Injection Molding

PROSES MANUFAKTUR“Injection Molding”

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Casting Re-Forming

PROSES-PROSES MANUFAKTUR

Forming Machining Joining

• Sand Casting• Die Casting• Shell Molding• Injection Molding

• Forging• Extrusion• Rolling• Coining• Drawing

• Turning• Drilling• Milling• Flame Cutting• Reaming• Grinding

• Welding• Screw• Rivets• Pins• Clips• Keys

ThermosetA plastic that, when cured by application of heat or by chemical means, changes into a substantially infusible and product

General properties: more durable, harder, tough, light.Typical uses: automobile parts, construction materials.

Examples:Unsaturated Polyesters: varnishes, furnitureEpoxies & Resins: glues, coating of electrical circuits,composites: fiberglass

INJECTION MOLDING PROCESSIntroduction Polymer/Plastik

ElastomersGeneral properties: these are thermosets, and have rubber like properties.Typical uses: medical masks, gloves

Examples:PolyurethanesSilicones

ThermoplasticsA plastic that can be repeatedly softened by heating and hardened by cooling through a temperature range characteristic of the plastic, and that in the softened state, can be shaped by flow into an article by molding or extrusion

General properties: low melting point, softer, flexible.Typical uses: bottles, food wrappers, toys, …

INJECTION MOLDING PROCESSIntroduction Polymer/Plastik

Polyethylene: packaging, electrical insulation, milk & water bottles

Polypropylene: carpet fibers, bumpers, microwave containers

Polyvinyl chloride (PVC): cables cover, credit cards, car instrument panels

Polystyrene: spoons, Styrofoam

Acrylics (PMMA: polymethyl methacrylate): paints, fake fur, plexiglass

Polyamide (nylon): textiles, gears, bushing, bearings

PET (polyethylene terephthalate): bottles for acidic foods like juices, food trays

PTFE (polytetrafluoroethylene): non-stick coating, dental floss

INJECTION MOLDING PROCESSEXAMPLE

• Almost unlimited variety of part geometries

• Plastic molding is a net shape process– Further shaping is not needed

• Less energy is required than for metals due to much lower processing temperatures– Handling of product is simplified during production because of lower temperatures

• Painting or plating is usually not required

INJECTION MOLDING PROCESSPlastic Shaping Processes are Important

Polymer is heated to a highly plastic state and forced to flow under high pressure

into a mold cavity where it solidifies and the molding is then removed from cavity

• Produces components almost always to net shape

• Typical cycle time 10 to 30 sec, but cycles of one minute or more are not

uncommon

• Mold may contain multiple cavities, so multiple moldings are produced

each cycle

INJECTION MOLDING PROCESS

Injection Molding

• Complex and intricate shapes are possible

• Shape limitations:

– Capability to fabricate a mold whose cavity is the same geometry as part

– Shape must allow for part removal from mold

• Part size from 50 g up to 25 kg (more than 25 KG), e.g., automobile bumpers

• Injection molding is economical only for large production quantities due to high

cost of mold


Injection Molded Parts

Two (or four) principal components:

1. Injection unit

– Melts and delivers polymer melt

– Operates much like an extruder

2. Clamping unit

– Opens and closes mold each injection cycle

3. Injection mold

4. Control system


Injection Molding Machine


INJECTION UNIT TYPE

A) RAM

B) RECIPROCATING SCREW

C) PREPLASTICIZER

Reciprocating Screw Injection Molding Machine• Resin is melt by mechanical shear and thermal energy

from heaters• The molten resin is conveyed to a space at the end of

the screw- collects in a pool• Here, the mold is closed

• The entire screw move forward and pushes the molten resin out through the end of barrel

• To ensure the resin does not flow backward, a check valve or nonreturn valve is attached to the end of screw

• Normally the screw will stay in the forward position, until resin began to harden in the mold

Reciprocating Screw Injection Molding Machine

• Retraction of the screw, create space at the end of the screw• Cooling of the part in the mold, until it can be removed• While the part is cooling, the screw turns and melts additional

resin



•Feed Zone: The portion of the screw that picks up the material from the feed opening (base of the hopper) and begins to soften the material as it is being conveyed. The flight depths are constant.•Transition Zone: The portion of the screw where the root diameter increases gradually resulting in the decrease of the flight depth. This causes the softened plastic pellets to further melt and compress eliminating any of the air pockets.•Metering Zone: The depth of the flights in this section are minimum but constant. This helps the accurate control of the melt discharge for the molding shot. In other words, it meters the amount of melt. The flight depths are constant.

CHECK RING

The check ring is used to prevent the plastic from being blown back during the injection phase of the molding cycle. It is similar to a non return valve

• Advantages• More uniform melting• Improved mixing or additives and dispersion throughout the resin• Lower injection pressure• Fewer stresses in the part• Faster total cycle


• Designing and making mold for injection molding is more complicated than making extrusion die

• Mold Parts – mold is placed in between stationary plate and the moveable plate

INJECTION MOLD

CLAMPING UNIT UNIT

TOGLEHYDROLIC

- Limited clamp force

- Lower costs- Lower horsepower - No feedback on load- Not controllable speed

- Fast

• High clamp forces• Higher costs• Higher horse power• Easily adjustable• Clamp speed controllable• Slow speed

PART DEFECTSHRINKAGE AND WARPAGE

What are shrinkage and warpage?

Shrinkage geometric reduction in size of the part. If the shrinkage is uniform, the part does not deform or change in shape, it simply becomes smaller Warpage when shrinkage is not uniform or if regions of the part shrink unequally

Shrinkage and consequently warpage is affected by processing condition.It is a complex function of machine settings, however a major factor is the pressure and time history during fills, packs and cools.

Shringkage

Warpage

PART DEFECTOTHER DEFECT

Burnt streaksIf the melt was damaged thermally by to high themperatures

Weld lineHigh viscosityIf themperature and preesure is not high enough, the corner of the flow front will not clompletely develope

PART DEFECTOTHER DEFECT

JettingJetting is caused by an undeveloped frontal flow of melt in the cavity

Incomplete filled partLong flow distance or thin wallInjected volume compound to small- Injection pressure not sufficient – premature frezzing

Injection Molding

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