Injection Molding Mold Design

INJECTION MOLD DESIGN JUNAID PARKAR 16POL 204 POLYMER PROCESSING II

TOC

CONTENTS ▸ Introduction ▸ Selection of Machines ▸ Mold Venting

INTRODUCTION

INTRODUCTION ‣ Buying a PIMM is not a small investment. ‣ Too much machine for the job at hand is wasteful. ‣ Too little machine does not get job done. ‣ Careful matching of the jobs needs and the attributes

MACHINE SELECTION

SELECTION OF INJECTION MOLDING MACHINE ‣ Shot Weight ‣ Clamping force ‣ Screw diameter ‣ Injection pressure ‣ Injection Speed ‣ Injection rate ‣ Plasticizing capacity

‣ Mold opening Stroke ‣ Maximum Daylight ‣ Platen Thickness ‣ Dry Cycle Time ‣ Electric Motor Rating ‣ Electric Heater Rating

MACHINE SELECTION

SHOT WEIGHT ‣ Weight of the plastic injected when the nozzle is free standing ‣ The plastic used is PS with a SG =1.05 ‣ Higher than the shot weight of the mold

MACHINE SELECTION

CLAMPING FORCE ‣ Maximum force the machine is capable of, to keep the mold close against cavity pressure during injection ‣ Insufficient clamping force gives rise to flash at the mold joint.

MACHINE SELECTION

CALCULATING CLAMING FORCE ▸ E.g. PS ▸ Flow Path Length (FPL) = 104 mm ▸ Wall Thickness (WT) =0.6 mm ▸ FPL:WT = 173:1 2 P=550 bar=561kg/cm ▸

▸ Clamping Force = Pressure x Projected Area

MACHINE SELECTION

L/D RATIOS ‣ High L/D ratio of 22:1 ‣ Compression in transition section ‣ Better mixing ‣ Uniform heating ‣ Medium L/D ratio of 20:1 ‣ For general application ‣ Low L/D ratio of 18:1 ‣ Shot weight is more dominant selection criterion ‣ Injection pressure is low

MACHINE SELECTION

INJECTION PRESSURE ‣ Maximum pressure in the barrel during injection ‣ Not the max hydraulic pressure ‣ Injection pr. ≈ 10 x Max. hydraulic pr.

MACHINE SELECTION

INJECTION SPEED ‣ Max speed of the screw that can be attained during injection ‣ Injection speed affects injection time. ‣ E.g. Thin walled articles require higher injection speed so that melt does not solidify before the cavity is filled ‣ Its better to have a machine than can deliver multiple injection speeds ‣ Accumulators can be used to increase injection speed ‣ Increasing motor and hydraulic pump size - 25% increase in injection speed ‣ Using an accumulator - 3 times increase in injection speed

MACHINE SELECTION

INJECTION RATE ‣ It is the max volume swept out by the screw per sec during injection ‣ It is expressed in cm3 /s ‣ Injection rate =Injection speed x 3.14 x (d/2)2 ‣ where, d = screw diameter in cm.

MACHINE SELECTION

SCREW SURFACE SPEED (SSS) ‣ Screw rotary speed by itself is not critical as screw surface speed. The two are related by screw diameter ‣ SSS = 0.052 x screw diameter x rpm. ‣ Each plastic material has a recommended maximum SSS which must not be exceeded ‣ For example PP and HDPE should not experience a SSS of higher than 800 mm/s.

MACHINE SELECTION

PLASTICIZING CAPACITY ‣ It is the amount of PS that a PIMM can uniformly plasticize in one hour at max screw rotary speed with 0 back pr. ‣ Since cycle time is longer than screw rotation time ‣ The shot wt. S (g) of a machine and its plasticizing capacity G (kg/hr) set a lower limit on cycle time tmin (s) as follows -

‣ t min=S x 3600 (G x 1000) ‣ It is important to match shot wt and plasticizing capacity in the case of ‣ fast cycling machines producing thin walled or close tolerance components.

MACHINE SELECTION

MOLD OPENING STROKE ‣ It is the displacement of the moving platen from mold close to mold open. ‣ Mold opening stroke determines the max height (H) of the mold part, the machine is capable of. ‣ The relationship is ‣ Mold opening stroke ≥ 2H + L ‣ Where, sprue length = L ‣ In hot runner system L = 0

MACHINE SELECTION

MAXIMUM DAYLIGHT ‣ The maximum opening between the fixed and moving platens when the clamp is wide open ‣ It is related to mold opening stroke and minimum/maximum mold height as follows. ‣ For a toggle clamp machines ‣ Max daylight = mold opening stroke + max mold ht. ‣ For direct hydraulic clamp machine ‣ Max daylight = mold opening stroke + min mold ht.

MACHINE SELECTION

SPACE BETWEEN TIE BAR ‣ Mold width must fit within the horizontal space between tie bars if the mold is lowered from above. ‣ The mold length must fit within the vertical space between tie bars if the mold is slit in from the side. ‣ Usually there is a clearance of ‣ 25mm on each side for smaller molds ‣ 50 mm on each side for bigger molds

MACHINE SELECTION

PLATEN THICKNESS ‣ The moving platen and fixed platen must have sufficient stiffness ‣ to transmit the force of tie bar to the mold with minimum deflection ‣ For a given geometry ‣ a flat platens deflection is proportional to the cube of its thickness ‣ Especially for the moving platen, a compromise has to be struck between weight and thickness.

MACHINE SELECTION

DRY CYCLE TIME ‣ Dry cycle time = mold closing time + mold opening time ‣ Dry cycle time is the ultimate cycle time as there is no cooling period.

MACHINE SELECTION

ELECTRIC MOTOR RATING ‣ The current per phase drawn by a three phase motor at its rated power is ‣ im (A)=motor power rating (kw) x 1000/ (3 x single phase power voltage (V) x efficiency x power factor) ‣ For most three phase motors ‣ Efficiency=0.88to 0.91 ‣ Power factor=0.84 to 0.88

MACHINE SELECTION

POWER DEMAND DURING THE MOLDING CYCLE

MACHINE SELECTION

ELECTRIC HEATER RATING ‣ Electric band heater along the barrel provides heat at start up ‣ It also supplements the heating by plasticizing (when screw rotates) during the molding cycle ‣ A higher rating per heater has the advantage of shorting the initial heat up time. ‣ ih (A)=[electric heater rating (kw) x 1000]/ [3 x single phase voltage (v)]

MACHINE SELECTION

SYSTEM PRESSURE ‣ The most common hydraulic system pressure used in injection molding machine is ‣ 140 bars ≈ 140 kg/cm2 ‣ This is limited by vane pump ‣ Higher system pressure ‣ 170 bar to 240 bar ‣ piston pump ‣ cleaner oil is required

MOLD VENTING

MOLD VENTING ‣ Why is it required ‣ Removal of air that is displaced by plastic material ‣ If not provided/insufficient molding venting ‣ Considerable compression of air ‣ Slow mold filling ‣ Premature plastic pressure buildup ‣ The locations that are difficult to vent effectively ‣ Vacuum is used with a solenoid operating valve

MOLD VENTING

ADVANTAGE OF USING VACUUM ‣ Material enters with a minimum back pressure ‣ Fills the cavity more rapidly ‣ Essentially no trapped air ‣ No voids in the part

MOLD VENTING

DIMENSIONS FOR DIFFERENT PLASTICS ‣ For thermoplastics

MOLD VENTING

DIMENSIONS FOR DIFFERENT PLASTICS ‣ For thermosets