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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Diaphragm Gate Design in Thermoplastic Mold

Actually this gate is another variation of side gate, but this gate have circumferential shape like fan gate,some time it also named as disc gate. basic modeling of diaphragms gate is like the picture below


this type applied when at the center of product has a trough hole, plastic flow when injections is similar to direct gate, but flow plastic material more constant and controlled becouse this gate have circular flat thickness. this gate punched out after ejection.

Design and Size
the following picture below shown basic part and size of diaphragm gate.

The gate height (t) is preferred about 0.1mm -0.17 mm
The circular runner height (T) is recommended about 5-10 mm depend on your size of product.
Land size (D-d) is about 1mm-5 mm each side.

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Design Gate sumary from hanser book

embed object below taken from ipaper, at the text book below will explain clearly how to design a good gate in plastic mold, to view more large click a little arrow at the top right, after click choose plus mark to see more large.

to see the embed object below, your browser must have installed adobe flash player 9

Read this doc on Scribd: hanser design of gates

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How to design fan gate in thermoplastic injection mold

fan gate, is a gate spreading toward in to cavity, basically this gate is thin similarly with previous gate (film gate) and is used in same purpose with film gate. with the gate construction it permits rapid filling of large parts or fragile section mold area with large entry area. it is used to create uniform flow into wide parts.
Purpose
1. thereby keeping the cross sectional area constant.
2. This ensures a constant and more even plastic flow into the cavity into a wide area at a slow injection speed, minimizing backfilling and reducing imperfections and stresses in the part.
3. used for thick-sectioned moldings and enables slow injection without freeze-off.
4. favored for low stress moldings or where warpage and dimensional stability are main concerns.
5.The melt velocity will be constant.
6.The entire width is being used for the flow.
7. The pressure is the same across the entire width

Construction and Design



side View

The gate should taper in both width and thickness, to maintain a constant cross sectional area, although at picture below thickness is doesn't tapered.



Dimension
the land thickness can be vary thin relative to the part thickness because the gate is very wide.the maximum thickness should be no more than 80% of the part thickness. if the parts very thin like 0,8 mm, you can also use 0.7 mm fan gate thickness, but when rubber is used as material parts, use 100 % of the thickness parts is more easy to control flow and product quality. The gate width varies typically from 6 mm up to 25% of the cavity length.in large parts is often use fan gate as wide as the parts it self.

Simulation Picture
all picture below based on injection simulation process, using material PS ASAHIPS 693 : Asahi Chemical
other data are
==== Machine setup ====

Melt temperature = 503.15 K
Mold temperature = 323.15 K
Fill time = 0.08 s
Packing/Holding time = 4.24 s
Cooling time = 7.72 s
Packing/Holding pressure = 12627300.00 Pa

==== Machine specification ====

Required clamp force = 27647.70 N
Required shot size (GPS) = 0.01 kg

Simulation results

mold time filling in seconds


time to cool after injection
from below picture we can learn where is the best position to make cooling layout, after you make cooling layout, enter the cooling simulation process to your parts. if the distribution is same in all shape of part (unless runner and gate) your design is good to realize.


Pressure distribution



all picture is enabled to enlarge, click the picture to enlarge.

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Film Gate Design (flash gate)

Film gate also called flash gate is very thin compare to another gate, this thin gate like film which have parallel runner before the gate, this type of gate used for straight edges. picture below shown film gate with main parts.


in generally this gate used in thin and flat requirement, like flat mobile phone cap, ipod cap and others. but sometimes this gate is not likely going to have a flat flow front, the make flow material more stable and flat, make temperature molds more higher than usual, but cycle time will also higher.

here is film gate from top view


Dimension
recommended dimension is approximately 0.2 mm to 0.6 mm in thick, the land area (parallel runner) is also be kept small approximately 0.6 mm - 1 mm depend the large and weight of product.

Other Design
picture below shown other design of film gate, with more reliable and easy in process.

look the different model of film gate, and different design of parallel runner.
picture below shown simulation of plastic flow from those design

and the last picture shown time that needed to cooling the melt plastic in mold


from the picture above we can choose the right place to make cooling pipe, the red color marks mean need more time to cool the product.

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Tab gate design in Mold

general construction of tab can see at picture below


basically melted material is fed to this gate from a separate primary gate (after runner), the function of the gate typically is employed for flat and thin parts to reduce the shear stress in the cavity, Tab gate able to used in various material plastic such us ABS, PS, acrylic, PP, PVC, Polycarbonate. SAN and other which has relatively low fluidity. at this type of gate, the high shear stress generated around the gate that trimmed after molded. this gate sometimes also called the collision gate.

Function in Product and General Rule
The thickness of tab gate should be the same as the part wall, yo can also make more thin, but when designed parts for cosmetic parts if preferred that the thickness is same with the product thickness.
rule number two is, make the gate as center as possible in the product side, but you must consider the product size and shape.
basically minimum tab width is about 6,5 mm and the tab thickness minimum is 75 % from the thickness product


for more picture and drawing view,please look at picture below

(Top View of Tab Gate)






(Front view of Tab Gate)





(Side View of Tab Gate)

each picture is able to enlarge by clicking the picture.

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side gate, edge dimension design

edge gate is the simplest type of gate and is used wherever the product can or must be gated from parting line. it's designed commonly for injection machine attended by operator, because the product must separated from the runner system by hand with the aid of side cutter.

Dimension
Picture below shown ideal configuration and practical gate, suggested dimension for general application of edge or side gate are


Runner width and Height for side gate or edge gate is preferred that height is about 0,7-0,8 from width of runner. the width of runner very depending on product size and weight, it from 3 mm until 7 mm in width ( product mass until 200 gram in maximum).
Gate width and height is about W=3h,with W is width and h is height (look at Herbert Rees, Mold Engineering, pg 159) but this rule can't applied for all part or product form, specially for thick product that have large in width such us panel and cover, when design the dimension of gate take the rule that height of gate is must about 75 % from the parts thickness, Suggested take 0,6 until 0, 7 from parts thickness, but specially for elastomer material take more large about 80-90 % from parts thickness.
T is gate width, for product mass until 200 gr, suggested take 1 until 5 mm in width , for the very large product some times have width until 12 mm.
C is Land of gate from runner, the length is should be no more than 1,5 mm. also preferred make taper before gate land, in that picture taper are change with R in both side, make R, taper or not is depend on side gate size and form.


Picture 1.A

Picture 1.B

both those picture shown edge gate or side gate with different design, application, parting line, taper and R system. taper from bottom runner to gate land have various dimension, the taper in picture 1A is enough large, about 30-45 degree, in picture 1B taper is about 10-20 degree.

beside those type of side gate, they are still various type of side gate or edge gate, will upload in other post.

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Side gate, edge gate design

as we know Gate In injection molding, is the channel through which the molten resin flows from the runner into the cavity. Generally, it is small and solidifies first. The restricted opening through which the plastic enters the mold cavity.

Standard Gate
this type of gate have various name, the common name are standard gate, edge gate and side gate, this is the most typical gate in molds, which is applied to almost all types of plastics. look at picture below, it's single side gate from sprue until gate and product (click the picture to enlarge)



how about the parting line?
the are about 3 methods that we can choose when design parting line,
1. core side process,this type of parting line only core side that's must be machined , picture below shown side gate design with flat parting line in core side process.
this type parting line is useful applied in product without taper in side of product, but when product have taper in side (like picture below) it will undercut and need slider to form the side of product.

2. cavity side process, this type of gate design parting line is one method to hide the marks of gate after injection process. at this type runner channel machined in cavity side, then a little gate machined at core side. this type of gate parting line is more good and more cheap than number one when product have taper like picture below.

3. cavity and core side parting line, this type parting line for gate and runner is not common to use, when use this type parting line runner and gate, you must use cylindrical type of runner of semi cylindrical type of runner.

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Direct Gate or Sprue Gate

What is Gate ?
The entrance through which molten plastic flows into the cavity of the mold from the runner. This is where the flow or melting of molten plastic is controlled by control type of gate or gate dimension. It is positioned at the thickest area of a part.
other function of gate is the gate makes it easier for the runner, which will be unnecessary after molding, to be separated from the part.is recommended to consider gating options early in part design.

Direct Gate
direct gate or sprue gate we can define as connecting directly end sprue to the molding and used as a gate,
this type of gate is commonly used for single cavity molds, where sprue feeds the material to cavity molds directly rapidly with minimum pressure drop.

this type of gate is applied to molding of boxes or cups like bathtub, plastic bucket, and others, particularly large sized products. like at the picture above, shown cups product with direct gate before cutting.

Dimensions and Construction
from picture below we can concluded that direct gate is very easy to design,

from picture above the construction of direct gate is one parts with the product, after injection process, the sprue will cut manually.
parting line between cavity and core can directly placed at top surface of product or more lower from top surface product, it very depending of product shape and characteristics.
starting sprue diameter is controlled by nozzle of injection

- then, make sure the diameter of sprue orifices must be larger than nozzle exit diameter, choose from 1-2 mm or about 1/32 inch.
- give taper at the sprue from orifice diameter to product about 1-3 degree, it is will made the product demolded from cavity side more easy.
- in general the diameter of the gate much greater than the thickness of the product.

Advantages
- Low cost of design.
- The injection pressure was reduced due to direct cavity filling.
- Simple way to design gate and runner system.
- easy design and maintenance.

Disadvantages
some disadvantage of direct sprue are:
- since excessive stress is left easily around the gate, cracking often starts around the gate.
- when using non standard sprue tapers, will more expensive with little gain.
- gate mark left on part surface after runner of sprue trimmed of.
- This gate type can only produce one part per shot.
- The gate has to be cut manually by a cutter.
- Not applicable in little size product.

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