Sunday, 20 September 2020
ENGINEERING CONCEPTS

What are the ejection techniques in injection moulding

Ejection is a process of removing the plastic material from the mold. This are of different types to remove the material from mold.

Ejection techniques:

Pin ejector:

  • Simplest and most economical method.
  • Location and number of ejector pins depends on component shape and size.

D-shaped ejector pin:

  • Flat sided ejected pin.
  • Made by machining of flat on standard ejected pin.

Blade ejector:

  • Basically a rectangular ejector pin.
  • Slender parts such as ribs are ejected by this method.

Blade ejection

Sleeve ejector:

  • Molding by means of hallow ejector pin or sleeve.
  • Ejections of small cylindrical Molding.
  • Provide positive ejector around a local core pin in forming and hole in the mold.

Value ejector:

  • Large diameter ejector pin.
  • Used for ejections of large component.
  • Ejector applies ejector force on to the inside surface of the molding

Air ejector:

  • Ejector force is supplied by compressed air.

Stripper bar ejector:

  • Ejector element pushes against the bottom at the molding.
  • Effective area of ejection is greater.
  • Suitable for walled box type moldings.

Stripper plate ejector:

  • Mold adopted for ejecting circular box moldings with thin wall sections.

Use of injection moulding

Injection molding is use to prepare the materials like plastics or rubbers particles. Here injecting of plastics articles into mold with high Temperature.

Injection Molding

Injection molding Operation sequence: 

  1. Mold closes: As the screw begins to forward for injecting.
  2. Cavity fills: Screw moves forwards.
  3. Cavity is packed: As the screw moves continuously forward.
  4. Cavity’s cools: AS to screw begins to retract.
  5. Mold open’s: for ejection.
  6. Mold closes: Next cycle begins.
  7. Feed section: Transports material from hopper to heated portion of barrel.
  8. Compression section: Plastic granules are compressed to Homogeneous melts.
  9. Metering section: The final mix and heating of material into homogeneous melt carried  out in metering section.

Parts of mold:

Two cavity injection mold:

  • CAVITY INSERTS: External form of component is made on the cavity
  • CORE INSERT: Internal form of component made on the core.
  • GUIDE BUSH AND PILLAR: Used for Align two halves and guide pillar are fitted on cavity plate.
  • Guide bush are fitted on the core plate.
  • Used as a 4 sets in a mold.

Parts of injection

SPRUE BUSH:

  • Transports the material form machine nozzle to cavity through runner and Gate.
  • Has a tapered conical shaped hole.

REGISTER RING: 

  • Fitted on the top plate and its purpose is to align machine nozzle and spur bush.

EJECTOR ROD:

  • Mounted with Threads on the ejector back plate and is guided in ejector rod bush.
  • Trans’mitts the knockout of the ejector assembly to function.

ROD BUSH:

  • Mounted on bottom plate and act’s  guide to ejector rod.

PUSH BACK PINS:

  • Push the ejector assembly to its rear position as the mold is closed.
  • mold used in injection molding.
  • Machine having Hydraulic actuating system do no require this.

SPRUE PULLER:

  • When the molds open its positively pulls the sprue from sprue bush.
  • Common pulling method is an a undercut recess situated directly opposite to bush.

SUPPORT BLOCK:

  • Sandwiched between support plate and bottom plate  to form a space.
  • Which allows ejection system to function for the ejection of molded.

Types of plates

  • Cavity plate
  • Core plate
  • Top plate
  • Bottom plate
  • Support plate
  • Ejector retainer plate
  • Ejector back plate

CAVITY PLATE: Stationary part of mold.

CORE PLATE:

  • Part of movable  part of mold.
  • Holds core insert, Guide bush and supports ejector pins sprue puller and return pins.

TOP PLATE:

  • Plate holds all stationary parts of mold to the fixed pattern of machine.
  • It holds directly or In directly cavity plate, Register ring, Guide pillar and Cavity insert.

BOTTOM PLATE:

  • It holds a movable parts of mold to the movable patten of the machine.
  • Holds directly or in directly all the movable part of the mold.

SUPPORT PLATE:

  • Mounted behind the core plate to keep this plate from bending under high pressure during injection.

EJECTOR RETAINER PLATE:  

  • Holds the return pins,ejector pins and Sprue puller.
  • Clamped together with ejector back plate to form a unit.

EJECTOR BACK PLATE:

  • Clamped together with ejector retainer plate to form a unit.
  • To act as a backup  plate  for the pins mounted in ejector retainer plate.