Influence of injection temperature and speed on product shrinkage

Time：2023-02-09 08:57:32 / Popularity： / Source：

1. Two temperature conditions that are not conducive to solving shrinkage problem

1) Mold temperature is too low to solve shrinkage problem

Shrinkage problem of hard plastic parts (surface shrinkage and internal shrinkage cavity) is caused by fact that space left by concentrated shrinkage cannot be fully replenished by melt from direction of water inlet when melt shrinks when it cools. Therefore, factors that are not conducive to feeding will affect us to solve problem of shrinking.
Most people know that mold temperature is too high to cause shrinkage problems, and usually prefer to lower mold temperature to solve problem. But sometimes if mold temperature is too low, it is not conducive to solving problem of shrinkage, which is not noticed by many people.
Mold temperature is too low, melted glue cools too fast, and slightly thicker glue position farther from water inlet, because middle part cools too fast, feeding channel is blocked, and melt cannot be fully replenished in the distance, which makes shrinkage problem more difficult to solve. Shrinkage problem of thick and large injection molding parts is particularly prominent.
Furthermore, mold temperature is too low, which is not conducive to increasing the overall shrinkage of injection molded parts, increasing concentrated shrinkage, shrinkage problem is more serious and obvious.
Therefore, when solving more difficult shrinkage problem, it will be beneficial to remember to check mold temperature. Experienced technicians usually touch surface of mold cavity with their hands. Each raw material has its proper mold temperature. For example, shrinkage problem of PC material can be improved if hole is properly sealed, but if mold temperature is too high, problem of shrinkage of injection molded parts will occur.

2) Too low melt temperature is not conducive to solving shrinkage problem

Also, most people know that if melting temperature is too high, injection molded parts are prone to shrinkage. If temperature is appropriately lowered by 10~20℃, shrinkage problem will be improved.
However, if injection molded part shrinks in a thicker part, adjust melt temperature too low, for example, when it is close to lower limit of injection melt temperature, it is not conducive to solving shrinkage problem, and even more serious. The thicker piece, the more obvious it is.
Reason is similar to that of mold temperature being too low. Molten glue condenses too quickly, and a large temperature difference that is conducive to feeding cannot be formed between shrinking position and nozzle. Feeding channel at shrinking position will be blocked prematurely, and solution of problem becomes more difficult. It can also be seen that the faster condensation speed of melt glue, the less conducive to solving shrinkage problem. PC material is a raw material that condenses quite quickly, so its shrinkage cavity problem can be said to be a big problem in injection molding.
In addition, too low melting temperature is also not conducive to increasing amount of overall shrinkage, resulting in an increase in the amount of concentrated shrinkage, thereby aggravating shrinkage problem.
Therefore, when adjusting machine to solve difficult shrinkage problem, it is also very important to check whether temperature of melt is too low. In addition to looking at thermometer, it is more intuitive to check temperature and fluidity of melt by air injection.

Influence of injection temperature and speed on product shrinkage,

2. Too fast injection speed is not conducive to solving problem of serious shrinkage

To solve shrinkage problem, the first thing that comes to mind is to increase injection pressure and prolong injection time. But if injection speed has been adjusted very fast, it is not conducive to solving shrinkage problem. Therefore, when shrinkage is difficult to eliminate, it should be solved by reducing injection speed.
Decreasing injection speed can make a large temperature difference between molten glue walking in front and water inlet, which is conducive to sequential solidification and feeding of molten glue from far to near, and it is also conducive to shrinkage position farther from nozzle. Getting higher stress supplements can go a long way towards problem solving.
Due to reduction of injection speed, temperature of melted glue in the front is lower, speed has slowed down, injection molding part is not easy to generate a sharp edge, injection pressure and time can be increased and longer, which is more conducive to solving problem of serious shrinkage.
In addition, if last-stage end-filling with slower speed, higher pressure, longer time, pressure-holding method of gradually slowing down and pressurizing are adopted, effect will be more obvious. Therefore, it is also a good remedy to use this method from later stage of injection when it is not possible to use a slower speed injection at the beginning.
However, it is worth reminding that filling is too slow, but it is not conducive to solving problem of shrinkage. Because when cavity is filled, melted glue is completely frozen, just like temperature of melted glue is too low, there is no ability to feed shrinkage in the distance.

Gud Mould Industry Co., Ltd