Technical Papers by Prabodh C. Bolur

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

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

TECHNOLOGICAL TOOLS FOR
PART DESIGN, MOULD DESIGN ∓mp; FABRICATION OF MOULD

- Prabodh C. Bolur , B.SC.,B.E.,MIE

Introduction

Technology means scientific study of any subject. It also means new or improved methodology of carrying out any activity with improved quality and productivity by application of scientific and engineering principles.

There is no room for miracle or magic in technology. Each and every phenomenon has its scientific reasons. You have to understand these reasons if you want to master the technology. You must have good understanding of basics of Physics and engineering.

Injection moulding of plastics involves Polymers, Mould, machine, material, handling, part handling etc. Technology of polymerisation to produce Plastics granules is the subjects of Chemical Engineers. Injection moulders have to understand certain characteristics of polymer like response to shear ,stress and temperature, viscosity as function of shear and temperature and pvT characteristics.

Injection moulding process need not be a mysterious process. You can develop skill to visualise what is happening inside screw barrel and mould during the entire injection moulding process provided you understand the following basics of injection process.

• Moulding cycle (Cavity pressure profile),
• pvT diagram,
• Shear thinning of plastic,
• Flow mechanism -i.e. Simultaneous Flow and Freeze of melt with formation and fountain flow,

You must also understand that root cause of most of the quality problems lies with three balancing tricks in the process.

• Flow balance,
• Heat balance,
• Uniform freezing,

Reasons for failures

RAPRA Technology Ltd. has developed series of software under Knowledge Based System. This software takes in to account the effect on properties of polymer under working condition of moulding. During the study of failed plastic parts (over 5000 cases) it was discovered that over 40% of failures on account of poor part design and another 40% of failures were ofn account of poor selection of material or grade of material. Remaining 15-20% failures were due to other reasons like processing condition and poor specification of the machine selection for moulding.

Numerous failures of plastic parts were examined and it was found that environmental Stress Cracking was the major cause of failure in majority of cases. Failure due to stress cracking can occur in the presence of aggressive chemical in service environment and moulded-in stress are present in the moulded part. If one of them is removed from the scene, then the stress crack does not occur. Therefore, it is necessary to remove the moulded in stressfrom the moulded plastic part. The moulded in stress or residual stressin a moulded part can be due to followingthree reasons.

• Unbalance flow causing overpacked regions in the part,
• Non uniform freezing of plastic melt during the filling phase of moulding cycle,
• Overpacking during folloe up pressure phase of moulding cycle,

We can take care of moulded- in stress while evaluating the design of the part with the help of CAE. However, the properties of the polymers are effected under certain working condition, such cases can be detected with the help of software called SENSAN (Sensitivity analysis) at the design stage itself.

Normally plastic parts are not for load bearing application. However, for example, plastic chair is a very good load bearing application of plastic. Here, the understanding of fatigue and creep behabviour of plastic is relevant. Please note that fatigue and creep behaviour of plastic are more complex than that of metals. SENSAN provides useful information about material for certain environment (chemical ∓mp; temperature) at design stage of the part.

SENSAN is available with

• PLASCAMS- material database designed to help selection of material based on its properties.
• Rover Electronic Data book, - Chemical resistance data sets.
• WINSNAP - a computer tool for design of snap-fit elements in the assembly of plastic parts.

STEPS FOR PART ∓mp; MOULD DESIGN

PLASTICS PART ∓mp; MOULD DESIGN

• Defining End-Use requirements ∓mp; test procedures.
• Create preliminary solid model with CAD.
• Initial material selection from material data base. Material database software SENSAN ∓mp; PLASCAM are available.
• Design part in accordance with material selected. - Design for Functionality- using CAD/CAE 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 computer aided mould design module CAMD 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 corpull if required by using CAD.
• Use 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, part drawings and bill of material.

TECHNOLOGICAL ADVANCEMENT IN THE MOULD MAKING

Great Technological improvement has taken place in Part design, Mould design and Mould fabrication during last 15-20 years mainly due to development of microprocessor and software technology. This technology (CAE - Computer Aided Engineering) is capable to producing total error free part design and mould design because it takes care of constrains of material and the process. Obviously the software is designed by using scientific and engineering principles and advanced mathematics. It is not based on intuition or guess or hunch. Therefore, mould making is now less of an art and it has become an applied science.

DIFFICULTIES AND BENEFITS OF CAE AND CAD TECHNOLOGY

In fact total quality and productivity of injection moulding depend more on perfection of part design and mould design.

• Layman's perception of injection moulding process is not enough to fully appreciate this technology. This technology demands good understanding of fundamentals of physics, injection moulding process and computer skill in handling CAE and CAD. It may be difficult to combine these skills in one persons to begin with. It is knowledge-centered technology. Therefore, it also demands teamwork and new work culture.

• It requires higher investment and higher salaried knowledgeable personnel.

• Design time is reduced to a great extent. Perfection is achieved in the first trial itself. Therefore higher investment is compensated by perfection in the first trial and further trials are not necessary.

• Standard pre-machined to size plates and guide pins are used to reduce the mould fabrication time.

What is CAE?

This is 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. It also presents variation of temperature, shear and stress over part surface and across thickness.

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

• Structural analysis : It gives information on structural deficiencies like moulded in stress.

There can be analysis for shrinkage, warpage, etc.

POWERFUL TOOL FOR MOULDER ∓mp; LEARNING ENVIRONMENT

CAE soft ware is a powerful tool for achieving perfection in Part design, Mould design and even for determining the optimum cycle time of the newly designed mould as well as existing mould. Therefore it is a most for all the mould makers, if they want to survive in their business in future. It reduces drastically the design time and enables you to get perfect moulding during first trial itself.

It is also very useful to moulder for getting the data of optimised processing for a given mould. It also enables you to know the shortcomings in the existing mould. That prompts you to take possible corrective improvements in the mould for improving the quality as well as productivity. If the shortcommings are impossible to correct on a given mould , then you do not waste time and money on correcting the mould by trial and error. In short, you will be able to assess the capability and performance of your existing mould. This powerful tool would save you from wasting time and money on irrational developmental as well as corrective activities.

In addition to all these, it provides learning environment for the users of software to master the moulding process in very precise engineering terms.

USAGE OF CAE

You will have to provide (input) the following information:

• Melt density,
• Heat capacity,
• Thermal conductivity,
• Viscosity of melts as a function of shear rate, temperature and pressure as determined by capillary rheometer.

(These details are available in the data bank of standard material of various manufacturers.)

• Temperature of mould,
• Cooling channels details, hot runner details of mould,
• Sprue brush details, hot runner details of mould,
• Part runner, gate details of mould,
• Time to fill, pack, cycle time,
• Maximum pressure at screw tip of machine,

You get the following output from the analysis:

• Pressure, temperature distribution in cavities and runners,
• Flow front advance, weld line location, air trap,
• Flow rate at any point and at any time,
• Freezing rate,
• Velocity profile,
• Velocity directions,
• Minimum and maximum clamp force during fill,
• Shear stress,
• Shear rate,

These results are shown as colour shaded pictures, colour plots, tables of data or wire form diagrams. There are set norms for acceptability of the result of these analyses. If the result of the analysis is with in the set norms then the part design becomes acceptable. Otherwise the designer has to modify the part design to overcome the constrains visible in the analysis. This way this analysis is iterative in nature.

Is it not an expensive showpiece?

The design of mechanical part involves quite accurate calculation 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 behavior under load condition is 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 involve in DEVELOPMENT of trouble - free moulds.

Plastic moulders are called up on to develop automobile parts on the basis of sketches at a very short notice. CAE can be a very useful tool as it can predict the quality problems during shaping up of part itself. CAE also ensures precision of part. It reduces the development time for moulds.

CAE technology is also referred as PREDICTION TECHNOLOGY. While developing the plastic parts, many unforeseen problems can occur that will have a major influence on delivery. These problems can be foreseen and eliminated by having an accurate predictive ability during the product development. The rectification of the problem can be swift and low cost. By integrating prediction into the design process in the early stages, most potential problems can be eliminated in virtual environment - created in computer -, with out any time to waste resources in a factory environment. The predictive ability of CAE enables elimination of problem at subsequence stages. In other words, CAE predicts the manufacturability of the part design and helps in eliminating the constrains of moulding the parts.

The form, fit, function and cost of plastic components can now be optimised, in terms of part weight, part strength and manufacturing cost.

This CAE software must be expensive. We may not be able to afford its high cost.

If it allows you to perfect your mould and production, improves quality with zero defect, then your initial expenses would be recovered quite soon. The cost of CAE is the one time investment for QUALITY and PRODUCTIVITY. With this you will be spreading scientific work culture in your organization which is desirable.

You will be able to run production automatically with out operator and may be with robot. Since quality is in built in part design and mould design, there may not be any need for separate quality supervisors. There id no need for post moulding corrections on mouldings. The production supervisors need not have technical background but he should be more a statistician and a good organizer of the shifts.

You will realise that with the employment of few people of good technical caliber and less number of unskilled personnel you will be able to improve the efficiency of your operations. The cost of CAE will definitely be justified when overall productivity, quality of production and operation is drastically improved.

MOLDFLOW ∓mp; C-MOLD are CAE software. They are available in modules. You need not buy all modules at one time. You may buy every year some modules and build up your R∓mp;D capabilities. In two or three years you might have mastered the technology.

CAE helps you to master the moulding process in greater depth. It deals with the quantified process prameters - direct as well as indirect. Only CAE software can make you expert in plastic moulding technology.

With the help CAE software it is possible to incorporating 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.

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

• Optimised dimensions for runner and gate and also placement of gates.
• Optimised cooling channel dimensions, flow rate of coolant, positioning of channels.
• Shrink corrected dimensions for core and cavity.
• Identifies warpage and its causes, which enables the designer to remove or minimise cause for warpage.
• Optimised process parameters ∓mp; reduced cycle time.
• Quick set up/start-up with out wastage.
• Zero defect parts possible with the first trial of new mould-reduced mould development time.

Now it is possible to evaluate the performance (process parameter for a combination of mould, machine and material) of existing mould and find out whether it is possible to improve. We can even know what can be best possible cycle time for a given set up. With this knowledge we need not waste time for doing impossible things on the given set of mould, machine and material.

COMPUTER AIDED DESIGN AND COMPUTER AIDED MANUFACTURING

CAD system 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 scaled and also duplicated easily. Colour graphics improves the clarityof assemblies. Different components, notes, dimensions can be put on different layers. These layers can be selectively presented with out loosing information.

SOLID MODELERS : It uses basic 3D shapes like blocks, cylinders, cones, toroids, spheres and prisms, and 3D edge based on construction madeby rotating line and arcs. These are added or subtracted until the model is over. It can calculate area, volume, and weight.

Well - known CAD system are Pro-Engineer, Unigraphics, Ideas, Cimatron, Solid Works, Solid Edge , Delcams PowerShape, Mechanical Desktop,.....

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 systm so that it can generate the tool paths automatically.

CAM software creates roughing and finishing toolpaths to optimise the productivity of CNC machine tools while at the same time, ensuring the highest quality matching of models and tooling.

CAM software offers high speed calculation, powerful integrated visualisation and verification facilities which allowthe user to compares alternative strategies with any combination of cutting tools and check all toolpaths before they are sent to the machine. As a result, machine idle time and wasted materials and resources are eliminated.

Well-known CAM software are Unigraphics, Cimatron, Delcams PowerMill, Command, Mastercam,.....

Actually it is imposible to imagine a good tool room with out such marvelous technological tools.

Global trend in development of Plastic part and Mould

Today, in U.S.A, Europe Canada and Japan, more and moe plastic parts design and mould design are carried out in solid modeling software. Solid modelling has replaced drawing board and usage of 2D sofeware.

When the part is designed in Solid, the same model can be used for CAE analysis, mould design and for CAM. This intergrated approach - of automated design for Solid modeling, Mould design and CAM.

CAE analysis software (Mould flow ∓mp; C-mould) Provides useful design parameters of part as well as mould in order to ensure perfect mouldability (manufacturability )of plastic parts. This analysis is carrired out after solid modeling and before design.

Speed of designing and time saving is because of parametric and associative nature of the solid modelers. They are some kind of in-built intelligence therefore, changes made to one elementof the mould design update the dimentions and locations ofall the related elements. For exeample, resizeing of a mould plate will also update and size and locations of all the holes bushings and other features assosiated with that plate.Changes also carry through to bill of materials and productions drawings providing greater time saving .

The mould design software is not a stand - alone package but is run within the specific CAD system.

Mould Design (CAMD) modules available are;

• Mold Expert, Q-Split ∓mp; MoldBase 3D with Cimatron,
• MoldCreator with Mechanical Desktop,
• PS-Draft with Power Shape,
• MoldMaker with Euclid,
• MoldWorks with Solid Works.
• XMD with Cadkey,
• VISI-Mold with VISI-CAD,
• TopMold with TopSolid,
• MoldWizard with UG/Designer.
• Pro/Moldesign with Pro/Engineer.

Mould design system have a great time saving feature in automatic extraction core and cavity from a part model. It produces parting lines and parting surfaces. A single value shrinkage obtained from CAE analysis is applied to core and cavity dimensions to get the perfection at the moulding stage. It easily generate detailed assembly drawings, sectional drawing and automatically produces the bill of materials which is linked to the database of few leading manufacturers of mould bases and mould components like leader pins, screws, guide pins, ejectors, support pillars, sprue bushes, etc., (DME-Hasco, etc.). Non-standard components can also be added by the users in the data bank.

CAM (Cimatron's NC Machining ) is designed to maximise productivity. It works on solid model data and generates automatically precise tool-paths for any complex shape for 2.5 to 5 axis milling, drilling, turning and wire EDM. It provides control over creation, modification and display of all NC operations. It also machinist to operate and generate tool path on the shop floor. Verifier performs QA checks on entire machining process.

How does the Mould Design software expedite the work?

1. After creating or importing a 3D-part model, you create core and cavities around that model, using automated routines. Shrinkage values obtained from CAE analysis is applied to compensate for the shrinkage in the ultimate mouldings.
2. There after you pick a mould base size in which core ∓mp; cavity insert fit. In fact, in Cimatron it recommends the suitable size of plate automatically.
3. Through a dialog / menu interface, you add all the tool's functional components - from gates, runners, ejection, cooling,(all from CAE system) and slides right down to individual screws, pins and bushes. As these components are added, the mold base software also creates the associated holes and features for those components, keeping track all the while of relation between features.
4. The mould base package creates four - view production drawings and an automatic bill of materials. These programs also set the stage for CNC code generation.

Experienced users of this software claim that there is in all 25 to 75% time saving.

How does CAM software work?

CAM-(Cimatron) id used for programming the solid model created earlier. After the completion of programming, it is sent to the CNC machine's computer located on the shop floor adjacent to CNC machine. CNC is computer Numerical Control. CNC machine Center picks up the programmed cutter tool and removes the metal from the job.

It is programmed to cut one region on the job at a time. It roughs out the edges, preparing them for semi-finishing stage. CNC machine continues to cut each region one after another till the whole job is finished.

During the next stage, the job is smoothened, but not entirely. At the last stage the job is completed with all the finest details and is made ready for assembly. Ball nose cutters with carbide tool insert, give good finish.

TOW motor picks up the job and places it on the CNC machine. Then x, y, z axis are picked out to match the design of the programmed shape. The machine picks up the required tool and performs the machining operation as programmed.

ASSEMBLY and TESTING of MOULD

Assembly of mould becomes quite easy as parts produced are very precise and standard mould bases and other components fit precisely with out any rework for fitment. Since mouldability is taken care of during part design, you will find that the mould produces good parts in the first trial itself.

It can be observed that by using new technologies7890- , you can achieve:

• Faster design and drafting through CAD and CAMD software.
• Good selection of most suitable material and grade of material for the defined service condition of plastic part, by using SENSAN software.
• Mouldability and precision through CAE software.
• Precision and perfection in manufacturing through CAM software.
• Ease of assembly of mould because of accuracy and precision of manufactured parts of mould.

There is some hesitation in absorbing the new technologies in our working system. This is because the new technologies are based on knowledge of physics and engineering and not based on hunch or intuition. These new technologies provide a learning environment and hence it is possible to absorb them by the people, who have good understanding of basics of physics and engineering.

In the design office, the draft-man's job has become easy, precise and much faster. Therefore, this job has been integrated with the job of design engineers. At the shop floor, the manual skill of the machinist is replaced by the programming skill of the CAM programmer. Experienced machinist can provide the strategy for machining while programming. CNC machines provide very good repeatability, whereas it is not easy to get good repeatability by manual skill. However, the experienced machinist can provide very valuable inputs to the machining strategy for CAM programming.

Previous Article / Next USEFULLINKS

Technical Papers by Prabodh C. Bolur

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)

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