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Weldlines, also called Knitlines, take on the appearance of a hairline crack on the surface of an injection molded part. During injection mold cavity filling, the plastic flow front sometimes must split to go around an obstruction. When the two flow fronts meet on the other side, the plastic next to the mold cavity walls is slightly cooler and the molecules cannot completely fill the gap. This causes the weldline. Steinwall (a longtime Paulson customer, a true innovator in employee training AND Plastics News 2011 Processor of the year – Hint: model their training and watch what happens) has a nice explanation of weldlines.
Back to our weldline problem. If we ask the plastic molecules in each flow front why they didn’t completely rejoin, they would say, “our viscosity (resistance to flow) was too high. We couldn’t flow any further”.
So as an injection molder, what do we need to do to satisfy the plastic molecules? We already know that there are only 4 basic processing variables that the plastic will react to: plastic temperature, plastic pressure, plastic flow rate, and plastic cooling rate (if you need to learn about the “4 variables” approach pioneered by Paulson, go here). Which one or more of these plastic variables would reduce viscosity?
1. Plastic flow rate – The best solution to a weldline problem is to increase the plastic flow rate. Faster plastic flow rate reduces the plastic’s viscosity resulting in a faster filling mold. There will be less cooling of the plastic during mold filling which will also help with the weldlines problem. And as a bonus, we will reduce cycle time, which means higher productivity and more profit.
2. Plastic pressure in the cavity – We can combine faster filling with an increase in cavity pressure. This will increase the force on the two plastic flow fronts when they meet beyond the obstruction in the mold. But be aware that an increase in cavity pressure will also increase part dimensions. The amount of the increase is approximately 1% if the cavity packing pressure is increased by 3,000 psi.
3. Plastic temperature – We could reduce the plastic viscosity by increasing the plastic’s temperature. The penalty for doing using this solution would be a longer cycle time. This is because the plastic will take longer to cool enough to hold its shape after ejection from the mold.
4. Cooling rate – The fourth adjustment we can make to change the plastic’s viscosity is to reduce the cooling rate (i.e. increase mold temperature). This will cause the viscosity to remain higher during mold filling. The penalty for this solution is also an increase in cycle time.
You may need to use more than one of these solutions together to get the result you want.
It is instructive to note that the solutions given above can be implemented by using several different machine control adjustments. But in almost all cases there is a small subset of adjustments that result in the most efficient (i.e. most profitable) solution. We have a complete program on how to solve various injection molding part defects “from the plastic point of view”.