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17Kunststoffe plast europe 3/2003
I N J EC T I ON MOULD ING ■ PE 1 02542
ERWIN BÜRKLE MATTHIAS SIEVERDING
I n automotive engineering, instrument panels, front-ends and underbody elements are increasingly being pro-
duced from glassfibre-reinforced poly- propylene. PP is replacing engineering plastics and metals from these applications because of the lower density, cheaper materials and recycling benefits. However, PP can only meet mechanical specifica- tions if the reinforcement with long glass- fibres increases its elastic modulus and im- pact strength.
The parts are made by either injection moulding or compression moulding glass- fibre-reinforced PP. In compression moulding, the starting material is usually semi-finished panel goods made from PP reinforced with glass mat thermoplastic (GMT). Classic compression moulding of GMT yields parts with excellent mechan- ical properties because of the length and isotropy of the fibres. Production of the GMT is very complicated, though. The se- mi-finished goods are therefore relatively expensive.
Thanks to recent developments, it is now possible to perform inline com- pounding of PP and glassfibres fol- lowed by direct compression. For all the advances made in the process technology, however, compression has major draw- backs in comparison with injection moulding. In most cases, the parts have to be remachined. As a rule, openings in compression-moulded parts can only be effected with downstream stamping. This generates production scrap and so adds to costs.
Injection moulding of long-glassfibre- reinforced parts from PP usually entails processing long glassfibre granules with the aid of modified plasticating units. By way of alternative to the processing of granules, Krauss-Maffei has launched a new injection moulding compounder (IMC) system, which manufactures injec- tion-moulded parts in a one-step process direct from the PP and glassfibre base ma- terials. The injection moulding com- pounder consists of a twin-screw extrud- er and an injection-moulding machine . The extruder melts the PP and mixes it with the glassfibres. The melt passes through a buffer region into what is called a shot-pot injection unit. From there, it is injected into the mould. Compounding directly during injection-moulding (IMC) eliminates the need for a semi-finished goods stage.
In the following, we compare the injec- tion-moulding machine and the injection moulding compounder on the basis of technical and economic criteria as an aid
for plastics fabricators to decide which of the two methods is better suited to their production tasks.
Behaviour of Fibre-reinforced Thermoplastics
Good fibre/matrix adhesion is crucial to a part’s mechanical properties. GMTs yield somewhat higher strengths and impact strengths than direct-processed moulding compounds or long-fibre granules (Fig. 1). The needle-punched mat structure, by virtue of the physical anchoring of the fi- bres and fibre filaments and the very good
Injection Moulding of Long-glassfibre-reinforced PP Process Comparison. Long-glassfibre-reinforced polypropylene parts are usually
made by injection moulding long glassfibre granules. A new one-step process
makes it possible to compound PP and glassfibres together for direct manufacture
as injection-moulded parts. Which of the two methods is better suited to a partic-
ular production task depends on the vagaries of parts production.
Krauss-Maffei Kunststofftechnik GmbH D-80997 Munich Germany Phone +49 (0) 89/88 99-0 Fax +49 (0) 89/88 99-3092 www.krauss-maffei.de
Fig. 1. Processing methods for long-glassfibre-reinforced thermoplastics
Translated from Kunststoffe 3/2003, pp. 47–50
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filament distribution, offers advantages which, however, compared with moulding compounds injected either direct or via long fibre granules, are lost if the flow paths in the compression process are long. The fact that injection moulding is better at introducing fibre orientation into the part can partly offset the disadvantage of the absence of needle punching if the design is suitable for the stress incurred.
Damage done to the fibre structure in composites can be used to draw conclu- sions about the processing method in- volved. It may take the form of fibre break- age, debonding and fibre pull-out. For full use to be made of a fibre’s strength, it must be longer than the so-called critical fibre length lC. Corresponding literature values for lC range from 1.3 to 3.1 mm for a fibre/matrix laminate of PP and glass. Use of special coupling (size) can lead to values of up to 0.9 mm.
The ratio of the current fibre length to the critical fibre length can be used to in- fer the quality of the fibre matrix coupling. If the current fibre length in the part is above the critical range, i.e. above lC, the fibres can be expected to break. If it is low- er than the critical value, fibre pull-out can occur. By that is primarily meant failure at the fibre/matrix interface, as can happen in chopped fibre compounds, where the usual fibre length is 0.2 to 0.6 mm.
Strictly speaking, the length of the re- inforcement fibre remaining in the fibre is of no relevance to the design. Mechanical characteristics, such as strength, rigidity and impact strength are more important for the design of a part. Although they are a function of the fibre length, their rela- tionship is highly complex. Analysing the fibre length alone, therefore, can only lead so far, although it is a practical parameter for obtaining trend information. Figure 2 is a normalised diagram of the change in rigidity, strength and impact strength as a function of fibre length.
Fibre Length in the Part
When long-glassfibre-reinforced PP is be- ing processed, it is important for the longest-possible fibres to be incorporated into the part because that produces the best mechanical properties in the com- posite. However, there is no way of pre- venting the fibres from breaking due to the mechanical application of stress, and thus being shortened, during compounding and injection moulding.The greatest dam- age to the fibres occurs while the melt con- taining the fibre is filling the mould (Fig. 3). Judicious mould design, howev-
er, can reduce the extent to which the fi- bres are shortened. The melting process greatly affects fibre length as well. There are major differences between injection- moulding machine and injection mould- ing compounder in this respect.
With the injection moulding machine, the initial fibre length is restricted by the size of the granules (as a rule 10 to 25 mm). Manufacturers of long glassfibre granules offer sheathed and pultruded systems (Fig. 4). In pultrusion, the fibres are wet- ted with matrix material in a melt bath and joined together into bundles. This has the advantage of impregnating the individual fibres very evenly with matrix material. In the case of the sheathed granules, the fibres and the matrix material are coextruded to- gether. The melting process in the injec- tion-moulding machine has to dissolve the fibre clusters and then wet the individual fibres with matrix material (Fig. 5).
The extent of the damage done to the fibres during melting decreases with de- crease in flow resistance. Large cross-sec- tion flow channels are kinder to the fibres. Screw configuration and the non-return valve should therefore be modified corre- spondingly when long glassfibre granules are processed.
When granules are injection-moulded, the fibres are subjected to the complete melting process. Mechanical stress on the fibres lasts a relatively long time. The start of plastication applies relatively large forces to the fibres since at that stage the matrix material has not yet completely
melted. Some of the fibres are trapped and exposed to high shear forces.
The size and the metering stroke of the screw additionally influence the fibre- damage mechanisms. A comparison of Figures 3 and 6 shows that the large screw with D = 165 mm causes much less da- mage to the fibres than the small one with D= 90 mm. Figure 6 also illustrates the negative influence of a longer metering stroke (s/D = 1.5 to 2.5) on the remaining fibre length. The scatter ranges show the influence of the long glassfibre granule structure (pultruded and sheathed).
By contrast, the injection moulding compounder melts the pure matrix mate- rial without fibres. The fibres are added to the melt later and are thus exposed to correspondingly less mechanical stress (Fig. 7). This method is kinder than melt- ing in the injection-moulding machine and leads to a higher average fibre length. The injection moulding compounder (IMC) offers the option of incorporating endless rovings directly into the melt in- stead of chopped strands. Although the rovings are broken into shorter pieces by the rotation of the screws, the resultant fibres are relatively long on average (see Fig. 3).
The price of the starting material is im- portant in the production of fibre-rein- forced PP parts. Long glassfibre granules for injection moulding may be cheaper than GMT semi-finished goods. Howev-
18 © Carl Hanser Verlag, München Kunststoffe plast europe 3/2003
I N J EC T I ON MOULD ING■ PE 1 02542
Fig. 3. Fibre lengths as a function of the processing section in injection moulding
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I N J EC T I ON MOULD ING ■ PE 1 02542
er, fabricators have to pay more for the