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Hi all,
We are currently using a foaming agent on a transfer tool from another company. The foaming agent is used to help eliminate the warp. When we cut the part in half we notice that there are not any voids in the part we are molding. When we cut the part in half that the customer supplied we are seeing lots of voids.
We are using a 1% endothermic foaming agent. Is the foaming agent not being activated upon injection? Is there a difference in the foaming agents?
What you are calling voids is most likely the foaming agent doing it's job. Your part does not look like the foam is activated or if it is then there is too much pack/hold pressure or time which prevents cellular formation. You do not want to activate the foaming agent too early in the barrel so it is recommended to keep the rear zone lower then you may typically. Then activate it in the middle zone with higher heat and front zone drop a little lower again to prevent to much foam activation in the barrel. For a PP I would set my temps. as such. 380-380-420-350 F.
Fill the part out to 95-99% full and let the foam pack the part out. Add just enough pack and hold improve overall appearance but not so much that it doesn't allow expansion in the cavity. Most often gate seal is also not used.
Don't cut the part with a saw as it melts resin into the voids left by the CFA. Score with a knife and then break. I'm curious as to what your set up is but as a "rule of thumb" you need to reduce the metering stroke about the same amount as the percentage of CFA your using. You need to leave enough volume in the cavity to allow for foaming to occur. High pack/hold psi for 1-2 seconds is all you can run or no room to foam. Do you have a shut off nozzle? You can lose the gas in solution with the resin w/o one. Slowest screw rpm you can use plus MAX back psi you can run. That also keeps the bubbles mixed into the melt stream. Also you should fill as fast as possible to keep bubbles from escaping the melt as you fill the cavity. Also too cold a steel temp can inhibit foaming as the part freezes too deeply into the wall stock.
Rick.
We use Hyrocercol at 2-3 %. Fill the part 95-99 % full and use NO holding pressure, but have some time. Let the foaming agent do its work.
On the money above - NO HOLDING PRESSURE or just enough to seal the gate. you can also run this with zero cushion!
Check the weight of your part is not heavier than your sample
This is the advice I used to give to technicians who were having issues with foaming agents, based on my own experience:
INJECTION MOLDING WITH FOAMING AGENTS
Foaming agents are used with polymers to remove sink marks and reduce warpage in thick section moldings.
The technique which can be used to maximize the effect of the foaming agent in the molding process is as follows:
1 . Set the shot (metering position) on the machine to make short shots (about 10% short).
2. Set the injection with a small transfer position (2.0 – 5.0mm) and just enough holding pressure to bottom the screw during injection.
3. Gradually increase the shot size until the mold is full.
This allows the gas in the melt to complete the filling of the mold with minimum pressure on the melt.
The use of high holding pressure and cushion can prevent the expansion of gas in the tool. It is better to use no cushion if the process allows.
Use of regrind - The foaming agent has no residual activity in regrind material, it can readily be used.
Moisture content of polymer - If the polymer is not dried adequately, the moisture in the material will enhance the effect of the foaming agent and may cause thickest areas of the surface to bulge.
% letdown of foaming agent - This will initially need to be determined by trial and error with a new product. Too little will not affect the sink and too much will cause bulging of the thicker sections.