Injection Moulding Guide

  • Injection Moulding Troubleshooting Guide

    Injection Moulding Troubleshooting Guide


    Injection moulding troubleshooting guide for compounds

    ProblemPossible CauseRemedy
    Dimensional inconsistency Inadequate packing Enlarge gate to prevent premature freeze-off
    ▪ Increase hold time
    ▪ Check heater bands / controllers
    Melt temperature variation Check for worn check ring  
    Shot to shot variation Maintain adequate cushion  
    Gate blush Melt fracture ▪ Add radius to gate
    ▪ Increase gate size
    ▪ Reduce injection speed
    Cold material ▪ Increase melt temperature
    ▪ Add cold slug well in runner
     
    Sinks or voids Gate freezing off or located improperly Check gate dimension and location
    Insufficient feed • Increase shot size  
    Holding pressure or time too low • Increase hold pressure or time  
    Surface imperfections (frosty surface or white marks) Wet material • Review drying procedures
    Cold melt/cold mould • Increase barrel temperature /mould
    temperature
     
    Injection speed too low • Increase injection speed  
    Sticking in mould Mould design

    • Increase draft in tool

    • Inspect ejector system

    • Check for undercuts

    Over packing • Reduce injection speed
    • Reduce injection pressure
     
    Weak weld-lines Incorrect gate location • Relocate gate or add overflow tab
    Injection speed or mould temperature too low • Increase injection rate and tool temperature  
    Insufficient venting • Improve cavity venting  
    Burn marks Hang-up in moulding machine • Remove and clean screw
    Contamination • Clean hopper dryer • Purge barrel  
    Shear heat • Reduce injection speed  
    Barrel or nozzle overheating • Check heater controls  
    Air trapped in cavity • Improve mould venting  
    Flashing Excessive vent depth • Review mould venting
    Misaligned platen • Align platen  
    High injection pressure • Reduce injection pressure  
    Inadequate clamp tonnage • Use a larger machine  
    Warped parts Ejection problem • Increase ejector pin area
    • Reduce mould temperature
    • Increase cooling time
    • Check for uniform wall thickness
    Poor part design • Add ribs or part thickness to improve
    stiffness
     
    Orientation of material • Change gate location  
    Excessive shrinkage • Increase part packing  
    Part temperature differential • Check mould cooling system  
    Brittleness Weld-lines • Increase melt temperature
    • Increase injection pressure
    Poor part design • Eliminate sharp corners  
    Moulded-in-stresses • Increase barrel/nozzle temperature  
    Overheating • Reduce barrel/nozzle temperature  
    Wet material • Review drying procedure  
  • Machine clean up & Shutdown Guide

    Recommended purge materials

    The proper selection of a purge material for a given compound depends highly on the melt temperature of base resin in the compound you wish to purge.

    When purging dark compounds a 20–30% glass filled version of the purge material will help scour the barrel and will aid in removing the pigment residue. The glass reinforced version should be followed by its unfilled analog to eliminate any glass residue that may remain.

    It is important to purge barrel of the injection moulding machine with a purging material. Shriram Axiall also manufactures purging material. Same can also be achieved with GPPS, ABS or Acrylic before and after the use of compounds. The injection moulding machine must not be shut down with the material left in the barrel. If material is left in the barrel, it will degrade and the screw and barrel will have to be removed and cleaned mechanically.

    The use of polyethylene and polypropylene are not recommended as they are immiscible with compounds.

    Warning!

    compounds must never come into contact with Acetal copolymers within an extruder or moulding machine. The two materials will rapidly degrade; traces quantities will be enough to cause degradation.

    Shutdown procedure

    The recommended procedure for shutdown after moulding compounds differs depending on the length of shut down.

    • Complete shutdown At the end of a moulding run, all traces of compound should be thoroughly purged from the barrel with a purging material. After the purge appears clean, the screw is left in the forward position. Barrel and nozzle heats can then be turned off. Shriram Axiall also manufactures purging material. Same can also be achieved with GPPS, ABS or Acrylic. A neutralizing agent should be sprayed on the mould surface, knockout pins, sprue bushing and moving slides. After neutralizing, wipe clean and apply a good rust prohibitor on all exposed surfaces.
    • Breaks in production When the production is halted for short periods of time or in case of power failure; material cools and solidifies in the barrel. When power is restored, special startup procedures should be used. Initially, the heater bands should be turned on low heat, 93–121°C (200–250°F) and should be retained there until the material in the barrel has had time to warm through. The heater band setting should then be increased to slightly below normal operating temperatures. As soon as the machine reaches the higher temperatures, the screw should be manually jogged; and as soon as the material moves, the temperatures should be raised to those for normal moulding conditions. The remaining material is then purged from the barrel with one of the recommended thermoplastic materials. At this time, the earlier grade being used can be reintroduced into the barrel and production can be resumed.
    • Material changeover Immediate changeover to a dissimilar compound requires complete purging of the plasticating cylinder. compounds should not come in contact with acetal or acetal copolymers in the injection moulding machine. The two materials are not compatible and will react violently when mixed.
  • Injection Moulding Processing Guide

    Material handling and preparation

    • Drying Compounds can normally be processed without pre-drying. However, during conditions of high humidity, moisture can condense on the pellets, causing problems such as excessive splay, foaming, low weld line strength, or other surface defects. If a moisture problem is suspected the compound can be dried using a hopper mounted dehumidifying air dryer. If a hopper mounted dryer is not available, then a tray oven can be used.
      Drying conditions should be 75°C for 1 hour. Material depth in each tray should not exceed 5 mm. Prolonged or overnight drying or higher temperatures are not recommended. Most glass filled compounds require thorough drying prior to injection moulding to assure maximum properties in the moulded part. The hopper dryer should be equipped with a diffuser cone to ensure proper distribution of air flow and allow "plug flow" of material through the hopper.
    • Regrind The use of reclaimed sprues, runners, and non-contaminated parts is permissible with most Shriram Axiall composites depending on the colour and physical property requirements of the finished part. Use a conventional scrap grinder equipped with 0.25 in. to 0.375 in. (6.4–9.6 mm) screen. A maximum of 20% regrind can be blended with virgin material without appreciable loss of properties or colour. If end use properties are important or critical part tolerances are required, the use of any regrind should be fully qualified prior to production. Be sure to keep regrind free of contamination, and dry before recycling. Blend, regrind with virgin material before adding it to the hopper dryer. Regrind may require longer drying times as the reground compound will tend to absorb more moisture than virgin pellets due to the larger surface area of the regrind.
    Machine preparation

    Prior to start of the production, the moulding machine barrel should be thoroughly cleaned by purging with a purging material. Shriram Axiall also manufactures purging material. Same can also be achieved with GPPS, ABS or Acrylic and foreign particles or discolouration in the air shot should be looked at. Any evidence of contamination in the first several moulded parts may suggest that the cleaning procedure should be repeated.

      • Clean both mould halves thoroughly using a good recommended cleaner.
      • Make sure all vents are thoroughly cleaned and free of any pre-applied rust preventative, dirt or other material build-up.
      • Address all water leaks and check for adequate water flow through the tooling.
      • Clean and polish sprue bushing. Check for rough spots.
      • Check nozzle and sprue orifice for proper match up.
      • Review your process set up sheet for proper mould temperature settings. If none exists refer to this manual or call Shriram Axiall Technical Services Group.
      • Set barrel temperature controllers to desired or recommended temperatures.
      • Reduce injection and back pressures and screw RPM to the lower end of their operating ranges.
      • Injection and back pressures should be checked and set during purging or after start of moulding Cycle.
      • Melt temperatureThe compound stock temperature should be monitored during the normal processing cycle with a needle pyrometer by taking air shots through the nozzle. If a small shot size makes accurate readings difficult, take several shots and use the first purge to preheat the needle prior to subsequent readings. Usually the melt temperature for varies in the range of 165–180°C depending on the machine configuration.
      • Injection speed & pressure Lower first stage pressure can be expected for unfilled compounds. The first stage (boost) pressure influences the surface quality, orientation, and mechanical stressing of the melt, therefore, excessive pressure should be avoided.
        Injection pressure is in the range of 50-75% of the maximum available. Holding pressure in the range of 50–75% of first stage pressure will provide compression of the melt in the cavity for replication of the mould surface. Holding pressure should be set just high enough in order to prevent voids, sink marks and control shrinkage.
      • Back pressure Screw back pressure should be kept as low as possible with 2–4 bar usually being sufficient. The back pressure serves to displace air in the screw feed section, improve melt homogeneity, and facilitate heating of the melt. Adjustments to the back pressure will be apparent in the stock temperature developed. A surplus of back pressure will result in overheating of the melt and excessive fiber breakage.
      • CushionA minimal amount of cushion should be provided. Normally a range of 0.125"–0.25" (3.2–6.4 mm) cushion will allow for adequate compensation of shot to shot variation. A small cushion of material must be maintained ahead of the screw to compensate for part shrinkage as it cools under holding pressure, thus preventing sink marks. Ideally, the screw should only reach the full forward position after material movement has ceased.
      • Screw speed A screw speed between 30–60 rpm is sufficient for most compounds. Slightly higher rotation speeds can be used for small diameter 1.5" (38 mm) screws. Optimum screw speed selection can be reached by adjusting screw return stoppage to occur just prior to the mould open sequence. High screw speeds can result in over heating of the melt and increased residence time in the moulding machine.
      • Mould temperatureAccurate mould temperature control is essential for optimizing cycle time and finished part quality. compounds are usually run with mould heater-coolers between 20–65°C (70–150°F). Higher mould temperatures usually give improved surface appearance, better material flow, improved weld line integrity and lower part stress. Cooler mould temperatures give shorter cycle times. High pumping rates of the temperature control medium with minimal line and coupler restrictions will improve temperature control in the tool and optimize the combination of part quality and cycle time. Running the "B" half of the mould cooler than the "A" half usually facilitates part ejection and removal.
        Large cores or small diameter core pins may require lower temperatures or special cooling control to aid part ejection (prevent sticking). Higher mould temperature increases crystallinity, shrinkage and heat distortion temperature. This also decreases moulded in stresses and impact strength values.

    Mould release agents

    The application of external mould release agents to tool surfaces should be kept to a minimum. Initially, part sticking can be corrected by making adjustments to pressure, temperature or cycle time. In addition, draft angles, mould surface finish, and ejector pin area may require attention. Intermittent part sticking can often be overcome by moderate application of a mould release spray. Since some mould release sprays may have a detrimental effect on the properties and surface appearance.

    Care should be taken to minimize the possibility of rusting and corrosion when putting a mould into storage. The mould to be stored should be thoroughly coated with a commercially available spray to neutralize any residual acids present, dried and then sprayed with a commercial rust inhibitor, lubricant or mould release agent. At the end of a work day or over a weekend, it is also advisable to neutralize spray followed by a commercially available mould protective solution of the mould core and cavity.

Extrusion Guide

  • Extrusion Troubleshooting Guide

    Extrusion Troubleshooting Guide


    ProblemPossible CauseRemedy
    Voids on the entire wall of jacket or insulation
    • Melt zone too far forward in barrel to force entrapped air back through hopper
    • Inadequate screw design for low back-pressure extrusions
    • Too-large construction for extruder capacity
    • Increase screen pack — more or finer screens.
    • Decrease space between wire guider and die. Use long land dies.
    • Preheat feed stock to a constant temperature prior to extrusion.
    Rough and grainy surface Humid weather and prolonged storage of feed stock in damp locations Dry material in hopper dryer installed on extruder. Prevent moisture pickup by:
    • Minimizing inventory during humid seasons.
    • Locating storage areas in warm dry locations.
    • Storing materials in tightly closed containers. Move water bath closer to die line.
    Voids resulting in chunky surface roughness
    • Excessive stock temperature
    • Lower temperature of controlling barrel heat zones and crosshead.
    • Decrease screen pack – fewer or coarser screens. 1-40 and 1-60 minimum screen pack recommended.
    • Adjust screw cooling. Increase space between wire guider and die.
     
    • Extended runs without cleanouts or screen changes
    For extended runs without adequate shutdowns for cleanouts, maintain stock temperature under 380°F. Provide for screen change at least every eight hours.
     
    • Poor streamlining or flow in approach and crosshead
    Decrease crosshead temperature
     
    • Dirty or pitted dies, guiders, and breaker plates
    Maintain dies, guiders, and breaker plates so they are clean, highly polished, and free from rust, pits, burs, and any buildup.
     
    • Improper startup and shutdown procedures
    Standardize startup, shutdown, and cleanout procedures to provide:
    • Adequate bleeding before beginning new run.
    • Continuous bleeding at low RPM with screw cooling turned off and barrel heats decreased during temporary shutdowns.
    • Sufficient flushing of barrel with head open before shutting down for extended periods.
    • Proper cleanup of tools while still warm. Tools and dies should never be burned out to clean.
    Large blisters or bubbles in
    insulation
    • Moist or oily conductor
    Clean, wipe, and preheat the conductor
     
    • Conductor with a loose strand
    Check stranding of wire to be sure it is tight and uniform
     
    • Wet marker thread
    Preheat marker thread in oven or dry by passing it over heat lamp while running.
    Fine pimples Inadequate screw design for low back-pressure extrusions
    • Increase screen pack - more or finer screens.
    • Decrease space between wire guider and die.
    • Use long land dies.
    • Adjust screw cooling.
    • Raise temperature of controlling barrel heat zones
    Lumps with different textured materials Break in screen pack. No guards on hopper. Careless handling of feed stock or of material containers Flush out extruder barrel and thoroughly clean head. Avoid storing feed stock in open containers. Take care when emptying bags to keep paper out of hoppers.
    Raised lines on surface Damage to die due to improper handling and care Thoroughly clean die. Polish until all depressions are removed.
    Holes or splits in jacket Die opening too large - out of proportion for construction requiring excessive drawdown Raise crosshead and die temperatures. Cut RPM
    Fluctuation in rate surging Hot feed stock used with no provision to keep temperature constant Let feed stock cool to room temperature. Hold feed stock in extruder hopper to constant elevated temperature ±1°C.
      Poor flow due to poor granulation or foreign contamination. Dry blend with poor flow characteristics. Poorly designed hopper and hopper throat Hand feed or carefully watch feeding of material. Use hopper cooling (if available). Avoid contamination
        Maintain barrel temperature at hopper between 150–200°F. Adjust warm water to ensure cool screw surface in feed section.
    Low rate High back-pressure die with deep-flighted screw Decrease screen pack to minimum, normally 1–40 and 1-60. Increase space between guider and die. Increase temperature of crosshead.
      High-pressure screw with slow pitch or relatively shallow final flight depth - a screw designed basically for another thermoplastic Limit screw cooling to feed section. Increase barrel heats in front sections.
      Clearance between screw and barrel too great. Surface of barrel smoother than surface of screw. Polish surface of screw. Roughen surface of barrel.
  • Extrusion Processing Guide

    Extrusion process converts the feed stock which may be in the form of powder or granules into a supply of homogenized melt at the correct temperature. Typical products made by extrusion are sheets, film, pipes, cables, laminates, etc.

    The extrusion process is carried out through an extruder. The main element of extruder is a rotating screw. It is a helically flighted shaft which when rotated mechanically works and advances the material being processed. The other element of extruder is the Barrel.It is a cylindrical housing in which the screw rotates.

    The screw generally contains three main sections:

    • Feed Zone: It takes the feed stock from the hopper and feed it to the next zone – compression zone. The design of feed zone should be capable to supply sufficient material to metering zone.
    • Compression Zone: This zone comes next to the feed zone and there is a gradual increase of root diameter of the screw upto the start of metering zone. Here the material is homogeneously melted and become sufficiently viscous. Compacting action also takes place to push any occluded air back to feed zone and also to improve its thermal conductivity.
    • Metering Zone: This zone acts as a metering pump and from this; the material goes to die system at constant volume and pressure. Enough care is taken so that the material remains as free flowing and does not melt and sticks to the die.

    The extruder also has following provisions:

    • Vent: It is an opening at an intermediate point in the extruder barrel to remove entrapped air and volatile matter from the material being processed.
    • Barrel Cooling: Jacket is provided around each zone of barrel and water passes through them for cooling thus helping to control temperature of the material. Some extruders have screw cooling arrangement also.
    • Breaker Plate: It is a metal plate possessing holes and is fitted between the end of screw and the die. It increases back pressure inside the extruder, improves mixing and also drops pressure at the die end. Generally a wire mesh is put along with the breaker plate to stop unwanted foreign particles entering the material.

    Material handling and preparation

    Drying: Compounds can normally be extruded without pre drying. However, during conditions of high humidity, they should be dried using a hopper mounted dehumidifying air dryer or a trey/ oven. Drying condition should be 75 – 80 deg C for one hour.

    Machine Preparation: Before starting the machine, its screw and barrel should be thoroughly cleaned by purging with cleaning compound. If the extruder is being started after a gap of 10–15 days, its screws should be taken out from the barrel and thoroughly cleaned with cotton emery paper, etc.

    Barrel should be cleaned with brass wire mesh or brush and cotton. The die and breaker plate holes, etc. should also be cleaned. After the cleaning is complete and extruder is properly drained, die should be fitted along with breaker plate. Water tank, conveying pipes, granulating unit, screens, packing trey should also be cleaned thoroughly.

    Temperature of various zones and die should be set according to the material. The hopper should hold the preheated material and after achieving the temperature, machine should be started at low rpm till it comes out of the die and then gradually rpm should be increased. Material come out of die holes in the form of strands which are passed through a water tank for cooling and are then cut into granules. These are stored in a trey for packing. Alternately, strands are cut into granules as soon as they come out of the die and are conveyed through pneumatic line to packing trey.