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To ensure uniform fill, it is critical that the feed system (sprue, runners, and gates) be balanced. This depends on the size and location of the gates and is often determined by experience. Advances in mold filling simulation software have provided an additional tool for analysis prior to the manufacture of the tool. Finetuning may be required and is generally done by utilizing a series of short shots, observing the fill pattern, and making minor adjustments, as required. For multi-cavity tools utilizing single gates and a hot runner system, adjustment of the temperatures of the inpidual gates may be used to balance the overall fill pattern.
In high-speed, thin-wall molding, it is common to provide cooling around the gate to remove the heat produced by the high shear rates. This may be supplemented by the use of inserts fabricated of high conductivity alloys, such as berylliumcopper, in these critical areas.
Mold cooling
Although mold cooling is extremely critical to cycle time, warpage, molded-in stresses, mold-filling, etc., the sizing and layout of the cooling pattern are often overlooked and neglected aspects in the initial stages of tool design.
The cooling layout should be considered in relationship to the thickness profile of the part and the general filling pattern in order to provide adequate cooling in critical sections and not overcool others which may cause part warpage. In areas where coolant flow may be restricted due to part geometry i.e., bosses, the use of inserts fabricated from high thermal conductivity alloys, such as beryllium-copper, should be considered.
In all cases, cooling channels should be sized in relation to the available coolant flow to ensure turbulent flow which is much more effective for heat removal than lowering the temperature of the coolant. Routine inspection and acid-cleaning of cooling channels are recommended to maintain the coolant flow velocity and minimize pressure drops. Ideally, the temperature differential between coolant inlets and outlets should be about 2°F. Jumpers between cooling circuits should be avoided in order to reduce temperature differentials in the coolant.
The utilization of low pressure-drop manifolds, valves, fittings, etc. and in-line flowmeters and temperature indicators are also good practices to provide information regarding the efficiency and condition of the cooling system.
Ejection devices
The ejection of injection molded parts is most commonly accomplished by air, vacuum, pins or stripper plates. Depending on part design, combinations of these systems are used for rapid positive ejection. Care should be taken in selecting ejection surfaces because of aesthetic and moldability requirements. Wherever possible, the part should be ejected off the core. For small, thin-walled moldings that may shrink onto the core, air ejection through the core is usually adequate for part removal. On some products with threaded or undercut features, collapsible, retractable or unscrewing cores are used.
Spiral flow measurement
The relative processability of an injection molding resin is often determined by its Melt Index (MI) or Melt Flow Rate (MFR). This involves measuring the relative flow of the molten resin through a specified capillary in a calibrated laboratory instrument, while maintaining the molten resin at 190°C (374°F) and 43.5 psi for Polyethylenes or at 230°C (446°F) for Polypropylenes.
Figure 32. Broad MWD (left) and narrow MWD (right) spiral flow
Melt index is a good measurement of a resin’s relative flow properties at low shear rates, but only for resins of the same molecular weight distribution (MWD). Under actual injection molding conditions, differences in MWD will affect the resin’s melt viscosity (flow characteristics) at high shear rates. Temperatures, pressures and shear rates of actual molding do not conform to those of the MI or MFR test methods.
Equistar has a number of unique manufacturing processes available which allow the control not only of the melt index and density, but also MWD. This capability results in a better overall balance in resin properties and processability. Because melt index and MWD play a key role in performance in actual enduse applications, Equistar has utilized “Spiral Flow” (SF) as a more practical method of measuring and comparing a resin’s performance using realistic processing conditions.