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Figure 33. Effect of MWD on spiral flow of HDPE (all materials have MFR = 5 gms/10 min,)
Spiral flow measures the flow length when molten resin is injection molded into a long, 0.0625" radius, half-round spiral channel (Figure 32). The higher the spiral flow number (SFN), the easier the resin is to process. The melt temperature is monitored and maintained at 440°F (227°C) and injection molding is conducted using a constant pressure of 1,000 psi (7000 kPa). Spiral flow is a more realistic measurement than melt index because it is run at a much higher shear rate allowing resins of similar MIs and different MWD to be compared at realistic conditions. The broader MWDs resins exhibit lower melt viscosity (higher SFN) at higher shear rates than narrower MWD resins with similar melt indices (Figure 33).
Since it does not take into account the effects of MWD, relying only on the melt index can be misleading. For example, ALATHON® ETP H5057, a broad MWD, 57 meltindex resin for thin-wall HDPE applications, exhibits flow properties similar to many narrow MWD resins in the 75 to 80 melt index range.
Equistar has established the use of spiral flow as a specification for all high-flow (30 melt index and above) HDPE resins and has begun reporting the spiral flow number for each lot on the Certificate of Analysis (COA). This allows the molder to compare the spiral flow of an incoming lot of resin with the SFN of the lot on-hand and readily estimate how the new lot will process relative to current production. For example, if the current lot being run has a SFN of 20 in. and the new lot has a reported SFN of 22 in., the new lot can be processed at either lower temperatures and/or at a faster production rate. Only minor adjustments in either melt temperature and/or injection pressure may be required to compensate for SFN variability from lot to lot.
General injection molding operating procedures
Prior to starting up the injectionmolding machine, be sure to have the following available:
• Safety glasses for all personnel assisting in the start-up.
• Loose fitting, heavy-duty insulated work gloves.
• A large metal container or cardboard for collecting melt produced during the start-up procedure.
• Soft beryllium-copper, bronze, or aluminum tools for use in removing any plastic from the nozzle area.
Always refer to the manufacturer’s operating manual for any specific start-up and shutdown procedures. Refer to the Equistar suggested resin startup conditions (Table 4) for general guidelines to use in starting up an injection molder on polyolefins.
General safety
As with any process involving energy and mechanical motion, injection molding can be a hazardous operation if appropriate safety procedures aren’t well documented and followed. (Refer to the Manufacturer’s operating manual.)
Mechanical, electrical, and hydraulic interlocks are critical to the safe operation of any piece of processing equipment. In some cases, these interlocks may need to be bypassed while performing set-up and maintenance functions. Under no circumstance should this be done by non-qualified personnel. In order to assure utmost safety during normal operation, interlocks should never be bypassed.
Keep all molding equipment and the surrounding work areas clean and well maintained.
Hydraulic leaks should be repaired immediately to eliminate safety hazards. Hydraulic lines, valves, fittings and hoses should be checked periodically per the manufacturer’s recommendations.
Good housekeeping is essential. Loose pellets, tools, oil, etc. on and around the molding machine can cause accidents, damage to the equipment, or contamination of the parts.
Heat
High temperatures are necessary in the injection molding process. Always use heat-resistant gloves, safety glasses and protective clothing. Modern injection molding machines have warning signs identifying specific hot areas on molding machines; do not ignore these signs. Keep the splashguard in place during purging and when the machine is operating.