Category Archives: Injection molding

Function:

• sprue bushing must be adapted to the machine nozzle used
• exact coupling must be ensured

• nozzle mouth almost always has to be smaller than the diameter of the sprue bushing

• due to high forces of the nozzle contact sprue bushings are therefore made from hardened steel

  

Risk:

• wear of sprue bushing
• leakage causes pressure drop and causes filling problems

Design:

Function:

1. capture any cold slugs that are formed in the nozzle

2. pull the sprue when the mold is opened and the part is ejected

Risk:

• passed Cold slug cause poor quality in parts

   

Design:

Function:

• provide molten material to mold
• Hotrunners are basically an extension of the machine nozzle into the mold (Pass Through Channels)
• important that the hot runner system is dimensioned in relation to the shot volume
•  no hold-up spots are formed (causing degradation of material)

Risk:

• Sealing is difficult as well because the melt doesn’t have any self- sealing at a gap formed between hot walls.

• In case of mass leakage into insulating air gaps troubles will arise because of a 10 time higher heat transfer

• difficulties with color change restrict usage of internally heated hot-runners
• Thermal expansion of various components needs to be taken into account 

Design:

externally heated hotrunners:

• circular flow cross-section in the center of a heated tube or block.
• The heating elements should care for maintaining the temperature of the melt which means that the insulation to the outside should get particular attention.
• The main advantage of externally heated hotrunners is a better defined and better controllable mass flow

• manifold is insulated using an all-around air gap (8-10mm)

   

• heating elements are positioned in the center of the runner channel:  heating rods or so called “torpedos” with cartridge heaters inside
• self-insulation of solidified molding compound
• inexpensive but difficult to guarantee a safe mass exchange

Insulated Runners :

• Melt cools to form an insulating layer of solid plastic along the walls of the runner.
• The insulating layer reduces the diameter of the runner and helps maintain the temperature of the molten portion of the melt as it awaits the next shot
•  runner volume does not exceed the cavity volume, all of the molten polymer in the runners is injected into the mold during each shot.
• This full consumption is necessary to prevent excess build-up of the insulating skin and to minimize any drop in melt temperature.

 Advantages:

• Less sensitivity to the requirements for balanced runners.
• Reduction in material shear.
• More consistent volume of polymer per part.
• Faster molding cycles.
• Elimination of runner scrap — less regrind.
• Improved part finish.
• Decreased tool wear. 

Needle shut-off nozzle :

popular due to advantages:

• good appearance of gate
• can be individually operated
• equipped with multiple gates and even no weld lines are appearing between the gates because of sequential opening and closing of the nozzles (“cascade filling”)

“cluster nozzles”:

• For very low cavity distances multi-nozzles are appropriate

• Multi- needle shut- off nozzles with grouped needle drive but individual nozzle

Function:

• simultaneously produce molded parts in two levels, while the parts fall out of only one level in single-parting plane molds.

• clamping forces remains the same, same machine can be used with double output

• number of parts ejected per shot is almost doubled.

•  two ejector sides and a centrally located hot runner system

• hot runner works with

1. central sprue bushing, a so-called “snorkel” 

2. with two opposing needle shut-off nozzles

• synchronization with racks
• often very heavy center is guided with massive columns and sometimes with additional support of wheels
• Disadvantages: high cost and much higher daylight opening necessary

Sample:

Source:   www.acomold.com

Function:

• simplest, most common type of molds for injection molding
• the fewest number of moving parts
• cheaper to manufacture than more complex designs

• Both mold halves each have a separately controllable temperature control system

   

Basic Design:

Nozzle Side = Cavity half (fixed side):

Ejector Side = Core half (moving side):

Function:

• cold runner system

• advantage in that molded parts and sprue spiders are autmatically ejected from two different opening areas (parting planes) of the mold (no separation needed)

• uses tear-off pinpoint gates

• retaining undercuts hold the gate on the fixed mold half,

• gate tears off from the part

•  close cavity spacing

  greater design freedom by possibility of various sprues

• Latches or pull-tabs determine the path to drive up plates

• actuator:

• mostly driving up movement of the machine
• hydraulic cylinders
• air cylinders

Function:

• simplest, most common type of molds for injection molding
• the fewest number of moving parts
• cheaper to manufacture than more complex designs

• Both mold halves each have a separately controllable temperature control system

   

Basic Design:

Nozzle Side = Cavity half (fixed side):

Ejector Side = Core half (moving side):

• Function:

• positive or negative change in the geometry at the inner or outer diameter is called an undercut.
• Parts with internal undercuts are fundamentally harder to demold

Design:

Elastic Stripping:

• molded part is demolded by a simple stripping  during opening of the mold without a slider.
• works with tough and elastic materials :

• polypropylene (PP),
• polyethylene (PE),
• soft polyvinyl
• chloride (PVC),
• or elastomers.

• frequently used ejector features are stripper plates.
• Ejector pins, push-out sleeves, or air may also be used.

Applications:   screw caps. medical and packaging area

Demolding through Sliders:

• Auxiliary parting planes can be generated with sliders to release partial molded part areas that have an undercut
• Sliders location either into the nozzle or ejector side
• At mold opening the slider runs to the side of the mold axis at an angle of 90°.
• A maximum deviation of 7° in both directions should not be exceeded otherwise the slider could lock.

• The movement of the slider is carried out mechanically by a sloping or control bolt, also called the positive control.

  movement of the slider can take place before, during, or after the opening of the mold,

• Actuator:  springs, air, or hydraulic cylinders.

• applications:  

   

Demolding with Jaws (Split Molds):

• two or more jaws completely enclose the molded part and form the mold cavity. T
• Actuated mostly with hydraulic cylinders or mechanically using springs
• Jaws should beintegrated into the cooling circuits of the molds.
• Split molds can be combined with all known
• manifold systems and gate variations.
• applications:  bottle crates and car batteries. 

Collapsible Cores:

diameter :   15 mm < d  < 500 mm

Applications: thread or thread-like undercut on covers used for resealable containers

Demolding Internal Thread:

• demolding by rotation of the threaded cores
• mold part must be secured against rotation:

Unscrewing when the Mold is Closed:

• controlled by a guide thread and retracted
•  rotation is avoided by outer geometry

• Advantage: Molded parts are molded without damaging the last thread.

  Disadvantage:  threaded core has to be rotated forward in spraying position =>  increase of the cycle time

Unscrewing for an Attached Stripper Plate:

1. stripper plate is attached to the nozzle side
2. rotating cores are not retracted
3. machine opens parallel to the thread pitch
4. stripper plate is pressed against the nozzle side with spring force.
5. The injection molding machine drives up
6. parts are ejected by the stripper plate

Advantage:

• No expensive design of the threaded cores or expensive lead screws are required.
• No loss of time occurs by turning back the threaded cores.

Disadvantage:  

• A portion of the closing force is lost by the counter force of the springs.

Unscrewing the Stripper Plate with Spring Force:

cores do not run back but pressed back with the spring force of the support plate

 molded thread in the injection molded part takes over the task of the lead screw

 molded thread is strongly loaded.

danger, that the last gear can be damaged

Advantage: 

•  manufacture is cost-efficient

 Disadvantage:  

• thread is of lower quality.

Function:

• provide molten material to mold
• Hotrunners are basically an extension of the machine nozzle into the mold (Pass Through Channels)
• important that the hot runner system is dimensioned in relation to the shot volume
•  no hold-up spots are formed (causing degradation of material)

Risk:

• Sealing is difficult as well because the melt doesn’t have any self- sealing at a gap formed between hot walls.

• In case of mass leakage into insulating air gaps troubles will arise because of a 10 time higher heat transfer

• difficulties with color change restrict usage of internally heated hot-runners
• Thermal expansion of various components needs to be taken into account 

Design:

externally heated hotrunners:

• circular flow cross-section in the center of a heated tube or block.
• The heating elements should care for maintaining the temperature of the melt which means that the insulation to the outside should get particular attention.
• The main advantage of externally heated hotrunners is a better defined and better controllable mass flow

• manifold is insulated using an all-around air gap (8-10mm)

   

• heating elements are positioned in the center of the runner channel:  heating rods or so called “torpedos” with cartridge heaters inside
• self-insulation of solidified molding compound
• inexpensive but difficult to guarantee a safe mass exchange

Insulated Runners :

• Melt cools to form an insulating layer of solid plastic along the walls of the runner.
• The insulating layer reduces the diameter of the runner and helps maintain the temperature of the molten portion of the melt as it awaits the next shot
•  runner volume does not exceed the cavity volume, all of the molten polymer in the runners is injected into the mold during each shot.
• This full consumption is necessary to prevent excess build-up of the insulating skin and to minimize any drop in melt temperature.

 Advantages:

• Less sensitivity to the requirements for balanced runners.
• Reduction in material shear.
• More consistent volume of polymer per part.
• Faster molding cycles.
• Elimination of runner scrap — less regrind.
• Improved part finish.
• Decreased tool wear. 

Needle shut-off nozzle :

popular due to advantages:

• good appearance of gate
• can be individually operated
• equipped with multiple gates and even no weld lines are appearing between the gates because of sequential opening and closing of the nozzles (“cascade filling”)

“cluster nozzles”:

• For very low cavity distances multi-nozzles are appropriate

• Multi- needle shut- off nozzles with grouped needle drive but individual nozzle

Function:

The position of the ejector pins is regulated by the geometry of the molded part.

To prevent markings, it has to be ensured that the pins push against ribs, shoulders, and hidden molded part surfaces.

Risk:

•  If the venting is inadequate there is a risk of filling problems of the cavities.
• Surface burns may also occur on the parts where the compressed air is trapped.
• reduce physical properties at weld lines
• Unsatisfactory design:
  Error
  You are trying to load a table of an unknown type. Probably you did not activate the addon which is required to use this table type.

   

Design:

Ejector Pins:

• most common design
• Standard DIN 1530
• standard ejector pins with the largest possible diameter (about 20 mm) have to be chosen to avoid impressions
• Ejector pins can also provide ventilation (flattened areas)
• pins must be secured against rotation

Sleeve ejector:

• for ring shaped parts  -> tubular ejector

Ejector plate:

• for flat parts
• avoid deflecting by sufficient thickness
• Ejector plates of bigger molds are heated to avoid sticking of the ejector pins as a result of thermal expansion

• demolding force is introduced over a large area

   

  

Mushroom Ejector:

• used for thin-walled packaging containers ( yogurt cups)
• located centrally on the molded part
• raises part about 10 – 15 mm, part is then ejected with air

Air Ejector:

•  nozzles and slots are attached supplied with compressed air,
• mainly used in molds for thin-walled parts.

Actuation of ejector plates:

1. Ejecting by directly attached hydraulic cylinders,

2. Ejecting by a piston in the press table,

3. Ejecting using a stripper mechanism in the press