Technical Papers by Prabodh C. Bolur

Technological Solutions for Quality in Injection Moulding of Plastics(1998)

Technological Tools for Part Design, Mould Design ∓mp; Mould Fabrication.(1999)

Understanding Selection of Injection Moulding Machine.(1980)This paper was part of authors lectures at CIPET since 1980. It has been regularly updated.

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

Continued from previous page.

DESIGN STEPS

PLASTICS PART ∓mp; MOULD DESIGN

• Defining End-Use requirements ∓mp; test procedures.
• Create preliminary sketch.
• Initial material selection from material data base..
• Design part in accordance with material selected. - Design for Functionality- using CAD software with surface modeling.
• Final material selection from material data base..
• Use CAE software to simulate meltflow, shrinkage analysis, warp analysis, stress analysis.
• Use results of CAE analysis and modify design from manufacturing point of view.
• 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.
• 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.
• Design mechanism for undercut, thread and or corepull if required by using CAD.
• 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.
• Incorporate details of cooling circuits in mould design in CAD.
• Incorporate ejection system in mould design in CAD.
• 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:

  • 2D SYSTEM is the simplest of all. It replaces the drawing board with a computer system. It can create engineering drawings. When drawing needs modification it can be carried out with out redrawing the entire drawing.
  • 3D INTERACTIVE GRAPHICS SYSTEM : This enables the designer to produce 3D assemblies. It has capability to ZOOM- IN on any details. It can also rotate the models to enable view the assemblies from different directions. Isometric views can be produced easily. Parts can be scaled and also duplicated easily. Colour graphics improves the clarity of assemblies. Different components, notes, dimensions can be put on different layers. Theses layers can be selectively presented with out loosing information's.
  • SOLID MODELERS : It uses basic 3D shapes like blocks, cylinders, cones, toroids, spheres and prisms, and 3D edges based on constructions made by rotating line and arcs. These are added or subtracted until the model is over. It can calculate area, volume, weight.

  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

  • PART is well designed from the consideration of
  • Functional needs
  • Service condition
  • Mechanical loading and duration of loading
  • Polymer melt behaviour ( flow, shrinkage, response to shearing )
  • MOULD is well designed by considering
  • Melt behaviour ( flow, shearing and shrinkage) in deciding type, size and location of gate, type, size and configuration of runner system, possible sink mark due wall thickness variation, warp due to differential shrinkage, possible weld line position and it's strength, size of core and cavity.

• Balanced heat exchange system ( design of cooling circuit) between heat source (melt) and heat out ( cooling media in cooling channel and environment) to achieve uniform temperature on mould surface. This establishes process stability.

• Suitable mechanism of mould to take care of parting line, undercuts, threads, core movements and ejection system.

• Most suitable steel of sufficient size for different parts of mould ( to avoid undersign).

• Mould parts are fabricated by suitable metal cutting / removal process and suitably heat treated, finished, matched and assembled perfectly. Importance of heater / limit switches assemblies are not ignored.

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:

• Optimised dimensions for runner and gate and also placement of gates.
• Optimised cooling channel dimensions, flow rate of coolant, positioning of channels.
• Optimised wall thickness profile for the part.
• Shrink corrected dimensions for core and cavity.
• Identifies warpage and it's causes which enables the designer to remove or minimise cause for warpage.
• Optimised process parameters.
• Quick set up / start-up with out wastage.
• Zero defect parts possible with the first trial of new mould.
• Under service condition of load and temperature, the creep performance of part can be tested.

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

All this results in trouble free production and enhanced ∓mp; 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

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Technical Papers by Prabodh C. Bolur

Technological Tools for Part Design, Mould Design ∓mp; Mould Fabrication.(1999)

Understanding Selection of Injection Moulding Machine.This paper was part of authors lectures at CIPET since 1980. It has been regularly updated.

Technological Solutions for Quality in Injection Moulding of Plastics(1998)