Plastic Threads Technical University of Gabrovo Yordanka Atanasova Threads in plastic products can be produced in three ways: a) by direct moulding with thread punch or die; b) by placing a threaded metal insert; c) by removing chips How are plastic threads produced? When designing plastic products with threads produced directly in the moulding process, we should account for the shrinkage of the material during the cooling process, which is different for the different types of plastics. This should be taken into consideration when producing thread punches necessary for moulding of internal threads and the dies shaping the internal threads. Placing a thread metal insert is required in the production of plastic nuts when the screw or the bolt is made of metal or in the case of frequent screwing or unscrewing of the nut. Removing chips is a method for producing plastic threads of small diameters and high batches, which cause frequent failures of thread punches. When producing threaded plastic products, the thread profile may vary, but unless the service conditions require a different type, metric thread is recommended. Metric thread can be represented through a triangular profile having a depth Н, the crest of which has been cut at a distance Н/8, and the root is at a distance Н/4 (Fig.1). Geometrical parameters of the thread Fig.1 The rated dimensions of the thread are: External diameter d the diameter of the round surface circumscribed round the crests of the screw turns; Internal diameter d1 is the diameter of the cylinder formed by the surfaces of the roots; Plastic threads 1
Mean diameter d2 is the diameter of the cylinder at the surface of which the root width is equal to half of the theoretical thread pitch; Thread pitch s is the distance between the parallel sides of two adjacent turns, measured along the thread axis; Depth of thread h is the depth of the plane cut rated profile formed by the output triangular profile having a depth Н; Angle of thread α is the apex angle at which the sides of the profile are located; Length of winding l is the length of contact area between the surfaces of the screw and the nut measured along the axis. For standard threaded joints l 0,8 d. The deviation of the actual thread profile from the rated one is limited by adopting a set of tolerances. The set of tolerances for threaded joints is structured in such a way as to allow contact of surface areas only along the side parts of the turn, contact in the area of thread crests or roots is ruled out. Dimensioning Depending on the angle of thread α threads can be classified as metric threads, inch threads, pipe threads, etc. The sharp profiles of metric and inch threads lead to internal stresses and lower strength. Therefore threads of pitch s less than 0.4 mm are not recommended. In case of threaded products made of elastic plastics (PEHD, PELD, PP), the depth of thread should be sufficient to prevent loosening or failure of the joint. The minimum allowable thread diameter for parts made of thermoplastics, such as ABS, PMMA, PE, PP, is 2,5 mm. The minimum thread pitch depends on its diameter and is chosen according to the data given in Table 1. The rated bending radius of the thread root rn = H/6 = 0.144 s. The minimum permissible bending radius rmin = H/8 = 0.108 s. To increase the strength of plastic threaded joints, bending radii lower than the rated ones should be avoided, especially when dimensioning bolts and screws. The length of winding l should not exceed its diameter d more than twice. To improve the grip, wings can be added at the thread start / l1 / and termination / l2 /. /Fig.2/ The height of the wing l1 at the thread start for pitch s < 0.75mm is 1.0 mm, for s = 0.75 mm it is 1.5 mm and for s >1.5 mm it is 2.0 3.0 mm. The height of the wing l2 at the thread termination depends both on the pitch and the diameter of the thread. For d < 30 mm and s < 1.0 mm l2 = 2 mm, and for s > 1.0 mm l2 = 4 mm ; for d >30 mm l2 = 6.0 10.0 mm. There are rules determining the minimum space between the threaded hole and the end of the part b1. It depends on the thread diameter /Fig.3 /, Table 2. To preserve the product strength, there are also norms determining the minimum space between the threaded holes b2, as well as the bottom thickness b3 in the case of blind (closed) threads. Plastic threads 2
Table 1. Thread dimensioning, mm Thread diameter External d Mean d2 Internal d1 Pitch, s Depth of thread, h 2,5 2,208 2,013 0,45 0,243 3,0 2,675 2,459 0,50 0,325 4,0 3,546 3,242 0,70 0,379 5,0 4,480 4,134 0,80 0,433 6,0 5,350 4,918 1,00 0,541 8,0 7,188 6,647 1,25 0,676 10,0 9,026 8,376 1,50 0,812 12,0 10,863 10,106 1,75 0,947 14,0 12,701 11,835 1,0 1,082 18,0 16,376 15,294 2,5 1,353 24,0 22,051 20,752 3,0 1,624 30,0 27,727 26,211 3,5 1,894 36,0 33,402 31,670 4,0 2,165 42,0 39,077 37,129 4,5 2,435 Fig.2 Table 2 Thread diameter d, mm Space b1, mm Space b2, mm Thickness b3, mm Up to 3.0 2.0 1.3 2.0 From 3.0 to 6.0 2.5 2.0 3.0 From 6.0 to 10.0 3.0 2.5 3.8 Above 10 4.3 3.8 5.0 Plastic threads 3
When the plastic parts have several threads of different diameters, to simplify the design of the mould, it is recommended that all threads have the same pitch. The following requirements have to be observed in the design of threaded products (Table 3). Table 3 Design of threaded products Non technological Technological Note s The end of threaded products should have a wing of pitch s for improved strength R s should have a wing for improved strength and be without a groove should have a wing for improved strength and be without a groove s in a blind nut should not reach the bottom to avoid stress concentration R 1,5 in a cap should not reach the bottom to avoid stress concentration Plastic threads 4
Non technological Technological Note M16x1,5 M16x1 Multiple threads should have the same pitch for easy ejection M12x1 M12x1 Threads should be positioned outside the product outline to avoid deformations Threads should be positioned outside the thickened places to avoid deformations The major norms refer to metric threads of diameter from 1 to 180 mm. When producing the threads of plastic products, the crest edges of the internal and external thread can be rounded by rimax, which is calculated using the relationships rimax = 0.054 s for s < 0.5mm, and rimax = 0.034 s + 0.02mm for s > 0.5mm. The shape of the root of the external thread can be rounded or flat. When designing the moulding service, it is the rounding or the plane at a distance of H/6 = 0.144s from the profile crest that is primary. The following rules are applied for the main dimensions of threaded plastic products: the diameters and pitches should correspond to the data given in Table 1; for diameters less than 4 mm the pitch should not be less than 0.45 mm, for diameters greater than 16 mm the pitch should not be less than 0.5 mm, for diameters greater than 18 mm it should not be less than 0.75mm and for diameters greater than 36 mm it should not be less than l mm; for diameters from 3 to 8mm the use of particularly great pitches is permissible (Table 4). These norms are valid both for plastic metal joints and for plastic plastic joints. Plastic threads 5
The tolerances for thread diameters refer to the normal length of winding N or to the entire length of the thread, if it is less than the normal length of winding. Two types of fits are used in threaded plastic joints slide fits and fits with guaranteed clearance. Tolerances and fits for threaded plastic products In the case of external threads the fit is produced in the basic hole system with deviation H/g or in the basic shaft system with deviations G/h (Η and G are margin tolerances for internal thread). Any combinations of margin tolerances are permissible for internal and external threads, but the same degree of accuracy is preferred. Table 4 Plastic threads of particularly great pitches Diameter, mm External d Mean d2 Internal d1 Pitch, s,mm Depth of thread, h, mm 3.0 2.480 2.134 0.80 0.433 4.0 3.350 2.918 1.00 0.541 5.0 4.026 3.376 1.50 0.812 6.0 5.026 4.376 1.50 0.812 8.0 7.026 6.376 1.50 0.812 The margin tolerance width is determined according to the accuracy chosen. The length of winding also has to be taken into account when choosing the fit short M, normal N or long L. The margin tolerances for internal and external threads correspond to three degrees of accuracy medium, rough and very rough (Table 5). For lengths of winding M and L the use of additional margin tolerances is allowed (Table 6). Table 5 Margin tolerances of threaded products Degree of accuracy Margin tolerances Screws Nuts Medium 6h 6g 6H 6G Rough 8h 8g 7H 7G Very rough 10h 8h 9Н 8Н Plastic threads 6
Table 6 Additional margin tolerances for threaded products depending on the length of winding Degree of accuracy M Length of winding L Margin tolerances Screws Nuts Screws Nuts Medium 7h6h 7g6g 7H 7G Rough 7h6h 7g6g 6H 6G 9h8h 9g8g 8H 9G Very rough 9h8h 9g8g 8H 8G The accuracy of threads produced by injection moulding or pressing from materials exhibiting different shrinkage can be determined using diagrams drawn out of experience. The degrees of accuracy shown in these diagrams can be achieved when there are certain ratios in shrinkage variations, length of winding and diameter provided the thread punches and dies are produced with pitches that are corrected using average shrinkage. If the threads are produced in moulds with rated (uncorrected) pitches, the desired degrees of accuracy can be achieved only if the length of winding is reduced, because for great lengths the thread accuracy is reduced due to shrinkage of the material along the axis. Strength is also reduced. To achieve the same strength of the turns as the strength of the product walls, the length of winding must be optimal. Taking into account the shrinkage, the admissible number of turns n can be determined using the formula where т is the number of turns; ΔS variations in plastic shrinkage; Smax maximum plastic shrinkage. n = mδs 2S max Accuracy of threaded joints Plastic threads 7