Simulation outperforms trial and error

In the process of determining the costs and the selection of a materialand a machine for a moulded part, the trial-and-error or empirical approachis still prevalent today. Yet more and more, moulders are using softwaretools for this purpose, which provide more rapid and effective assistancein answering the typical questions posed by injection moulders.

Which is the most suitable material? Attempts are obviously made to savematerial and design the part to give it the thinnest possible walls. However,it must be kept in mind that different groups of materials will permitdifferent minimum wall thicknesses to be achieved.

What type of injection moulding machine is required? Thinner walls requirea higher injection pressure, since the flow channels become smaller. Andunder certain conditions, this can necessitate the selection of a biggerinjection moulding machine.

How long are the cycle times? Thinner walls have a positive impact on cycletimes, since the residual cooling times are shorter.

Material, of course, needs to be adapted to the flowability required forthe thinner-walled geometry. Such considerations as these accumulate andassume the proportions of a complex set of underlying questions. Softwaretools to simulate the injection moulding process and to perform cost analysishelp find a rapid solution here.

From CAD to Costing

The possibilities opened up by simulation of the injection moulding processand by costing are illustrated by the design of a 10-liter household bucket.The bucket, which is shown in Figure 1, was designed in PT/Modeler (ParametricTechnology Corp., Waltham, Massachusetts, USA) and produced by Tetra GmbH,Herzogenrath, Germany. A stereolithography file of the geometry was usedfor the simulation.

TABLE I.
Characteristic Values
of the Materials Used

Polypropylene and polyethylene represent suitable materials for this typeof household commodity, so a comparison was conducted on the basis of thesetwo materials. The characteristic data of PP and PE (Table I) have beentaken from the Campus 4.0 database. The strength of PP is approximately17 percent higher than that of PE. In this case, the density of the PPis approximately 6.8 percent lower than that of the PE. To achieve thesame level of strength in the moulded part, the wall thickness of the PPbucket would need to be designed at 1.8 mm throughout and that of the PEbucket at 2.1 mm throughout. The gate is positioned in the center of thebase. The bucket is designed to withstand hot water at 80 C.

TABLE II.
Process Parameters

The cycle time is a crucial parameter when it comes to costing. The temperaturesin Table II have been selected in such a way that the shortest possibleresidual cooling times result. A lower melt temperature can be selectedfor PP than for PE.

With the Cadmould-Rapidmesh program, the filling time (Figure 2) and fillingpressure (Figure 3) were worked out from the stereolithography file ofthe bucket in a matter of seconds, with the filling pressure being calculatedon the basis of structural viscosity and temperature dependence.

The material, wage, and machine and mould costs all go into the costingof the moulded part. Costing is similarly based on the following assumptions:a production figure of one million items per year, with material costsof DM 2/kg (US$ 1.13) for PE and DM 2.2/kg (US$ 1.24) for PP. The mouldcosts are the same for both moulds. Identical setup times are assumed forthe machines and moulds. Identical personnel numbers for setup and productionare assumed as well.

TABLE III.
Simulation and costing
results

Costing was performed with Simcon's cost calculation software, which runsunder Windows 3.1 and later versions. The cost calculation program canbe used to facilitate the compilation of quotations, to establish the costof optimizations in the injection moulding field, and to conduct a rapidcheck on incoming offers. All the data of relevance to costing are storedin databases (material, mould, machine, and wage databases). Table IIIsummarizes results obtained from the simulation and the calculation.

Despite the higher material costs and the higher hourly rate for the machine,the PP bucket works out to be less expensive than the PE bucket even beforeit has been optimized in flow engineering terms. What then might an optimizationhold in store? Once again, simulation is seen to outperform trial and error,and precise costing is seen to be better than making a rough guess. Furthermore,the cost savings achieved with this example would easily have paid forall the tools presented here, plus a new PC.

Contact Information:
Simcon Kunststofftechnische
Software GmbH
Mr. Hendrik Genoske
Kaiserstrasse 100
D-52134 Herzogenrath
Germany
Tel: +49 (2407) 5088
Fax: +49 (2407) 59453
e-mail: [email protected]