7 Venting of Molds

7 V e n t i n g of M o l d s During mold filling the melt has to displace the air which is contained in the cavity If this cannot be done, the air can prevent a complete filling of the cavity Besides this the air may become so hot from compression that it burns the surrounding material The molding compounds may decompose, outgas or form a corrosive residue on cavity walls This effect can occasionally be noticed in poorly vented molds at knit lines or in corners or flanges opposite the gate Burns usually appear as dark discolorations in the molded part and render it useless If this residue is not carefully removed again and again, it may cause irreparable damage to the mold from corrosion and abrasion Table 7.1 [7.1] summarizes the major consequences of inadequate mold venting Table 7.1 Consequences of inadequate mold venting [7.1] For injection molding For the molded part For the mold Burn marks due to diesel effect Abrasion (leaching) through Irregular processes through combustion residues in the blockage of venting channels combustion gas —> diesel effect Structural defects/surface defects through detachment of the polymer from a structured mold wall Corrosion by aggressive gases Longer cycle times due to —> diesel effect increased back pressure in the cavity Overpacking due to injection pressure set too high when vents clogged Mold coated by combustion residues in the combustion gas -> diesel effect Short service life of machine due to higher loading Displacement of weld lines due to changes in vents (e.g through increasing clogging of venting elements) Mold exposed to direct heat due to strong air heating during compression (diesel effect) => hardening of outer layer (varying with steel grade) Escaping gases during compusting of the polymer (diesel effect), may be harmful to health, depending on material Entrapped air (voids) Increased cleaning of venting elements Longer setup time through higher scrap rate Incomplete mold filling Higher repair and maintenance costs Greater need for pressure due to increased back pressure in the cavity Reduction in strength, especially at weld lines Injection molding machine has higher energy requirements For systematization of the subject "mold venting", a distinction is drawn between "passive" venting, where the air escapes from the cavity due to the pressure of the incoming melt, and "active" venting where a pressure gradient is created to artificially remove the air 7.1 Passive Venting Most molds not need special design features for venting because air has sufficient possibilities to escape along ejector pins or at the parting line This is particularly true if a certain roughness is provided at the parting line such as planing with a coarse-grained grinding wheel (240 grain size) This assumes filling the cavity in such a way that the air can "flow" towards the parting line A grinding direction radial to the cavity has been successfully tested (Figure 7.1) Air pockets must not be created Figure 7.1 Special ground sections in sealing surface [7.2 to 7.4] The configuration of the part, its position in the mold, and its gating have a considerable effect on venting This can be best demonstrated with some examples The tumbler in Figure 7.2 is gated at the bottom The confined air is pushed towards the parting line and can escape there Special provisions are not necessary The design of Figure 7.3 is different The tumbler is gated on the side During mold filling, the melt flows around the core first, closes the parting line and then rises slowly pushing the air ahead into the bottom of the tumbler Here the air is compressed and overheated To prevent this, special design steps are necessary The same holds true for the mold in Figure 7.4a The enclosed air in the rib cannot escape because the melt first crosses the base of the rib Figure 7.2 Tumbler gated at bottom, favorable gate location Figure 7.3 Tumbler laterally gated, poor gate location for venting Figure 7.4 Part with rib [7.3]: a Air cannot escape from rib section and is trapped (1), b Remedial measure: Additional parting line (2) In both cases adding a joint can provide venting of the cavity The tumbler mold (Figure 7.3) can obtain an additional joint if the bottom piece is made separately (Figure 7.5) or a cylindrical insert is used, which lets the air escape A mark on the bottom of the tumbler, of course, cannot be avoided If it is a nuisance, it can be converted into a decorative line [7.5] Figure 7.5 Laterally gated cup Mold with additional parting line: Venting through parting line (left) or venting pin (right) In the case of a rib (Figure 7.4a), an additional joint for venting is obtained by dividing the forming inserts into two pieces (Figure 7.4b) For the presented solution it was assumed that the air can escape through joining faces, but this is feasible only if the faces have sufficient roughness and the injection process is adequately slow to allow the air to escape This solution fails for molding thinwalled parts with very short injection times Here, special venting channels become necessary In the case of the center-gated tumbler (Figure 7.2), a solution is found by machining an annular channel into the parting-line plane into which the air can escape via one or more venting gaps during injection, and then from the mold through a venting channel Dimensions for these channels can be taken from Figures 7.6 and 7.7 A logical development of this approach is that of continuous venting [7.6, 7.8] or peripheral venting [7.9] The annular gap is not interrupted by flanges but is continuous Penetration by the melt is prevented by adjusting the gap width so as to just prevent ingress of melt Usual gap widths vary from 10 urn to 20 urn in accordance with the polymer employed [7.8] Figure 7.6 Cup mold with annular channel for venting [7.6] Detail X Detail Z Figure 7.7 Mold venting through venting gaps and annular channel [7.7] Figures 7.8 and 7.9 demonstrate another option for venting molds with large surface areas by using a set of lamellae One has to bear in mind, though, that these venting elements leave marks on the molded part and may interfere with cooling lines Packs of lamellae are also of advantage if multiple gating is needed and an exact location of knit lines cannot be determined Reference is made here to Section 5.9 where it was demonstrated how knit lines and locations of possible air trapping can be Detail Z Figure 7.8 Venting with a set of lamellae [7.7]: a Spring, b Venting channel through lamellae, c Connection of venting channel with the outside Detail x Figure 7.9 Venting with sleeves [7.10] predetermined with the help of the filling image method The safest way to rule determine such potentially harmful points is to perform a computer simulation (see Chapter 14) Porous inserts of, e.g sintered metals that open out into free space, have not proved suitable because they more or less clog rapidly, the rate depending on the molding compound being processed [7.7] If the sintered metal opens into a cooling channel, however, completely new perspectives open up for the use of sintered metal mold inserts In a manner analogous to the water-jet pump, the existing cooling water circuit can provide active venting of the cavity [7.9, 7.11, 7.12] The water from the circuit draws the air out of the cavity via the sintered metal inserts, without water entering the cavity If ejector pins are located in an area where air may be trapped, they can usually be used for venting Venting can be facilitated by enlarging the ejector pin hole (Figure 7.10) This solution offers an additional advantage If needed, compressed air can be blown into the hole to support demolding In addition, the gaps are cleaned by the movement of the pins Frequently, so-called venting pins are employed They can be grooved (Figure 7.12) or kept 0.02 to 0.05 mm smaller in diameter (material dependent) than the receiving hole for a length of mm (Figure 7.11) [7.10] A venting channel follows, in which the air can expand and from where it reaches the outside through an axial groove A design according to Figure 7.13 is called "self-cleaning venting pin with ejector function" in the literature The definite advantage is based on the precise centering of the ejector, which ensures a defined venting gap [7.7] In multi-cavity molds or in molds with multiple gating, venting should already start in the runners so that air there cannot get into the cavity This prevents extensive degradation of the material by burning and the gate system can be ground and reused without major loss of quality As one can see from Figure 7.14, the same rules for venting channels apply here as for the venting at the end of the flow path There is finally the question left about the size of the venting gap To avoid flashing, a certain gap width, which is plastic-specific, cannot be exceeded after mold clamping The critical gap width for specific plastics is as follows [7.10, 7.13-7.15]: Figure 7.10 Design of holes for ejector pins permitting improved venting of cavity [7.6]: Enlarged hole about mm below the cavity wall surface Figure 7.11 Fluted venting pin [7.6] Figure 7.13 S elf-cleansing venting pin [7.7] Figure 7.14 Venting of runners [7.7] Air exit for venting Air inlet for ejection Figure 7.12 Trapped air Venting pin [7.6] Crystalline thermoplastics: PP, PA, GF-PA, POM, PE amorphous thermoplastics: PS, ABS, PC, PMMA for extremely fluid materials 0.015 mm, 0.03 mm, 0.003 mm If the venting gap is considered a rectangular diaphragm, and one assumes the validity of the laws of dynamics of gases, then, with the volume of part and runners and the injection time, the flow rate can be determined, which has to flow across the venting gap to vent the mold [7.16, 7.17]: (7.1) V VM VR tj Flow rate, volume of molding, volume of runner system, injection time If the flow rate is equated to the admittance of the assumed rectangular diaphragm (venting gap), the width of the gap can be calculated from the equation: (7.2) (7.3) (7.4) L = V[m /s], A = b • h cross section of gap [cm2], b = width of gap [cm], h = height of gap [cm], TK =temperature of air [K] For a molded part with a total volume (part + runner volume) of 10 cm3 which is produced with an injection time of 0.2 s Equation (7.2) results in a gap width of 12.5 mm if one assumes that a gap height of 0.02 mm does not yet cause flashing and the air temperature is 293 0K (20 C) Of course this venting gap has to be located where air trapping can be expected and not some place else Several points of air trapping are anticipated, then the mold has to be equipped with several vents with the sum of their cross sections at least equal to the predetermined cross section 7.2 Active Venting Aside from active venting via the cooling water circuit, which was already mentioned above, partial or complete evacuation of the cavity prior to the injection process is possible This type of venting is used in the injection molding of microstructures, since conventional venting gaps are too large for the extremely low-viscosity plastic melts used, there, and would clog [7.18] In thermoset and elastomer processing, there are applications that require evacuation of the cavity, primarily to improve the accuracy of reproduction and the molded part quality [7.19] The structure of a vacuum system is shown schematically in Figure 7.15 [7.20] The circuit diagram also contains a vacuum accumulator This is connected in series if the evacuation of large volume parts is to be accelerated; furthermore, the power consumption of the vacuum pump can then be reduced overall Evacuation of the molds is only efficient, however, if the complete mold is sealed off Due to the many moving parts on the mold, such as slides, ejectors, etc., this is extremely complicated and virtually impossible to achieve The mold is instead surrounded with a Circuit symbols as set out in DIN 28401 Gas filter, general Change in diameter or pipe Rotary slide vacuum pump Movable line Pp i e screw joint Vacuum measurement, vacuum measuring cell Ground-in ball-and-socket joint Small flange connection Conical ground joint Vacuum measuring device with digital display, recording Figure 7.15 Circuit for a vacuum unit [7.20] closed jacket or box that has just one parting line This type of construction for a microinjection mold is shown schematically in Figure 7.16 To an extent depending on the type of demolding, this design requires either no other or very few moving parts projecting out of the vacuum space; these, however, can be readily sealed [7.21] xStdtiq nbfv mdd fm|f Vacuum tank Vacuum pump WomymldMll O-ring or V-ring Figure 7.16 7.3 Evacuation of the mincroinjection mold [7.1] Venting of G a s Injection Counter-Pressure Molds Structural foam parts frequently have a rough surface as the decomposition of the blowing agent starts immediately with injection into the mold This can be avoided by suppressing the foaming of the blowing-agent-containing melt through injecting against a pressure that has been generated in the mold cavity prior to injection This counterpressure must be precisely as large as the blowing agent pressure The mold also has to be sealed (Figure 7.17) This may be done with the aid of heat-resistant seals (O-rings) To fill the mold with gas it is best to arrange a collection channel around the mold cavity that is connected via gaps to the cavity (Figure 7.18) These gaps and the collection channel allow the gas to escape again as the mold is being filled [7.22-7.24] To keep the counter-pressure constant during injection and also to ensure venting, the collection channel is connected to magnetically-controlled pressure-control valves Accumulator channel Molded part Venting gap Seal Figure 7.17 Schematic diagram of the gas counterpressure process [7.21] Figure 7.18 Mold for injection molding a decorative wood-carving imitation under gas counterpressure The mold cavity wall consists of electrolytically deposited hard nickel shell The dimensions of the gap required for filling and venting the mold are obtained with the aid of Equation (7.1, passive venting) If complete venting of the mold is not possible, e.g into blind holes, spring-actuated venting pins may be used (Figure 7.19) The required amount of pressure is readily adjusted manually via the pre-tension of the spring Figure 7.19 Schematic diagram of a venting pin References [7.1] [7.2] Notz, K: Entliiftung von SpritzgieBwerkzeugen Plastverarbeiter, 45 (1994), 11, pp 88-94 Weyer, G.: Automatische Herstellung von Elastomerartikeln im SpritzgieBverfahren Dissertation, Tech University, Aachen, 1987 [7.3] DE PS 198 987 (1961) Jurgeleit, H F [7.4] DE PS 1231 878 (1964) Jurgeleit, H F [7.5] Stoeckhert, K.: Werkzeugbau fur die Kunststoffverarbeitung 3rd Ed., Carl Hanser Verlag, Munich, 1979 [7.6] Giragosian, S E.: Continous mold venting Mod Plast, 44 (1966), 11, pp 122-124 [7.7] Sander, W.: Formverschmutzung (Formbelag)-verschleiB und Korrosion bei Thermoplastwerkzeugen Paper presented at the 2nd Tooling Conference at Wurzburg, October 4-5, 1988 [7.8] Rees, H.: Mold Engineering Carl Hanser Verlag, Munich, 1995 [7.91 Allen, P.: A non-traditional approach to mold cooling and venting SPE Injection Molding Div Conference, Columbus, OH, Oktober 20-22, 1981, pp 71-77, Confer 831 [7.10] Hartmann, W.: Entliiften des Formhohlraums Paper at the VDI Conference, Niirnberg, December 6-7, 1978 [7.11] Smith, B.: Venting is vital British Plastics and Rubber, May 1986, p 22 [7.12] Water-line venting saves the job Plastics World, 46 (1988), 3, pp 27-28 [7.13] Ufrecht, M.: Die Werkzeugbelastung beim Uberspritzen Unpublished report, IKV, Aachen, 1978 [7.14] Huyjmans, H.; Packbier, K.; Schurmann, E.: Trial run with a two-cavity mold with 12 gates at NWM, s'Hertogenbosch, 1978 [7.15] Stitz, S.; Schiirmann, E.: Measurements of deformations of injection molds at H Weidmann, Switzerland, 1976 [7.16] Wutz, M.; Hermann, A.; Walcher, W: Theorie und Praxis der Vakuumtechnik Vieweg, Braunschweig, Wiesbaden, 1986 [7.17] Speuser, G.: Evakuierung von SpritzgieBwerkzeugen fur die Elastomerverarbeitung Unpublished report, IKV, Aachen 1987 [7.18] Rogalla, A.: Analyse des SpritzgieBens mikrostrukturierter Bauteile aus Thermoplasten Dissertation, RWTH, Aachen, 1997 [7.19] Meiertoberens, U.; Herschbach, Ch.; MaaB, R.: Verbesserte Technologien fur die Elastomerverarbeitung Gummi, Fasern, Kunststoffe, 47 (1994), 10, pp 642-649 [7.20] Michaeli, W; Weyer G.; Speuser G.; Kretzschmar, G.: Entliiftung von Formnestern beim SpritzgieBen von Elastomeren Kautschuk + Gummi, Kunststoffe, 44 (1991), 12, pp 1146-1153 [7.21] Winterkemper, A.: Entwicklung eines Werkzeugkonzeptes ftir das SpritzgieBen von Mikrostrukturen Unpublished report, IKV, Aachen [7.22] Semerdjiev, S.; Popov, N.: Probleme des Gasgegendruck-SpritzgieBens von thermoplastischen Strukturschaumstoffen Kunststoffberater, 4/1978, pp 198-201 [7.23] Eckardt, E.: SchaumspritzgieBverfahren - Theorie und Praxis Kunststoffberater, 1983, 1/2, pp 26-32 [7.24] Semerdjiev, S.; Piperov, N.; Popov, N.; Mateev, E.: Das Gasgegendruck-SpritzgieBverfahren zum Herstellen von thermoplastischen Strukturschaumteilen Kunststoffe, 64 (1974), 1, pp 13-15