Why Against-Pressure injection molding is fast growing?
Let we start at the beginning of the process, when the designer apply simulator software, MoldFlow or one of its competitors, to check the injectability of the piece. To perform the simulation the software make a number of assumption (or default value), that are responsible for a certain amount of deviance, from the output of the simulator, and what the user will notice in the reality. Among others, the two main assumptions are:
1) A perfect symmetry between the two surfaces of the mold, side A and side B. The same temperature, the same roughness, the same heat transfer coefficient, in short the two surfaces treat the entry polymer in the same way, throughout the filling and the cycle time.
2) During the filling, the air in front of the melt flow has a constant atmospheric pressure of 1 bar. As the melt flow enters into the cavity so an equivalent amount of air flows out throughout the venting system, without increasing the pressure against the front flow.
These two simplifications and others minors, are necessary to perform the analysis and are responsible for a kind of “side effect”, or myths, that deter the molders to involve these two assumptions as causes of troubles.
For instance: Did it happen to you to try in a hurry an unfinished mold, just to get some sample to reassure the customer, and find the mold perfectly running, no problem in filling, sink marks, warpage, etch? And receiving the mold, after some weeks, with the right textures, a marvelous venting system, cooling system, clean parting line, etch…, all done as experience suggest; and discovered that there is no way to convince the mold to work again?
Well, if you are a deductive person, confident in your acquire experiences, nerves of steel, not easily influenced by the cheerful remark of your supervisor – You make the usual mess, you work well but absently with an unfinished mold, and you didn’t registered anything, and now you are in the shit up to the neck. That is your own lookout!
You will find quickly the way to produce if you first think that they have delivered a totally new mold. New, naturally, only from a rheological point of point of view, and you have to work out a way to produce with it, sometime with the help the mold maker.
When all is set, and you have mastered what it happened – mainly working on the point 1 and 2 – you will have make a big step in understanding part of the dark side of our arrogant technology.
Injection molding start talking about pressurizing the mold, before filling the cavity, when they try to improve the quality of the surfaces of the pieces made in structural foam. By mean of foaming agents, add to the raw materials, you can get lighter pieces with some remarkable advantages and one big disadvantage: the poor quality of the surfaces.
You recognize at the pieces made in structural foams these advantages:
- Raw material saving: up to the 35%, in direct function of the wall thickness. That normally shouldn’t be < 5 mm.
- Absence of internal frozen stress, hence pieces free of warpage or deformation of any kinds;
- Absence of sink mark of any kind, ribs and bosses, also the external sharp corners are tight as expected.
- By the pressure of the foaming agent, you push the material in the periphery, this increase the ‘Inertial Module’ of the molded object, this compensates for the weight reductions, and you get the same mechanical strength of the integer piece.
- Now the one big disadvantage: the poor quality of the surfaces, everywhere ruined by silver streaks.
This big disadvantage was put right by pressurizing the cavity so that to prevent the gas to perforate the fountain flow during the injection. As the pushing of the blowing agent is at the most 10 bars, so, pressurization between 12 to 15 bars is enough to get clean surfaces. This remedy has had a high cost: the raw material saving drop down between 3 to 7 %.