Plastic Injection Molding Process

PRESENTATIONON

By Prakash Chand Sharma

ME- Vth (B)

INTRODUCTION

• It is a manufacturing technique for making parts from thermoplastic and thermosetting materials.

• The plastics used in the injection molding process are liquefied at temperatures between 220° C and 270° C.

• Molten plastic is injected at high pressure (ranges 70MPa to 200MPa ) 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 .

By Prakash Chand Sharma

Injection Molding Machine

By Prakash Chand Sharma

Hopper The thermoplastic material used in the plastic injection molding process is supplied in the form of small pellets. These pellets are loaded into the hopper, then gravity-fed into the barrel and the screw assembly.

BarrelThis is a chamber in which the reciprocating screw is located. The barrel is heated by electric heater bands. .

By Prakash Chand Sharma

Nozzle The nozzle forms a sealed connection between the barrel and the sprue bushing of the mold. The temperature of the nozzle is usually set near the plastic’s melt temperature. The mold’s sprue bushing has a concave radius. The nozzle fits into this radius with the aid of a locating ring.

Reciprocating ScrewThe reciprocating screw compresses and melts the plastic material. It also moves the material toward the die. The reciprocating screw is divided into three zones: the feeding zone, the transition zone, and the metering zone.

By Prakash Chand Sharma

• 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).

By Prakash Chand Sharma

Clamping

Powder or granules from a hopper into a steel barrel with a rotating screw. The barrel is surrounded by heaters The screw is forced back as plastic collects at the end of the barrel .

Initially both parts of the mold clamped(locked) and this clamping force is given in tonnage.

By Prakash Chand Sharma

Once a sufficient charge of melted plastic has accumulated a hydraulic ram forces the screw forward injecting the thermoplastic through a sprue into the mouldcavity.

Injection

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

By Prakash Chand Sharma

Ejection

Pressure is kept on the mould until the plastic has cooled sufficiently and thet time taken called hold on time.

After completion of cycle time the mold opened and the component can be ejected manually or by robotic arms.

By Prakash Chand Sharma

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

By Prakash Chand Sharma

PVC Polystyrene

HDPE GPPS

Various products made by various types of plastics

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Process capabilities :

• High production rates

• Good dimensional control

• Cycle time range 5 to 60 sec’s

• Mold materials- tool steels, beryllium - Cu, Al

• Mold life- 2 million cycles (steel molds) 10000 cycles ( Al molds)

Machines :• Horizontal or vertical machines• Clamping – hydraulic or electric

By Prakash Chand Sharma

 

Advantages of Injection Molding 

 High Production rates Design flexibility Repeatability within tolerances Can process a wide range of  materials Relatively low labor Little to no finishing of parts Minimum scrap losses  

Disadvantages of Injection Molding 

 High initial equipment investment

 High startup and running costs possible

 Part must be designed for effective molding

 Accurate cost prediction for molding job is difficult

THANK YOU

By Prakash Chand Sharma