Foaming Thoughts
Understanding of the basics of the process is needed. What is meant is that in injection molding process material is injected and than packed with machine pressure to achieve the finished part size. Pressures up to 30,000 psi are used on the plastic to achieve this. When foaming for weight or density reduction you have to think in terms of a low-pressure system. The material needs to foam. This relates to lower clamp pressures and no packing pressure since we wish to reduce the amount of material in the cavity and finished part. The real process key is in short shooting the material into the cavity / tool and allowing the foaming action to take place thus producing the required part to size. If the addition of the foam were for sink control than the use of a high-pressure system model with pack and hold pressure would be used.
The foaming process being discussed is by the addition of a chemical-blowing agent. It is realized that some of these steps can be used for most other foaming processes on the market today, but for this discussion chemical-blowing agents (CBAs) is the form of foaming. There are two basic groups of CBAs, an exothermic or endothermic. The names refer to the reaction, endo meaning absorb heat and exo meaning gives heat off. The other factors not discussed are the cell size and shape given by the two types and combination thereof. (This can be of a benefit and should be discussed with the suppliers of the foaming agents). These groups can be used individually or in combination.
Venting:
Venting is critical; vents should total a minimum length of 30% of the perimeter. If it is noticed that the vents are clogging up very quickly and or build-up in the exhaust channel, check the depth and width of these areas. The process of foaming is a chemical reaction, which produces a gas, causing the foaming and some other by-products, which must be exhausted from the mold along with the air in the cavity. Some of the gases produce have been detrimental to some types of steels; check the MSDS sheet for your particular grade and type. The vent land should be as short as possible and the exhaust channel as deep and wide as possible to allow easy breathing of the tool. Gases coming off the process contain all sorts of things, and in some cases we have seen clean vents around the part but 20 mm from the part there is a build up that clogs up the vent exhaust. This is an example of the vent exhaust being too small in volume to handle the gas, and or sometimes the mold is too cold thus causing the gas and particulate to condensate.
Clamping:
The clamp tonnage should be set as minimum as possible, this can be as low as 1/4 ton per inch of projected area. This allows for breathing of the mold: allowing air and gases to escape. Basically we are allowing the mold to open a bit if necessary upon injection so as allow escape of gases. The concern would be with the runner or feed system blowing open the tool and not the cavity since we are only filling to 90% or less.
Melt Temperature
CBAs work within an activation temperature range. It is important to select the correct blowing agent for the material / application. Below the specified temperature, the chemical reaction is not going to happen, too hot and the reaction may start too soon or burn itself up in the barrel. Each agent has its range and percentage amount for a specific density reduction; these are starting points for the material. The set points are given by the manufactures.
Screw return:
The set up of the injection screw recovery is important to a properly mixed shot. An analogy would be shaking up a bottle of carbonated fluid. If we shake a bottle up and than let it set for a period of time the bubbles / gas will dissipate back so we can open the container without a problem. In molding, if we drive the screw back too soon the reaction, which starts, in the barrel, will continue as we wait for the next cycle start signal. This than could lead to loosing some of the foaming potential, and or upon injecting the material into the cavity the desired results are not achieved. What is needed is to have the screw return just prior to mold opening by 1/2 to 1 second.
Drooling:
Drooling tends to be a big concern and problem. Shut-off nozzles works great but most custom molders do not have this option. The best way is to retract the screw prior to mold open (1/2 second to 1 second) and use a bit of suck back. Further to reduce the nozzle temperature slightly so that you may get a bit of freeze off. Use a slightly longer nozzle with the heater band placement acting as your control. Installing an extremely small nozzle orifice, this may freeze off in less time. The small orifice can aid in shearing of the material upon injection both for flow (lowering of the viscosity) and adding a bit of temperature kick, (approximately 10 to 40 F increase).
Balance:
In multi-cavity tools and family molds that are to be foamed for weight reduction, they need to be balanced as close as possible. If the shot is short and the parts were compared by weight to their respective full part, the weight percentages would be within 5%. The gates must be identical in height, width and land length. Sometimes smaller gates than normally are used, and do work better on foamed parts. Corners within the runner system need radiuses and the runner must be vented extremely well. Remember that we are using a low-pressure in the cavity and any air that can be eliminated helps in the process. If we have an unbalance in fill, we tend to over-pack one or two cavities and this causes us to get different density parts, thus using too much material to run the job. Exploring the use the Melt FlipperŽ would be a wise decision on multicavity tools.
Cooling:
Each cavity must have the same cooling rate. If we do not cool properly we can end up with extended cycle times while we use the tool to cool one cavity, while seven others are ready to be ejected. The idea is to build up a skin on the part that is thick enough to dissipate the heat from the center of the part upon ejection. At this point the part could be ejected and dropped into a water bath /cooling table. If using a water bath keep the water moving or circulating over and around the parts; otherwise, the water will act as an insulator and the part well not cool properly. If the use of water is not allowed an extended cycle time should be anticipated. This is due to the chemical reaction, and heat transfer occurring within our parts and even the percentage of foam used. What needs to happen is to build up a skin that is thick enough for rigidity purposes. This thickness than can dissipate the heat and not loose its stiffness properties to a point that the reaction which is going on causes the stretching or blowing up of the part from the original shape.
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