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 have good understanding of basics of physics and engineering.

Injection moulding of plastics involves polymers, mould, machine, material, material handling, part handling etc. technology of polymerization to produce plastics granules is the subject of chemical engineers. Injection moulders have to understand certain characteristics of polymers 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 the skill to visualize 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 plastics,
	Flow mechanism, i.e. Simultaneous Flow and Freeze of melt with skin formation and foundation 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 of plastic parts

Rapra Technology Ltd. has developed series of software under Knowledge Based Systems. This software takes into 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 on 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 selected 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 when the presence of aggressive chemicals in the service environment and moulded-in stress are to be found 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 plastic part. The moulded-in stress or residual stress in a moulded part can be present due to following three reasons.

	Unbalanced flow causing overpacked regions in the part.
	Non uniform freezing of plastic melt during the filling phase of moulding cycle.
	Overpacking during following 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 used for load bearing application. However, for example, the plastic chair is a very good load bearing application of plastic. Here, the understanding of fatigue and creep behaviour of plastic is relevant. Please note that fatigue and creep behaviour of plastics are more complex than that of metals. SENSAN provides useful information about material for certain environments (chemical & temperature) at the 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 books, Chemical resistance data sets.

WINSNAP-a computer tool for design of snap-fit elements used in the assembly of plastic parts.**

Steps for part and mould design

	Defining end-use requirements and test procedures.
	Create preliminary solid model with CAD. IGES files to be provided for CAE analysis.
	Initial material selection in from material database. Material data base software SENSAN and 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 database. (SENSAN & PLASCAM)
	Use CAE software to simulate melt flow, shrinkage analysis, wrap analysis, stress analysis.
	Use results of CAE analysis and modify design from manufacturing (mouldability) point of view. Update the solid model for CAM program.
	Use results of these analyses 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 (CAMD) Software 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 care and cavity.
	Design mechanism for under cut, thread and or core-pull 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 CAMD.
	Incorporate ejection system in mould design in CAMD.
	Get printout of mould assembly and part drawings.
	Use CAM software to program CNC operations to produce core and cavity blocks.
	Assemble and test the mould.

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 constraints 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 in now less of an art and it has become an applied science.

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

	Superficial knowledge 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 man to begin with. It is knowledge-centered technology. Therefore, it also demands team work 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 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.

It 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 plastics 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 stresses.

There can be analysis for shrinkage, warpage, etc.

CAE software 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 must for all the mould makers, if they want to survive in their business in future. It reduces the design time drastically and enables you to get perfect moulding during trials itself.

It is also very useful to the moulder for getting the data of optimized processing for a given mould. It is also enables you to know the shortcomings in the existing mould. This prompts you to take possible corrective improvements in the mould for improving the quality as well as productivity. If the shortcomings 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 a learning environment for the users of software to master the moulding process in very precise engineering terms. It enables you to learn more details of the process in an iterative manner. It provides a learning environment for you and you will be thrilled to discover the causes of many mysteries of moulding. You will be delighted to know more and more complexities of the process and you will be wondering how you thought in the past that moulding is the simplest of all process.

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 an pressure as determined by capillary rheometer.

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

	Temperature of mould.
	Cooling channels details of mould.
	Sprue bush 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 form the analysis:

	Pressure, temperature distribution in cavities and runners.
	Flow front advance, weld line location, air trap.
	Flow rate at any point and at anytime.
	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, color pots, tables of data or wire form diagrams. There are set norms for acceptability of the result of these analysis. If the result of the analysis is within the set norms, then the part design becomes acceptable. Otherwise the designer has to modify the part design to overcome the constraints visible in the analysis. This way the analysis is iterative in nature.

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 the thumb rule prevails while designing plastics parts. Dimensional stability of plastic parts and creep behaviour under load condition is quite complicated. They cannot 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 trouble free moulds.

Plastics moulders are called upon 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 the part itself. CAE also ensures precision of the 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 a virtual environment-created in the computer – without any need to waste resources in a factory environment. The predictive ability of CAE enables elimination of the problem at subsequent stages. In other words, CAE predicts the manufacturability of the part design and helps in eliminating the constraints of moulding the parts.

The form, fit function and cost of plastic components can now be optimized, in terms of part weight, part strength and manufacturing 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 without an operator and may be with a robot. Since quality is inbuilt in part design and mould, there may not be any need for separate quality supervisors. There is no need for post moulding corrections on mouldings. The production supervisor need not have a technical background, but he should be more a statistician and a good organizer of the shifts.

You will realize 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 efficiency of operation is drastically improved.

Mouldflow & C-Mould are CAE software. They are available in modules. You need not buy all the modules at one time. You may buy every year some modules and build up your R&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 parameters-direct as well as indirect. Only CAE software can make you expert in plastic moulding technology.

With the help of CAE software, it is possible to incorporate quality at the part design stage itself. It identifies problems with part geometry and enables to find a solution to the problem. It enables to perfect the part geometry and makes it 90%-100% mouldable.

It provides the 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 parameter and reduced cycle time.
	Quick set up/start-up without 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 an existing mould and find out whether it is possible to improve. We can even know what can be the best 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.

CAD systems have been available for about 20 years now. 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 without 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 scaled and also duplicated easily. Colour graphics improves the clarity of assemblies. Different components, notes, dimensions can be put on different layers. These layers can be selectively presented without losing information.
	Solid modellers: It uses basic 3D shapes like blocks, cylinders, canes, 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, and weight.

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

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.

CAM software creates roughing and finishing toolpaths to optimsie 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 allow the user to compare 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, Cilmatron, Declams Power/Mill, Command, Mastercam etc.

Actually, is it possible to imagine a good tool room without such a marvelous technological tool?

Today in USA, Europe, Canada and Japan, more and more plastics parts design and mould design are carried out in solid modelling software. Solid modelling has replaced the drawing board and the usage of 2D software.

When the part is designed in solid, the same model can be used for CAE analysis, mould design and for CAM. This integrated approach – of automated design to manufacturing - saves lot of time. All the leading software offers integrated modules for solid modelling, mould design and CAM.

CAE analysis software (Mouldflow and C-Mold) provides useful design parameters of part as well as mould in order to ensure perfect mouldability (manufacturability) of plastic parts. This analysis is carried out after solid modelling and before mould design.

Speed of designing and time saving is because of parametric and associative nature of the solid modellers. They have same kind of in-built intelligence. Therefore, changes made to one element of the mould design update the dimensions and locations of all the related elements. For example, resizing of a mould plate will also update the size and location of all the holes, bushings, and other features associated with that plate. Changes also carry through to the bill of materials and production drawings, providing greater time saving.