Packing of material along flow path in injection moulded container S. Aasetre and E. Andreassen The distribution of material (weight) along the ow path in an injection moulded cylindrical container was quanti ed by cutting the container into ve segments. Moulding conditions were varied and eight diVerent polypropylene grades were used. The total weight increased and the weight distribution remained nearly uniform when increasing the packing pressure. Increasing the packing time above a certain limit, however, mainly packed material close to the gate. The weight distribution was also aVected by material parameters, such as shear viscosity and degree of nucleation. The shrinkage and the compressive strength of the container were related to the weight distribution. In particular, the compressive strength was highly correlated with the weight of the segment at the end of the ow path. PRC/1752 © 2002 IoM Communications Ltd. Mr Aasetre is with Borealis AS, N-3960 Stathelle, Norway and Dr Andreassen (
[email protected])is with SINTEF, PO Box 124 Blindern, N-0314 Oslo, Norway. Manuscript received 2 August 2001; accepted in nal form 5 December 2001.
INTRODUCTION Materials for injection moulding are developed continuously to satisfy the market with regard to product properties and economical production.1 Some important issues for polypropylene (PP) are improved balance between stiVness and impact strength, reduced shrinkage and warpage, and improved properties in terms of ow and clarity. Demoulding performance and cycle time are also important. From a materials point of view, the cycle time is usually governed by the kinetics of nucleation and crystallisation. A wide range of nucleation agents are used for tailoring commercial PP grades. One de nition of optimal injection moulding could be as follows: a process that minimises the cycle time and weight of a product, while keeping dimensions and other product properties within speci cations. Thinner walls lead to reductions in both product weight and cycle time, but how do nucleating agents, which reduce the cycle time, aVect the thin wall processing performance? In a previous unpublished study of products with long ow paths and relatively thin walls, it was found that the shrinkage of PP increased with increasing degree of nucleation. This posed a question about how modi cations such as nucleation agents aVect the packing of the cavity, and the nal material (weight) distribution along the ow path. The present paper reports results of measurements of weight distributions along the ow path and discusses how these distributions are aVected by material parameters (copolymerisation, shear viscosity, nucleation) and moulding conditions (injection rate, melt temperature, packing pressure, packing time). Product properties such as shrinkage and mechanical properties are discussed in terms of the weight distributions. A large number of experimental and theoretical studies have been published dealing with injection moulding of polypropylene and the resulting microstructures and product properties, including shrinkage and warpage.2–5 However, there are still unresolved 20
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issues, partly owing to the complex solidi cation mechanisms and microstructures, which depend on material parameters and moulding conditions, and partly owing to the thermomechanical interactions between the polymer and the mould, which may lead, for example, to quite diVerent shrinkage in the in plane and thickness directions.6 Special packing techniques, such as shear controlled orientation injection moulding (SCORIM), have also been explored.7,8
EXPERIMENTAL Materials, mould, and processing Eight diVerent commercial PP grades produced by Borealis were used in the present study. Some relevant properties of these materials are given Table 1. Note that, from a materials point of view, the grade BE375P is the centre of the experimental design and this grade was subjected to an extended set of moulding trials. The moulded product was a 10 L container (bucket), with height 252 mm, bottom diameter 230 mm, and top diameter 266 mm, see Fig. 1a. The wall thickness was 1·4 mm at the bottom, with a gradual decrease Table 1
PP materials used
Grade
Type*
MFR†
Nucleation level
Impact strength
BC245P BE170M BE375P BE376P RE220P HE125M BH345P BJ355P
C C C C R H C M
3·5 13 13 13 13 13 45 100
medium none medium strong strong none medium medium
high medium medium medium low very low medium low
* C = heterophasic copolymer, R = random copolymer, H=homopolymer, M=miniblock copolymer. † Melt flowrate (g/10 min) at 230°C with 2·16 kg load. DOI 10.1179/146580101125000475
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a ( × ) ahead of screw; ( ) bottom of container; ( along height; ( ) top of container
2
b
1
a Photograph of moulded container and b same container after cutting into five segments to quantify weight distribution along flow path
towards 1·2 mm at the top. The cavity was lled from a single gate at the centre of the bottom. Hence, the ratio ow length/wall thickness was ~300 for this product. A Mold-Masters hot runner with a pneumatic needle valve was used. The containers were moulded with a Netstal 1570/ 300-MP-Sycap machine. This machine has a maximum clamping force of 3 MN, a screw with a length/ diameter ratio of 25, and a maximum screw speed of 0·5 m s – 1 (corresponding to an injection time of 0·24 s for the product studied ). The moulding parameters that were varied are given in Table 2. The packing pressure referred to in the present paper was calculated from the measured hydraulic pressure, i.e. converted to pressure ahead of the screw. The cavity pressure was recorded at three positions along the ow path, and some typical pressure curves are shown in Fig. 2. Note that in this Table 2
Moulding conditions
PP grade
Melt temperature, °C
Injection time, s
Packing pressure, MPa
Packing time, s
BE375P BH345P BJ355P all other
220, 260 220 220 260
0·6, 1·2 0·6 0·6 0·6
35, 55, 75 35, 55, 75 15, 35, 55 35, 55, 75
0·5, 1·5, 3, 5 0·5, 1·5, 5 0·5, 1·5, 5 0·5, 1·5, 5
) halfway
Typical pressure recordings (BE375P, packing conditions 55 MPa for 3 s); pressure ahead of screw estimated from measured hydraulic pressure; cavity pressure measured with sensors at three positions along flow path
gure, and generally at high degrees of packing, the pressure near the gate (bottom of container) did not drop to atmospheric pressure after removing the packing pressure, but remained at a certain plateau. With the packing pressure used in Fig. 2, the plateau was higher for a packing time of 5 s, while it was absent for a packing time of 1·5 s. The non-decaying pressure near the gate could be due to overpacking. In addition to the parameters in Table 2, containers were also moulded with some intermediate packing pressures and packing times, but only total weights and weight distributions were assessed for these cases. The cooling time (i.e. the time between releasing the packing pressure and ejecting the part) was 12 s in all the trials, to ensure the same screw residence time, so that the melt temperature would not vary with the packing time. This relatively long cooling time was chosen to reduce variations in mould temperature when varying the packing time. The total time with the mould open was also held constant. (In addition to the moulding trials of Table 2, the packing conditions of BE375P were also varied while maintaining the cycle time constant, i.e. with shorter cooling times – and screw residence times – for longer packing times. This gave results similar to those reported below.) The mould temperatures were 5 and 20°C for the moving and stationary halves, respectively. Not all materials were moulded with the same set of processing conditions. A lower melt temperature was used for the two high melt owrate (MFR) materials, to avoid over lling. For comparison the ‘centre material’, BE375P, was also moulded using this lower melt temperature. Furthermore, the highest packing pressure was not used for the material with MFR 100, to avoid ashing and problems during demoulding. Finally, BE375P was also moulded with a reduced injection rate, to compare this with the reduced melt temperature. In the following sections, data for standard injection parameters (injection time 0·6 s and melt temperature 260°C) are reported, unless otherwise stated. Plastics, Rubber and Composites 2002 Vol. 31 No. 1
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Packing of material along flow path in injection moulded container
Shrinkage, compressive strength, and weight distribution The shrinkage relative to the mould dimensions was measured for the height of the container, as well as the top and bottom circumferences. The compressive strength of the containers (maximum axial force prior to collapse, also known as ‘top load’) was measured in a Zwick 1464 testing machine, using a compression speed of 1 mm min –1. The weight distributions were quanti ed by cutting the containers into ve segments with approximately the same weight (see Fig. 1b), using the following procedure. The container was placed upside down on the horizontal table of a drilling machine. A hole with diameter 10 mm was drilled through the bottom centre, and the container was fastened to the drilling table with a nut on a bolt through this hole. A knife with a thin sharp blade was mounted on the table at a selected height and with a free movement towards the container. The container was then rotated in contact with the knife, leaving a circular track, but not cutting through the container wall. All containers were marked at the same height before the knife was moved vertically to the next position. Finally, the containers were cut along the four circular tracks, and the ve segments were weighed. With the procedure outlined above, diVerences in the height of the container (due to shrinkage) mainly in uence the measured weight distribution via the bottom segment. However, considering the data obtained, the weight fraction in the bottom segment was not higher correlated with height shrinkage than the weight fraction in the adjacent segment. Hence, the eVect of container height on the measured weight distribution was small compared to the other eVects considered. All measurements were performed three weeks after moulding. Three parallel measurements were made for all combinations of material and moulding parameters. The weight of three ‘parallel’ segments typically varied by 0·2 g (~0·4%). This was partly due to cutting errors and partly due to shot to shot variations in the injection moulding process. RESULTS AND DISCUSSION Total weight As expected, the total weight of the container increased with increasing packing pressure and packing time, as shown in Fig. 3. The latter eVect was small at long packing times. As a rst approximation, the relationship between weight and packing conditions can be described by a simple logarithmic model weight= a + b ln(packing pressure) + c ln (packing time) where a, b , and c are constants. The weight diVerence between containers with high and low degrees of packing, i.e. 75 MPa for 5 s and 35 MPa for 0·5 s, varied with material parameters, injection time, and melt temperature. The largest weight diVerence (316 276 g =40 g) was observed for BH345P, which had the highest MFR value (lowest shear viscosity) among those materials packed with Plastics, Rubber and Composites
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( ) BH345P, 75 MPa; ( ) BC245P, 75 MPa; ( s 35 MPa; ( ‚ ) BC245P, 35 MPa
3
) BH345P,
Total weight of container for different packing times and packing pressures; materials were moulded with different melt temperatures, see Table 2
75 MPa. The smallest diVerence (299 275 g=24 g) was measured for BC245P, which had the lowest MFR value. The BE375P was moulded with two diVerent injection rates and melt temperatures. When the degree of packing was low, slow injection and low melt temperature lead to higher weight. However, when the degree of packing was high, the opposite eVect was observed. Slow injection or low melt temperature can aVect the packing by two competing mechanisms. A colder melt has higher viscosity and solidi es earlier. The latter may reduce the need for packing, since the total amount of solidi ed polymer in the cavity is higher at a given time. On the other hand, a more viscous polymer may be harder to pack, i.e. to transport along the ow path. Hence, in the present study, when the degree of packing was low, a colder melt gave a higher weight due to earlier solidi cation. When the degree of packing was high, the plateau weight was reached earlier with a cooler melt, i.e. the potential for packing was lower. A comparison between the two unnucleated materials with equal MFR – HE125M ( homopolymer) and BE170M (heterophasic copolymer) – showed a clear diVerence. Containers produced with the homopolymer were on average 3·7 g lighter, see Table 3. However, the diVerence was small at the highest degree of packing. Generally, among the materials with equal MFR, the three heterophasic copolymers gave the highest weights. This could be due to a lower degree of crystallinity. The container weight was only slightly aVected by the degree of nucleation. On average, containers of the highly nucleated BE376P had the same weights as those of the unnucleated BE170M, see Table 3. However, some details should be noted. At the two highest packing pressures, the highly nucleated material gave the highest weights at high packing times, but the lowest weights at low packing times. The third heterophasic copolymer with the same MFR value (BE375P), had a medium nucleation level. However, on average and especially at medium packing times,
Aasetre and Andreassen
(
4
) 75, ( j
) 55, and (
Œ
Packing of material along flow path in injection moulded container
) 35 MPa
(
Weight distributions for different packing pressures (BE375P, packing time 1·5 s)
) 5, ( j
) 3, (
Œ ) 1·5, and ( }
23
) 0·5 s
5
Weight distributions for different packing times (BE375P, packing pressure 55 MPa)
6
As Fig. 5, but weights of segments normalised by total weight of container
this material gave lower weights than either BE170M or BE376P. This indicates that nucleation aVects weight by at least two diVerent mechanisms. Weight distribution A typical example of the eVect of packing pressure on the weight distribution is shown in Fig. 4. The weights of the ve segments increased by almost the same amount when increasing the packing pressure. There was, however, a slight tendency for relatively higher packing close to the gate ( bottom) at the highest pressures. The eVect of packing time is shown in Fig. 5. When the packing time was increased above a certain value, ~2 s, only segments near the gate gained further weight. This agrees with cavity pressure measurements in the top segment, which showed that the pressure started to decrease after ~1·5 s. Alternatively, the distributions can be analysed by plotting the weight fractions as in Fig. 6. The rst ring above the bottom (~180 mm from the gate) is a transition zone, above which the weight fraction decreased with increasing packing time. A comparison between Figs. 4 and 5 clearly shows that increasing the packing pressure is more eVective than increasing the packing time when it comes to transporting material towards the Table 3
end of the ow path. Figure 7 summarises the eVects of packing conditions. The weight fraction in the top segment decreases with packing time, but is independent of packing pressure, while the weight fraction in the bottom segment increases with both packing time and packing pressure.
Selected measurements averaged over all nine packing conditions
Material parameters*
Moulding conditions†
Total weight, g
Weight of top segment, g
C-3·5-1-3 C-13-0-2 C-13-1-2 C-13-1-2 C-13-1-2 C-13-2-2 R-13-2-1 H-13-0-0 C-45-1-2 M-100-1-1
STD STD LIR LMT STD STD STD STD LMT LMT/LDP
283·9 289·9 290·5 289·3 288·8 290·0 286·4 286·2 293·7 286·8
55·2 56·1 56·6 56·6 56·4 56·9 56·2 55·8 57·6 56·9
Shrinkage of container height, %
Shrinkage of top circumference, %
Shrinkage of bottom circumference, %
Shrinkage top circumference shrinkage bottom circumference, %
Compressive strength, kN
1·50 1·30 1·49 1·59 1·46 1·55 1·53 1·43 1·28 1·36
1·48 1·51 1·51 1·49 1·51 1·73 1·64 1·51 1·72 1·75
1·41 1·40 1·37 1·40 1·44 1·64 1·57 1·52 1·67 1·87
0·07 0·11 0·14 0·09 0·08 0·09 0·08 0·01 0·05 0·12
3·7 3·9 4·8 4·7 4·3 5·2 4·3 5·1 5·3 6·0
* Code describes polymer type, MFR value, nucleation level, and impact strength (see Table 1). † LIR=low injection rate, LMT =low melt temperature, STD=standard, LDP=low degree of packing (see Table 2). Plastics, Rubber and Composites 2002 Vol. 31 No. 1
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Aasetre and Andreassen
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7
‚ ) 35 MPa, 0·5 s; ( s ) 75 MPa, 5 s
Packing of material along flow path in injection moulded container
) 75 MPa, 0·5 s; (
) 35 MPa, 5 s;
Normalised weight distributions for different combinations of packing pressure and packing time for BE375P
The BE375P was moulded with two diVerent injection rates and melt temperatures. At low degrees of packing, low temperature or slow injection reduced the weight fraction in the top ring, while the weight fractions of the three segments closest to the gate increased. These eVects were small, but signi cant. When the degree of packing was high, the opposite eVect was observed. Slow injection in particular, led to a reduced fraction in the bottom segment, while increased fractions were measured in the rst and second ring, see Fig. 8. For both packing conditions the total weight (see above) increased when the weight fraction in the bottom segment increased. When increasing the MFR value (reducing the shear viscosity), the weight distribution tended to shift slightly from the bottom towards the top, as shown in Fig. 9. Nucleation seemed to give a similar eVect, see Fig. 10. Regarding the type of PP, the homopolymer HE125M deviated somewhat from the other materials with the same MFR value (with or without nucleation). A higher weight fraction in the rst ring, and a lower fraction in the top, was obtained with this homopolymer. In all three cases (MFR, nucleation, and copolymerisation), the
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Œ
(
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Shrinkage Typical shrinkage data as a function of packing time are shown in Fig. 11. Note that an increase in packing time above ~2 s hardly reduced the shrinkage of the top circumference. This agrees with the weight distributions reported above, i.e. long packing times did not contribute to better packing far from the gate. The diVerence between the highest and lowest shrinkage values increased with increasing packing time. It is well known that inhomogeneous shrinkage may lead to warpage of injection moulded products. When plotting these shrinkage measures as function of packing pressure, as in Fig. 12, it can be seen that the shrinkages of the top and bottom circumference may be equal at a certain packing pressure. At high packing pressures, the top experienced the highest shrinkage, i.e. the degree of packing decreased along the ow path. With a low packing pressure, on the other hand, the degree of packing may be lower near
(
No. 1
) BE375P, MFR 13
shift in the weight distribution towards the top was accompanied by an increase in the total weight (see above).
) 1·2 s injection time
Normalised weight distributions for different injection rates (BE375P, packing conditions 75 MPa/ 5 s)
Plastics, Rubber and Composites
) BC245P, MFR 3·5; ( Œ
Normalised weight distributions for materials with different MFR values (packing conditions 55 MPa/ 1·5 s)
10
) BE170M, no nucleation; ( Œ
) BE376P, strong nucleation
Normalised weight distributions for materials with different degrees of nucleation (packing conditions 55 MPa/ 1·5 s)
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Packing of material along flow path in injection moulded container
) top circumference; ( j ) height; ( ‚ ) bottom circumference Shrinkage as function of packing time for BE375P with 55 MPa packing pressure
(
11
the gate, due to the hot incoming melt. At a certain packing pressure all three shrinkage values may be equal (e.g. at ~45 MPa for BE375P with 1·5 s packing time as in Fig. 12). With short packing times, the diVerence between the three shrinkages values was quite small, even for the highest packing pressure. The shrinkage was aVected by injection rate and melt temperature, as studied with BE375P. The height shrinkage increased when either the melt temperature or the injection rate was reduced, see Table 3. The shrinkage of the bottom circumference, on the other hand, was reduced by these changes in moulding conditions (the melt temperature eVect was only signi cant for low degrees of packing). The shrinkage of the top circumference was also reduced by these changes, when the degree of packing was low. At high degrees of packing, the eVects on the top shrinkage were reversed (although small). The net result of these eVects was that lower melt temperature or slower injection increased the diVerence between the highest and the lowest shrinkage measurement. The diVerence between top and bottom shrinkage was highest for the case with slow injection, see Table 3. The height shrinkage (parallel to ow) was more sensitive than
) top circumference; ( j ) height; ( ‚ ) bottom circumference Shrinkage as function of packing pressure for BE375P with 1·5 s packing time
(
12
25
the bottom shrinkage (perpendicular to ow) to melt temperature and increased with decreasing temperature, in agreement with results reported by Jansen.3 The shrinkage of the height (parallel to ow) was lower for the two high MFR materials, especially when the degree of packing was high, compare, for example, rows 4 and 9 in Table 3; (the high MFR materials were moulded with a low melt temperature, but the MFR eVect seemed to be stronger than the (opposite) melt temperature eVect.) The shrinkages of the top and bottom circumferences generally followed the opposite trend. In particular, the two high MFR materials diVered from the rest by giving lower shrinkage of the height and higher shrinkage of the top and bottom circumferences. This indicates that the anisotropy in shrinkage may be higher with these materials. The material with the highest MFR value (BJ355P) diVered from the rest by giving higher shrinkage at the bottom than at the top at all packing conditions (the highest degree of packing for this material was 55 MPa/5 s, for which the top and bottom shrinkages were equal ), see Table 3. The shrinkage was more sensitive to packing conditions for the materials with high MFR values. The shrinkage increased with the degree of nucleation. The variation in shrinkage from one position to another was lowest with medium nucleation, at low and medium degrees of packing. At the highest degree of packing, medium and strong nucleation levels performed equally in reducing the shrinkage variation. The homopolymer (HE125M) generally showed higher shrinkage than the heterophasic copolymer with the same MFR value and nucleation level (BE170M). At a certain degree of packing with the homopolymer, the top shrinkage ‘crossed’ the bottom shrinkage (the former being highest at higher degrees of packing). At and above this intermediate degree of packing, the homopolymer had lower variation among the three shrinkage measures than the copolymer. Finally, it should be noted that a high pressure may expand the cavity,9 allowing more material to enter than with nominal cavity dimensions, and thereby aVecting weight and shrinkage indirectly. This eVect was not considered in the present study. Compressive strength The compressive strength generally increased with increasing degree of packing as shown in Fig. 13. However, for most materials the compressive strength levelled oV and in some cases even decreased at high packing times. At high packing pressures, this negative eVect of packing time disappeared when reducing the cooling time, i.e. the screw residence time. The negative eVect remained at low packing pressures and the buckling position moved towards the top of the container (decreasing thickness) with increasing packing time (the point of maximum in ection was typically 60–70 mm from the top). Hence in this instance, the reduction in compressive strength was due to a change in buckling pattern/position (the initial compressive stiVness did not decrease with increasing packing time). These phenomena could be studied further by nite element structural analysis10 coupled with simulations of the injection moulding process. However, Plastics, Rubber and Composites 2002 Vol. 31 No. 1
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(
13
) 75, ( j
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Packing of material along flow path in injection moulded container
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Compressive strength of BE375P for different packing times and pressures 14
the complex physics involved in this process, especially related to the solidi cation of semicrystalline polymers, makes it diYcult to simulate the packing of all volume elements with adequate accuracy. It is often found that cavity pressures are only moderately well predicted.11 The compressive strength increased with increasing MFR value, see Table 3. The material with the highest MFR value gave the highest compressive strengths, although the degree of packing was lower. This was probably related to the stiVness, which is known to increase with increasing MFR, and maybe also the miniblock architecture. The weight distribution was also favourable, but other materials gave higher total weights and top segment weights. The material with the lowest MFR gave the lowest maximum compressive strength. For this material the compressive strength was not much aVected by packing pressure. Among the materials with equal MFR value, the highest compressive strengths were achieved with the strongly nucleated heterophasic copolymer and the homopolymer. Strongly nucleated materials and homopolymers are known to have high stiVness. Interactions and correlations The shrinkage and the mechanical properties of injection moulded products are determined by material parameters per se (e.g. type of nucleation and impact modi cation) and the interaction between material parameters and moulding conditions. This interaction results in ow induced local properties (including anisotropy) and a certain weight distribution in the mould. One way to obtain further insight into the relationships between material parameters, processing conditions, and product properties is to analyse how these entities are correlated. In this study, the correlation between the diVerent measurements (total weight, segment weights, shrinkages, and compressive strength), when varying the packing conditions, was investigated by calculating correlation coeYcients (linear regression) for the data sets obtained at nine diVerent packing conditions (three packing pressures and three packing times). As an example, the correlation coeYcient r for Plastics, Rubber and Composites
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Correlation between two entities measured for BE375P (data for three different packing pressures and three different packing times)
the two data sets in Fig. 14 is 0·988. The average correlation coeYcients given below r are average values for 10 regressions, i.e. eight materials plus two extra conditions for BE375P ( low melt temperature and slow injection). As expected, the total weight is strongly related to the individual segment weights ( r =0·98 for the correlation between total weight and bottom segment weight). The three entities total weight, height shrinkage, and bottom shrinkage are also highly correlated for all materials (absolute values of r in the range 0·95–0·97). The weight of the top segment is the entity that is highest correlated with the compressive strength ( r =0·90). For comparison, correlating compressive strength to total weight and bottom segment weight yields r = 0·84 and 0·79, respectively. The top weight is also correlated with the three shrinkage measures ( r in the range 0·88 to 0·89), although the bottom weight has slightly higher correlation with shrinkage. The correlation coeYcients are high for high MFR materials, and vice versa. The lowest correlation coeYcients for the low MFR material are those involving the compressive strength and this is mainly due to the low sensitivity of the compressive strength to packing conditions for this material. If the low MFR material is omitted from the average, r for the correlation between top weight and compressive strength increases to 0·94. Finally, a comment about the relationship between top weight and top shrinkage when varying the material parameters. A negative correlation might be expected, which was the case when varying the packing conditions. However, for the data set in this study there is an overall positive correlation. The two respective columns in Table 3, for example, give a correlation coeYcient of 0·70 (if the two data sets for BE375P with low melt temperature and slow injection are omitted, the correlation coeYcient increases to 0·83). Hence, it is not the amount of material that governs the shrinkage in this case. The microstructure,
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Packing of material along flow path in injection moulded container
including degree of crystallinity and molecular orientation, may be important. The correlation between top weight and height shrinkage is weak, with a correlation coeYcient (calculated from Table 3) of 0·3 ( 0·4 if the two non-standard data sets for BE375P are omitted ). For comparison, the correlation between top weight and compressive strength, based on Table 3, is positive (r =0·70), as when varying the packing conditions. CONCLUSION The properties of the container were aVected by the weight distribution along the ow path. In particular, the compressive strength was highly correlated with the weight of the top segment at the end of the ow path. The weight distribution was aVected by moulding conditions and material parameters. Increasing the packing pressure generally packed all segments evenly. On the other hand, increasing the packing time above a certain value mainly packed material close to the gate and the time available for packing was relatively short for this product with rather thin walls. A high MFR value and a high degree of nucleation were favourable for obtaining a high weight fraction in the top segment. Changes in melt temperature and injection rate also had some eVect on the weight distribution and the shift along the ow path was either positive or negative, depending on the degree of packing. With a single gate at the bottom, as is usual for this type of product, a high packing pressure may be favourable for the product’s mechanical properties. The material consumption can be reduced by preferentially packing the weakest part of the product. Furthermore, thinner average wall thickness and more eVective packing could reduce the cycle time. On the
27
negative side, a higher packing pressure may require a larger and more expensive machine. Furthermore, tooling costs may be higher, and residual stresses in the product could become detrimental. Alternatively, an optimised material distribution could be achieved with a small machine by underpacking an overdimensioned mould. However, with this method it would still be hard to pack material in critical areas far from the gate, and it would be diYcult to control shrinkage and warpage. ACKNOWLEDGEMENTS The authors gratefully acknowledge support from Borealis AS and the Norwegian Injection Moulding Forum. REFERENCES 1. m. gahleitner, w. neissl, p. pitkanen, p. jaaskelainen, and b. malm: Kunstst. Plast. Eur., 2001, 91, (4), 59–60. 2. p. zipper, a. janosi, w. geymayer, e. ingolic, and e. fleischmann: Polym. Eng. Sci., 1996, 36, 467–482. 3. k. m. b. jansen: Int. Polym. Process., 1998, 13, 309–317. 4. p. m. gipson, p. f. grelle, and b. a. salamon: J. Inject. Mold. T ech., 1999, 3, 117–125. 5. s. j. liu: Plast. Rubber Compos., 2001, 30, 170–174. 6. g. titomanlio and k. m. b. jansen: Polym. Eng. Sci., 1996, 36, 2041–2049. 7. g. kalay and m. j. bevis: J. Polym. Sci. B, Polym. Phys., 1997, 35, 241–263. 8. g. kalay and m. j. bevis: J. Polym. Sci. B, Polym. Phys., 1997, 35, 265–291. 9. d. delaunay, p. lebot, r. fulchiron, j. f. luye, and g. regnier: Polym. Eng. Sci., 2000, 40, 1692–1700. 10. c. fernandes, s. grimaldi, and a. macchetta: Plast. Rubber Compos. Process. Appl., 1993, 19, 29–37. 11. k. m. b. jansen, d. j. van dijk, and e. v. burgers: Int. Polym. Process., 1998, 13, 99–104.
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