How to injection mold cyclic olefin copolymers.This new family of clear engineering thermoplastics made its first big splash Big Splash could refer to:
The first substantial wave of molded applications for the Topas family of metallocene-catalyzed cyclic olefin copolymers Cyclic Olefin Copolymer (COC) is an amorphous polymer made by several polymer manufacturers. COC is a relatively new class of polymers when compared to polypropylene and polyethylene. (COOs) is just now arriving on the market. Some limited molding activity with these new transparent engineering thermoplastics dates back to 1969, but injection molded (and injection-blow molded) medical and optical applications are only now starting to appear in force. Emerging uses include optical lenses, light guides, microtiter plates, syringe bodies, tubing connectors, catheter manifolds, and lids. Ticona introduced Topas COCs five years ago as a lower-cost alternative to other COC See chip on chip. materials produced by non-metallocene technologies. The first commercial plant (66 million lb/yr) started up in 2000. Metallocene-catalyzed COCs offer excellent optical properties, high flow, and ultra-low moisture absorption. In addition, their density is 3% lower than that of polystyrene and 15% less than polycarbonate A category of plastic materials used to make a myriad of products, including CDs and CD-ROMs. and acrylics. Up to now, these properties have been exploited mainly in extruded film and sheet. For injection molders, COCs offer glass-like transparency and excellent barrier to water vapor with high HDT HDT Heat Deflection Temperature (plastics) HDT High Dose Therapy HDT Heatpipe Direct Touch (Xigmatek) HDT Heat Distortion Temperature (plastics) HDT Henry David Thoreau and good electrical properties. COCs are amorphous polymers with high modulus and surface hardness. Chemically, they resist hydrolytic hy·drol·y·sis n. Decomposition of a chemical compound by reaction with water, such as the dissociation of a dissolved salt or the catalytic conversion of starch to glucose. degradation, aqueous aqueous /aque·ous/ (a´kwe-us) 1. watery; prepared with water. 2. see under humor. a·que·ous adj. acids and bases, and most polar and oxygenated solvents. In injection molding injection molding n. A manufacturing process for forming objects, as of plastic or metal, by heating the molding material to a fluid state and injecting it into a mold. , they provide high precision, and they flow well to fill complex, thin-walled parts with low shrinkage and warpage. Because COCs' stiffness is 25% greater than that of polycarbonate, they lend themselves to molding thinner walls. COC properties & molding Injection molding grades of COC from Ticona have a glass-transition temperature ([T.sub.g]) between 70 and 185 C (158 to 365 F). As a result, their heat-deflection temperature (HDT) ranges from 75 to 170 C (167 to 338 F). Grades with [T.sub.g] above about 140 C (284 F) have a flexural flexural pertaining to the flexure of a joint. flexural deformity fixation of joints in flexion. In the newborn called contracted calves or foals. modulus in the vicinity of 500,000 psi and elongation elongation, in astronomy, the angular distance between two points in the sky as measured from a third point. The elongation of a planet is usually measured as the angular distance from the sun to the planet as measured from the earth. at break of 3% to 4%. Those with a lower [T.sub.g] have a lower modulus but greater elongation (up to 10%). Post-molding shrinkage of COCs typically falls between 0.4% and 0.7%. Shrinkage is greater with higher [T.sub.g] grades and tends toward lower values as cavity pressure rises. Shrinkage in the flow direction is only slightly greater than that in the transverse To cross from side to side. direction, so very flat parts can be molded. COCs' good stiffness and flow means that parts can have walls down to 1 mm or less. The resin replicates extremely fine mold features. One precision COC laboratory component, for instance, has complex channels as small as 10 microns. COCs also faithfully reproduce sub-micron surface patterns, such as those found on diffraction gratings and optical data-storage media. Only neat grades of Topas COC have been offered until now, but glass-reinforced, mineral-filled, flame-retardant (UL94V-2 and V-0), and impact-modified grades have been developed. Ticona has also made grades containing stainless-steel fibers to impart electrical conductivity. These new modified grades will open up potential in business machines, communications equipment, and automotive parts like headlight reflectors and air-conditioning fans. Equipment considerations COCs are usually molded on conventional machines having general-purpose, low-compression screws with ratios of 2.2:1 to 2.5:1. Screws are typically shallow single-flighted or barrier models made from standard tool steels. The screw should homogenize homogenize /ho·mog·e·nize/ (ho-moj´in-iz) to render homogeneous. homogenize to convert into material that is of uniform quality or consistency throughout; to render homogeneous. the melt gently because excessive shear heating can cause yellowing and optical inhomogeneity in·ho·mo·ge·ne·i·ty n. pl. in·ho·mo·ge·ne·i·ties 1. Lack of homogeneity. 2. Something that is not homogeneous or uniform. Noun 1. . A high-compression screw can generate too much shear and even shatter the pellets. Optical parts are best processed in machines with higher screw L/D L/D Labor and Delivery L/D Lethal Dose L/D Lift/Drag (ratio) L/D Low Dynamic L/D Limiter/Discriminator L/D Loading / Discharging Rate (shipping) ratios, which tend to produce less splay and improve recovery times with lubricated lu·bri·cate v. lu·bri·cat·ed, lu·bri·cat·ing, lu·bri·cates v.tr. 1. To apply a lubricant to. 2. To make slippery or smooth. v.intr. To act as a lubricant. grades. Barrels do not need venting because of the resin's negligible water absorption. COCs can be used with all types of nozzles, although free-flow, open types are best because they allow for easier sprue sprue, chronic disorder of the small intestine caused by impaired absorption of fat and other nutrients. Two forms of the disease exist. Tropical sprue occurs in central and northern South America, Asia, Africa, and other specific locations. pullout pull·out n. 1. A withdrawal, especially of troops. 2. Change from a dive to level flight. Used of an aircraft. 3. An object designed to be pulled out. Noun 1. . The resin also can be used with nearly all gating styles. Fan gates are recommended because of their relatively low shear. Gate size should be generous enough to prevent excessive shear heating or premature freeze-off. Gating preferably should be into the thick end of a part and gate diameter should be at least 60% of the wall thickness. COCs with [T.sub.g] above about 280 F should be run on oil-heated tooling. Grades with a lower [T.sub.g] can use a circulating hot-water system for mold-temperature control. Tools can be made of standard mold steels but should be polished well to eliminate surface imperfections. This is vital for optical applications, where a finish down to a quarter of a wavelength of light may be required. Because of COC's low shrinkage, draft angles should be as large as possible--especially for parts with longer bearing lengths--to avoid distortion and demolding difficulties. Sticking can be a problem on parts with a shallow draft. Draft usually falls between 2[degrees] and 5[degrees], though draft as low as 0.5[degrees] has been used successfully in some cases. Draft is affected by part length and must be higher with textured surfaces. Molders switching to COC from a material with greater shrinkage should modify the tool so it has sufficient draft for part release. If the tool was previously used for polycarbonate or acrylic, it can typically run COC without any modification. Molding guidelines COCs absorb almost no water and do not need drying. If the resin does become wet, surface water can be driven off by about 2 hr of heating at 54[degrees] F below the resin's HDT. When feeding COC pellets, allow them to warm up in the feed throat to 45[degrees] to 54[degrees] below the HDT by throttling back on the cooling water. This makes the pellets less brittle and therefore less likely to generate fines that can affect part homogeneity. Screw speed is usually in the moderate-to-fast range (50 to 150 rpm), depending on screw diameter and barrel size. The smaller the screw, the higher the rpm needed. Take care to homogenize the melt well, but without undue shear heating. Cylinder temperatures should be matched to the Tg of the grade used (see table). Melt temperatures are typically 200[degrees] to 215[degrees] F above the Tg and usually fall between 464 F and 572 F. (Molders tend to run at the high end of this range.) So far, COCs have been molded almost exclusively on reciprocating-screw injection presses. Though there is little industry experience with COCs in two-stage screw/plunger systems, their thermal stability should be adequate for use in such a system. Mold temperature for COCs should not be more than 90[degrees] F below the Tg for lower-temperature grades and not more than 54[degrees] below Tg for high-heat grades. Warmer molds minimize silver streaks, decrease molded-in stresses, and preserve clarity. Because COCs set up rapidly and their viscosity rises steeply as they cool towards Tg, molds should be hot enough to minimize weld-line formation. Injection moldable COCs have melt flow rates between 1 and 56 g/10 min at 260 C. Shot size should be one-quarter to one-half the capacity of the barrel. COCs can withstand barrel residence time of about 15 min at normal molding temperatures. If process interruptions are likely to be longer, maximum barrel temperature should be reduced to 302 F to avoid yellowing. Normal fill times are about 1 to 2 sec and cycle times typically extend from 10 sec for small, thin parts to 360 sec for parts 0.5-in. to 1-in. thick. Fill times for optical parts maybe extended to 5 or 10 sec when using a mold temperature within 50[degrees] F of Tg. Residual stress Residual stresses are stresses that remain after the original cause of the stresses (external forces, heat gradient) has been removed. They remain along a cross section of the component, even without the external cause. in parts can be limited by increasing injection speed in steps and by using mold temperatures as close to the resin's Tg as possible. For low-stress parts, you should also avoid overpacking and long hold times. Try reducing holding pressure in stages to between 4000 and 7000 psi. Given COCs' low shrinkage, lubricated grades and mold-release agents can help in removing parts from a mold, especially if long cores are present. Choose mold sprays carefully to ensure they do not leave part surfaces hazy. Mold release can be helped by maintaining holding pressure a bit longer so the part relaxes before it is extracted. COC parts emerge from the mold with smooth surfaces and need little or no deflashing or finishing. They have a soft surface until they cool completely, so just-molded parts should be handled carefully. If possible, degate COC parts when they are still warm. For degating cold parts, use a band saw because snipping can cause fractures. COCs retain mechanical properties well over multiple regrind cycles and as much as 20% regrind can be used in many parts. Reground COC may yellow slightly during molding, so regrind cannot be used in optical applications or where very high clarity is needed. When molding optical parts, use a high mold temperature and keep injection speed as fast as possible to prevent flow marks. On the other hand, slower injection may be needed to overcome sinks, voids, and birefringence Birefringence The splitting which a wavefront experiences when a wave disturbance is propagated in an anisotropic material; also called double refraction. In anisotropic substances the velocity of a wave is a function of displacement direction. in thick optical parts. Some suck-back may be needed to prevent nozzle drool. Higher backpressure--as much as half of a machine's maximum pressure--helps improve optical properties but should not be so high as to yellow the part or hinder screw refraction refraction, in physics, deflection of a wave on passing obliquely from one transparent medium into a second medium in which its speed is different, as the passage of a light ray from air into glass. . Low holding times and pressures are also usually good for optical parts. Holding time varies from 30 to 60 sec, depending on part thickness. Cooling time (Law) such a lapse of time as ought, taking all the circumstances of the case in view, to produce a subsiding of passion previously provoked. - Wharton. See also: Cooling can last from 60 to 120 sec. Longer cooling time helps reduce stress level in the part. Total cycle time for optical parts is typically 2 to 5 min. COCs should be processed in hot molds to maximize flow length and weld-line strength and prevent shock freezing that can embrittle em·brit·tle tr. & intr.v. em·brit·tled, em·brit·tling, em·brit·tles To make or become brittle. em·brit parts and make them prone to stress cracking. Hot molds also reduce haze, splay marks, and streaks and improve surface quality. Mold temperature is often the determining factor in reducing haze in optical parts. Each COC grade has an optimum mold temperature that minimizes part haze. Streaking can be controlled by using the proper balance between injection speed and mold temperature. Bubble formation and nozzle drool can be reduced by using moderate backpressure back·pres·sure n. Residual pressure opposing the free flow of a gas or liquid, as in a pipe or an exhaust system. (between 20% and 30% of maximum). If discoloration dis·col·or·a·tion n. 1. a. The act of discoloring. b. The condition of being discolored. 2. A discolored spot, smudge, or area; a stain. Noun 1. occurs in parts that need extreme clarity it helps to use a nitrogen blanket over the hopper to keep oxygen away from the resin. Even small amounts of contaminants in COCs can cause bubbling, black specks, haze, and smoky swirls in clear parts. To prevent this, molders should thoroughly purge other materials from the barrel and feed areas and from material-transfer systems. It is best to remove the screw and clean both it and the barrel. Secondary operations COCs can be finished by printing, coating, and metalizing as well as joining and machining. COCs are nonpolar nonpolar not having poles; not exhibiting dipole characteristics. materials, so relatively few materials stick to them. Before a COC part can be printed or coated, it needs corona or plasma pretreatment pretreatment, n the protocols required before beginning therapy, usually of a diagnostic nature; before treatment. pretreatment estimate, n See predetermination. to functionalize its surface and allow bonds to form. Metal coatings Metal coatings Thin films of material bonded to metals in order to add specific surface properties, such as corrosion or oxidation resistance, color, attractive appearance, wear resistance, optical properties, electrical resistance, or thermal protection. are an exception: Vacuumdeposited aluminum, chromium, silver, nickel, and other metals adhere well to COCs without pretreatment. COC parts can be solvent bonded using a 15% solution of COC in cyclohexane cyclohexane (sī'kləhĕk`sān), C6H12, colorless liquid hydrocarbon. It is a cyclic alkane that melts at 6°C; and boils at 81°C;. It is nearly insoluble in water. , heptane hep·tane n. A volatile, colorless, highly flammable liquid hydrocarbon, C7H16, obtained in the fractional distillation of petroleum and used as a standard in determining octane ratings, as an anesthetic, and as a solvent. , or other aliphatic aliphatic /al·i·phat·ic/ (al?i-fat´ik) pertaining to any member of one of the two major groups of organic compounds, those with a straight or branched chain structure. al·i·phat·ic adj. solvent. Some commercial adhesives, such as those based on polyurethane, also work well with this polymer. COCs can be ultrasonically welded to depths of 0.5 to 1.0 mm in 1 to 3 sec. Vibration amplitudes are typically 15 to 35 microns. In machining COC parts by cuffing, drilling, and diamond turning, cuts should be conservative and made at relatively low speeds. This keeps the applied force low, so the COC does not shatter, and it prevents excessive heat build-up. Common machine oils attack COCs and can cause cracking, so coolants must be water-based and machine surfaces and tools must be free of oil. NEED TO KNOW MORE? Ticona Summit N.J. (800) 838-4882 www.ticona.us.com [GRAPH OMITTED]
INJECTION MOLDING TOPAS COCs
(TYPICAL START-UP CONDITIONS)
Grade 8007 5013 6013 6015 6017
Cylinder Temp, F
Feed <140 <230 <230 <230 <230
Rear 374-428 410-464 446-482 464-518 464-518
Center 392-464 446-509 464-518 482-554 482-572
Front 428-482 446-509 464-518 482-554 482-572
Nozzle 428-482 446-482 446-500 464-500 464-586
Melt Temp., F 374-482 464-518 464-518 500-572 500-572
Mold Temp., F 104-158 230-266 230-266 230-300 266-388
MACHINE SETTINGS FOR INJECTION MOLDING COC
Injection Pressure, psi 7000-16,000
Hold Pressure, psi 6,000-10,000
Injection Speed Moderate to Fast
(0.5 to 6 in./sec)
Backpressure, psi >50 for GP molding
>100 for improved optics
Screw rpm 50-200
Screw Suck-Back None
Cushion Small (4 mm typical)
Screw Type Low compression preferred
Nozzle Type Free-flow
Drying Not normally needed
RELATED ARTICLE: Troubleshooting COC Injection Molding Burn Marks * Check for contamination * Decrease injection rate * Improve venting in mold cavity * Increase gate size * Clean tooling and nozzle tip Dimensional Variability * Confirm full screw recovery in allotted al·lot tr.v. al·lot·ted, al·lot·ting, al·lots 1. To parcel out; distribute or apportion: allotting land to homesteaders; allot blame. 2. time * Check for leaking check ring * Maintain uniform feed and cushion * Fill mold as rapidly as possible * Check machine hydraulic and electrical systems for erratic performance Discoloration * Purge heating cylinder * Lower material temperature * Provide additional vents in mold * Move mold to machine where shot size is 50% to 75% of capacity * Lower cylinder temperatures to 338 F during extended periods of stoppage Nozzle Drool * Lower nozzle temperature * Lower material temperature * Use nozzle with smaller orifice orifice /or·i·fice/ (or´i-fis) 1. the entrance or outlet of any body cavity. 2. any opening or meatus.orific´ial aortic orifice * Use reverse-taper or positive shut-off nozzle Nozzle Freeze-Off * Insulate nozzle from mold * Increase nozzle temperature * Decrease overall cycle time * Increase mold temperature * Use nozzle with a larger orifice Splay Marks, Streaks & Blisters * Check for contamination * Eliminate screw decompression decompression /de·com·pres·sion/ (de?kom-presh´un) removal of pressure, especially from deep-sea divers and caisson workers to prevent bends, and from persons ascending to great heights. * Shorten overall cycle * Increase injection pressure * Raise mold and material temperature Sticking in Cavities * Decrease injection pressure * Decrease injection speed * Maintain proper cushion/decrease feed * Increase mold-closed time * Lower mold temperature * Check for undercuts and/or insufficient draft Sticking in Core * Decrease injection pressure * Decrease hold pressure * Minimize cushion * Decrease mold-closed time * Decrease core temperature * Check for undercuts or insufficient draft Poor Weld Lines * Raise mold temperature * Raise melt temperature * Increase injection speed * Relocate gate to alter flow pattern * Provide overflow well at weld area Unmelted Pellets * Change to a machine with plasticating capacity adequate for shot size and cycle speed required * Raise barrel temperatures, especially at rear zone * Increase screw speed * Increase backpressure * Check for proper screw compression * Change to a larger L/D barrel Warpage/Part Distortion * Equalize e·qual·ize v. e·qual·ized, e·qual·iz·ing, e·qual·iz·es v.tr. 1. To make equal: equalized the responsibilities of the staff members. 2. To make uniform. temperature in both mold halves * Check uniformity of ejection ejection /ejec·tion/ (e-jek´shun) 1. the act of casting out or the state of being cast out, as of excretions, secretions, or other bodily fluids. 2. something cast out. 3. * Check handling of parts after ejection * Increase hold time * Alter injection pressure * Increase cooling time * Reduce material temperature * Try differential mold temperatures to counteract warp At Ticona in Summit, N.J., Dr. Ronald R. Lamonte is a development associate; Dr. Donal McNally is a product marketing manager; Ken Music is a technical service engineer; and Doug Hammond Doug Hammond (born December 26, 1942) is a free funk/avant-garde jazz drummer, professor, composer, poet and producer from Tampa, Florida, U.S.A. His first major release was Reflections In The Sea Of Nurnen on Tribe Records. is a development engineer. |
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