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MFGT 142Polymer Processing

Injection Molding Tooling

Professor Joe Greene

CSU, CHICO

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Chapter 21: Dies and Molds

• Overview– Injection Molding Molds

• Mold parts

• base

• sprue, runners, gates

• cavities,

• mold materials,

• construction of mold

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Injection Molding Costs• Dies and Molds are an integral part of shaping process for

plastics.• Dies and Molds are expensive.• High cost is a product of

– very accurate dimensions of die components

– time required to machine mold

– computer data and cutter path machining

– complexity of part with lifters, slides, and tool actions

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Injection Molding Molds• Mold Bases (Figure 21.4)

– Built to have one parting line for one mold opening

– Built to have more than one mold opening- stacked molds• Used when relatively small parts are made with large machine• Parts are created on two mold plates• Like having two or more molds running simultaneously

– Mold bases mounted to platens by mold clamps or die bolts

– Locating ring assists in aligning the sprue bushing with the injection machine

– Bases hold two plates • Front Cavity (A plate)• Rear Cavity (B plate)

– Resin flows through sprue cavity and reaches a pin that is inserted into B plate when the mold is closed.

– Pin is mounted on ejector plate and ensures runners stay on B side

– Several types of sprue end types as shown in Figure 21.5

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Mold Bases

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Ejection System• Ejection system is driven by power system separate from the one that opens and closes the

mold.• The ejector plate, to which the ejector pins are attached, advances after the mold has been

opened.• Ejector pins move through the support plate and the B plate and into the back of the cavities

to push parts out of cavities.

• Stripper plate mold is used when ejector pins are not used to push part out of cavity by moving against a feature in part.

• Bases can allow for complicated features– Slides or lifters when have undercut sections in part which have to move out of the way before mold can

open.– Threaded features.– Machine hydraulics drives slides and lifters.

• Cooling is achieved through channels in cavities. Important

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Injection Molding Runners• Runners– Distribution system for the resin from the sprue to the cavities.– Runners must be large enough to fill the cavities even with the resin cooling along the runner walls.– Materials with high viscosities (low MFR) runners should be large enough to prevent gate freeze-

off, e.g., PC has a large viscosity and should have a larger runner than nylon which has a low viscosity.

– Runners should be designed to minimize shear, e.g., acetal can decompose if subjected to excessive shear.

– Length of runner should be minimized – When secondary runners are used as in multi-cavity molds, the flow should be streamlined (Figure

21.7). Streamlining minimizes shear.– When the same part is made in a multi-cavity design the runners should be balanced and fill at the

same time with similar pressures. (Figure 21.8)– When cavities are producing different parts, balanced runner design is more complicated and flow

analysis should be used.

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Runners• Shape

– Round channels give best flow characteristics but are difficult to machine.

– Semicircular channels are less expensive but should have these rules:• depth of channel is at least two-thirds the size of the width

• sides are tapered 2º to 5º.

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Gates• Gate is the end of the runner and the entry path into the cavity.

– Shape of gate strongly affects the ease with which the part is removed from the runner system.

– Gate is the most restricted point in injection path and first to freeze-off– Designs

• Edge gate: small rectangular opening at end of runner channel. Low cost gate.• Submarine gate: starts from the edge of the runner and goes into cavity edge at an angle. Gate is

sheared off at part ejection. Creates high shear.• Tab gate: formed by connecting the runner directly into the cavity with no reduction in runner cross

section. Used for large parts.• Fan gate: made by reducing only the thickness and not the diameter of the runner channel as it goes

into the cavity. Used for parts of intermediates size and when reinforcements can’t flow though edge gate.

• Ring gate: Used to make hollow cylinder parts. Covers the entire top of the cylindrical part so the resin flow is downward into the walls of the part. Used to minimize weld lines.

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Gates

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Lifters and Slides• UNDERCUTS • (ref: http://www.paralleldesign.com/moldability_101/index.htm)

– Undercuts are those portions of the part that can not be pulled in the line of draw. If an undercut were to be machined into a mold without a mechanism to relieve it, the part would be destroyed upon mold opening or ejection.

– Slides are usually used for relieving external undercuts or to allow for zero degree draft on part exteriors.

– Their motion can be driven by one of several mechanisms. Any slide mechanism must also employ a locking device to hold the slide against the ravages of molding pressures in excess of 10,000 psi

– Undercuts can usually be divided into two categories, internal and external.• SLIDES are used to pull external undercuts and

• LIFTERS to pull internal undercuts.

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Peckerslide• Referred to in prudish circles as a cam pin or angle pin, this is the most

common and versatile of all slide mechanisms. – It is driven by the opening of the press. The action of the angled pin withdrawing

from the angled hole drives the slide back.

– Springs hold the slide in the retracted position. Upon press closing the pecker pin returns to drive the waiting slide back almost to molding position.

– Finally then the lock seats against the back of the slide, driving it to its home position and clamping it there for the duration of the next shot.

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Lifters• Lifters are used primarily for relieving internal undercuts or for zero draft

internal faces.

• Their motion is driven by the molding press pushing on the ejector plates. A simple side action lifter withdraws at 90 degrees to the line of draw, simultaneous with ejection.– The angle of the lifter passing through the core frequently confuses the novice into

thinking that it pulls the feature at an angle.

– Relative to the core the lifter indeed moves at an angle.

– Relative to the part however a lifter moves directly sideways because the part is

being ejected forward at the same rate as the lifter.

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Mold Materials• P-20 Pre-hard steel. Rc 29-36. The most common tool steel used, it is a favorite among mold makers for

its durability and machining qualities.• H-13 Air hard steel. Rc 46-54 Great for high production cavities and cores, slide bodies, lifters, and gibs.• S-7 Air hard steel. Rc 54-56 Used for many of the same applications as H-13 however somewhat less

stable and more prone to cracking. Great for long wearing gibs and guides.• M-2 High speed steel. Rc 60-62 used where tough rigidity is required such as tall thin core pins or blades.• 420 SS stainless steel. Rc 49-53 Best for achieving high polish finishes. Also for corrosive polymers such

as PVC.• Beryllium Copper. Primarily used for inserting small areas in tools that may be prone to over heating.

The thermal characteristics of beryllium copper make it the best choice for wicking heat out these areas.• 7075 T-6 Aluminum. Generally used for short run or prototype tooling, however the hardness, wear

resistance and ability to take a good polish has caused many a short run mold to outlast its projected life.• Lamina. "Lamina" is a name brand material that is used for wear plates. This material is a bronze over

steel laminate that imparts long life to moving parts with a minimum of lubrication. In addition to wear plates it is commonly used for large locking surfaces.

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Mold Materials