Predrying

Under unsuitable conditions of storage and transportation SAN, and hence Luran also, absorbs moisture. As a result of this, surface defects can arise in processing. We therefore recommend that Luran be dried for two to four hours at about 80 °C prior to processing. Suitable dryers for this purpose include dehumidifying dryers and circulating air dryers.  Compatibility with other thermoplastics Luran 358 N is compatible as a melt with Luran 368 R and Luran 378 P is compatible only with Luran 388 S. In the molten state, Luran is incompatible with most thermoplastics such as polyolefins, polystyrenes and polyamides. Admixture of even small amounts of these materials causes streaking and results in finished parts with slate-like layers and low strength. This should be borne in mind especially when changing materials and when using regrind. Luran 348 Q, 358 N and 368 R are compatible with ABS.

Self-coloring

Self-coloring of Luran is now well-established technology. Volumetric or gravimetric metering equipment is generally recommended for the self-coloring process. This method permits precise matching of the desired shade – even with difficult colors.

Luran molding compounds can be processed on all commercially available injection molding machines. Due to their amorphous structure they have a wide processing range, good thermal stability during processing and low shrinkage. Finished parts made from Luran, therefore, have a low tendency to warp.

Injection unit

Screw geometry

In order to achieve optimum product properties, the plasticator must supply the melt in an ideal manner and as uniformly as possible with regard to both timing and position. Shallow-flighted, three-section screws equipped with a non-return valve at the tip of the screw have proven to be effective for processing Luran (see Fig. 19).  The overall length of these screws is 18-22 D (length relative to the diameter), the length of the feed section being approximately half the length of the screw. The ideal screw length for self-coloring – to optimize pigment dispersion – is 22 D. To maximize homogeneity, mixing elements may also be used (static or dynamic). The screws are single-flighted and constructed with a constant pitch which, as a rule, amounts to 1 D. For trouble-free production and acceptable quality, the maximum metering stroke should be limited to 3 D. 

Injection nozzle

Free-flow nozzles can be used for processing Luran but shut-off nozzles have advantages when it is necessary to operate with high back pressures or when stringing is to be avoided.  Thick-walled moldings require extended cycle times. If the molding compound is not removed completely from the nozzle aperture, in such cases it can quickly cool there and give rise to defects on the surface of the molding on the next shot. Mechanically or hydraulically operated needle valve nozzles have proven effective in such cases. However, the loss of pressure in such nozzles is not inconsiderable.

Injection mold

Gate and mold design

All the usual types of gate can be employed for processing Luran.  The relevant construction guidelines as set out in VDI 2006 apply to the design of the mold.  The gates should be of adequate size. If the runner and gate cross sections are too small, the melt temperatures and injection pressures must be set unnecessarily high and this can give rise to streaks on the surfaces of the molding or to burn marks. If the melt solidifies prematurely in the gate, voids and sink marks form on the molding because the volume shrinkage of the melt cannot be sufficiently compensated during the holding pressure phase.

Draft angles

An inclination on one side of 1:100 or 1:30 is sufficient for the draft.  If the mold is polished in the longitudinal direction, a draft as low as 1:10 may be possible in favorable circumstances.  In the case of textured surfaces, the draft must be greater.

Use of inserts

Metal inserts can be encapsulated but they should be heated to 80 -120 °C prior to insertion so that internal stresses are kept as low as possible. Good anchorage is achieved by means of knurls, grooves or the like. Care has to be taken that the edges of the metal are well rounded.

Mold temperature control

Shrinkage and the mechanical and thermal properties of the molding can be controlled within limits by the surface temperature of the mold.  Higher cavity surface temperatures result in better weld line strengths and lower internal stresses and hence lower tendency to warpage.  In practice, mold surface temperatures of between 40 and 80 °C are proven to be reliable. Even at cavity surface temperatures at the upper end of the range, Luran products have the advantage of rapid solidification due to their high heat resistance. This allows short cooling periods and hence efficient cycle times.  When the geometry of a molding requires it, separate temperature control of the two halves of the mold (core, cavity) can be advantageous for prevention of warpage.

Fig. 19: Screw geometry – Terms and dimensions for three-section screws in injection molding machines

Injection molding parameters

Processing temperature

Luran grades are normally processed at melt temperatures between 210 and 260 °C. The temperature range between 230 and 250 °C is preferable. When processing is carried out in the upper range of temperatures, residence times have to be kept short in order to prevent yellowing or degradation of the material. The melt temperature should be monitored with a stick-in thermometer.

Shrinkage

Molding shrinkage is generally between 0.4 and 0.7 %. After shrinkage is usually negligible. Although shrinkage is primarily a property of the material, it is also affected by the shape of the molding and by the processing conditions. Accordingly, shrinkage can be highly variable in different parts of a molding. In zones, in which for example a high holding pressure prevails, even shrinkage levels of 0 % can be achieved.

Warpage

Warpage is caused by differences in shrinkage perpendicular and parallel to the direction of melt flow. Low-warp or warp-free moldings can be produced by selective control of the temperatures of individual parts of the mold (e. g. core and swage). Thus, for example, warping of housing walls towards the inside can be counteracted by means of a low core temperature and a high cavity temperature.

Flow characteristics

The range chart “Product features, applications, typical values” includes MVR data for individual grades of Luran. Fig. 20 shows the flow properties measured by the spiral flow test of Luran 358 N and 388 S as a function of melt temperature. This is not a standardized test but it allows comparison of the flowability of products of the same type.

Injection molding of glass-fiber reinforced Luran

Luran 378 P G7 can be processed on injection molding machines under much the same conditions as Luran without glass fibers, i.e. the processing temperatures recommended for most applications lie between 230 and 260 °C. The mold surface temperature should not be set lower than 60 °C. Molding shrinkage is generally between 0.1 and 0.4 %. When designing injection molds it has to be borne in mind, particularly in relation to undercuts and drafts, that Luran 378 P G7 is a very rigid plastic. If Luran 378 P G7 is to be processed over an extended period, we recommend the use of a wear-resistant plasticating unit.

Luran 378 P G7 is also highly suitable for extrusion. If the product is to be processed over an extended period, wear-resistant plasticating units are advantageous. In the case of extrusion, it is again recommended to dry the material before processing or to employ plasticating units with vacuum venting.

Fig. 20: Flowability as a function of melt temperature; mold: 10 x 2 mm test spiral, mold surface temperature: 60 °C

Methods of joining; surface treatment

When bonding with solvents, the differences in resistance of the Luran grades to aromatic hydrocarbons become noticeable.  While Luran 358N and 368R can be readily bonded using toluene, Luran 378P and 388S require the use of more powerful solvents such as ethyl acetate, dichloroethylene or cyclohexanone.  Parts made from Luran 358N and 368R as well as 378P and 388S can only be reliably bonded to parts made from the same material or to parts made from the same group using solvents.  When bonding problems arise, we recommend that you contact the adhesives industry which also supplies special adhesives.  Semi-finished products and moldings made from Luran can be welded by the hot plate, spin and ultrasonic welding methods. Preliminary tests are necessary in individual cases.  Luran readily and durably accepts print and paint.

Safety precautions in processing

When the products are correctly processed in well ventilated work areas no harmful effects on the health of those engaged in processing have been observed. Depending on the processing conditions, traces of the following substances may be liberated: styrene, or possibly a-methylstyrene, and acrylonitrile. The respective threshold limit values in the workplace have to be observed. The MAK (maximum allowable concentration) and TRK (technical guide concentration) values laid down in TRGS 900 apply in Germany (TRGS = Technical rules for hazardous substances). More information can be found in our safety data sheets (www.plasticsportal.eu). Experience shows that when Luran is correctly processed and suitable measures for ventilation are in place the concentrations are well below the aforementioned threshold values (see also Staub-Reinhalt. Luft 43 (1983), p. 376 et seq.).  As a general rule, inhalation of vapor-phase degradation products, which can arise when the material is overheated or pumped off, is to be avoided. Our safety data sheets for Luran and for special grades provide additional information.