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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Fill time runner balance on 6 cavity of mold

runner balance is important when we works with more than 1 cavity in mold design, unbalancing runner can cause various like unfill mold (short shot), weld line, air traps, burning and others.

from previous post that talk about design of runner balance in 6 cavity, now let's prove it by simulation of filling time.

Unbalance Flow Definition
Unbalanced flow is plastic completely filling some flow paths in the mold before other flow paths have filled. we can see from result of simulation, if color is same in cavity (for example at the end and gate) that mean it have good flow.

1. Unbalance Runner System


from picture above we can consider that flow of plastic in various cavity doesn't fill in the same time, it can cause flashing, short shots, high cycle time, density differences throughout the part, warpage, air traps and extra weld lines.
flashing will occur in cavity that fill earlier, becouse cavity that fill earlier will continue receive plastic flow until other cavity filled (it depend on injection pressure and cycle time set up).
Short shots happened at cavity number that fill more slowly then others cavity then the pressure and cycle time doesn't enough to fill the last cavity.

2. Balance Runner of 6 cavity
although this runner layout is not the best for 6 cavity layout, it better in fill time than runner layout before. look at the picture below


by changing the layout we can get runner that more balance with previous, from cavity number 1 until 6 the fill time is almost same.

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Runner Balance Layout for 6 cavity of mould

Picture below shown layout model for 6 cavity product, star layout used to connecting 3 cavity, then connected again with straight line. distance between one cavity to another is same

Top view


3D view


becouse flow at nearest sprue is faster than other, although the distance from the branch to gate is same, the flow of plastic will not good, nearest sprue will fill first and not same with another.

Improved runner layout
so we need another layout, below is improved runner layout for 6 product cavity, although is not the best, but it's more improved than first. but disadvantage of this runner layout is consume more material.

Top view


3D view

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Runner system, flowing part of plastic injection

After material injected by nozzle, runner systems will convey the molten material from the sprue to the gate. basic runner system consist of sprue, main runner, sub runner, cold slug, and gate. since the design of the runner system has large effect on molding quality, molding efficiency, cycle time, cooling time and other factor, it is important key when design of mold.

Runner System Construction

Sprue
sprue will contact first time with melt plastic, at the injection process, sprue always contact with nozzle from the injection machine, so consider when design the sprue consider the ball radius of nozzle, make sure it isn't make undercut on sprue.The dimensions of the sprue depend primarily on the dimensions of the molded part in particular the wall thickness, too little sprue will make sprue cold faster and make insufficient flow.





Main Runner
First time when design runner system, determine number of cavities, shape and gate types, then arrange the cavity layout,at the multi cavity of mold balancing of runner is important, balancing ensures virtually equal flow of plastic trough each gate.
To achieve balancing runner layout must be though,

Sub Runner
this part will continues flow from main runner until the gate, the dimension of sub runner and how much sub runner system will be, is very depending on how much cavity of the mold, and how the layout, what the layout use herring bone, H type, or star type. at the picture above use H type runner layout.

Cold Slug
when runner layout bent, clod slug may be required to continue plastic flow of better quality to the cavities.The cold slug well helps the flow of material through the runner system by stopping colder, higher viscosity material moving at the forefront of the molten mass entering into the cavity.The length of the well is usually equal to or greater about 1,5-2 time than the runner diameter.

Gate
Basic purpose of the gate is to limit the flow of the melted plastic, but it must allow that melted plastic flow and fill the cavity easily.
considering the fluidity of the material, gate should be made near the center of gravity of molding or the line to make flow uniform and melted material even, then make the gate thick but still considering melted plastic material, becouse thin part will cooled easily and faster.
the gate type is very various depend on part shape, mold layout, mold system, etc, here some explanation about design those gate type:
1. Direct gate
2. Side, edge gate
3. Tab gate
4. film gate
5. fan gate
6. Disc gate

Runner geometry
they are various geometry that we can use in runner, such us
1. full round
2. half round
3. trapezoid
use full round and trapezoid shape is recommended in various mold. Half round runners are not recommended because of their low volume to surface ratio
here some area calculation of cross section of runner (source : http://www.dsm.com)


Runner dimensions
The diameter of a runner depends on its length in addition to the part volume, part flow length, injection machine capacity, and gate size, Generally runner must never be smaller than the largest wall thickness of the product, it's usually make from 3 mm until 15 mm. picture below shown relation between runner diameter and maximum runner length (source : http://www.dsm.com)

The runner should be large enough to minimize pressure loss, yet small enough to maintain satisfactory cycle time, Large runners are not economical because of the amount of energy that goes into forming, and then regrinding the material that solidifies within them.

Runner layout
In general they are three type of runner layout
1. "H" bridge (branching) runner system
2. Standard (herringbone) runner system (conventional runner)
3. Radial (star) runner system
look at picture below shown standard runner and H bridge type runner.

(Source: http://www.hotmold.cn)
and below is star layout runner system (both bottom picture)
(source : http://www.dsm.com)
Balancing the runner system ensures that all mold cavities fill at the same rate and pressure, if the product have not similar product geometry, it is recommended to use mold analysis software like mold flow or C-Mold.

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