Filling the Mould

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Filling the Mould

• The movement of plastic begins at the nozzle
• It passes through the sprue bushing
• At the base of the sprue bushing is the runner
  which delivers material to the cavity
• At the cavity, plastic passes through a
  restriction called a gate

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Nozzles

• The nozzle is not part of the mould but rather it
  is part of the moulding machine
• Nozzles are supplied in two varieties
• Spherical nose
• Flat nose

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• Spherical nozzles are very common
• They provide the best seal line contact between
  the nozzle and bushing
• Because of the force applied to a line of
  contact, hobbing and peening can occur
• A hardened nozzle and sprue bush can reduce or
  eliminate this

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• Flat nozzles overcome the peening problem
• They do transfer more heat from nozzle to mould
• They are also more difficult to seal because of
  the larger surface area of contact

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• The radius of the sprue bushing is typically
  1/32 larger than that of the nozzle to ensure
  good seating
• The orifice diameter of the sprue bushing is
  typically 1/32 larger than that of the nozzle to
  allow for mis-alignment

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Extended Nozzle

• An unusually thick A plate would create an
  excessively long sprue bushing
• This leads to extra waste in the longer sprue
• Pressure and temperature drops also occur in the
  longer sprue bushing
• An extended nozzle can be employed to shorten the
  bushing length

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• An extended nozzle prevents the need for an
  excessive sprue
• The extended nozzle needs heater bands to
  maintain temperature
• The help maintain temperature, the nozzle is
  surrounded by at least an inch of air
• Insulation can be added if needed

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Nylon Nozzle

• Nylon is a difficult material to mould because it
  has a low viscosity when melted
• This allows it to penetrate even the smallest
  crevice
• To prevent the material from leaking out of the
  nozzle, the temperature is kept just below the
  drool temperature

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• If the material freezes, it can be withdrawn from
  the nozzle because of the reverse taper design

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Sprue Bushing

• The sprue bushing provides a controlled path to
  deliver material into the mould
• It is part of the mould itself
• It is made of nickel-chromium steel that is
  hardened and tempered to resist wear and shock

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• The sprue bushing is press fitted into the A
  plate
• The shoulder prevents it from being pushed away
  from the nozzle
• Because of the nozzle, it cannot back out either

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• The bore of the bushing is tapered
• It can have between 2 and 6 included angle
• The bore is smooth ground to allow easy ejection
  of the sprue
• The O orifice diameter is sized 1/32 larger
  than the nozzle

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• The actual size is dependant upon the part size
  and area

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• While a sprue can be cut directly into the A
  plate, it is most common to use a bushing
• Direct use of a sprue occurs most often when the
  mould is designed for parting line injection

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Sprue Puller

• A sprue puller is designer to ensure positive
  removal of the sprue from the bushing
• The sprue puller stops below the parting line and
  creates a pocket called a cold well
• The cold well collects the first material that
  enters the mould and prevents it from clogging
  the gates

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• There are three styles of sprue puller
• Z hook
• Reverse taper
• Groove type