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In general, >95% of the part weight(s) should be delivered during the injection step. The final part weight is achieved via the packing and holding step. Packing pressures are typically about half the level of the injection pressure and serve to achieve final part weight(s) and also to allow time for the gates to freeze off before plastication can begin for the next shot.
During the injection and packing steps, coolant (typically a mixture of ethylene glycol and deionized water if ²45°F) is circulating through the mold. The coolant takes heat out of the mold and therefore out of the part via conduction. Optimum cooling is achieved when the water is in turbulent flow. In general, an increase in coolant flow rate will remove more heat than a decrease in coolant temperature.
Figure 39. Effect of melt index of polyethylene resin on injection temperature
As resin flows into the mold, the material in contact with the mold surface solidifies very quickly forming a skin layer and an inner flow ‘channel’ through which material continues to flow (Figure 40). As more heat is taken from the plastic by the mold, the flow channel is reduced to the point that no more material will flow. The optimum process parameters should be chosen to allow complete mold fill and pack before the flow channel solidifies completely. Keeping the melt channel open allows for better packing of the extremities of the part. There are also differences in part temperature depending on proximity to the gate.
Uniform cooling to the mold occurs when the coolant makes one pass through the mold (no looping or connecting of flow channels) and there is only one temperature controller. During even cooling, the gate area is always the hottest area of the part because throughout the injection and packing steps, molten material continues to flow through the gate. The extremities of the part tend to have the lowest temperature since the polymer melt has transferred heat as it has flowed through the cavity.
Figure 40.
Differences in part temperature can lead to differential shrinkage and therefore warpage. There are two ways to minimize temperature differentials either through differential cooling or an increase in injection rate.
Differential cooling involves directing coolant towards the gate area, which has the highest part temperature and away from the extremities where the part temperature is lowest. The typical method is to reduce the coolant flow to the cooling channels nearest the extremities and open up the valves to the channels nearest the gate. Directing the coolant flow from the hotter portions of the tool to the extremities may also be effective in some cases.
As covered previously, increasing the injection rate will shorten the injection times allowing for a more uniform part temperature.
Because the cooling step is usually the longest time period in the injection molding cycle, a cold mold temperature is generally recommended. Because of the wide range of mold and part designs, it is very difficult to specify mold temperatures. A typical range of mold surface temperatures for polyolefins is 70-125°F (20-50°C), which requires coolant temperatures of 32-50°F (0-10°C). Materials with lower melting temperatures, such as EVAs, will be at the lower end of the range while the higher melting materials, such as HDPE and PP will be at the higher end. Cold molds, however, tend to give a less glossy surface finish and will restrict the flow of resin within the mold. A cold mold can also lead to a higher amount of molded-in stress within the part. Warmer mold temperatures will increase the gloss level and may also improve resin flow by constricting the melt channel less, at the expense of increased cycle times.
The injection molder and the mold design typically fix the amount of time needed for the mold to open, eject the part and then close again. Mold open time can be reduced by using only enough daylight to allow the part(s) to fall freely, reducing the amount of time required for air assists and also the number of times the ejector pins activate.