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1. Gates and RunnersChapter 10 Professor Joseph Greene
All rights reserved
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
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.
4. Gate Location and Number per Cavity 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
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
5. Gate Location and Number per Cavity 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)
6. Gate Location and Number per Cavity 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)
7. Gate Location and Number per Cavity 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.
8. Gate Location and Number per Cavity 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.
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
10. Gate Location and Number per Cavity 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.
11. Gate Location and Number per Cavity 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).
venting the closed end
limited space for suitable cooling channels
limited space for ejection from this side
Core shift is less of a problem
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
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
Diameter d= 4 - 10 mm
Height h= 0.75t (for t<0.75mm), or h=1.0t for 0.75mm < t < 1.5mm
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
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
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
16. Hot Runner Gate Types and Configurations 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
17. Hot Runner Gate Types and Configurations Another Open Gate (Fig 10.19)
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
18. Hot Runner Gate Types and Configurations 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
19. Hot Runner Gate Types and Configurations Annular Gates
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
20. Hot Runner Gate Types and Configurations Annular Gates
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.
21. Hot Runner Gate Types and Configurations 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.
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.
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
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.
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.
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
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
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
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.
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)
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
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
31. Hot Runner Gate Types and Configurations Gate Shape and Size
Role of Shearing in Gate Diameter Sizing
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.
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
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
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:
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
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.
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
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
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
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.
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
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.
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)
Gate diameter small
Runner extensions rounded
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
Distance D of sucker depends on flexibility of plastic
Runner extension and runner should be as small as possible
41. Cold Runner Gate Types & Configurations Tunnel (sub) Gates
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
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
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.
43. Cold Runner Gate Types & Configurations Cold Runner Ejection and Pullers
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
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.
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)
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
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
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 opposite gate are required similar to dimples for hot runner gates for similar reasons
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
48. Cold Runner Gate Types & Configurations General Comments in Three-Plate Molds
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
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
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
50. Cold Runner Gate Types & Configurations Hot Runner Molds
Important points for HR technology
Distribute hot plastic to all gates of the mold at the same temperature and pressure as it was in the nozzle
Heat losses due to radiation and direction contact (conduction) with the cold mold must be replaced with heaters. Insulation helps
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
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
Quantity of plastic
Color or product
Method of gating