Plastic Injection Molding

Injection Molding

3 major functional units; injection, mold, clamping

Plastic Injection Molding

• is a manufacturing technique for making parts from thermoplastic and thermoset materials

• In contrast to the extrusion (which makes continuous parts of constant cross section), injection molding make discrete parts (with complex and variable cross section)

• Molten plastic is injected at high pressure into a mold, which is the inverse of the desired shape.

• The mold is made from metal, usually either steel or aluminium

• widely used for manufacturing a variety of parts, from the smallest component to entire body panels of cars

• The key to success in injection molding are to have;– Proper machine for good melting and injecting

of the resin– The proper resin to appropriate part

performance– A good mold for part definition and removal– Proper operation for efficient molding cycle

(mold cycle depends on the design of the mold and manufacturing parameters)

Plastic Injection Molding

• The most commonly used thermoplastic materials are;– polystyrene (low-cost, lacking the strength and

longevity of other materials)– ABS or acrylonitrile butadiene styrene (a co-polymer

or mixture of compounds used for everything from Lego parts to electronics housings)

– nylon (chemically resistant, heat-resistant, tough and flexible - used for combs)

– polypropylene (tough and flexible - used for containers

– PVC (more common in extrusions as used for pipes, window frames, or as the insulation on wiring where it is rendered flexible by the inclusion of a high proportion of plasticiser).

Injection Unit• Purpose: to liquify the

plastic materials and then inject the liquid into mold– Resin is introduced

through hopper– Some machines can

have several hoppers (to fed filler, colorants, other additives)-Injection molding act as mixer

– However, due to limited size of barrel, mixing capability is poor

• From hopper – hole (feed throat)

• Barrel made of heavy steel cylinder to withstand the pressure and temperature involved in melting the resin

• 2 types of system used in injection molding;– Reciprocating screw- similar to extruder screw

but with unique reciprocating action– Ram injector

Injection Unit

• Design of screw- similar to an extrusion screw• 3 sections;

– Feed section- to advance the resin– Compression section- to melt the resin– Metering section- to homogenize the resin and pump it forward

• The screw of injection molding machine is shorter than extruder, L/D ratios are 12:1 and 20:1

• Low L/D ratios suggest the mixing is less efficient in the injection molding machine

• The compression ratio (diameter of root at feed zone to the diameter of root at metering zone) often in the range of 2:1 and 5:1

• Low compression ratio means less mechanical action is added during melting process

Injection Unit

• Important measure of the size of an injection molding is weight of resin that can be injected, called shot size

• Typical shot size range from 20g to 20 kg

• Since shot size depends on the density of the plastic, PS has been chosed as the standard for rating the machine

Injection Unit

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

Reciprocating Screw Injection Molding Machine

• Advantages– More uniform melting– Improved mixing or additives and dispersion

throughout the resin– Lower injection pressure– Fewer stresses in the part– Faster total cycle

Reciprocating Screw Injection Molding Machine

• In this type of injection molding, the resin is fed from a hopper into the barrel, and heated through thermal energy from the heaters

• The molten resin is collect in a pool in a barrel celled injection chamber

• The molten resin is then push forward by the action of plunger (ram or piston)

• To five better mixing, the molten resin is pushed past a torpedo/spreader, impart shear to the melt

Ram Injection- Injection Molding Machine

Molds• 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

• The connection from the injection unit to the mold is through the nozzle

• The channel that run through the stationary plate of the mold is called the sprue channel (material that is in the channel is called the sprue)

• The solid sprue is removed from the finished part assembly after the part is ejected from the mold

• Resin flow from the sprue through the runner (connecting channel) to the mold cavities

Molds

Mold Bases• Assembly of various

mold parts

• Mold bases can be purchased as entire units, then the cavities are cut from A & B plates

Runners

• Distribution system for the resin from the sprue to the cavities

• Flow characteristics (viscosity), temperature and other factors are important in determining the runner diameter and length

• If the diameter of the runner is too small or the length is too long,the resin can freeze in the runner before the mold is completely full

• If the runner system is too large, excess material would be ejected and too much regrind created

• If the resins have a high viscosity, larger runners are needed compared to low viscosity resin

• The optimum flow of the resin through the runner system depends on the shape and diameter of the channel

• Round channel give the best flow characteristics but difficult to machine

• Machining cost can be reduce by machining one side of the mold plates

• Better shape where the depth of the channel is at least two-thirds the size of the width and the sides are tapered between 2 to 5º.

Runner Channel Shapes

Secondary Runners

• Secondary runner channel are used for multicavity molds

• The flow into the secondary channel should be streamlined (angle in flow direction)

• The streamlined minimizes shear on the resin

Gates

• The end of runner and the entry path into the cavity

• The gate shape can also affect the filling of the cavity, dimension and properties of the parts

• Gate is the most restricted point in injection molding system, i.e. for reinforcement and filler + polymer systems

Gate Design

• Small rectangular opening at the end of the runner channel, connect to the edge of cavity

• Edge gate can be below the parting line if the channel and part are also below the parting line

• Or it can be symetricaly about the parting line, if the runner channel and part are at both side of parting line

Edge gate

Submarine Gate

• Starts from the edge of the runner, and goes into the cavity edge at an angle

• It narrows to a point as it moves from the runner to the cavity

• The advantage; separation of the parts and the runner is automatic

• Disadvantage; gate cannot be used for some resins because of high shear

Submarine Gate

Tab Gate

• By connecting the runner directly into the cavity with no reduction in runner cross-section

• Used for very large parts where a reduction in flow would disturb the resin’s flow pattern and might result inadequate flow into the cavity

Fan Gate

• Made by reducing the thickness and not the diameter of the runner channel as it goes into the cavity

• Used for intermediate size, and when reinforcement in the resin cannot flow through the edge gate

Ring Gate

• Used to make hollow cylinder parts

• The ring gate covers the entire top of the cylinder part so that the resin flow is downward into the wall of the part

Cavities

• Are actual molding locations

• Resin enter the cavities through gate, fills the cavities, and cools to form the solid. The parts are ejected and finished

• Cavities are the heart of the molding process, and must be precisely prepared

• The shape of the cavities determines the shape of the part

Materials & Product Consideration

• Almost all thermoplastic can be injection molded

• Resin with low melt viscosity is required; so that the flow through runner, gate, cavity – easily done with minimum injection pressure

• Resin with injection molding grades have low molecular weight and narrow molecular weight distribution

Shapes

• Hollow parts can be created by allowing the moveable plate to protrude into the cavity of the stationary plate

• Threads can be placed on the inside of a part by using a core pin that is inserted into the cavity where the threads are desired

Shapes

• A hollow part with a hole on the side is even more complicated (the core pin is used)

• The core pin slide into position after the mold is closed

• The core pin seals against the surface of the moveable plate, prevent flow of resin into the area

Shapes