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

Plastic Injection Molding 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)

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

Injection Unit 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

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

Reciprocating Screw Injection Molding Machine 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

Ram Injection- 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

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

Molds 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

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

Shapes 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