CAE DS – Injection Casting Design Wall Thickness Technical University of Gabrovo – Yordanka Atanasova Wall thickness has an important effect on the performance and operational charac‐
teristics of injection moulded materials, e.g. occurrence and magnitude of internal stresses, accuracy of machining and the period of time that the product has stayed under pressure during moulding, etc. Why should wall thickness be noticed? On the one hand, the non‐simultaneous running of the process of plastics setting in the mould and the process of product cooling after ejection cause uneven shrinkage not only in products of sharp transition, but also in products having uniform wall thickness. When the walls in a product vary in thickness, uneven shrinkage takes place, which causes bumps, deformations and cracks. The greater wall thickness leads to greater shrinkage, and hence to the occurrence of gas cavities and greater deformations. The internal stresses that build up as a result can cause destruction of the product. The same effect is observed not only in the case of non‐simultaneous setting, but also when the polymerization is not completed, a phenomenon which is more common in thick‐wall products. Therefore determining the optimal thickness of the product walls and bottom is Determining the very important. Moreover, when the wall thickness is above the optimal one, the wall thickness product strength is not improved, only the time required to keep the product in the mould under pressure is increased. This causes additional thermal stresses to occur in the thinner walls of the product. Furthermore, exceeding the optimal wall thick‐
ness leads to unreasonably high material consumption and increases the risk of deformations. Wall thickness depends on a number of factors, such as: −
−
−
−
−
−
type of plastic; size, shape and use of the product; design of the shape; strength requirements; consumption of material etc. It is advisable to determine wall thickness according to the product dimensions / Table 1/. Wall Thickness ‐ 1 CAE DS – Injection Casting Design Table 1: Dependence of wall thickness on dimensions Rectangular product of dimensions, mm Cylindrical product of dimensions, mm Wall mm 20 х 20 х 20 Ø20 х 20 0,5 50 х 50 х 50 Ø 50 х 50 1,0 80 х 80 х 80 Ø 80 х 80 1,5 150 х 150 х 150 Ø 150 х 150 2,0 180 х 180 х 180 Ø 180 х 180 2,5 250 х 250 х 250 Ø 250 х 250 3,0 300 х 300 х 300 Ø 300 х 300 3,5 400 х 400 х 400 Ø 400 х 400 4,0 thickness, The minimum wall thickness is mainly determined according to strength and flow ability of the plastic. Plastics manufacturers provide information about the ratio of wall thickness а and the length of the melt flow course l (Fig.1). a, mm
6
I
5
4
II
3
2
1
III
0
0
100
200
300
400
l, mm
500
Fig.1 Wall thickness according to plastic flowability І – low‐
flow ability plastics (РС, РVС), ІІ – medium‐flowability plastics (ABS, PMMA), ІІІ – high‐flow ability plastics (РР, РEHD, PELD) The minimum wall thickness also depends on the length of the melt flow course into the mould and can be determined from Fig.2. l, mm
200
150
100
50
0
0
0,5
1
1,5
2
2,5
a, mm
Fig.2 Minimum wall thickness а depending on l, mm Wall Thickness ‐ 2 CAE DS – Injection Casting Design Fig.3 Wrong Correct Note Local thickenings should be avoided The rim /flange/ should have the same thickness as the wall Thick‐wall products should be released from excessive material. Otherwise shrinkages and gas cavities occur and the production cycle is extended a
a
1,1a
1,1a
1,2a
1,2a
a
Wall Thickness ‐ 3 CAE DS – Injection Casting Design The average wall thickness of injection‐moulded products is 0.8—З mm, reaching up to 6 mm for larger products. Modern technologies and machines allow the manufacture of products having smaller wall thickness – from 0.8 to 0.3 mm, but they require the material to have uniform properties. The thickness of the solid sections at the bottom and the local thickening of the product should not exceed the wall thickness more than twice, i.e. it should not be more than 10 – 12 mm. Fig.3 shows typical examples of correct moulding of wall thicknesses for different products. It can be seen that the uniformity of the walls can be achieved by remov‐
ing or rational distribution of the material in thicker places. It is particularly important to remember this when replacing metal products by plastic ones. Some materials shrink in quite a different way in relation to the melt flow direction. Therefore, to compensate for transverse shrinkage and to preserve the correct shape, for instance in injection casting of disk‐like products, it is recommended to increase the thickness in direction of the rim by about 25%. Measures for avoiding wall distortion a b Fig.4. Design of large vessels Large vessels are protected from distortion by thickening the walls by 25% from the corners to the centre, and by thickening the bottom from the centre to the rim by 25% as well (Fig.4а). Alternatively, deliberate outward drawing of the walls can be used (Fig.4b), which makes them less pliable and partially compensate for the shrinkage. Another way of avoiding distortion is the use of different temperatures in the punch and the die (Fig.4c). The effect of the difference in temperatures in the punch and the dies and the measures against distortion are shown in Fig. 5. Box‐like products exhibit distortion of walls relative to the bottom,(draft). This is the result of either the difficulty in removing the temperature from the corners, which increases the shrinkage, or shifting the melt flow along a smaller rounding radius, which increases the heat removed from the inner walls. Wall Thickness ‐ 4 CAE DS – Injection Casting Design t1
t1
t2
t1>t2
a2
a2
a2
r1
a1>a2
Fig.5. Effect of temperature on product shape and design: а – cooling and distortion, b – measures against distortion a2
r2
r2
r1
H
This defect can be corrected by thinning the transition between bottom and the walls or by preliminary change in the draft angles. The transitions between the walls should be smooth, which is also important for the product strength. Wall Thickness ‐ 5