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Technical Papers by Prabodh C. Bolur

Technological Solutions for Quality in Injection Moulding of Plastics

Technological Tools for Part Design, Mould Design & Mould Fabrication.

 

TECHNOLOGICAL SOLUTIONS FOR TOTAL QUALITY IN INJECTION MOULDING OF PLASTICS PRABODH C.BOLUR

There are SMALL factories in Europe where virtually number of machines work with robot - without operators. Manpower is used for movement of finished parts for packing or further operations.

Since injection moulding is an automatic mass production technique, it is meaningless to use cheaper, inferior and imperfect moulds on expensive machine. Once the mould is fixed and process parameters are set on the machine for desired quality, the production should go on and on till the production schedule is over . This can be possible only if all concerned with operations understand the strength and significance of four ‘M’s.

MACHINE which gives excellent repeatability of set parameters - for which machine control is responsible. Machine should be able to reproduce machine parameters repeatedly with out any error. The machine should have microprocessor controls which is capable of giving

These features of multistep control of machine parameters can - if suitably set - can have influence on quality of moulding.

The machine is not responsible for

There should be periodical machine preventive maintenance practice so that there should be no unscheduled interruption of production.

MOULD which has to mould part.

When above mentioned factors are considered before manufacturing mould , then only it can produce parts without interruption in production. There should be scheduled for lubrication, inspection, servicing of cooling system of mould.

MATERIAL including additives of excellent quality standards and consistency are used. Knowledge of material characteristics and it’s properties essential.

MAN who is able to visualise and understand what is happening inside the mould and plasticising screw assembly. He should be able to visualise forces acting and its influence on various component of mould. He should have clear understanding of fundamentals of the subject - strength of materials of metal and peculiarities of plastics.

Mr. William J. Patton- a moulder and inventor, graduate of University of Manitoba in Mechanical Engineering ,Canada, Author of PLASTICS TECHNOLOGY : Theory, design & manufacturing says in the PREFACE of his book:

" Plastics are more complex material than metals and therefore, are less understood (even by technical personnel) and not always successfully applied...

Normally we are familiar with metals and we tend to view plastics as a type of metals. Metals are hard, stiff and elastic, they corrode where as plastics have none of these characteristics. All metals have similar behaviour but one plastic does not necessarily behave like any other plastic...

Different types of same plastic may be totally unlike. Solid Urea-formaldehyde is a hard material while urea-formaldehyde foam is the softest material. Polycarbonate is very tough material, but it is not tough if a notch is cut in it or wipe it with a solvent...

Plastics are exciting materials to use, but they are deceptive in ways that no metals are. Therefore, skilled practicing engineers should have imagination and caution in working with plastics... "

 

QUALITY RELATED PROBLEMS

It should be realised that every intricate shape need not be perfectly mouldable. Therefore, there exists problems which can be recurring processing problem or quality related problems. To solve these problems we should have in-depth knowledge of plastics technology. It can be realised that the root cause of all processing and quality related problems lies in part design and mould design.

The problems may be due to

  1. defective part design from manufacturing point of view.

  2. defective mould design from plastic melt characteristics and processing point of view.

  3. poor machine specification or poor machine controls.

  4. inconsistent material quality.

The problems belonging to (a) can not be solved by searching for solution in (b), (c), (d).

The problems belonging to (b) can not be solved by searching for solution in (a), (c), (d).

The problems belonging to (c) can not be solved by searching for solution in (a), (b), (d).

The problems belonging to (d) can not be solved by searching for solution in (a), (b), (c).

The solution for problems have to be in it’s own domain.

All theses quality problems - listed below- are not new. They existed right from the beginning and they continue to exists. Obviously this shows men concerned have not applied their mind to the problems and hence they are repeated. CAE analysis software like MOLDFLOW can help in detecting these problems at part design stage itself and it enables to correct the defect in computer even before mould is fabricated. The useful data obtained from the analysis can be used to design perfect mould.

Let us first list the problems of moulding.

MOULDING PROBLEMS

MOULD related

MACHINE related

SINK MARKS

  • Part design related- non uniformity of wall thickness .
  • Injection speed profile hold on pressure profile can minimise to some extent.
  • Gas injection Moulding technique can be considered.

    • WARP

    • Use technique to improve part stiffness with part design.
    • Random distribution of mould surface temperature. Cooling circuit design related.
    		 

    WEAK WELD LINES

    • Part design related.
  • injection speed can influence.

    • IN-CONSISTENT CRITICAL DIMENSIONS

    & Fluctuating quality during long production run.

    • Unequal shrinkage due to variation in mould surface temperature.
    • Cooling circuit design related.
    • Core & cavity dimension related. Correct shrinkage is not considered for core / cavity dimension.
  • Melt temperature, Injection speed & hold-on pressure can have little influence.
  • Unstabilised process due to unbalanced heat exchange in mould & unbalanced melt flow & high %age utilisation of shot capacity - more than 3D injection stroke.

    • DEGRADATION

    • Check dimensions of runner system for excessive shearing.
  • Excessive Residence time due to use of oversized injection unit.
  • Lowering barrel temperature & injection speed can have some influence

    • EJECTION DIFFICULTY

    • Part design related -Unbalanced melt flow. Use flow leader to reduce unbalance in flow.
    		 

    POOR IMPACT strength

    • Radius at projections & sharp corners as it act like notch from where crack propagates.
    		 

    IN-CONSISTENT WEIGHT.

    		 
    • % age utilisation of shot capacity is closer to 90% resulting in inconsistent melt quality- if metering stroke is more than 3 times screw diameter. Borderline case for shot weight.

    IN-CONSISTENT FILLING

    • Increase gate size to reduce pressure drop across the gate.
  • Available maximum injection rate is not adequate for flow ratio of part being moulded . Borderline case for injection rate.

    • STRESS CRACKING

    • Melt flow is unbalanced. Part design related.
  • Over packed part. Set correct pressure profile during pressure phase.

    •  

     

    MATERIAL RELATED PROBLEMS

    SINK MARKS

    • Filled polymer can reduce sink mark.

    POOR MECHANICAL STRENGTH

    • Consider high molecular weight polymers.

    • Consider filled polymers. Pre heat polymer in dehumidifier if required for polymer as recommended for polymer.

    DIMENSIONAL VARIATION

    • Check shrinkage characteristics of material.

    DEGRADATION OF MELT

    • Check thermal stability of polymer as well as pigments and other additives if any used with material.

    DIFFICULTY IN FILLING

    • Consider polymer of higher MFI.

       

    PERFECT MOULDABILITY

    A 100 % balanced flow in mould results in 100% perfect mouldability. If say 60% balanced flow part will definitely have some area overpacked while filling unbalanced region of part. Overpacked parts have quality problem like - dimensional inaccuracy, unequal shrinkage, distortion while ejection, moulded-in stresses resulting failure due to environmental stress cracking. Since overpacked parts have quality problems, part designer should try to achieve balanced flow by using flow leader / flow deflector in the part design.

    It should be noted that all these major operational problems- requiring frequent alteration of process parameters on machine - have their roots in

    Therefore, obviously if these factors are considered while freezing part design and mould design then we can have moulds which once set on the machine, will keep on producing quality parts with out any intervention of supervisor or operator.

    In fact production shop supervisor should be more busy with statistics of production and SQC then frequent adjustment machine settings.

    Therefore, it is a challenge for practicing engineers to design plastic parts, keeping in mind the

    Keeping in mind the flow & shrinkage behaviour of plastic melt the mould has to be designed to ensure

    The Injection Mould can be considered good only if it produces consistently excellent parts with out any quality problems as well as processing problems at an economic cycle time on long production run. Mould plays the highest and most important role in producing good quality parts.

    Mold design and manufacturing technology has gone through many changes in the seventies and eighties, from designing simple moulds with spurs and runners, molding polypropylene and polyethylene, to the present complex moulds incorporating runnerless moulding systems to produce parts from expensive engineering plastics for the automotive electronic and industrial markets.

    Highest level of economic productivity with very high level of quality can be achieved when solution for all the possible quality problems are considered while designing the part and mould. This means establishing the quality at the part design stage through the integration of Design for Functionality (DFF) and Design for Manufacturability (DFM). The possibility of achieving this objectivity is certainly a function of the knowledge of the technological process and experience- knowledge database. This is possible with the help of CAE software programs like MOLDFLOW.

     

    COMPUTER AIDED ENGINEERING

    This is a software for analysis program that enables the designer to test the design before it is produced. This software bridges the gap between the part designer and mould designer. As explained earlier the plastic part requires the following basic analysis:

    There can be analysis for shrinkage, warpage, etc. Since it predicts defects in part, it is also referred as PREDICTION TECHNOLOGY.

     

    The design of mechanical part involves quite accurate calculations of stress, strain, bending moment, heat transfer, .... whereas the formulas for plastic parts are quite complex, therefore thumb rule prevails while designing plastics parts. Dimensional stability of plastic part and creep behaviour under load condition are quite complicated. They can not be easily estimated manually. Therefore it calls for the use of Computer Aided Engineering - based on sound engineering principles. Now PC and CAE software prices are affordable to even rationally thinking small entrepreneurs involved in DEVELOPMENT of troublefree moulds. These CAE software like MOLDFLOW are available since late 70’s. CAE is an extension of CAD capabilities.

    With the help of MOLDFLOW we can carry out various types of analysis (by simulating) like Melt flow pattern, Fill time, Filling temperature, Filling pressure, Hold-on pressure, Volumetric shrinkage, Shrinkage all over the part, Temperature distribution along the mould surface and also across the wall thickness, Weld lines air traps, Deflection under stress. MOLDFLOW provides specific norms for each of these analysis to determine the acceptability of the results. The problematic results are to be corrected by proper interpretation of the results till the result becomes acceptable. In this manner the possible quality problems are eliminated at part design stage itself.

    MOLDFLOW’S CAE analysis software -in modules -for following analysis are available:

    MOLDFLOW’S

    FUNCTION

    GRAPHIC DISPLAYS

    BENEFITS

    MF / VIEW

    • Quick Pre- Processing 3D- wire frame - MODELING of part. Models can be rotated, scaled, zoomed & panned enabling easy view.
    • Automatic finite element mesh generator for analysis.
    • Animation capability makes it easy to quickly understand and communicate the intricacies and complexity of the moulding process and its effect on resulting component.
  • Improved team information,
  • Communication and project control
  • Speedy modeling.

    • MF / FLOW

    • Simulates plastic melt flow through out injection moulding cycle.
    • Interactive flow analysis quickly establishes a set of processing conditions that forms a moulding window of injection time, mould temperature and melt temperature for a given part and material.
    • Within this window acceptable parts can be produced.
    • Automatic runner balancing.
    • Automatic wall thickness profile for balanced melt flow.
    • Establishes mould temperature and melt temperature desired
    • Calculates filling profile and packing profile
    • Potential problem areas such as weld lines, air traps, short shots are identified and can be corrected on the computer.
    • The extent of unbalanced melt flow can be identified and corrected by incorporating flow leader or flow deflector on the part.
    • Flow analysis is based on accurate and reliable material database of over 4000 polymer grades. Thermal, rheological and PVT data are available in the data base.

    • Pressure, melt temperature,
    • Shear stress, shear rate,
    • Dynamic fill pattern, Flow direction,
    • Frozen layer distribution,
    • Maximum holding pressure,
    • Weld lines and air traps,
    • Temperature, shear rate, velocity, viscosity through thickness
    • Frozen layer thickness over time for all elements
    • Molding window,
    • Viscosity, volumetric shrinkage,
    • Pressure and temperature over time for all nodes,
    • Clamp tonnage,
    • Flow angle,
  • Improved quality,
  • Faster production,
  • Wider processing
  • Window, dimensional
  • Accuracy and
  • Material saving.

    • MF / SHRINK

     

     

     

     

     

    MF / SHRINK

    (cont.)

    • Shrinkage analysis based on effects of processing and material data.
    • Predicts shrinkage variation across the mould and parallel / perpendicular to flow direction so that core / cavity dimensions can be refined to compensate for these variations.
    • Critical dimensions need not depend on critical adjustment of machine parameters.
    • This enables to get critical dimensions with in tolerance.
  • Shrinkage values in x, y, z axes
  • Out of dimension tolerances and confidence intervals,
  • Shrinkage variation across the parts
  • Error distribution for shrinkage allowances
  • Average shrinkage allowances
  • Dimensional accuracy report
  • Mould dimensions between any two points on part
    • Closer control of dimensional accuracy,
    • Reduced mould adjustment costs
    • Faster mould commissioning.

    MF / WARP

    • 3D - Warpage analysis enables to predict causes of warpage and optimise design and processing.
    • Calculated shrinkage is incorporated into structural analysis to calculate part warpage.
    • Core & cavity dimensions can be refined to get Fit for mating parts.
    • Linear Buckling, small displacement & large deflection under load can be predicted.
    • Volumetric shrinkage
    • Elemental parallel / perpendicular shrinkage
    • Elemental principal stresses and strains
    • Material orientation direction
    • Fiber orientation direction
    • Elemental Von-Mises stresses
    • Total deformation
    • Deformation and deflection in x, y, z axes
    • Deflection history at any node
    • Direction of principal strains
    • Deflected component shape with exaggeration factor
    • Buckling mode shape
    • Mechanical properties
    • Faster production,
    • Better dimensional stability,
    • Elimination of warpage in service

    MF / STRESS

    • Structural analysis linked to effects of plastic melt flow during injection moulding to mechanical properties.
    • Predicts moulded-in stress, warpage,
    • Predicts stress and deflections on load, warpage,
    • Calculates load required to cause buckling.
    • Examines the behavior of parts subject to load that may cause permanent deformation.
    • Predicts creep behavior
    • Calculates load required for buckling
  • Deflections in x, y, z directions
  • First and second principal stress / strains and directions
  • Load and constrains
  • Deformed shape,
  • Orthotropic moduli and tensile strength
  • Poissons ratio
  • Fiber orientation
  • Mode shape
  • Node deflection versus applied load
  • Non linear material displays
    • Reduced material usage,
    • Consistent structural performance,

    Fit for purpose parts.

     

     

    MF / COOL

    • 3D heat transfer analysis enables to predict optimum cooling time.
    • Optimises cooling circuit design to achieve uniform mould surface temperature with in minimum cycle time.
    • Predicts flow rate of coolant, size of cooling channels, positioning of cooling channels.
    • Contributes in reducing cycle time.

    For cavity

    • Cavity surface temperature distribution
    • Distribution of Temperature difference across opposite walls of cavity
    • Distribution of average plastic temperature at ejection time
    • Distribution of maximum plastic temperature at ejection time
    • Distribution of relative position of peak temperature at ejection time
    • Distribution of frozen layer thickness
    • Through thickness temperature profile for each cavity element

    For mould

    • Surface temperature distribution on top and bottom sides of inserts and parting planes
    • Distribution of temperature difference across insert and parting plane surfaces
    • Temperature of mould external surface of cooling circuit
    • Pressure drop along each cooling circuit
    • Flow rate in each cooling circuit
  • Improved mould design,
  • Consistent quality and
  • shorter production cycles.

    • MF / OPTIM

    • Sets optimum process conditions for a given machine, mould and material combination.
    • To keep melt front velocity constant it computes the optimum Injection speed and pressure profiles.

     

     

    		 
  • Reduces trial & error time for optimisation by manual method.
  • Optimisation Injection speed & pressure profile with out this software is very time consuming.

    • DESIGN STEPS

    PLASTICS PART & MOULD DESIGN

      1. Defining End-Use requirements & test procedures.

      2. Create preliminary sketch.

      3. Initial material selection from material data base..

      4. Design part in accordance with material selected. - Design for Functionality- using CAD software with surface modeling.

      5. Final material selection from material data base..

      6. Use CAE software to simulate meltflow, shrinkage analysis, warp analysis, stress analysis.

      7. Use results of CAE analysis and modify design from manufacturing point of view.

      8. Use results of these analysis to get optimised runner and gate size, placement of gates, placement flow leader / deflector to balance the flow with gradual pressure gradient while injection.

      9. Use CAD with database of standard mould plates and components of desired steel for mould design. Use results of earlier CAE analysis to get shrinkage compensated dimensions for core and cavity.

      10. Design mechanism for undercut, thread and or corepull if required by using CAD.

      11. CAE software to design cooling circuit to get uniform mould surface temperature. Obtain details for size and location of cooling channels and flow rate of coolant with entry and exit temperatures.

      12. Incorporate details of cooling circuits in mould design in CAD.

      13. Incorporate ejection system in mould design in CAD.

      14. Get printout of mould assembly and part drawings.

      .

      MOLDFLOW analysis can also help

      • to evaluate the performance of existing moulds with a given material on a given machine by generating optimised machine parameters. It can also detect the quality problems.

      • to modify - to the extent physically possible - the mould design so that performance of mould is improved in terms of quality and productivity.

      • to test the performance of moulded parts under load, if required.

      • to determine the right specification for the Injection moulding machine.

      • to determine the right material specification, may be you require a very special grade. This can be (proprietary grade) developed if material manufacturers co-operate.

      COMPUTER AIDED DESIGN

      CAD systems are available for about 20 years. There are three types of CAD systems:

       

      CAM - Computer Aided Manufacturing:

      Computer aided manufacturing is the automatic machining of parts by numerically controlled machine tools. CAM system can be integrated with CAD systems so that it can generate the tool paths automatically.

      SUMMARY

      The mould can be considered GOOD only if

      • MOULD is well designed by considering

      With the help of MOLDFLOW software it is possible to incorporate quality at part design stage itself. It identifies problems with part geometry and enables to find solution to the problem. It enables to perfect the part geometry and makes it 90% - 100% mouldable. With the help of MOLDFLOW we can carry out various types of analysis like Melt flow pattern, Fill time, Filling temperature, Filling pressure, Hold-on pressure, Volumetric shrinkage, Shrinkage all over the part, Temperature distribution along the mould surface and also across the wall thickness, Weld lines air traps, Deflection under stress. MOLDFLOW provides specific norms for each of these analysis to determine the acceptability of the results. The problematic results are to be corrected by proper interpretation of the results till the result becomes acceptable.

      It provides following useful parameters for mould design - which is carried out with CAD software:

      The perfection of moulding is thus, ensured during part and mould design itself. The quality is incorporated in the design itself. The key benefits of CAE software can be summarised below:

      SUMMARY OF BENEFITS

      • Reduced mould development time and cost
      • Decreased number of mould trials
      • Achieve faster mould start-up
      • Reduce material cost
      • Consistent part quality from multi-cavity moulds
      • Minimised injection pressure and clamp tonnage
      • Locate & Optimis gate size
      • Balanced & optimised runner system
      • Balanced & optimised wall thickness profile
      • Locate weld lines and air traps
      • Reduced cooling time
      • Reduced cycle time
      • Predict short shots
      • Identify hot spots
      • Predicts sink marks
      • Predicts linear shrinkage
      • Predicts part warpage
      • Identifies mechanisms contributing to warpage

      All this results in trouble free production and enhanced & uniform part quality. The perfect part development time, mould design and mould fabrication time can be drastically reduced with the help of CAD, CAE and CAM software. With this technology there is no need to produce prototype parts and it eliminates the delay on account of (at-least 2 or 3 ) mould trials and corrections (required before taking up for production )- when mould is made with conventional methods.

      MOLDFLOW analysis can also help to evaluate the performance of existing moulds with a given material on a given machine by generating optimised machine parameters. It can also detect the quality problems. It can also to determine the right specification for the Injection moulding machine and material specification

      Technical Papers by Prabodh C. Bolur

      Technological Solutions for Quality in Injection Moulding of Plastics

      Technological Tools for Part Design, Mould Design & Mould Fabrication.