Runners and Gates

RUNNERS AND GATES Information contained in this presentation is to be used by Seneca College Students while registered at the College Only and can not be reproduced.

All injection mould (plastic and die-cast) have Runners.

A runner is a channel which connects the gate(s) of a mould to the sprue.

A runner is a channel which connects the gate(s) of a mould to the sprue.

When ever possible runners should be: 1. as short as possible 2. have the same length for multi-runner Runners should always be radially spaced leading from the sprue.

Branch runners are employed when it is impossible to generate a ideal runner

http://www.injectionplasticmold.com/wp-content/uploads/2009/01/naturally_balanced_runner6.gif

Runners must always be polished. Why is polishing important on the runner if the runner (cold) is always trimmed off. Runner should always be as small as possible. Why? Runners must be large enough to carry sufficient material to the cavity of the mould.

The two most frequently used runner shapes are: 1. trapezoidal 2. round

http://www.injectionplasticmold.com/wp-content/uploads/2009/01/cross_sectional_runner_p1_6.gif

Trapezoidal: +ve. Easy to machine -ve. Increased surface friction Round: +ve. Reduced surface friction Increased volume -ve. Difficult to machine into both sides of the mould

Gate: The narrow and shallow portion of the runner as it enters the cavity.

It is important that gates be located where they are easily trimmed and not easily seen on the finished product. Gates should be located so that the flowing material hits what? The opposing surface as soon as possible after entry into the cavity.

Note, material entering into: thick section, ribs, parallel wall sections or any open area will result in visible shrinkage marks on the finished product. Gates are made as small as possible. Small gates facilitate the removal of the piece part from the runner and sprue.

Small gates also promote proper flow of molten material prevent strains and weld marks.

Weld marks normally result from poor gating and venting. What is a weld mark?

http://www.injectionplasticmold.com/wp-content/uploads/2009/01/cross_sectional_runner_p1_6.gif

For asymmetrical mould gravities designers cannot rely on radially spaced runners and gate. Designers must make changes to the gates to allow the cavities to fill uniformly. How do you think this is achieved?

Cold Slug Well When material is injected into the mould the leading of the injected material is somewhat cooler. Why is this a problem? Cold Slug well is located opposite the sprue to trap this cooler material allowing the hotter material to fill the cavity of the mould.

Cold Slug wells are also found at the end of branch runners.

Sprue Puller An undercut is normally machined into either the cold slug well or the ejector pin immediately under the cold slug. What is the purpose of this undercut? This undercut will pull the sprue out of the sprue bushing, thus allowing the mould to cycle again.

http://www.globalspec.com/reference/50137/203279/Chapter-17-Runner-and-Gate-Design

Fan gate: A fan gate is used with acrylic material. This type of gate improves the flow of acrylic material by spreading the material into the cavity.

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Ring gate: This type of gate is used to feed material into long cylindrical parts with thin wall section.

http://www.injectionplasticmold.com/wp-content/uploads/2008/12/internal_ring_gate6.gif

Disc gate: The disc gate is used to fill a cavity from a hold located inside the part. The material from the sprue spreads out evenly and all sections of the cavity are filled simultaneously.

http://2.bp.blogspot.com/_oF_1Qcfku_w/SDBDRVUq8YI/AAAAAAAAAfQ/ZnqIINQzXdI/s400/disc-gate.jpg

Direct gate: This gate allows the injected material to flow from the sprue directly into the cavity.

Tab gate: This type of gate generates a better flow pattern by having the tab take up any strains and imperfections in the flowing material. http://4.bp.blogspot.com/_oF_1Qcfku_w/R--s2bvduCI/AAAAAAAAAcw/SiRdaYQC-jc/s400/tab-gate3.jpg

Pin point gate: The pin point gate is generally used in three plate mould construction with multiple cavities and having trapezoidal runner. http://4.bp.blogspot.com/_oF_1Qcfku_w/RuVaduEmQgI/AAAAAAAAAHY/RuJ1kQH7wPc/s400/mold.gif

Tunnel or Submarine gate: There are many advantages to using the tunnel gate. The gate is sheared off as the piece is ejected from the mould. The gate does not have to be located on the parting line and it is possible to gate on boss/deep sections of the mould. Venting is often not a problem because the cavity fills from the bottom up-thus allowing the cavity to vent at the parting-line.

Tunnel or Submarine gate cont’d Tunnel gates very in diameter from .020”-.050” and have a land approximately .040”.

http://www.rtpcompany.com/info/molding/design/graphics/tunnelgate.gif

Overflow well If the part design causes problems of weld lines, an overflow will is provided. The overflow well is generally located opposite the gate.

Venting: All plastic mould must be vented. Venting is achieved by machining vent grooves into the cavity at the parting line which will carry the gases out the side of the mould. These vent grooves are approximately .005” deep and 1/8” to 1/4” wide and are located opposite the gate.

Venting cont’d Injection mould are vented to get rid of any air that might be trapped in the cavity by the flowing material. In addition to vents, air in injection mould escapes around ejector pins, loaded inserts, and at the parting line. A properly vented mould will prevent incomplete piece parts, burn spots on the part, and will produce excellent weld.

Slides: Slides are used to produce undercuts on moulded parts.

SIDE OPEN POSITION

SLIDE IN CLOSED POSITION

HORN PINS/LOCKING PINS

Horn pins are used to move a slide backwards and forward and to also lock the slide in position when material is injected in to the mould cavity. If a slide moves backwards; this movement is called blowback.

Slide Movement Which method of moving a slide do you think is cheapest to incorporate in your mold design? Regardless of the movement method used all slides and core-pull must be designed with a housing. What is a housing? And what is its purpose?

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There are two components which make up a slide housing. • •

Back plate Side plates

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Slides must also have carriers.

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The carrier allows for attachment of the slide insert. The carrier must have guide pin bushing and dowel pins to allow for proper alignment of the slide to the mold.

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Wear Plates

What is a wear plate? Where is a wear plate located?

• When ever possible a wear plate should always be designed with lubrication in mind. – Grease grooves

It is always important to use as hard a steel as possible when designing a wear plate. The designer must never use the same hardness on both members rubbing against each other. Also the alloys used might also affect how the mold functions.

• The fit of a wear plate is also critical to the function of a mold – Size of the wear plate – The ride-height of the wear plate

• The surface of the wear plate is also very important – The attachment method used – Surface finish of the wear plate