Ejection - Polymer Design and Tooling - Lecture Slides, Slides of Mechanics of Materials

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Ejection

Chapter 12

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• Definitions Ejection

• Basic Requirements for Any Ejection Method
• General Ejection Guidelines
• Ejector Pins and Sleeves
• Strippers
• Air Ejection
• Multiple Ejection Stroke
• Special Ejection Methods
• Two-Stage and Multistage Ejection
• Molding Surface Finish
• Sequence of Ejection
• Collapsible Cores
• Unscrewing Molds Docsity.com

Basic Requirements

• Rule of ejection
  • As mold opens, products must stay on side from which are ejected
    • Sometimes ribs, bosses, undercuts keep part on core side
  • Molds have election actuator on clamp (moving) side of machine
• Ejection Features
  • Mold with ejector pins shown in Fig 12.
    • Knock-out pads are used to keep the machine ejectors level with the moving platen surface for ease of mold installation.
    • Stripper plate is actuated from the ejector plate (Fig 12.2) used if ejector plate cannot be directly reached (Fig 12.3)
  • Length of Machine Ejectors
    • If mold can be inserted by moving it toward the moving platen,
      • there is no need for knockout pads, and
      • the machine ejectors can reach all the way to contact the ejector or stripper plate (Fig 12.4)
    • Less expensive than adding the knockout pads
    • More difficult mold installation Docsity.com

General Ejection Guidelines

• Stroke, Clearance, and Product Height
  • Must have clearance between cores and the cavities to permit the products to fall freely.
  • Stroke should be about S=2.5H (H=product height) for deep products
  • Method of alignment affects required stroke

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Venting

• Venting is very important where a vacuum occurs

• Especially for cup-shaped and flat products
• Fig 12.8- A cup-shaped product is pushed off the core
• For flexible material, PE, it may return to original shape
• For a brittle material, PS, it may break.
• An ejector near the center may act as a vent

• Where to Eject Relative to the Product

• Guidelines
  • For deep products, eject at the points where product is stiffest
    • Fig 12.10- Do not eject at center of cup on inside
  • For products with deep ribs, eject at the points where it is stiff
    • Fig 12.11 Docsity.com

Venting

• Where to Eject Relative to the Product

• Guidelines
  • Shallow ribs (where the height is less than 1.5 times thickness) usually do not need ribs but need venting to prevent implosion.
  • For pin ejection of shallow products:
    • Distribute ejectors to lift product without tipping.
    • Place ejectors at lowest point for ribs or bosses.
    • Use ejector pins and sleeves as natural vents.
  • Land should be not longer than 2D (Diameter of pin). (Fig 12.12)
  • Witness lines are round marks left on the part by ejector pins.
  • Ejection of bosses depends upon the shape and solidity of boss.
    • No need for ejection for solid bosses that are shallow (depth=width or D) and there is good draft angle (at least 5° per side) Docsity.com

Ejector Pins and Sleeves

• Pins and sleeves can be used or any shape of product

• Advantage of ejectors versus stripper plate
  • Inexpensive and easy to use
  • Good natural, self cleaning venting in areas where the air would otherwise be trapped and require vent pins
• Disadvantage of ejector pins and/or sleeves
  • Area where product is pushed is relatively small
  • The area must be well cooled (stiff) so as not to bend
  • Witness lines
• Advantage of strippers (plates, rings, bars) or air ejection
  • Surface of ejection is large and vestige is smaller.
  • Only used where product at the parting line has proper contour
  • Air is simple without out moving parts but limited to cup shapeDocsity.com

Clearance (Fit) and Length of Land

• Clearances for ejector pins and sleeves

• Specified by pin manufacturer per vent sizes in Chap 11
• Clearance of pin in bore
  • Too tight a fit may bind the ejector causing it to break.
  • Too loose a fit may create flash
• Length of land
  • Land should not be longer than 2D for small pins D up to 6mm
  • Land should not be longer than 1.5D for large pin sizes
  • Too long lands are costlier to produce and may result in poor venting
  • Too short a land may cause excessive flashing and may cause bore to wear out more rapidly.

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Size, Finish, and Shape

• Shape of ejector pin (or sleeve) hole in core

• Holes must be funnel-shaped from the rear to facilitate

entry of the pins (sleeves) during assembly (Fig 12.17).

• Selection of ejector pin sizes

• Make ejector pins as large as possible.
• Bore diameters above 6mm are cheaper to produce than

smaller ones.

• Avoid sizes 3mm diameter and smaller, especially if

length is 50 times diameter.

• Sizes smaller that 3mm (2.5,2.0, and 1.5mm) are stepped

down from 3 mm, which extends to 75mm

• Long, slender pins may collapse under ejection force and

could wear faster and need to be replaced often Docsity.com

Special Ejector Shapes

• Very small ejectors required for bottom of ribs,

• Better to provide a flat ejector pin instead of a round pin
• Example,
  • Width at the bottom of the rib is 1.5mm, it is better to have a flat ejector (1.5mm x 5mm). The area facing the plastic in the round pin is 1.77 mm 2 , whereas for flat pin is 7.5mm 2. - Larger surface area is less likely to penetrate the plastic during ejection - pin is stronger and will last longer
  • Design a slot for the special shape pin.
    • Use EDM into the solid core or use two-piece construction. Fig 12.
    • Rear of the core under the slot is bored out for clearance of the shank of the pin from which a special ejector was made.
    • For a 2-piece construction, a shallow U-tube is machined into one portion of core.

6 mm standard pin size

6 mm 1.5 mm 1.5 mm

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Ejector and Ejector Retaining Plate

• Two forces tend to deflect the ejector plate:

• Ejection and injection

• Ejection Force

• Forces depend upon
  • Finish of core: rougher the core = more force
  • Draft angle: smaller draft = more force
  • Undercuts required from product: heavy undercuts = high force
  • Undercuts required when product remains in cavity due to:
    • Vacuum under bottom of product, e.g., containers.
    • Stickiness of plastic, e.g., some adhere more to steel.
    • Shrinkage forces in cavity, e.g. pins in cavity or too warm a core.
    • Symmetrical products about parting line, e.g., records, dinner plates, etc
  • Max ejector force is 6-10 tonnes (metric tons =1000kg) small
  • Max ejector force is 10-16 tonnes for larger machines Docsity.com

Ejector and Ejector Retaining Plate

• Injection Force

• Ejector pins are loaded at their face with the injection

pressure during injection, packing, and hold stages.

• The bigger the area subjected to pressure, the greater the force transmitted through the ejector pin to the ejector plate. - High forces can cause the pins to deflect and bend.
• Stop pins under or near heavily loaded pins, the effect of plate deflection due to these forces can be eliminated.
• Ejector plate is more often a plate rather than a simple beam, with ejector pins well distributed over the entire surface. - Complicated plate theory is actual condition. - Simple beam calculations due not apply. - Plate deflection, f , must be kept to a minimum. - Rule: f < 0.1 mm is acceptable. Docsity.com

Ejector and Ejector Retaining Plate

• Ejector Retainer Plate

• Purpose
  • Hold the ejector pins (or sleeves) on the ejector plate.
    • Bad practice to thread ejectors into ejector plate to eliminate retainer.
  • Sometimes several small retainer plates rather than one large plate are used on one ejector plate.
  • Heads of the ejector plate and return pins or sleeves, should float with lateral play in the retainer plate for proper alignment.
  • Fig 12.
  • Thickness of retainer plate at the head need never be more than 3mm.
  • Axial clearance should be very little (0.1mm)
  • Radial clearances, HC and SC, should be a minimum of 0.5mm Docsity.com

Ejector and Ejector Retaining Plate

• Preventing Ejector Pins from Turning

• Must be prevented from turning when
  • Front of pin in the cavity is shaped to match the product.
  • It carries engraving.
  • A large pin is used for ejection and partly as a return pin.
• Preferred method is to key the head of the pin.
• Fig 12.23 A and B
  • B is preferred, but milling the cutter in view B is smaller and thereby slower.
  • Thickness of the ejector retainer plate does not add anything to the strength of the ejector plate against deflection.
  • Spacing the screws close to groups of ejectors and return pins, few screw are usually necessary.

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