1Kunststoffe plast europe 5/2005

Translated from Kunststoffe 5/2005, pp. 51–56

ERWIN BÜRKLEJOCHEN MITZLER

Injection moulding machines are, with-out a doubt, complex high-tech products.Nevertheless, their importance for themanufacturing process – here, the injec-

tion moulding process – should not beoverestimated [1]. It is only the appro-priate combination of machine, mouldand processing technique that providesthe necessary framework for economicalproduction. Moreover, the more chal-lenging the product design and func-tionality, the more complex are the com-ponents from which the products are as-

sembled. This development is closelylinked to the processing technology, thecomplexity of which grows with that ofthe components.

It will probably never be possible todetermine when “processing technology“first appeared. This question is compa-rable to that of which came first, thechicken or the egg. Fact is that at some

Injection Moulding Processes

Integrative Processing Technology. As one of the most important processing

technologies, injection moulding has experienced, by far, some of the greatest ad-

vances in recent decades. From its origins based on simple moulding of thermo-

plastic resins, a product-specific processing technology has been developed that

has as its objective production of complex parts via a combination of various indi-

vidual processing techniques.

– Yesterday, Todayand Tomorrow

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2 © Carl Hanser Verlag, München Kunststoffe plast europe 5/2005

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point end users and processors alike de-cided to incorporate several functions in-to components, while at the same timeeliminating additional assembly opera-tions. This led initially to “special pro-cessing techniques“ that, after severalyears, became “normal“ processing tech-niques.

Functional Integration via theMulti-component Technique

A prominent example can be found in themulti-component technique, which wasintroduced over 15 years ago. Processingof two or more plastic melts in a singlemould, however, can be accomplished invarious ways. Depending on the applica-tion, terms such as multi-material or mul-ti-colour moulding, sandwich mouldingand in-mould assembly are employed. Asa rule, material-bonding, adhesive con-nections are the objective. If, however,combinations of incompatible materialsare involved, adhesion can be achievedthrough use of primers, flame treatment,corona- or plasma treatment (Fig. 1). Thedesired properties are the deciding factor,for instance,■ sink-free surfaces,■ combination of different colours,■ improved tactile properties (haptics),■ integration of seals or vibration-damp-

ing elements.Currently, multi-material injectionmoulding is concentrated primarily onintegration of functions and improve-ment of surfaces. Development projectsare focussed on combinations of ther-mosetting (cross-linking) resins or ce-ramics materials with thermoplastics.

Foam Moulding Improves Part Properties

The direct gas injection technique thathas been known for about 35 years – orig-inally, a version of structural foammoulding – is intended to achieve densi-ties of 0.3 to 0.6 g/cm3, produce partswithout sink marks and reduce clampingforce requirements. To accomplish this, ablowing agent is injected at high pressureinto the melt-containing screw channel.After expansion to form a gas, foam struc-tures result. A few well-known applica-tions from the automotive industry in-clude decorated panels, coverings, hous-ing parts and also connectors.

In a modified form and using im-proved technology, this technique is cur-rently experiencing a renaissance as mi-cro-structural foam moulding. As the

result of advancements in chemical blow-ing agents, the physical and chemicalfoam moulding techniques are compet-ing with one another today. The benefitsof each technique include (Fig. 2)■ cycle time reduction (–20%),■ weight reduction (–10%) and■ improved dimensional properties.Lower melt viscosity is a side effect of gasincorporation, which permits longerflow paths and clamping force reduc-tions of up to 30%. The opportunitiesresulting from this have not yet beenexhausted.

Surface Improvement Tech-niques – Internal and External

Two additional and related low-pressureinjection moulding techniques, back in-jection and back injection compressionof decorative materials in the mould,weredeveloped to production readiness at theend of the 1980s / beginning of the 1990s.In-mould laminating, as these techniques

are also called, represents an alternativeto the usually more involved adhesivebonding approach.

While pillar covers, door inserts andsimilar applications can be backmouldedon modified conventional injection ma-chines, backpressing initially requiredspecial vertical machines to permit dep-osition of the melt in the open mould. Fora long time, this principle served to pro-duce large decorated door and luggagecompartment panels as well as instru-ment panel coverings. In the meantime,back injection compression has been de-veloped for use on horizontal machines,as a result of which the above-mentionedproducts can now be produced on con-ventional machines (Fig. 3).

High-quality surfaces are no longer re-served exclusively for luxury automobiles.The interiors of mid-sized and compactcars are also expected to impart an im-pression of quality, for instance, a pleas-ant, leatherlike feel to the surface or soft-touch effects.

Fig. 1. Injectionmoulding with in-lineplasma pretreatment

Fig. 2. Cycle times for foaming

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3Kunststoffe plast europe 5/2005

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Processors and automakers wish tohave a reliable method capable of high-volume production that requires no sec-ondary finishing operations and is suitedto complex, highly three-dimensionalsurfaces with tight radii.

While originally developed for pro-duction of automotive interior compo-nents, both techniques are now employedfor numerous other applications such asoffice furniture, backmoulded films forelectronic displays and the front halves ofmobile telephones. They also serve as thebasis for backmoulding of paint films.This technique is suitable for productionof exterior auto body panels and couldreplace the complicated wet paintingprocess. If the required surface quality ofthe film can be achieved, both the re-quirement and wish of automakers forshort model runs, variety, flexibility andmodularity would be met.

Development projects with texturedfilms in colours matching those of the ve-hicle are already in process, as is initial se-ries production [2].The film quality is crit-ical, for which reason the number of con-crete examples of applications is limited atpresent.The weak link is colour matching,i.e., matching the film colour to that ofother painted parts, and providing a com-plete range of colours for the line of vehi-cles. In addition to Class A surface quali-ty, the absence of dust and foreign parti-cles must be established.Even though a fewimportant fundamental aspects are stillmissing, the use of plastic components forauto exteriors is getting closer.

Transparency with FunctionalityThanks to Injection Compression

For more than 30 years, injection com-pression moulding, which, by the way, isalso a low-pressure process, has served toproduce warp-free thermoset parts. Forrelatively flat parts the surfaces of whichmust be very dimensionally accurate aswell as for optical lenses and inside mir-rors in cars, it is superior to convention-al injection moulding. More recentprocess variants are suited to productionof other high-quality, stress-free parts, forinstance data media.

With the development of suitable poly-carbonates (PC), new automotive appli-cations have become possible, for in-stance, headlamp lenses (1993) and thefirst automotive glazing (1998) in theSmart car. This led to additional processvariants for injection compression in-volving clamp motion, breathing, wedgesand rotary platens (turntables) for pro-

duction of two-component glazing(Fig. 4). With its expansion compressiontechnique, Krauss-Maffei has developeda method for producing large transpar-ent PC parts. The current size limit is1,5 m2 in series production. The benefitsof plastic glazing are obvious, for instance■ relatively great design freedom,■ opportunities for integration of func-

tions,■ significantly lower weight and ■ high impact strength.Unconventional design ideas such as win-dows with tight radii (cockpit windows)for automobiles have failed to date in glassbecause of the limited design freedom

and the associated lack of a high-volumeproduction technique. Given the right in-jection moulding technique, this situa-tion could change in the near future withPC (Fig. 5).

Injection Moulded Long-fibre Reinforcement

There are increasing attempts to replaceGMT (glass mat-reinforced) parts withlong-fibre- reinforced thermoplastics viadirect processing (D-LFT-ECM), in-the-press processing or injection moulding oflong-fibre-reinforced pellets (LFG-IM) aswell as via direct compounding (D-LFT-IM) (Fig. 6). Compared to classical GMTprocessing, screw injection, whether oflong-fibre-reinforced pellets or rovings,is considerably simpler, since cutting tosize, loading and heating the mats is elim-inated.

Long-fibre-reinforced pellets havebeen processed on injection mouldingmachines for about ten years now as a wayof producing parts with improved me-chanical properties (Fig. 7). This becamepossible through the development of spe-

cial fibre-preserving screw geometries,suitable non-return valves, along withnozzles and hot runner systems.

The development of direct compound-ing on injection moulding machines wasbased initially on economic considera-tions. The objective was to complement, ifnot replace, the involved GMT process [3].Today,the process that was first introducedseven years ago has achieved even greaterimportance than previously. It providesprocessors with the ability to develop in-dividual formulations with customers. Atthe same time, production can react moreflexibly to changes with an injectionmoulding compounder (IMC).

In direct compounding, the matrixcomponents and reinforcing fibres(chopped glass fibres or rovings) are feddirectly to the injection moulding ma-chine, where they are compounded andprocessed into the moulded part in “oneheat“ (Fig. 8). This allows the processorto customise the system of matrix, fibreand adhesion promoter, for instance, inorder to match the fibre content and fi-bre length in the part exactly to the spe-cific technical requirements. In this way,very long fibres, whether in mats, fabricor unidirectional strips, can significant-ly improve the property profile notice-ably.

With direct compounding, the valueadded at the processor increases signifi-cantly, keeping in mind that the potentialof the process is far from being exhaust-ed (Fig. 9). If long-fibre reinforcementalone is not adequate for parts subject tohigh loads, the load-critical regions canbe optimised by inserting additional re-inforcing fabric, which can subsequentlyundergo backmoulding. How such in-serts, prepared as necessary, are trans-ported, positioned and held in the mould

Fig. 3. Production-ready decorated automotive parts

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is known from experience withbackmoulding and backpressing.

Additional potential for thistechnology can be found in fillingwith large amounts of fillers,blending of resins and even reac-tive extrusion with subsequentpart forming.

A definite plus of this one-stepprocess is the greatly reduced ther-mal history of the materials to beprocessed. It is even conceivablethat this may be the only way toprocess thermally sensitive mate-rials or material components.

“New Surfaces” with Improved Properties – the SkinForm Process

For many years, the expression “syner-gies“ was overused and more often wishthan reality in the plastics industry. Ittook a long time for various processingtechniques to come together and ulti-mately create completely new possibili-

ties. In a joint project involving a proces-sor, material supplier and Krauss-Maffeias machine builder, a new technology forproduction of moulded parts with ahigh-quality surface has been developed,the so-called SkinForm Process (Fig. 10).

In this process, injection moulded sub-strates are overflooded with a soft cast PUsystem in a single process step requiringno secondary finishing operations. Theprocess utilises two mould cavities andpermits localised changes in wall thick-

Fig. 7. Door modules injection moulded from long-fibre-reinforced pellets(LFG-IM) (source: M. Schemme, FH Rosenheim)

Fig. 8. Front end injection moulded via injection moulding compounder(D-LFT-IM)

Fig. 4. Roof glazing based on two-component technology Fig. 5. Design study for plastic window (source: Freeglass)

Fig. 6. Market development for long glass-fibre-reinforced polypropylenes (Europe) (source: M. Schemme, FH Rosenheim)

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5Kunststoffe plast europe 5/2005

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ness. The PU component is partiallyfoamed, thus providing the surface withcertain soft-touch regions. Since theprocess replicates the surface of themould cavity, various surface structures(graining, decorative stitching and thelike) can be reproduced exactly. A newdesign effect that was previously not at-tainable with soft painting of mouldedparts or in-mould laminating (back-moulding) has now become possible. Afew important benefits of the process are ■ pleasant leather-like feel to the part

surface,■ high scratch resistance in spite of very

thin surface layer,■ suitable for highly three-dimensional

surfaces with tight radii and openings,■ no distortion of the decoration or loss

of graining.The process is already suitable for high-volume production, with costs no higherthan for a soft-painted part. As a result of

this initial development, many promisingnew applications are currently being con-sidered, especially where new surfacequalities are required.

Retain and Refine Know-how

Research and development in the plasticsindustry occur largely in a customer-ori-ented and product-specific manner.Considerations are focused on themoulded parts and their production, thatis, on the process technology. Ideally, theend user, processor, material supplier andmachine builder are striving to achievethe same objective of combining or de-veloping the appropriate materials, ma-chines and processes. This requires closecooperation among trusted partners thatshould not end after success has beenachieved.

The next objective must be to retainand refine the acquired know-how, since

a trend that is known to observersfrom other sectors is becoming in-creasingly apparent even in the plas-tics industry – especially with regardto processing technology: patentingof already known, yet still importantprocess steps in order to subsequent-ly make them available only after pay-ment of expensive licensing fees. Apossible consequence of this phe-nomenon, already known from theconsumer software branch, could bethat important advancements eitherbecome uneconomical or are blockedcompletely because to the licensingfees that must be paid. To permit thiswould be to rob the plastics industryof the momentum that has charac-terised it as one of the most innova-tive branches of industry. ■

REFERENCES1 Bürkle, E., Sieverding, M.: Process Technology for

Developing New Markets, Kunststoffe plasteurope 94 (2004) 10, pp. 40–45

2 Ludwig, H.-J.: Folientechnik im Automobilbau –ein Vergleich VDI-K-Tagung (2005) Kunststoffe im Automobilbau

3 Jensen, R: Making Intelligent Use of Synergies,Kunststoffe plast europe 91 (2001) 9, pp. 40–45

THE AUTHORSDR.-ING. ERWIN BÜRKLE, born in 1942, is Manag-

er of Advanced Development, New Technologies andInjection Moulding Processing at Krauss-Maffei Kun-ststofftechnik GmbH, Munich.

DIPL.-ING. (FH) JOCHEN MITZLER, born in 1973, isHead of Product Management for injection mouldingmachines at Krauss-Maffei.

Fig. 10. Synergies inthe SkinForm produc-tion cell

Fig. 9. Classification of long-fibre-reinforced thermoplastics (source: M. Schemme, FH Rosenheim)

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