Step by Step Mold Design (part 2), Insert Design

continuing with the previous post, now let's make some insert in core side, this insert very simple and easy in machining process. from the core block side we divide at the circle line of side of bowl, why ?, because we will eject the product using stripper plate, stripper plate construction will eject the product by touching the thin face in all around of bowl.

insert function
there are two function in of insert in this mold construction
1. to provide re change part, if the bowl shape doesn't good, we don't need to change all core side, but only the insert part. so this system is more cheaper when the part need good shape and good tolerance.
2. provide gas vent, and stripper construction, because the product need gas vent in bottom the side, and gas vent in stripper is not good, gas vent only can make with insert in core.

below is construction of insert part


Gas Vent Dimension and construction
gas vent can divide into 2 part, the accurate tolerance and not accurate tolerance part, the dimension of accurate tolerance is depending from plastic material that injected in molding, but generally is about 0,005 mm until 0,020 mm


Collar
at the end of insert part, there was a collar to prevent part move in Z axis, the dimension of collar is about diameter + 2mm and height is 4 mm

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step by step mold design of ejector plate type

PRODUCT
by sample complate design of bowl i'll try to explain easily how to design a mold, start with simple product a bowl like picture below


by looking the bowl, we can consider that bowl only need core cavity side without undercut release.

PARTING LINE
choosing the right parting line is important, parting line will separate core side and cavity side, becouse we only need core and cavity, the parting line is only one, between cavity and core, to make it, first,make the block then from subtract the product and split it by parting line that you define before


some CAD software also have instant cavity and core making feature, split and sweep feature so make the mold is easy by using those feature, after split it into cavity and core we have cavity side and core side of the mold.

Cavity Side

and below picture is core side


now we have cavity and core side, it still rough, without bolt, without gate, ejector and other part, the next step is make some insert part.

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shringkage of mold calculation, mold calculator

as the request, lot of people need engineering and calculation of injection mold and mold design, so i try to make little software, the name is mold calculator, it's freeware so download it and you can publish it, now it still under development, in this beta version this software only provided various shrinkage formula.

what is shrinkage ?
Shrinkage is inherent in the injection molding process. Shrinkage occurs because the density of polymer varies from the processing temperature to the ambient temperature
There are linear and volumetric shrinkage. Both methods are difficult to measure accurately.
see more information about shrinkage and what the main thing that affecting shrinkage in http://plastics-engineering.blogspot.com/search/label/shrinkage

Relation Between Injection parameter setting and shrinkage
picture below shown relation between shrinkage and injection machine parameter.


Mold Calculator software beta 1
calculating shrinkage is basic when designing mold, so
i try to make little software, more than 4 year never learn or make some program again, then now i still learn .net frame work, as the practice i try make this software, becouse I'm not professional software designer please don't hesitate to contact me, give some advice both in programming and mold calculation.

this software is still under development, on this version you can calculate, shrinkage value, dimension after and before shrinkage, and exact shrinkage scale to input in 3D modeling software.

to download those mold calculator, i have 2 version, In Indonesian language version and English language version, download at

English version

Indonesian version

you also need download .net framework 3.5 from Microsoft

Basic Formula of Shrinkage

D' = D x(1+shr)
with
D' is dimension after shrinkage
D is dimension before shrinkage
shr is shrinkage (in persen %)

so to calculate shrinkage we can use formula

shr = (D'/D)-1

from this basic formula we can develop by substitute with other formula

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3D and Drawing ejector simple construction in mold with video

simple and cost effectively ejector system i try to design, it using very minimum part, using this ejector system just have 3 ejector pin, by understanding minimum part to make ejector system, we can develops more complex ejector system. see picture below



this ejector system is same with previous post, i just completely the picture with isometric picture, drawing sample and 3D movie, so i hope we could more understand in ejector system. below is drafting picture of those 3D picture



little different from previous post, this ejector system use long ejector rod, ejector rods type is very depend the type of machine, stack mold, basic injection mold have different ejector rod type, although the basic system is same.



video above shown simple 3D model of ejector system, at those video is not include bolts that connected ejector plate and ejector baking plate, also bolts that connected bottom plate to core plate.
i hope by using those video is more easy to learn mold design.

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Ejector Component List for ejector pin type

Each type of mold have their own ejector system characteristics, such as : one step ejector, two step ejector, stripper ejector system, pin and blade ejector system, gas ejector, and vacum ejector. from various ejector system that available all of that type have main part or main component. picture below shown common and main component for ejector system. this image size about 700X500 px, click the picture to enlarge.



picture above shown side view of ejector system, the target is to eject the product (black) from core insert after mold open. when the ejector system work and push the product to outer parting line the side view picture is like below



Ejector working system
with above system, how could the ejector push the product? below i will try to explain step by step
1. after cavity of mold injected with plastic material, and cooled moldbase will open, and product have to eject from the core insert.see the open condition of molds.
2. after mold base in open condition like picture below

rod from injection machine will push the ejector rod,because ejector rod connected to ejector plate, ejector plate will move forward.
3. ejector guide pin will guide those moving, to prevent scratch and guide the moving more easily use ejector guide bush, ejector pin and ejector guide bush will have little friction.
4. ejector pin clamped between two ejector plate, because that ejector pin will also move forward push the product.
5. after product take it out from cavity, mold base will moving close again.
6. and because at the return pin have spring, spring and closing mold will guide ejector back to the position (1)

Ejector Component List
this list is standard minimum to make ejector system
1. Ejector plate, place for ejector and ejector guide bush, shoulder bolts and return pin.
2. Ejector Backing Plate, place for clamping the ejector and push back the ejector.
3. return pin, push the ejector plate and ejector backing plate to first position.
4. spring in return pin, to make sure and holds the ejector plate so it's always in back position, when in assembling process, it can lift up the ejector pin, and ejector plate so the position always in back, give the little preloads to ejector plate to make sure the ejector plate position.
5. shoulder bolts, first time lock in assembling molds
6. ejector pin, eject and push the product from core.
7. ejector guide pin, guide the moving of ejector plate.
8. ejector guide pin bush, to make sliding and prevent large contact when moving.
9. support pillar, to prevent large deflection of core plate when injecting the plastic material process held.
10. ejector rod, contact with rod in injection machine, this also passing force from injection machine to ejector plate.
11. ejector bolts, to tie up between ejector plate and ejector backing plate.

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How to Calculate Clamping force

as we know clamping unit purpose is to keep mold closed against force when part injected with plastic material, because of that clamping force at least more than injection force, we can applied this clamp pressure both in injection machine with hydraulic or with mechanical toggle.

Injection molding machines are usually characterized by the tonnage of the clamping force that they provide. The clamping force, indicates the amount of force that the clamping unit can apply to the mold to keep it securely closed during the injection of the molten plastic, picture below shown injection machine from Nissei with type NEX80 (80 ton)



formula
the clamping force is proportional to the projected area of the moulding and runner, and must be opposed by the clamping force. although a proportion of the pressure produced by the injection cylinder is transmitted to the cavity, various losses occurring in the heating cylinder, nozzle and gate we can consider that force acts on the mold to make it open can be calculated from the following formula:

Force (lb.) = Pressure (lb/in2) x Projected Area (in2)

what is projected area ? projected area is area that one line with mold base parting line, for complete calculation, calculate the runner projected area and part projected area is more precision. see picture below to get the imagine og projected area


Clamping force calculator
below are clamping force calculator using java script, the script i get from http://www.custompartnet.com, with little modification so it's more suitable with blogger post page.

Clamping Force

Number of cavities: Part projected area (in²): Runner projected area (in²): Shot projected area (in²): Cavity pressure (psi): Separating force (tons): Factor of safety: Clamping force (tons):

this picture below also have good visualization to explain projected area, clamping force and injection pressure


if you prefer see original calculator go to this webpage Clamping force calculator
but why i set safety factor 0.8, the result will more lower than without safety factor, in clamping force those formula is before correction for pressure drop and other, so the injection pressure always more lower than those manual calculation.

or we can use graph or table to define clamping pressure, like this one from niir.org

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Polypropylene Injection Molding Problems and Solutions

some times when we try to inject plastic part, problems always occur, beginning with shot mold, flashing, and some times jetting.

data below i take from Exxon Mobil Chemical, will explain various Injection Molding Problems and how to solve that properly.

it's begin with

• sink mark
  
• void
  
• shrinkage
  
• weld line
  
• flash
  
• burning
  
• warp
  
• brittle part
  
• sticking in mold
  
• gate bush and various others
  

below if you prefer read it online

Injection Molding Problems and Solutions - Upload a Document to Scribd

but if you prefer download, link below maybe more useful to read and download
http://www.scribd.com/doc/6200927/Injection-Molding-Problems-and-Solutions

this paper is very good tutorial with simple and practice solution for those each problems

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Calculation and Designing Support Pin or Leader Pin

as we know in previous posting about main construction of mold, there was pin in leader component, this pin will support main construction and cavity plate (3) from deflection and guide the cavity part and core part when mold closing. here the picture of support pin or leader pin in 3 plate mold type.


Support Pin Main Part
we can divide main part of support pin into 3 type
1. Support pin it self, some mold engineer call support pin by leader pin or main support pin.
2. Bushings, how much bushing that we needed depend on type of mold base, for example for 3 plate type of mold base we need at least 2 bushings, one in stripper plate (2) and one another in cavity plate (3). bushings prevent support pin have direct contact with mold base plate, becouse mold base plate usually use more soft material and support pin is harder material, it can cause scratch if use in long time and mass production, bushings always use harder material with good tolerance to support pin. so the other advantage is mold can be opened and closed with precision. and it can use in mass production mold.
3. Collar, it to prevent plate of mold out from leader pin when puller bolt broken or others accident possibility, with collar give the mold construction more safe.
see the main part of support pin in picture below


Deflection Calculation
ok, to learn more how to calculate the deflection in support pin, when support pin receiving the weight of a mold base plate on their tips,support pins may
deflect due to a large amount of bending moment, to choose right diameter and how much the bending first we must know the force that affect support pin, see picture below


yes at 3 plate type minimum load is from weight of stripper plate and cavity plate, with simple calculation of deflection (although isn't precision) like formula below



to calculate more complex and precision add the force from support pin weight, collars weight, and 3 dimensional calculation methods.
below is some list and use full engineering link to calculate the deflection, radius gyration list, derivation of deflection formula and others
http://en.wikipedia.org/wiki/List_of_area_moments_of_inertia
http://www.doitpoms.ac.uk/tlplib/beam_bending/printall.php

then by calculation we can get the fine radius, after calculation I, maximum deflection is about 0.05 mm, more than 50 micro millimeter, use more large radius of support pin. or place more precision guide, like block guide, linear guide and others between cavity plate and core plate, it should be more precision when mold in closed position.

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Considering Factor that affecting gate size

when we design a gate for mold injection, we need a proper size of gate, to large size of gate will make unbalanced flow and defect in product. to small size of gate make cycle time high, unfil l of product and sink mark is appear easily in product.

although there was various factor that affecting gate size, here i list some common factor that affecting gate size.

Mass of Part
of course large mass of part need more large gate size, i still doesn't have list a good relation graph or table which various material and mass of part. Herbert Ress in Mold Engineering 2nd edition page 159 listed some suggested dimension for gate cross section but it just very general list without material and other kind of gate.

Size of Part
size of part also have large effect when we design a proper gate,longer flow length, larger cavity surface in injection mold of course need larger size of gate.

Temperature of Melt
when we use possible maximum temperature then cavity still cannot be filled, use more smaller gate, because with smaller gate more heat from heater can be transfered in resin, so shear stress of resin will decrease and viscosity also decrease, then resin will flow easily with small value of viscosity.

Nozzle Tip Position
don't use smaller length between nozzle tip and gate, because it will make freeze premature, if the construction of mold does not enable to make longer length, you can prevent that by making the gate more smaller.

Viscosity of Resin
viscosity of resin and shear stress of resin have large effect in proper gate designing. if we buy a resin, shear stress and viscosity data also available, use those data to calculate proper gate size. we can calculate average plastic flow by dividing shear volume with injection time. always use average plastic flow speed more than speed of sound, more than 340 m/s, if your value below the speed of sound it large possibilities that your product will shot (unfinish fill of product).

although other factor beside i write above there are some large factor like cooling speed, cooling temperature and injection time also have valuable effect when designing the gate, the main point was i write all above. from those main point i think we should design a proper gate in mold fast and effective.

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Pin Point Gate Construction

pin point gate is commonly used in 3 plate mold types, see explanation more about 3 plate more here, basically the pin point gate part is like picture below
































those picture is pin point gate picture, to make gate like that we can buy pin point gate bush in mold manufacture company or we make it by EDM (electronic Discharge Machine) Process, the special EDM torch for Pin Point gate also available in mold part markets.
Pin Point gate Bush
picture below shown simple pin point gate bush,




































the construction to assembly the pin point gate bush in mold is like picture below
straight, without extended bush, picture below shown, locating ring, pin point gate bush, runner lock pin, runner, product and mold base plate (top plate, stripper plate and cavity plate)



some times, we need extended pin point gate bush, why when plastic material have high shear rate, we need to assembly extended bush, so from diameter gate until runner plastic material will easy to flow the cavity.
see the construction at picture below


here some good link of standard component of gate,
http://www.misumi-europe.com/www/lang/catmold_eng/nav/006/hl/007/menu/catalogues/fn/21/content.html

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Opening Construction of 3 Plate Mold Type

previous post http://mould-technology.blogspot.com/2008/08/main-construction-pin-at-3plate-mold.html
we can see closed condition of three plate molds

picture below is shown three plate condition when open


basically there are three opening type in 3 plate molds base
1. Sprue Cut Opening
2. Runner Opening
3. Product Opening

below i try to explain in simple words
1. sprue cut opening
this opening will cut sprue from nozzle, so the feeding runner can take easily by arm robot or manually by hand, this opening use puller bolt and runner stripper plate to cut the sprue. the stripper plate will move after puller bolts pulled by cavity plate, remember that puller bolts also end mounted in cavity plate.
the opening length between 5mm until 10mm.
2. runner opening
this opening use tho open runner system, than runner will take by robot or manually,this opening will make space between stripper plate and cavity plate.
to make sure this runner opening safely, attach spring in puller bolts between stripper plate and cavity plate, or use other methods like attach spring in out of puller bolts. length of this opening is same with runner length + 20 mm.

3. product opening, as we know this opening is to open the product, make space so the product can be ejected from core.
this opening length is about length the product + length robot arm + 10 until 20 mm to make save space when product ejected.

Opening Sequence

actually to pull the cavity plate, so the cavity plate can pull the puller bolts than open the stripper plate have two types, using tension link and magnet.
OK, at this post let's learn when using tension link.
1. first Opening is Runner Opening
why ? becouse between stripper plate and cavity plate,some springs always attached at there, some mold designer attach at puller bolts, and the other attach in each side using pin and make some hole to attach the spring. but i prefer attach the spring in puller bolts, it more easy in process.(sorry at picture above i don't write some spring)
2. Second Opening is Product Opening
Parting Line between core - cavity plate always easy to open, when moving plate at the injection machine pull the bottom plate until core plate, it will make Product Opening open.
3. Sprue Cutting Opening
after moving plate pull the core plate, with tension link core plate will pull the cavity plate until cavity plate pull the puller bolt, and make some space to cut the sprue from nozzle.

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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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