Technological Solutions for Quality in Injection Moulding of Plastics by Prabodh C. Bolur-
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A Guide to Injection Moulding of Plastics

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Preface
Contents
Summary

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Selection of IM Machine

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Continued from previous page.

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

  • Unbalanced melt flow causing overpacked regions,

  • Unbalanced heat exchange in mould resulting in erratic quality problems related to dimensional accuracy and warpage.

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

  • Functional requirement and strength of material, creep behaviour and other characteristics of material.

  • Flow behaviour of plastic melt while flowing and freezing resulting in moulded-in stress and weld lines which causes failure of part in service.

  • Shrinkage behaviour during processing and thereafter resulting in warpage and dimensional variation and dimensional inconsistency in moulded part.

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

  • balanced and uniform melt flow by selecting right size of runner system and type / number of gates, Balancing melt flow by flow leader / flow deflector.

  • uniform mould surface temperature to take care of warpage and non-uniform shrinkage. This has to be done by designing cooling system so that heat flowing in and flowing out of mould, - after the desired mould surface temperature is reached, - is balanced.

  • adequate ejection mechanism.

  • more than adequate thickness of mould plates. Cost increase on account of increase in mould plate thickness is negligible. This ensures safety of mould and improved life of mould.

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:

  • Flow analysis : Filling analysis- to determine the extent of unbalance in melt flow, so that part geometry can be modified till the melt flow is at least 90% ( if not 100%) balanced.

  • Cooling analysis : It enables to design the cooling circuits which will give uniform temperature all around the mould surface.

  • Structural analysis : It structural deficiencies like moulded-in stresses.

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.
    • Previous page / Next page

    Technical Papers by Prabodh C. Bolur

    Understanding Energy Consumption in Injection Moulding Machine(1994)
    Understanding Heat Exchange in Injection Moulds (1994)
    Ideal Moulding Shop (2001)
    Understanding Selection of Injection Moulding MachineThis paper was part of authors lectures at CIPET since 1980. It has been regularly updated.
    Technological Solution to Injection Moulding of Plastics (1999)
    Technological Tools for Part Design, Mould Design and Mould Fabrication (1999)
    Extrusion of Thermoplastics(1998)
    Archives.

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