Eastman polymers ™ Processing and mold design guidelines
Proper mold design and machine setup are essential parts of a quality molding operation. This publication is intended to assist you in the design or conversion of injection mold tooling and in machine setup to process Eastman™ polymers. Investing in high-quality molds can reduce costs and increase profits over the entire life of the mold.
Contents Eastman™ polymers—Processing and mold design guidelines . . . . . . . 6 Part I Mold design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Design for moldability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Mold filling analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Stress concentration factors . . . . . . . . . . . . . . . . . . . . . . . . .
Hot drops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Valve gates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Processing conditions using hot runner systems . . . . . . . . . . . . . . . 20 Venting and ejection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Dryer troubleshooting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Injection molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Molding conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Trial preparation and operation . . . . . . . . . . . . . . . . . . . . . . . . . 32 Start-up . . . . . . . . . . . . . . . .
Eastman™ polymers Processing and mold design guidelines Part I Mold design Design for moldability gate. Imbalanced fill can result in underpacked areas or stalled melt flow fronts that become cool and difficult to restart, causing molded-in stress and nonfill conditions. • Appropriate weld line location Mold filling analyses are critically dependent on the viscosity/ temperature/shear rate relationship of the molten plastic.
Stress concentration factors Coring thick wall sections Stress concentrations are areas that, by the nature of their design, tend to concentrate or magnify the stress level within a part. This increase in localized stress may allow the part to fail prematurely by serving as a crack-initiation point. Design features that can serve as stress concentrators are: A part can rarely be designed with uniform wall thickness because of such features as ribs and bosses.
Ribs are generally spaced at a distance at least twice the wall thickness to allow enough steel between the ribs for adequate cooling. A draft angle of 1° is suggested to allow proper part ejection from the mold. Finally, be sure to add a radius at least 0.2 to 0.5 times the wall thickness at the base of the rib to reduce stress concentration effects. Cooling By designing parts so that they can be cooled properly, you can obtain lower cycle times and high-quality parts while reducing cost.
Mold shrinkage and warpage Core cooling techniques Key factors in minimizing warpage include: Methods of achieving proper core cooling include: • Uniform wall thickness • Baffles • Consistent mold temperatures • Bubblers • Highly conductive alloys A uniform wall: • Circular cooling channels around cavity and core inserts • Promotes even flow • Minimizes shear heating Any of these methods are suitable, provided the heat removal capacity is sufficient to maintain uniform temperatures down the lengt
A typical bubbler configuration is shown in Figure 6. A bubbler is very similar to a baffle, except water is supplied to the end of the core with a bubbler tube. Water returns from the core through the annular space between the bubbler tube OD and the core ID.
NOTE: • Reynolds number must be calculated for each area of the mold having different cooling line diameters. • A water line in parallel should have the actual flow rate recalculated if the measured flow occurs prior to branching. • A pressure differential of 0.138 MPa (20 psi) is typically needed to achieve a good flow rate. Table 1 Kinematic viscosity for water °C (°F) Viscosity, centistokes 0 (32) 1.79 4 (40) 1.54 10 (50) 1.31 16 (60) 1.12 21 (70) 0.98 27 (80) 0.86 32 (90) 0.
Why corners are critical cooling areas Figure 7 illustrates that there is approximately 3 times the steel mass on the outside of a corner than on the inside. Thus, it is much easier to remove heat from the plastic on the outside than on the inside because there is more steel in which to place cooling. This concept also holds true for cores in general: they are more difficult to cool than cavities, especially as the size of the cores decreases. To overcome this effect, good core cooling is critical.
Spiral cooling sprue insert Figure 10 Sprue design Taper 3.0° included angle High polish (in direction of draw if possible) Sprue 80 mm (3 in.) max. Generous radius Runner Figure 12 Spiral cooling sprue insert Sprue location Sprue puller Double-lead thread design Use ejector pin. Air poppet would cause hot spot and impede cooling. Sprue cooling IN OUT Sprue bushing In Figure 11, upper and lower cooling line circuits are shown around the sprue to aid in cooling.
Figure 13 High-conductivity sprue bushing 5.6 mm (0.219 in.) Figure 15 Improved cooling Performance alloy sprue bushing Provide cooling around sprue area 76.2 mm (3.0 in.) Shorten sprue bushing 2.5–3 D • Stainless steel nozzle seat • Reduces cycle times – Wear resistant – Nozzle heat resistance • Enhances release of molded parts • Rigid sprues for pickers or grabbers U.S. Patent 4,950,154 Mold/sprue cooling example Figure 14 shows a part with inadequate cooling.
Gate design Runner cross section Remember that flow efficiency in runners increases as the cross section approaches a circular shape. The most efficient runner is a round one, as shown in Figure 17. However, this requires machining both halves of the mold across the parting line. Typically, a compromise is reached with the half-round approach.
It is good practice to gate into areas where the flow path is continuous and smooth, rather than into notches or ribs. Streamlining the flow path helps maintain low shear. No sharp corners or sudden changes in thickness should be allowed. If a transition is needed from a thick sprue or runner to a thin wall, the change needs to be smoothly radiused over the available distance. Figure 20 Typical tunnel gate design • 50 mm (2 in.
Edge gate A gate land of 1.0–1.5 mm (0.040–0.060 in.) is suggested. A generous radius at the edge of the gate will yield improved flow characteristics and reduce gate blushing. In general, the gate thickness should be 0.5 to 0.80 times the part thickness. See Figure 22. Figure 22 Edge gate guidelines Part T 0.5T to 0.80T Generous radius Gate land 1.0–1.5 mm (0.040– 0.060 in.) Gating parts with maximum dimensions of 50 mm (2 in.) or less • Gate diameter 0.9 to 1.3 mm (0.035 to 0.050 in.
Hot runner systems Design guidelines Hot runner systems are common in applications using polyester materials. When properly designed, these systems can eliminate sprue and runner regrind, mold with lower pressures, reduce cycle times, and improve processing windows. The selection of a suitable hot runner system can vary greatly depending on the size of the part, polyester formulation, and part design.
Excellent thermal control at the tip of the hot drop is critical to proper operation of this type of system. There is a relatively small distance between the bottom of the hot drop, maintained at the desired melt temperature, and the cavity surface, which must be cooled to the desired mold temperature. Heat transfer from the heated drop to the surrounding mold steel is minimized with an insulated gap in the annular space between the hot drop and the mold steel.
It is important to maintain suggested tool temperatures at the interface with the part. An independent cooling circuit in close proximity as shown is always suggested. Another viable solution for temperature control is a water-jacketed insert. These are sometimes custom fabricated but are also available as standard items from some of the manufacturers. These usually result in a witness around the gate which may need to be taken into consideration.
Venting problems can also be attributed to improper location of vents. Venting problems can sometimes be solved by relocating the gate so that the last area to fill is shifted to an area that has better venting. Ejection systems in molds designed for Eastman™ polymers In general, Eastman™ polymers can run in molds designed for other polymers without the need for additional ejection. As with any other polymer, use plenty of ejector pins or ejection sleeves where practical.
Family molds Family molds contain two or more cavities that mold different parts. Eastman™ polymers are being used successfully in family molds. Like any other polymer, their flow into the individual parts must be balanced. All parts should fill evenly and equally. Otherwise, uneven packing will occur; some parts will be overpacked and highly stressed, leading to warpage, and other parts will be underpacked or not completely filled.
Using zero draft Zero draft is not recommended. It can cause the mold to lock up during ejection. It can also increase the cost of the mold significantly because of the additional mechanisms required for ejection. If zero draft is necessary, it can be more easily accomplished in cases where the cores are short, the parts are thick so that the walls will not tend to shrink tightly to the core, or sleeve ejectors are used.
Nicklon™ coating This alloy is 10.5% phosphorous-dissolved nickel with 25% PTFE suspended in solution. It is applied using electroless co-deposition. As the coating wears down, new PTFE particles are continuously introduced to the mold surface, maintaining lubricity over a long period of time. The coating is known to improve chemical resistance of the steel surface. However, because of the slightly porous nature of the co-deposition, in extremely harsh environments, the coating can be chemically stripped.
Clamping force While vented barrels have been used with limited success, they are not a substitute for proper drying. In addition, the vent should be kept clean when processing clear material. Volatiles from polymers can accumulate and carbonize in the vent; this can cause the polymer being processed to be contaminated with black specks.
Good temperature control of the nozzle is important. If a thermocouple at the threaded end of a long nozzle is controlling a heater band or bands at the other end, temperatures at the heater band end can be more than 55°C (100°F) higher than the thermocouple is able to sense. This can be checked by inserting a needle pyrometer to different depths in the nozzle opening.
Figure 31 Typical desiccant dryer Regeneration blower Process heater Regeneration air filter Regeneration heater Regeneration temperature control Desiccant carousel Aftercooler Return air filter Tray dryers. These dryers can be used only if they are supplied with air dried by a good desiccant bed system. Tray dryers with heating only (and no desiccant) do not adequately dry the pellets.
Drying time. Pellets to be dried need to be in the hopper at the conditions shown on the data sheets for each specific polymer. If the dryer is turned on from a cold start, it must warm up to the proper temperature and the dew point of the air must be reduced to –30°C (–20°F) or below before drying time can be counted. Choosing the hopper size is critical; only when the hopper size is adequate for the rate of processing will the proper residence time in the hopper be possible.
Moisture measurement Common dryer problems Dew point meters measure only the dryness of the air, not the dryness of the plastic pellets in the hopper. Use of the dew point meter along with measurements of temperature, airflow, and time can give an accurate indication of whether the plastic pellets are being dried properly. • Poor airflow caused by clogged filters Weight loss type moisture meters are instruments that measure the moisture inside pellets.
Dryer troubleshooting guide For more detailed information, see the “Troubleshooting guide” on page 39. Problem Corrective action Desiccant worn out or saturated Dry cycle machine or replace desiccant. Incorrect desiccant type Replace desiccant with type and size recommended by dryer manufacturer. Regeneration heaters burned out Replace heaters. High dew point Regeneration filter plugged Clean or replace filter. (wet air) Regeneration blower reversed Reverse electrical connections.
Injection molding Proper conditions and machine operations for molding Eastman™ polymers are discussed in this section. It includes sections on start-up, purging, use of regrind, and shutdown. The recommended processing conditions are listed on the technical data sheet for specific grades of Eastman™ polymer. These data sheets can be found online at www.eastman.com. Molding conditions Barrel and melt temperatures The first consideration in setting barrel temperatures is how much shot capacity will be used.
of the screw at the end of the shot indicates a leaking check valve. A leaking check valve will prevent a cushion from being maintained and can cause random short shots and shot-to-shot variability. Back pressure Typical back pressure is 7–10 bar (100–150 psi), though it may be as low as 3.5 bar (50 psi). To improve melt uniformity, increase melt temperature, or eliminate air entrapment (air splay), back pressure can be increased to as much as 28 bar (400 psi).
Production molding Some typical sources of contamination include: Production start-up • Heat transfer tape The processing window needs to be defined to establish a controlled molding process. The starting point for a production run is typically the same as the conditions used in the last molding trial or start-up run. The machine should be cleaned before production is started.
Part III Secondary operations Parts made of Eastman™ polymers can be assembled using a wide variety of joining techniques. Solvent bonding, UV-curable adhesives, and ultrasonic welding are the most common methods of joining medical devices and are suitable techniques for joining parts made of Eastman™ polymers.
Ultrasonic welding Ultrasonic welding advantages, shear joint vs. energy director Ultrasonic welding can be performed successfully using sheartype joints. Shear joints produce strong, hermetic bonds. Shear joints for Eastman™ polymers should closely follow the design suggested in Figure 32. In some limited situations, energy directors may work satisfactorily; but in most situations, the bond will not retain enough toughness for the application.
Ultrasonic staking Ultrasonic staking is an assembly method in which the controlled melting and reforming of a plastic stud is used to capture or lock two components together. This method employs the use of a plastic stud on one part, which protrudes through a hole or slot on the second part. The vibrational energy applied by the ultrasonic horn is used to melt the plastic; the plastic is deformed by pressure applied to the staking horn.
Other fastening techniques Bosses, snap fits, and locating features that take advantage of the ductility and toughness of these polymers should be designed using proper part design guidelines. Many Eastman™ polymers have the required ductility to perform well in swaging and staking operations. The material can be softened using a heated horn or ultrasonic energy, and in many cases, the material can be swaged “cold.” Conversions of metric/U.S.
Splay is an appearance defect in the surface of the part, usually appearing as trapped gas bubbles being smeared across the surface as the flow front moves to fill the part.
Troubleshooting guide: Molding Eastman™ polymers PROBLEMS Suggested remedies Do 1 first, 2, 3, etc.
“Reading” parts as they are molded Part failure: causes and analysis Often some very small marks and other visible features of a part can tell much about improvements needed in the molding process. It is usually best to examine the entire shot, including sprue, runner, and parts from all cavities when defining a problem in molding. It is also advisable to do this at the operating machine so that guesswork concerning molding conditions can be avoided.
Form 1 PRETRIAL PREPARATION Company______________________ Date Sch.____________ Material ____________________ Location______________________ Time Sch.____________ Pounds ____________________ Part______________________ Received?____________________ Trial Objectives (short demo, fast cycle, production run, etc.
Form 2 MOLDING CONDITIONS RECORD Company__________________________________________ Company Representative Date/Time_________________________________________ Part ______________________________________________ Mold No.__________________________________________ No. cavities________________________________________ Eastman Representative Machine No.
Form 3 Identifying problems that are causing scrap 43
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