Co Injection

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Co-injection ProcessTechnical Challenges and Solutions

2012Moldex3D R11

European

Webinar

Series


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Current position:

CoreTech System Technical Research Division

Education:PhD in Polymer Science from the University of

Connecticut

Specialization:

Polymer rheology, processing, and properties,polymer composite, biomedical materials,

degradable polymers, and plastics applications

in industrial design

Dr. Sun has researched parenteral drug packaging in Eli Lilly and Company. He hasalso worked with Teleflex Medical to develop a fully absorbable bone graft composite.He is now a senior research engineer supporting the development of advancedinjection molding CAE software and also in charge of material testing and measurement

in Coretech System company.

Shih-Po Sun, Ph.D.


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Contents

> Part 1:

Introduction to co-injection molding

Product common defects and solutions

Product development through CAE

> Part 2:

Case study 1: Earphone hookMeeting product spec how to resolve unwanted warpage

Case study 2: Car door panel

Prioritize gate location for ideal filling pattern


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Multi-material injection

Also called multi-component injection molding (MCM)

Processes of combining different material qualities and

properties in one component.

PP

TPVPC

In-mold assemblyCo-injection

PP

EVOH


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Multi-material injection family

Multi-material processes

Multi-component

Bi-injection

Co-injection

(Sandwich)

Multi-shot

Rotating tool

Core back

Transfer

Over-molding

Insert

Lost core


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Skin / core combinations

Handle, lever : Computer housing:

Soft feel skin / hard core Unfilled skin / core with conductive filler

Garden furniture: Yogurt pot:

Virgin skin / recycled core Pigmented skin / uncolored core


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Stages

Step 1

Empty cavity1st shot (skin): blue2nd shot (core): yellow

Step 2

Skin injection

Step 3Core injection

Step 4Second skininjection for

1-2-1 structure


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Todays objective - aesthetics

Structural product: co-injection is preferred toconventional structural foam or fiber reinforced plastics

because of its superior surface finish.A solid skin combined with a structural foam coreprovides the advantages such as low molded-in stress,sink-free parts without the skin surface defects

Quality surface is obtained in fiber reinforced plasticssince the fiber-loaded material is restricted in the core ofthe part.


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Todays objective - property combination

Combined properties: one can achieve a combination ofproperties not available in a single resin by combining

different materials.For instance:

An elastomeric skin over a rigid core will provide astructure with a soft touch.

A brittle material can be encased by a material of highimpact resistance or high ductility.


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Sustainability


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> Recycled core reduces costs

> Recycled core reuses waste

> Engineering core structure, e.g. fiber

foaming, enhances product strength or

performance

> Virgin skin provides quality aesthetic

finish for recycled or engineering core

> Elastomer skin improves surface touch

> No increase on cycle times comparing to

sequential or multi-component molding

Benefits of using co-injection molding


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Important processing considerations

> Strength Material adhesion

> Skin / core distribution Flow characteristics

> Specification Shrinkage / Warpage

> Performance Material selection

Related processing conditions:

Material properties, injection rate, skin-core transition ratio,

skin/core temperature and packing pressure


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Machine design - nozzle


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Machine design hot runner


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Machine design multiple gate


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Process Co-Injection Bi-Injection Insert Molding Over Molding Multi-Shot

Interface

Controllability Poor

Poor

Good

Good

Good

# of Nozzles 1 n 1 n n

# of Materials 2 2 2 2 2

Mold Complexity Low Low Low Medium High

PartComplexity

Simple Very simple Medium High Very high

Use ofPreforms

No No Yes No No

Source: http://www.isr.umd.edu

Multi-Material Molding ProcessComparisons

interface depends on the laminar flow of materials interface is controlled by the mold geometryn the number of separate materials being used


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How to turn single shot machine into aco-injection one?

Source: Krauss Maffei


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Co-injection machine design - Melt

Milacron


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Co-injection machine design Mold

Meiki

Master Mold Plate US patent 6196822


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Co-injection machine design - Screw

Spirex Twinshot


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Source: Engel

Material compatibility


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Material Properties

Viscosity f( , T, P, ) Volume f(T, P)&

Heat conductivity f(T) Heat capacity f(T)


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Relationship

between and u

or viscosity,

Thermal conductivity, kHeat capacity, Cp

Temperature-pressure dependent volume, V(T, P)

Modulus, G

( ) 0=+

u

t

Mass balance

Energy balance

&: =

+ TkTtTCp u

Momentum balance

gpDt

D +=

u

CAE procedureThermoset conversion balance

0=++ RjX&

Density,

Conversion rate,R

Flow behavior

Cooling, warpage analysis

Equations to describe melt flow


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Important questions

How does my change of design affectthe skin/core distribution?

What is my flow orientation for thecore layer? If the core is fiber filled.

Where is the area with high stress andtemperature?

How does the core layer affect overallwarpage?


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Skin / core evolution


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> Two materials can only be processed successfully by co-injection molding if their processing properties match.

Previous studies have suggested that the core/skinviscosity ratio should be between 0.5 and 5.

> Lower viscosity ratios would lead to breakthrough ofcore material.

> Higher viscosity ratios would lead to poor mold filling,finger like defects and a variable core thickness throughthe part.

Gomes, 2011, Polym Eng. Sci.

Viscosity limitations of 2 materials


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Effect of viscosity ratio

Yottha Srithep, 2008

viscous


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Higher

Lower

2nd

shotviscosity

co

mparedto1stshot

Skin-core ratio (1st shot volume / total volume)

80% 70% 60%

A

B

Material: PC

Effect of viscosity ratio


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Injection rate effect

Yottha Srithep, 2008


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Injection rate effect

Skin injection speed constantCore injection speed


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Material break through

Source: C-Mold


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Core filling ratio

Vcore/Vtotal


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Material break through prediction

Two opposing forces: viscosity vs. filling ratioLess viscous core requires higher skin filling ratio to prevent break through.


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Controlled break out technique

Lattoflex, Winx300

IF 99 design award


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Break through simulation


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Cornering effect

> Uneven distribution, uneven thickness

Flow around corners must be considered indesign phase to prevent thin inside corners.

Use a generous radius.

Source: Gas-assist injection molding:principles and applications


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Flow

behaviorPartdesign

Processingcondition

Materialselection

Mold

design

Design considerations

Quality

product

Material distribution vs. Design criteria?


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Questions

If I want to

Which design change is possible?


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Benefit of CAE

CAE is a must in predicting the spatial distribution of core and skin.

General knowledge is difficult to predict the simultaneous flow of two

materials.

Knowing the melt front evolution of both shots, CAE can:

Evaluate the flow front pattern to aid in part design and gate placement

Predict the extent of penetration of the core material and the skinbreakthrough point

Determine injection pressure and clamping force for proper moldingmachine selection

Determine the best switch point from skin injection to core injection


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Benefits of Moldex3D

Moldex3D can help users in all aspects of co-injection molding

process:

> For mold design, locate the optimum gate locations

> For cost reduction, evaluate the maximum core ratio without

causing break through.

> For structural analysis, provide skin thickness distribution.

> For quality control, pinpoint potential locations with high

temperature and stress.

> For spec requirement, precisely estimate shrinkage and

warpage concerning the interaction between skin and core

material.


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Case Study 1: Earphone hook

> The hook connects to the phone and mic assembly.

> The hook design has a flexible skin to provide better feel oftouch in contact with the ear.

> The hook should have enough stiffness to withstanddeformation.

> The hook dimension should meet specification.

O i i l i l d i


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Original sequential design

PP

TPE

U i i j ti


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Using co-injection process

Benefit:

> The co-injection process provides ideal material

combination of soft skin / hard core to meet designcriteria.

Concern:

> Part and process design to reach desired mechanicalstrength

> Warpage due to imbalanced core distribution

M lt f t l ti


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Melt front evolution

Skin ratio


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Skin ratio

90% 80%

50%70%

Break through occurred!

Skin ratio


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Skin ratio

Cornering effect


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Cornering effect

Core travels toward

inside of the corner

Shrinkage X-axis direction


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Shrinkage, X-axis direction

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Gate location design


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Gate location design

70% skin filling ratio


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Controlled break through


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Controlled break through


0.07 sBreak through occurred


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Skin ratio


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Skin ratio

Shrinkage, X-axis direction


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g ,

10

Moldex3D FEA workflow


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Element properties

Structural simulationsoftware

Material parameters

Flow analysis

+

Structural analysis result


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y

B.C.Fixed locationaround the joint

Load500g of force applied at the end

Deformation results


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Case study 2: door panel


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> The size of the door panel requires multiple gate mold design.

> The core amount should be maximized without surfacing.

Using co-injection process


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Benefit:

> Co-injection integrates rigid and flexible components into

a single molded structure with a single step.> The core should be completely contained within the

flexible material.

Concern:> Number and location of the gates to ensure balanced

melt flow and material distribution

> Warpage due to imbalanced core distribution

Information


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> Dimension

> Dimension710476 mm

> Thickness : 7 mm

> Volume

1660 c.c.

> Material

> Skin : TPO

> Core : PP

> Processing condition

> Time (total): 5 s

> Melt temperature: 260 C

> Mold temperature: 40 C

Hot Runner 12.0 8.0mm

Gate Type : circularDiameter8 mm

Single skin material filling


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Skin /core material filling


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Number of gates


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5 gates, 60% skin ratio 6 gates, 60% skin ratio

Increasing skin ratio


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6 gates, 70% skin ratio 6 gates, 80% skin ratio

Gate location modification


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Gate contribution


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Skin ratio


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Clamping force


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Skin in contact with the wall

Volumetric shrinkage


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Warpage, Y-direction


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Conclusions


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> Co-injection process brings design versatility to newlydesigned or existing products. While the transition to co-injection process from already own equipment is not

difficult, optimizing part design and processingconditions to achieve desired material distribution poseschallenges.

> CAE flow analysis is an indispensible tool providing

complete solution for managers, designers, andengineers to fully control the outcome of their fiberreinforced plastic product.

> Modules in Moldex3D work seamlessly together

providing invaluable information throughout the entiredesign, manufacturing, and usage phases.


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If you have questions, please type

into the chat box.

Contact Information


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Dr. Sun

[email protected]

Dannick

[email protected]

Vincent

[email protected]

Next Webinar


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TopicLatest Development in Long / Short Fiber Reinforced

Plastics Applications, Processing Techniques and

Simulation Technology

Goals Lean how to estimate fiber orientations to gain strategic

benefits by adopting CAE incorporated design.

Have an overview on the design work flow from part geometryall the way to dimension accuracy and mechanical properties.

Speed up your fiber product development with a clearer viewon most important design considerations and processingcontrols.

Time

9:00AM UTC -13 January,2012

Date post:	04-Apr-2018
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Co Injection

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