PETpla.net Insider 09 / 2016

CAPPING / CLOSURES 46 PET planet insider Vol. 17 No. 09/16 www.petpla.net 4.5.2.1 Properties HDPE is flexible, shows good weather resistance, is tough at low tem-peratures up to -60 °C, and is highly resistant to many acids, alka- lis, and solvents. It is quite easy to process and at the lower end when it comes to cost (fig. 4.35). Disadvantages of HDPE are its lower heat resistance and higher pro- pensity for stress cracking. Closures with high molded-in stress may fail the drop test many applications require. Closed-loop control of injection parameters is recommended to avoid this situation. Physical Properties Tensile Strengh 0.20 - 0.40 N/mm² Notched Impact Strengh no break Kj/m² Thermal Coefficient of expansion 100 - 220 x 10 -6 Max Cont Use Temp 65 °C Density 0.944 - 0.965 g/cm³ Resistance to Chemicals Dilute Acid **** Dilute Alkalis **** Oil and Greases **variable Aliphatic Hydrocarbons * Aromatic Hydrocarbons * Halogenated Hydrocar- bons * Alcohols **** Key * poor ** moderate *** good **** very good Figure 4.35 HDPE properties at a glance 4.6 Manufacturing Methods may get damaged or worn over time and should be checked on a regular basis. They often fail intermittently causing inconsistencies in the molded closures, which may lead to hours of fruitless troubleshooting. A cold or hot runner divides the single stream of molten plastic into the appropriate number of cavities. All modern tools use hot runners that heat the material continuously on its way to the mould and produce no scrap in the process. In contrast, plastic in cold runner channels solidi- fies during each cycle and this must be ejected. This is neither practical nor economical in beverage closure production. As the plastic in the water-cooled mould cools and shrinks, the screw keeps the so-called hold or packing pressure on the material and supplies enough molten resin to replace the material that shrank away. The screw then starts turning and keeps plasti- cising material while the tool opens and ejects the parts. During the ejec- tion, the caps are mostly “bumped” off, i.e., the undercuts created by the thread beads are pushed forward and this forces the caps to bulge out by the amount of the thread depth. To accomplish this, caps have to be fairly warm, around 70 °C. The tooling is made to such a dimension that the caps will shrink back to their desired dimension when they cool down. Caps may also be unthreaded by hydraulic or mechanical devices instead of being bumped, but the high cost and cycle time penalties these devices demand have led tool manu- facturers to come up with nifty solu- tions that make them unnecessary. The point where plastic is injected into the cavity (the “sprue point”) through a suitable injection nozzle is slightly raised as the plastic is cut off there. In order not to have a protru- sion on the cap surface that may scratch a consumer’s finger, modern caps slightly indent the area around it so that the protrusion stays shy of the surface. IM is the most flexible process, and closures of any kind including Caps are either injection or com- pression moulded. There are distinct advantages and disadvantages for either system. This chapter gives an overview of both methods with the goal of helping users to make the right decision on which system can be used when there is a choice. 4.6.1 Injection Moulding IM of closures is using one male core and one female cavity for each cap produced per cycle. These are mounted in configurations of multi- cavity tools up to 192 cavities with most moulds in the 24- to 64-cavity range. Most machines cycle hori- zontally open and close, and caps usually fall onto a conveyor after each cycle. Plastic pellets are fed into the machine via a hopper. If coloured caps are to be produced, the resin may be precoloired or, more likely, colour is fed just underneath the hopper into the resin stream in ratios of 0.5–4%. Colour comes as mas- terbatch pellets or micro-pellets or in liquid form. Liquid colour is some- what messy to deal with but allows very precise dosing and is especially suitable for very small colour percent- ages. Masterbatch and micropellets use LDPE as a carrier often at a ratio of about 50%. The resin is then taken by a rotat- ing extrusion screw and melts under the combined effects of friction and heat. Usually the screw both melts the resin and injects it into the tool. During the melt phase or recovery phase, the screw turns and moves backward. Once enough material is in front of the screw, the screw stops and injects the melt into the closed tool after the caps made in the previous shot have been ejected. Screw size must be chosen so that screw recovery time is shorter than cooling time plus tool moving time. This is to ensure that the machine does not have to wait for screw recovery to finish before start- ing the next cycle. A suitable check valve sits at the end of the screw closest to the mould. It allows material to pass to the front of the screw during recovery but prevents material from moving back during injection. Check valves Figure 4.34 The HDPE molecule