Chapter 10

Which plastics have the best flowability?

DuPont explores the use of polymers for thin-walled parts

Miniaturisation throughout the electronics industry sets a triple challenge for plastics processors.

In spite of having very thin walls, parts have to be mechanically robust, practically non-warping and dimensionally highly accurate over an extended period. For such parts, very free-flowing plastics are needed; in this league, liquid crystal plastics lead the pack.

When moulding engineering plastics, their melt flow properties determine how well the tool can be filled and what injection pressures and times are needed.

The melt must flow very freely, especially when moulding very thin-walled parts with wall thicknesses of less than 0.5mm, and for large area parts with long flow paths. Only with free-flowing resins will parts be completely formed, have good dimensional stability with low distortion and have superior surface qualities.

Determining flowability

In practice, the melt flow index (MFI) and the spiral test are the most significant ways to obtain a comparative evaluation of a plastic melt’s flowability.

The MFI is measured according to ISO 1133 or DIN 53735: a hot molten plastic is forced through a standard die of 2mm diameter at a certain pressure. The design of the test equipment, the dimensions of the heated pressure cylinder, of the piston and the die, as well as the test method, are specified in every detail.

The testing temperatures are pre-set in accordance with the usual melt temperatures of the various plastics. Usually, test weights of 2.16kg are used for free-flowing plastics and of 21.6kg for poor-flowing plastics.

Sections are cut off at constant time intervals from the melt strand extruded from the die and are weighed after cooling.

The amount of melt extruded in 10 minutes is calculated from the average (for example, 12g) and is given as the MFI, in combination with the test temperature (for example, 190ºC) and load (for example, 2.16kg); in the foregoing example, the MFI would be 190/2.16 = 12g per 10 minutes.

As MFI measurements are among the most common tests to be carried out, highly automated equipment is available to execute them. Even simpler than weighing the sections of the melt strand, is measuring the distance of piston travel in a certain time. This method yields the melt volume index (MVI), which is connected with the MFI as a function of the density of the melt.

MFI and MVI

On the one hand, MFI and MVI measurements are suitable for quality testing of incoming goods, and on the other to identify processing errors. Increasing MFI indicates material degradation through thermal overloading of the melt.

As regards processability, MFI allows only a rough classification of various grades of the same plastic resin, because the flow rate of the melt through the die is much less than the injection speed when moulding real-life parts.

The spiral test yields better information regarding processability. This test is done with a moulding tool, with a long spiral-shaped flow channel.

The melt is injected using the appropriate processing conditions for the material being tested (temperature of the melt and tool, injection pressure and speed).

The flow length/wall thickness relationship is calculated from the length of the flow strand and the wall thickness of the flow channel. This value is often used to describe a material’s processability.

Various designs of rheometer are suitable for further investigation or for more accurate measurement of the melt viscosity as a function of temperature, pressure and shear speed. A plastic melt’s viscosity can be measured at various temperatures and over a broad range of shear speeds with this kind of equipment.

Flow and mould filling

Figure 1: Thin-walled contact strips for circuit boards, moulded from one of the 6000 series of Zenite® LCPs, withstand the high temperatures during reflow soldering without warpage.

Figure 2: The good flowability of Zytel® 103 HSL polyamide allows short cycle times when injection moulding thin-walled headlight bezels.

Figure 3: The good flowability of Zenite® LCP allows connectors with flow path/wall thickness ratios of more than 100 to be injection moulded free of warpage and with high dimensional accuracy.

Liquid-crystal polymers (LCPs) often represent the most efficient solution for very thin-walled parts, with wall thicknesses down to 0.1mm.

Their melt flows very well and they have another remarkable feature: their flowability increases with increasing pressure, so that even the narrowest crevices are reliably filled. This property, known as structural viscosity, is useful, for example, when moulding the body of miniature connectors for computer and data technology (see Figure 1).

In the outer walls of such parts with a wall thickness of only 0.2mm and a height of about 22mm, the flow-length/wallthickness relationship is about 110 – and in spite of this, connectors made of Zenite® are cleanly moulded, without flash, are non-warping and possess the required stiffness, as well as very good long-term dimensional stability.

Thin-walled contact strips for circuit boards have to withstand mechanical stress when contact pins are pressed into place and they must keep their good dimensional stability, even when subjected to the thermal stress of reflow soldering at 260ºC.

The 6000 series Zenite® LCPs is optimised for this kind of application. Apart from their unchanged flow characteristics, these materials stand out by the fact that they resist the increasingly high temperatures of modern soldering processes even better without any undesirable blistering or warpage.

Thin-walled parts are not necessarily small; on the contrary, large area parts are often made with walls as thin as possible for cost and weight-saving reasons. An example of such a design is the Ford Transit’s headlight bezel (see Figure 2).

The Zytel® 103 HSL chosen for this application stands out through its good flowability, so that in spite of the bezel’s low wall thickness and long flow paths, it can be injection moulded with short cycle times.

Other reasons for the choice of this material were its high strength and impact-resistance. Temperatures in the headlight housing can reach 150ºC, making this resin’s good dimensional stability at high temperatures another important factor.

DuPont recently added Zytel® HTN53G50LRHF, an especially well flowing high-performance polyamide to its range.

Its 50% glass fibre reinforcement gives it high strength, stiffness and impact-resistance, and the melt’s flowability is around 20% better than that of comparable types.

This allows higher productivity in the production of complex, high-strength thin-wall injection moulded parts for the automotive and other industries, as well as for consumer goods.

Good Flowability

Connectors (see Figure 3) and similar miniaturised electronics components often have flow path/wall thickness ratios of more than 100 and the injection pressure has to be kept as low as possible, on account of the fine textures in the tool.

These requirements are best met with liquid-crystal polymers, or with types of polyamide or polybutylene terephthalate that offer particularly good flowability.

In general, that is to say also with large parts, good flowability can make a decisive contribution to lowering costs, because it makes it possible to design thinner part walls and thereby also reduce cycle times.

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The examples in this series of articles are intended to illustrate underlying principles and to explain the main influencing factors.

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