Moldflow

Written by IR Luc Janssen

Mareco Kunststoffen B.V. uses the most comprehensive and latest ‘Moldflow Plastics Insight’ finite element package to simulate the complex behavior of injection molded polymer melts. Analysis results are professionally interpreted using the years of experience and knowledge of our moldshop and injection molding department, and are used to predict and eliminate potential manufacturing problems before they even occur. In this way, excessive costs and time delays associated with manufacturing problems are avoided.

Analysis modules available:

Fusion
Flow (Filling and Packing)
Fiber
Cooling
Warpage
Optimization
Design of Experiments

By using the fusion (i.e. Dual Domain) capability, we can perform detailed analyses directly on thin-walled, surface models imported from a CAD package. After applying the boundary- and process conditions, Mareco can simulate the Filling, Packing and Cooling stages of the injection molding process and predict post-molding phenomena such as part Warpage. Using the Fiber module, we can also analyse the flow of fiber filled thermoplastics and isolate its influence on the part warpage.

The Optimization analysis automatically determines the optimum velocity and packing pressure profiles, based on the material data, mold geometry, and molding machine characteristics. Design Of Experiments (DOE) makes it possible for us to automatically setup and perform a sequence of analysis, varying specified parameters of the injection molding process. The results are statistically analysed to optimize the processing parameters, ultimately resulting in optimized part quality and cycle-time.

FLOW SIMULATIONS

A flow analysis simulates the filling and packing phases of the injection molding process to predict the flow behavior of thermoplastic melts within the part.A filling analysis calculates the flow front position from the start of injection to when the cavity is filled and gives a prediction of the filling behavior for a part with a given plastic material and a set of process conditions. Additional packing profiles are used to control the volumetric shrinkage distribution and to avoid overpacking.

Figures 1-A to 1-F show a visualization of the flow front propagation through the part as a function of injection time. Click on the image to view larger size. To view the fill-time animation (gif-file): click here


1-A: fill-time=0.28 sec	1-B: fill-time=0.37 sec	1-C: fill-time=0.46 sec

1-D: fill-time=0.56 sec	1-E: fill-time=0.65 sec	1-F: fill-time=0.71 sec

Analysis results are interpreted by our skilled people, using the many years of experience in mold design, combined with our expertise in mold manufacturing and injection moulding.

When used in the earliest stage of design, we are able to provide designers and engineers with the necessary insights and knowledge of how molten polymer propagates through the part cavity. By suggesting part/mold design modifications, in combination with material choice and processing conditions, we can assist you in constructing a part/mold which is fully optimized for the injection molding process.>This ultimately results in reduced tool lead-times, optimized cycle-times and reduced costs

Overview of FLOW simulation capabilities:

determine the optimal gate location(s)
optimized runner system design and balancing
predicts and visualizes the flowfront propagation
determines processing conditions such as;
- fill-time
- clamping force
- melt temperature
- gate freeze time
- packing time and -pressure
predicts weld line locations (manually controlled placement)
identifies potential air traps (mold venting locations)
determine maximum shear stress, shear-rate and pressure during filling
fiber or molecular orientation at core and skin (see section xx)
evaluate flow analysis results for different plastic materials and initial processing conditions

Figure 2: Clamping force versus injection time

FIBER ORIENTATION

The modeling of fiber-reinforced thermoplastics (composites) is more complex than in other applications, since the parts are usually thin and shorter fibers are often used.Other aspects, such as the 3-dimensional orientation of the fibers and the significant orientation variations across the part also contribute to the complexity of the problem.

Figure 3:

In injection-molded composites, the fiber alignment (or orientation) distributions show a layered nature, and are affected by:

filling speed
processing conditions
material behavior
fiber aspect ratio (ratio of the length of the fiber to the diameter)
fiber concentration (weight percentage)

Without proper consideration of the fiber behavior, there is a tendency to significantly overestimate the orientation levels. The results from a fiber orientation analysis can be used as input for a warpage analysis, providing more detailed elemental results, and considerably enhanced analysis accuracy.

COOLING SIMULATIONS

Cooling simulations are heat transfer analysis designed to analyze temperatures and heatflows in the entire inection-mold. The main results of a cooling analysis are temperature distributions in the plastic part and throughout the injection-mold, cooling time, and cooling circuit flow parameters (such as pressure or flow rate requirements).

Assuming that the filling and packing phases (see section 2) have been optimized, improving the cooling system performance can substantially reduce the cooling time (in most cases the cooling time comprimises for about 80% of the total cycle time so any reduction reduces the cycle time and ultimately the part-costs).

In general, the mold cooling system performance is affected by four items:


1)	Heat transfer from the plastic part to the mold cavity surface.
	This aspect is mainly influenced by:
	-	Material properties of melt and mold (specific heat, thermal conductivity)
	-	Temperature difference between melt and mold cavity wall
	-	Contact quality between plastic and mold wall

2)	Heat transfer through mold (from plastic/metal interface to the metal/coolant interface).

3)	Heat transfer through mold to coolant channels.
	This aspect is mainly influenced by:
	-	material properties of mold and coolant (specific heat, thermal conductivity)
	-	Reynolds number of coolant flow(i.e. laminar or turbulent flow)
	-	coolant inlet temperature
	-	coolant flow rate (influences convection between mold wall and coolant)

)	Cooling circuit layout and location.

In order to determine the efficiency of the designed mold cooling system, the analysis results are analysed using the extensive experience and knowledge of our injection molding department. In this way, potential cooling problems (ultimately resulting in reduced part quality and/or increased cycle-time) can be avoided and the cycle time can be optimized (reduced costs).All this can be done before the mold is even made !

WARPAGE SIMULATIONS

Warpage in an injection molded part is caused by shrinkage variations throughout the part.

Shrinkage of plastic part is driven by the volumetric change of material as it cools down from the melt state to solid. It must be noted that shrinkage itself does not cause warpage. A high uniform shrinkage will give a perfectly shaped part that is simply smaller in size. However, if regions of the part shrink unequally, stresses are created within the part. D epending on the part stiffness, this may cause the part to deform (i.e. warpage) or even crack in the long term.

The fundamental causes of shrinkage variations (and consequently warpage) are:


1)	Differential Cooling
	Non-uniform cooling causes shrinkage variations through the thickness of a part. Aspects which influence the shrinkage variations due to differential cooling are:
	-	thermal properties of the mold
	-	variations in wall thickness' of the part
	-	uneven temperature across the mold

2)	Differential shrinkage
	This results in shrinkage variation across the part surface (i.e. parallel and perpendicular to the flow). Differential shrinkage can be caused by:
	-	wall thickness variations
	-	gate location
	-	cooling line design and –parameters
	-	processing conditions (for example packing profile)

)	Material orientation effects
	This results in different amounts of shrinkage parallel and perpendicular to the flow. Shrinkage due to orientation effects can be caused by:
	-	polymer orientation (molecular or fiber)
	-	gate position(s)
	-	cooling line design and –parameters
	-	processing conditions

Using Moldflow, we are able to isolate each individual cause of shrinkage variations (see list above) and determine its effect and contribution on part warpage. By identifying the primary cause of warpage, we can suggest specific part- or mold design modifications, or optimize processing parameters (for example modified packing profile).

The part- and mold designer can improve the dimensional stability of a product by considering the overall contributers of part warpage. These contributers are devided into the following (genaral) groups:

Part design

polymer molecular structure (crystalline or amorphous)
polymer fillers
wall thickness
part stiffness/shape
wall thickness variations

Mold design

gate locations (critical in warpage)
gate design
runner design
cooling system configuration and layout
ejection system (prevent overstressing of part at ejection)

Processing conditions

mold temperature
melt temperature
injection time
packing time
packing pressure

EQUIPMENT

Mareco B.V. uses Moldflow Plastics Insight, version 3.1 release 2 (August 2002). The analysis modules available are:


	-	Fusion
	-	Flow (Filling and Packing)
	-	Cooling
	-	Warpage
	-	Fiber
	-	Optimization
	-	Design Of Experiments

SUPPORTED FILE FORMATS

The following table lists the supported file formats which can be directly imported into Moldflow:


	-	STL (Structural Triangle Language, often referred to as Stereo Lithography file)
	-	Parasolid
	-	STEP
	-	IGES
	-	ANSYS neutral file (*.ans)

In general, we can handle all file formats. Our CAD department has extensive possibilities to convert or even create CAD models ready for use in Moldflow.

Welcome at Mareco !!

Flow Simulation

Fiber Orientation

Warpage Simulation

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