Loose cavity inner undercut release method in injection Molding

one way to release the undercut is to loose cavity, the opposite of loose cores, loose inner cavity is used to remove the undercut in the cavity area, note the image below for more details,

1 = top plate
2 = runner stripper plate
3 = cavity plate
4 = core plate
5 = locking block for loose cavity
6 = angular pin for loose cavity system
7 = loose insert cavity

 

some designers also say that with the inner mold cavity slider, whatever his name is, in essence, functions and works the same way. when the first opening between the top runner stripper plate (2) with the cavity plate (3) top plate will pull locking block (5) together with the angular pin (6), aided by the spring on the insert cavity, this movement causes the loose insert cavity to move straight straight to the angular movement of the pin, as a result of friction between the angular pin with a hole in the loose insert cavity.

when construction is used, ie if your product has undercut formation on a position in the cavity region, angular corners and locking block commonly used ranged from 8 to 20 degrees, above the angle can cause angular cartilage pin was broken, because the force that fought too large.
Another thing to note is the stroke, you must ensure that the stroke of movement of loose cores must be secure, which is about the length undercut products coupled with the 5-10 mm.

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Main Construction Pin at 3Plate Mold

I take those topic, to start learning about kind of pin on 3 Plate Molds, becouse between 3plate and 2 plate mold have several different function of Pin.
Basically 3 Plate mold to support the main construction have 3 kind of Pin.Look at picture below


(click the picture to enlarge)

3 Part Pin are
1. Main Support Pin
2. Positioning Pin
3. Puller Bolts

those pin will support both opening and closing process of 3 plate type of injection Mold. the function each pin are

Main Support Pin
this pin is totally support cavity, and runner plate when molds in open,when we buy mold base plate from fabricator, they always have exact diameter of Support Pin that we needed, it was calculated, but if your part is precision ensure it by calculating the deflection of those pin caused by cavity plate and runner plate.
make sure the deflection of pin not larger than 0.1 mm, choose more large diameter of Support pin when deflection is large.

Positioning Pin
Because support pin doesn't precision when closing, we need more precision guide part, this part also named Guide Pin, becouse it will guide core and cavity matching together. use this part with the Bush, in high precision part, some times we need more than one Positioning Pin, we can also add block guide, taper block set, taper pin set and other high precision positioning tools.

Puller Bolts
puller bolts doesn't support any plate, this part actually include in opening part of three plate molds.
working together with tension link or magnet block, this part will make parting line at stripper plate and top plate, this parting line used to make sure that sprue was cut from nozzle.

Parting line Opening is about 5-10 mm, depend on large of molds.

look at picture below to learn the length of each Pin.

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Sprue bush cooling design and construction

some times sprue bush need cooling to cut the cycle time of injection process, when diameter of sprue is large cooling in sprue bush is important. some fabrication of mold part have their own sprue bush that containing cooling pipe. like picture below


from those picture some standard part mold company have their own methods to make inner cooling at the sprue bush, we can by cutting in some part that weld together again, or by other method.

the other way to make cooling in sprue is like picture below


by those type of sprue we change the entrance and exit flow by using stopper buffle, for example like picture below


or you can make the flow straight, the other cooling method in sprue is like picture below, by using this type is easy to attach the O ring, becouse in and out flow from below side that contact with moldbase. becouse when assembling of ring to moldbase is more easy by attach the O ring in Moldbase then put the sprue bush than attach the O ring in sprue bush and assembling it together in moldbase.

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Locating Ring Design and Construction in Injection Mold

Locating ring is simple part but, very important when mold base attach to injection machine, ok here the construction of locating ring after assembled in mold base.



















then after attach in injection mold, the position of locating ring is like below


Basic Rule Design
1. when design locating ring, first rule is look at your injection machine catalog or hand book, and find the dimension of locating ring. like picture below

from picture above we know that locating ring diameter is 200 mm and diameter of nozzle is 52 mm, so your locating ring outer diameter must same with those injection machine locating ring hole.

2. don't worry about the tolerance, at the injection machine tolerance always about minus 0.01-0.1 mm, then the locating ring tolerance if we buy in standard mold part company is about minus 0.01-0.05.

3D Modeling various Locating Ring
below is some picture of locating ring that was i sketch using Google sketchUp, now i provide two type of locating ring. to choose right locating ring in mould is depend on injection machine nozzle, locating hole diameter, nozzle length in other factor.


Second type





















Third type
this type have large chamfer from top to bottom in inner hole, those chamfer provide shape so nozzle can touch the sprue more precise and easier.


and below is sample picture of locating ring base on Mold Part Industries

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Sprue Bush Design in Injection Mold

The molten plastic injected from the injector nozzle will go through a sprue (sprue bush), a runner, and a gate and fill up in the cavity.
As the temperature of molten plastic is lowered while going through the sprue and runner, the viscosity will rise; therefore, the viscosity is lowered by shear heat generated when going through the gate to fill the cavity.
so learn Sprue bush design is important, becouse sprue bush is part that contact first time with melted plastic, bad design of sprue bush can make plastic product broken, especially sprue broken and runner broken.
picture below shown mold basic construction with sprue.



























below is 3D model of sprue bush with 2 bolts
































this type is commonly use in small mold size, until 350 ton injection machine is still used.

the other construction is 4 bolts, this sprue bush type commonly use in large mold type and mass production, more than 200 ton






























and below is sprue bush with ring attachment, this type use when top plate thickness is width, but some designer prefer use large chamfer in top plate so with first model sprue bush is still can be attach


































Sprue Bush Design Rule
1. Ball radius of sprue must larger than ball radius nozzle, so don't forget to check diameter of nozzle of injection machine, or you can look at injection machine catalog. for example when ball radius of nozzle of injection machine is 10 mm use ball radius sprue bush 11 until 13 mm.
2. Hole diameter inner must be larger than hole diameter nozzle, diameter nozzle is variating base on machine injection type and manufacture, is about 2 mm until 5 mm. use 0.5 mm more larger than hole diameter of nozzle.
3. check your plate thickness, make sure that length of sprue bush fit with the plate thickness.

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Other Joint Construction Betwen Cooling in cavity and Mold base with O Ring

What is O ring
o-ring is a loop of elastomer with a round (o-shaped) cross-section used as a mechanical seal or gasket. They are designed to be seated in a groove and compressed during assembly between two or more parts, creating a seal at the interface. (wikipedia.com)
O-Rings are torus-shaped (i.e. doughnut-shaped) objects made from elastomeric compounds such as natural or synthetic rubber, and are used to seal mechanical parts against fluid movement (air or liquid). O-Rings perform their sealing action by deforming to take the shape of their cavity, after being oversized to guarantee an predetermined interference fit.( efunda.com)


O ring typically used in one of two seal designs, axial or radial, in mold design to attaching cavity to mold base axial, face type are commonly used then radial type, because axial type is more easy to assembly in mold than radial type.
2D O ring drawing like picture below


Static seals exist where there is no relative motion between the mating surfaces being sealed.





at previous post about basic construction for joining cooling system in cavity and mold base , we can learn how to joint pipe from mold base of mold to cavity site.

various construction can be develop from basic construction, previous post basic construction is fast and cheap construction model, but it isn't good when you in large mold and mass production mold, why? becouse production mold must have endurance until 1000 000 shot.

Picture below shown develop construction of joining cooling pipe








blue color indicate mold base side and yellow color indicate cavity side.
basic rule from those picture are :
1. T is height of O ring
2. part of O ring that will receive pressure form cavity plate. 15-30% from it's height is best. this little pressure give to prevent leak when water flow in cooling system.
3. t2 value is same with t, but t2 is horizontal, becouse O ring always made from elastomer plastic, by little pressure we can add O ring to O ring hole.
4. L is Gap between cooling channel and O ring hole.

O-Ring Design Considerations
1. Proper Squeeze
•Compression expressed as a percentage of the free-state cross-sectional thickness:








•Face Seal: 20-30%
•Static Male/Female: 18-25%
•Reciprocating: 10-20%
•Rotary: 0-10%
2. In static seals, where the O-Ring is not in axial motion in the bore, the recommended maximum compression is approximately 40%.
3. The O-Ring must be compressed by a predetermined amount, and this compression determines the O-Ring cross-section diameter.
4. The O-Ring inner diameter is typically chosen to be close to the groove's inner diameter; by selecting it to be slightly less than the groove's inner diameter, the O-Ring will stretch and hug the groove.
5. The Groove Width must be larger than the O-Ring cross-section diameter, to accommodate the radial expansion of the O-Ring when it's axially compressed in the gland.
6. stretch,
- Excessive stretch can overstress material, thin cross section, and reduces % squeeze
- % cross section reduction due to stretch about half of the % ID stretch
7. Sharp Corners, make R in cornet to prevent damage during seal installation.
8. Pressure and Clearance Gap.
Most elastomeric seals are designed to operate within ambient pressure to about 1,500 psi. At very high pressures, the seal must have sufficient strength to resist extrusion into the clearance gap. The chart at right illustrates the recommended limits of the combination of clearance gap (diametral), seal hardness, and pressure differential. picture below shown graph between clearance gap and pressure



















9. When it is said that an elastomer is good for an application it is meant that some compounds which include that material are acceptable. Not All. For instance, some compounds of EP are good for brake fluid applications, but most are not acceptable.
10. DO NOT use a lubricant composed of the same material as the O-ring. For example, a silicone lubricant should NOT be used with a silicone O-ring.
11. Avoid using graphite-loaded compounds with stainless steel, as they tend to pit the stainless steel surface over time.

useful reference
http://www.rlhudson.com/O-Ring%20Book/designing-static.html
http://www.pspglobal.com/application-limits/04-watery-substances.html
http://en.wikipedia.org/wiki/O-ring
http://www.efunda.com/DesignStandards/oring/design_guidelines.cfm#axial
http://www.marcorubber.com
http://www.allorings.com/gland_static_axial.htm

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Construction of Cavity Cooling and Mold Base Cooling, O ring

here the basic construction of joining cavity cooling and mold base cooling, we can use from simple construction to joint cavity and mold base cooling with one O ring, or we can use more complex construction like using special plate, and various O ring seal type. here is basic construction to joint the cavity cooling and mold base cooling.


both circular type and straight type cooling can be joint use those construction,
those construction use in half circular type of cooling system like picture below


click the image to see more large

simple joint just need one O ring or rubber seal, O ring pocket is preferred make in horizontal face from assembly, becouse it's make assembly and machining process mold more easily,
basic rule and tolerance to prevent leak in O ring are like picture below













at the cross section of cavity and mold base cooling construction, blue color indicate mold base part, and light yellow indicate cavity block, green is indicate hole or pipe of cooling. light black is O ring before penetrate with cavity block.
however is the simplest construction to join cooling cavity and mold base.

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Straight Type Cooling for Cavity

Basically cavity system cooling can divide into two type
1. straight type
2. Circular type

straight type system, usually have in and out of water pipe in not same side of mold or cavity. look at picture below

although the pipe that cool of insert part doesn't straight, but if inlet and outlet cooling fluid is not in same side we can consider that those type of cooling is straight. red color pipe indicate inlet and blue color pipe indicate outlet.

second picture below is still same with previous but this picture include the cavity block



yellow part at each side is plug to prevent unwanted flow of cooling, pink block at the center is core insert part. with attach to core block using bolt. Block with blue color is core block before inserted to mold base.

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Basic Functions of Mold Base Parts

A mold plate is any plate required to built a complete mold, in all plate use for molds have the main requirements, they are :
1. Good corrosion resistant
2. Good Machinability.
3. Easy to clean
3. Good heat transfer.
4. Homogeneity of steel.
5. Easy ability.
6. Low Cost.

at picture below you can see 3 plate mold base type with closed position, basically 3 plate type and 2 plate type has some main plate, see explanation about mold construction type to understand more,


explanation below based on number at those picture
1. Fixed Clamping Plate or Top Plate
Holds The fixed side of the mold to attached at the fixed platen of the injection machine. at this plate will attach locating ring, eye bolt, and sprue bush.
2. Runner Stripper Plate
this plate only used in 3 plate molds type, the function is to cut resin from nozzle in top of sprue bush, and pull the runner by runner locking pin.
3. Fixed Mold Plate or Cavity plate.
use to hold cavity side of product, leader pin, support pin, Puller bolts, and Angular pin when slider attached.
4. Movable Cavity Plate or Cavity plate
it used to attach core side of product, return pin, leader bush and slider core if needed.
5. Back up Plate or Support plate
used to support cavity plate, attach the hole for return pin's spring, and cooling channel when in cavity plate can not make it.
6. Spacer Block
Mounted between the movable clamping plate (bottom plate) and the movable cavity plate to give space and allow the ejector plate to move when ejecting the
part. the required length of spacer block depend on ejector stroke that needed to eject product.
7.Ejector retainer plate
to hold the ejector, Z pin, shoulder bolts,and give space to ejector leader pin and support pillar.
8. Ejector Plate
Pushes the ejector pins and return pins, connected with ejector rods.
9. Movable Clamping Plate or Bottom plate
Holds The movable side of the mold like spacer block, support plate, cavity plate and ejector mechanism to the movable platen of the injection machine.


and look at picture below, shown 3 plate molds type in opening position, but at picture below, Bottom plate or movable clamping plate is number 7, number 8 is ejector retainer plate, and number 9 is ejector plate.

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Top Plate, Stripper Plate,Sprue Bush and Runner Lock Pin Design

at the picture below you will see a set of Top plate until stripper plate design related with runner flow, at this picture support pin exclude. the mold type in this picture is 3 plate type.

the runner system accommodates the molten plastic material that coming from barrel, and guide it into mold cavity, the dimension, configuration, shape, and system connection with mold cavity will affect filling process.



as see from that picture, red plate indicate isolated plate, it's doesn't matter you use isolated plate or not, but you must see more in injection machine, if it needed isolated plate give it. number 1 and 2 in black plate it's indicated top plate for number 1 and stripper plate for number 2, those picture shown 3 open mold type.
after injection, in ejection process stripper plate will open because puller bolt pull it( not include at those picture).

runner ejector set will needed to push runner, after ejection, but if you use arm robot or manual take the runner by hand operator it doesn't needed. runner ejector set always contains spring, those spring will push pin in ejector set and will push the runner.

runner lock pin function is to pull runner when ejection begin still in top plate, if runner adhere in stripper plate, the robots or operator will difficult to take the runner, because no ejection system for runner in stripper plate.

when design the runner system and ejection system for runner in 3 plate type of mold you should use those mold accessories.

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When we need slider parts in Mold Design

second previous post i have post about the slider parts and construction in complete mold base , then now i give a little brief when we need slider in mold design, basically we design mold from our customer, first is we must look the parts that we will make by plastic mold, look mare at the drawing so we can know about tolerance and critical point of parts, seconds is define the parting line, first parting line is between core and cavity parts, and after that if the parts have side form we must define slider parting line, look picture below



from those parts we know that we need slider parts, side hole and other shape that can not form by core and cavity became slider parts. below is picture in 3D modeling,look at the hole shape in side, those shape need slider part.



and how the construction of slider, more about construction mold you can see by clicking mold construction or molds parts in quick topic, basically here i give slider 3D modeling and parts name, those parts names i take from Misumi standard mold design


you can use this tutorial for education and others purpose but still write copyright from this site.

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Slider Design and Basic Construction

picture below show slider basic construction in Mold, various construction and litle part available in slider but the basic construction of slider is same, look picture below, sorry if this picture is too grainy, i'll make a new one.


the main part from those picture is mold plate, number 1 until 10 is shown the plate of mold
plate 1 is top plate
plate 2 is gate opener plate, retainer plate, and others name depend in fabrics
plate 3 is cavity plate
plate 4 is core plate
plate 5 is backing plate
plate 6 is support plate
plate 7 is ejector plate
plate 8 is ejector retainer plate
plate 9 is bottom plate
plate 10 is runner plate

and the slider part name are
g for angular pin, this pin always have certain degree, usually between 10-30, angular pin will transfer vertical movement to horizontal movement, others construction, angular pin not from the top but from bottom, this kind of construction is rare to use now.
h is slider core, when contact with angular pin, this part also the core from slider construction, at this part we can attach insert part that mold the shape of plastic.
i is slider locking block, we can depend in angular pin when mold closed, slider pin is too week, so the holder of pressure from inside mold during injection process is locking block.
k is slider lock plate, for non mass production you do not have to attach this part, but if the mold used in mass production, i suggest to use this part, because slider locking block and slider core always have friction, then when this part broken or got abrasive is will need expensive cost to repair it, so use lock plate is better, is cheaper.
l(L) is slider insert, this part which form the plastic product, it's attach in slider core

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Mold part and construction

Many type of mold in the platic industry but generally mold devide in 3 big section there are :

Two plate mold
Two plate mold basically when opening the product after plastic injection process mold just devide in two parts core side and cavity side, this type molding only have one parting line, product parting line with located between core side and cavity side
Look at picture below for more detail


Three plate mold
Basically when product out after injection process this mold type divide in three parts, beside that, this type mold have three parting line, first between top plate and runner stripper plate, second are between stripper plate and cavity plate, the last is parting line product that located between core and cavity (see picture below)


hot runner mold (runner less mold)
this type basically similar with two plate mold but not same, hot runner mold always heating the runner, so runner will not drop out with product, so it is some times called runner less mold, there is two big parts when opening after injection, core side and cavity side. Look at the picture below


Mold Construction
look at the picture, the parts of mold are



1. Top plate (plate 1)
2. Striper plate (plate 2)
3. Runner plate (plate 10)
4. Cavity plate (plate 3)
5. Core plate (plate 4)
6. Spacer Block (plate 6)
7. Bottom plate (plate 9)
8. Ejector plate (plate 7)
9. Ejector retainer plate (plate 8)

other function from all parts will explain in other post

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