moulding Flashcards
rotational moulding process
a pre-deterined quantity of thermoplastics granules / powder are put into the mould
mould goes onto an oven and spun on two axes
thermoplastic is forced to the sides of the mould by centrifugal forces
removed from heater ans stil rotating until cool
removed from mould and flash removed
rotational moulding materials
HDPE
LDPE
rotational moulding producrs
kayaks
plastic safety barriers
cones
extrusion process
granules of thermoplastic are fed into the hopper
granules feed along the archimedean screw past the heaters where they soften
softened thermoplastic of forced through a die in a continuous length
product travels through water baths to cool
part is cut to the required length
extrusion products
guttering
window frames
extrusion materials
PVC
extrusion blow moulding process
a parison of thermoplastic is extruded into the mould
mould is closed and air is blown into the parison, blowing it up so that it fills the mould
allowed to cool, mould opened and the flash removed
extrusion blow moulding products
gas containers
bottles
extrusion blow moulding materials
HDPE
injection moulding process
a 2 part die is made from die steel and set up so one part can move
polymer pellett funneled into machine through hopper
plastic melts and is fed through by the screw to turning
plastic is injected into the die
water is pumped around the mould to cool the piece rapidly
cooled piece is pushed out with ejector pins and collected
sprue and flash are removed and the component is fetlled
pieces are checked for air bubbles and cracks - may be measured
design considerations for injection moulding
uniform wall thickness
ribs for injection
avoid sharp corners
avoid blind holes
draft angles
rotational moulding disadvantages
long cycle time
rotational mould advantages
low cost machinery and tools
large productions
rotational mould process
a 2 part hollow mould is made from die steel
a specific volume of polymer powder is placed inside the mould
the mould is loaded into an oven and rotated on 2 axes
the mould continues to rotate as the polymer melts and forms an outer coating
the mould is removed from the oven and continues to rotate and it is cooled with water
the piece is removed from the mould and the moulds are reused for steps 2 - 5
flash is removed and the piece is smoothened and fettled
the final piece is checked visually - make sure there are no air bubbles
what is compression moulding used on
thermosetting polymers
compression moulding process
mould is made of 2 parts with highly polished and high carbon steel
the top part is attached to hydraulic ram
the measured polymer is placed in the bottom mould
the hydraulic ram closes the mould
polymer is heated and pressure is applied
the heat is turned off and cooled so the thermoset polymer can cure
the part is ejected and removed
disadvantages of compression moulding
long cycle time
limitations on size and complexity
advantages of compression moulding
low capital outlay
low waste
