Shear rate: Greater than 1,000 sec -1

        • Shear stress Table 10.1

    • Empirical Analysis: Use the formula to determine gate diameter d

      • A is surface area, N and C are constants

    Hot runner gate types and configurations33 l.jpgSlide 33

    Hot Runner Gate Types and Configurations

    • Establish a Proper Gate Size

      • Empirical Analysis: Use the formula to determine gate diameter d

        • A is surface area, N and C are constants

          • thickness of product:

    Cold runner gate types configurations l.jpgSlide 34

    Cold Runner Gate Types & Configurations

    • General Features

      • Cold runners are used for small production runs

      • Cold runners cost less than hot runners, but have scrap

      • Preferences

        • Smaller gates and shorter lands are preferred

        • Tapered lands are important to ensure plastic will break at part

        • Sharp corner at entry will cause an even break

          • A straight section between 0.1 and 0.2 mm

        • Identical gate size is important for multicavities to insure equal flow to cavities and for interchangeability.

    Cold runner gate types configurations35 l.jpgSlide 35

    Cold Runner Gate Types & Configurations

    • Edge Gate Fig 10.30

      • Simplest form of gate and used when part can or must be gated at parting & where self-degating is not required

      • Guidelines

        • Sharp configuration in figure on the left of Fig 10.30

        • Flat portion in figure on the right of Fig 10.30

        • Gate width and height: W= 3h; Length of land: L = 0.5 to 0.8mm

        • Taper should have included angle of 30° or 15° per side, preferably 60° or 30° per side. Too large and angle weakens the cavity, too small an angle has bad flow of plastic

        • Cross section of gate: Draft angle of 5° in Fig 10.31

      • Dimensions for Edge Gate

    Cold runner gate types configurations36 l.jpgSlide 36

    Cold Runner Gate Types & Configurations

    • Fan Gates (Fig 10.32)

      • Variation of the edge gate with the gate as follows

        • Width, W, much greater than 3h. W may be 10 mm or more

        • Gate height, h, may be only 0.1 mm

        • Constant cross section as width is increased and h is decreased as the resin flows from runner to part through the gate.

      • Used when edge gating is OK, but vestige is avoided

        • Example

          • Suggested edge gate size is 0.5x1.5mm, or 0.75 mm2, then a comparable fan gate would be at least the same cross-sectional area, or h x W = 0.75 mm2, with W = 7.5mm and h = 0.1mm.

    Cold runner gate types configurations37 l.jpgSlide 37

    Cold Runner Gate Types & Configurations

    • Diaphragm (disk) Gate (Fig 10.34)

      • Variation of edge gate and is a circumferential fan gate.

      • Simplest form, the gate is a disk, where

        • Width, W, equals the length of the inside circumference

        • Height, h, may be 0.1 to 0.15 mm

        • Shape is similar to the fan gate, with an angled passage from a disk or circular runner and ending in a short straight section.

        • Located on inside or outside of product (Fig 10.33)

        • Can be used with cold sprue of hot runner

        • Two runners bring plastic to a distributing, circular runner, which is concentric with the product; the diaphragm gate connects it with the cavity.

        • This design is preferred to s solid disk due to large mass of runner

    Cold runner gate types configurations38 l.jpgSlide 38

    Cold Runner Gate Types & Configurations

    • Tab Gates (Fig 10.36)

      • Used with 3-plate molds when two or more products of different shapes are produced in one mold.

        • Some products are pin point gated directly in the top of the 3-plate mold, while some are edge gated.

        • Basic runner system is 3-plate

          • Cavities re gated from the edge have tabs in the P/L outside the cavity

          • 3-plate drop feeds the tab, which is connected to the cavity with a gate

          • Hot runner mold has a tab gate required when hot runner gates are not allowed in part.

    Cold runner gate types configurations39 l.jpgSlide 39

    Cold Runner Gate Types & Configurations

    • Tunnel (sub) Gates (Fig 10.37)

      • Used in two-plate molds to provide automatic, in-mold separation of the product from the runner.

      • Cross-section is similar to edge gates, except the that tunnel gate passages are usually circular (Table 10.3)

      • Suggestions

        • Gate diameter small

        • Land small

        • Runner extensions rounded

    • Product stays with core as mold opens and shears gate off

    • Taper gate so that it is pulled out of the cavity as mold opens

    Cold runner gate types configurations40 l.jpgSlide 40

    Cold Runner Gate Types & Configurations

    • Tunnel (sub) Gates (Fig 10.37)

      • Suggested dimensions for Tunnel Gate

      • Not suitable for following

        • Difficult to separate family mold products after ejection

        • If products must be kept on runner for inspection

        • If products are shipped with the runner for customer inspection

      • Design guidelines

        • Distance D of sucker depends on flexibility of plastic

        • Runner extension and runner should be as small as possible

    Cold runner gate types configurations41 l.jpgSlide 41

    Cold Runner Gate Types & Configurations

    • Tunnel (sub) Gates

      • Design Guidelines

        • Steel between cavity wall and runner extension is very thin and easily damaged, e specially if operator tries to remove cold slug.

        • Steel selection for mold must be tough rather than hard

          • H13 hardened to 46-49 RC is a good choice.

          • Carburizing steels are not recommended

      • Examples of Tunnel Gates

        • Very shallow products (lids) Fig 10.39

        • Completely flat products (disks) Fig 10.40

        • Deep products (containers) Fig 10.41

        • Multiple Tunnel Gating for slender products (vials or needle barrels) Fig 10.43

        • Curved or Submarine gating when side entrance is unacceptable

    Cold runner gate types configurations42 l.jpgSlide 42

    Cold Runner Gate Types & Configurations

    • Three-Plate Gates (Fig 10.45)

      • These gates are self-degating

      • Keep gates as small as possible to minimize vestige

    • Cold Runner Ejection and Pullers

      • Required to remove runners in cold runner molds

      • Guidelines

        • Runner must be positively ejected

        • Keep cross section and mass of runners small

        • Use ejector pins for runners, without pulling. Ejector pin must never extend into the runner channel.

        • Pulling the runner using ejector pins only

        • Pulling the runners with suckers. Runner is retained with sucker Fig 10.51. D is standard ejector pin size, W is larger than D.

    Cold runner gate types configurations43 l.jpgSlide 43

    Cold Runner Gate Types & Configurations

    • Cold Runner Ejection and Pullers

      • Guidelines

        • Pulling the runner with sucker pins.

          • As the mold opens, the runner stays on the stripper plate.

          • The stripper plate then strips the runner off the sucker pins.

          • Important that the runner not remain with stripper plate

          • The sucker pin must enter the runner by the amount of P (Fig 10.52)

          • Stripper plate is usually hardened steel. Hardened bushings are recommended around the sucker pins.

          • Diameter of sucker pin should be as large as possible and selected from standard pins sizes (4, 5, 6, 8 mm)

          • Pulling force is dependent on surface area of the head and on the angle A

            • If pin diameter is large, the angle can be smaller and there is less risk of breaking the plastic when the stripper moves forward.

            • Ideally diameter of sucker pin should be less than ejector pin

          • Common problems are that sucker pins fails to pull the runner or head breaks

    Cold runner gate types configurations44 l.jpgSlide 44

    Cold Runner Gate Types & Configurations

    • Cold Runner Ejection and Pullers

      • Flow of Plastic around Sucker Pin Heads

        • Important to make sucker pins as large as possible, which can restrict flow of plastic in runner.

          • Runner must be widened to increase flow. (Fig 10.54)

      • Location and Number of Ejectors, Suckers, or Sucker Pins

        • Keep the number as small as possible

        • Stiffer plastics require fewer pins

        • Important to provide adequate cooling around thicker runners

        • Keep suckers as large as possible and reentrant angle as small as possible

    • Placement of Suckers Near a Tunnel Gate

      • Suckers positioned close to runner extension

        • As mold opens, the runner and the drop are held in the core by sucker

        • The gate shears off and the runner flexes for soft plastics.

        • The gate shears off and the plastic breaks at sucker for hard plastics.

    Cold runner gate types configurations45 l.jpgSlide 45

    Cold Runner Gate Types & Configurations

    • Cold Runner Molds

      • Main advantage is lower cost compared to hot runners

    • Two Plate molds (Sec 10.1, 10.3, 10.4)

      • Sprue Gates

        • Cold sprue leads directly from machine nozzle into cavity space

        • With Outside Center Gating (OSCG) the cold sprue is short.

        • Diameter of sprue opening must be at least 1mm larger than the hole in the nozzle to prevent a hook and possible failure to pull the sprue out.

          • Machine nozzles have D = 3mm, the sprue > 4mm

        • ISCG is used to eliminate outside vestige.

          • The sprue must enter the core and is becomes very long.

      • Simple runner for 2 or more cavities (Fig 10.61)

        • Length of sprue should be kept to a minimum

        • Nozzle can go right into runner to same space

        • Short sprue length of 15mm

    Cold runner gate types configurations46 l.jpgSlide 46

    Cold Runner Gate Types & Configurations

    • Cold Runners in Three-Plate Molds Fig 10.63

      • Use a 3-plate mold if:

        • Product must be gated at the top, rather than the edge or side

        • The ejection is with floating stripper rings that are separated by stripper plate by a gap.

        • Cosmetic reasons require it because of smaller gate.

        • Self-degating is required.

        • Number of cavities is critical and the cavities can be spaced closer because no space is needed for the runners.

        • Clamping force would be reduced because no area is needed for the runners as in a 2-plate mold

    Cold runner gate types configurations47 l.jpgSlide 47

    Cold Runner Gate Types & Configurations

    • General Comments in Three-Plate Molds

      • More complicated and more expensive than 2-plate molds

        • Very similar to hot runner in design except for gate area and ejection mechanism

        • Can be converted readily to a hot runner mold

      • Dimples

        • Dimples opposite gate are required similar to dimples for hot runner gates for similar reasons

      • Runners

        • Runner layout should be balanced and the flow path be short

        • Straight line runners in X and H are best

        • Cross sections should be trapezoidal cut into cavity

        • Size and Xsection of main and branched runners are found with Moldflow and should be as small as possible

    Cold runner gate types configurations48 l.jpgSlide 48

    Cold Runner Gate Types & Configurations

    • General Comments in Three-Plate Molds

      • Drops

        • Drops should be as small as possible to minimize mass

        • Drop length is function of mold layout and should be short

        • Drop diameter depends on plastic flow characteristics like runner

        • Draft angle is function of finish, better finish = smaller draft

          • Poor finish will prevent plastic from pulling out

          • With small angle (<5° per side), the finish must be excellent (draw polish)

          • Commercial sprues reamers have a draft angle of 1° 19’ per side which creates a very large sprue.

        • With small products and large number of cavities, the runner system can weigh more than the parts.

        • Simplest drop design, the cavity and the runners are cut into the cavity plate. This is done if cavities are simple and replaceable

    Cold runner gate types configurations49 l.jpgSlide 49

    Cold Runner Gate Types & Configurations

    • Drops (continued)

      • More complicated drops

        • The cavities are inserted through-bores in the cavity plate. Fig 10.65B

        • The cavities are set into pockets in the cavity. Fig 10.65C

        • The cavity insert are set in a pocket, but they extend into the cavity plate

          • Fig 10.65B

        • Preferable to make the sprue in the same part as the cavity itself (Fig 10.65A, B, and D)

        • Fig 10.65C is not practical

    • Number of Drops per Cavity

      • One gate is sufficient otherwise get weld lines and venting issues

      • For large parts more than one gate might be necessary if:

        • The volume of the plastic is so large that one gate would slow down fill

        • The products are oddly shaped causing the flow length to be large.

      • Weld lines can be improved with thickening area and adding vents

    Cold runner gate types configurations50 l.jpgSlide 50

    Cold Runner Gate Types & Configurations

    • Hot Runner Molds

      • Important points for HR technology

        • Distribution

          • Distribute hot plastic to all gates of the mold at the same temperature and pressure as it was in the nozzle

        • Heat losses

          • Heat losses due to radiation and direction contact (conduction) with the cold mold must be replaced with heaters. Insulation helps

        • Start-up temperature

          • Plastic in HR is cold at the beginning and must be heated with large capacity heaters. Heat-up time should be 15 to 30 minutes.

        • Sealing and Heat expansion

          • Temperature differences between plastic in HR and in cold mold can be 100 F to 300 F and can cause alignment problems between

            • HR nozzles and mold gates

            • Seals in HR

    Cold runner gate types configurations51 l.jpgSlide 51

    Cold Runner Gate Types & Configurations

    • Hot Runner Molds

      • Important points for HR technology

        • Cleanliness of plastic- Dirt can plug up a HR system.

          • Exclusive use of virgin material is best

          • Filers can be used with 2 stage machines

        • Nozzle insulation

        • Quantity of plastic

        • Color or product

        • Method of gating

        • Power requirement


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