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From The Plastic’s Point of View – Splay

injection molding splayAnother post on injection molding part defects and how the plastic would describe them. This time we’re going to take a look at Splay.

Splay is sometimes also called “silver streaks” because of the appearance of silvery scratches across the surface of the injection molded part. Splay is primarily a cosmetic defect although it can affect the strength of the finished part.

Unlike the other molding defects we have discussed in previous blog posts, this is a molding problem not related to the four basic plastic processing variables.  However, your plastic still has some things to say about Splay.

First, what causes Splay in injection molded parts?  Splay is caused by tiny gas bubbles that are dragged across the surface of the part when the mold cavity is filled.  Splay is usually caused by water that is absorbed within the plastic granules. So the plastic would tell you “Hey, we’re kind of wet in here. There’s no way we can make you a good part covered in water.”

The actual process that takes place while the plastic is being heated in the heating barrel is that the moisture in the plastic turns to steam inside the heating barrel. That steam presents as bubbles of hot air in the melt.

Plastic materials that are hygroscopic absorb moisture. Two examples of hygroscopic materials are Nylon and Polycarbonate. The plastics will absorb moisture from the surrounding air. There only needs to be a very tiny amount of water in the material for steam to form during molding. As the mold fills, the steam migrates to the surface of molded part. It is these steam bubbles that you see as Splay.

The solution to a Splay problem is, of course, to keep the plastic dry. Moisture can get into the plastic in many ways. For example, bringing cold plastic granules into a warm room will cause splay because moisture will condense on the surface of the granules as the plastic warms up.

So any molded part is susceptible to Splay. But the ones most susceptible are parts molded from the hygroscopic plastics.  Plastic dryers are typically used at molding machines to dry materials before they feed into the hopper. However, dryers can and do either fail or operate at a reduced efficiency. This is probably the most common cause of moisture in the plastic.

Another cause of Splay is degradation of the plastic. This is caused by heating the plastic too long or heating the plastic at too high a temperature. In this case the plastic would probably say something like “Whoa! Turn down the heat. We’re getting gas.” Degraded plastic gives off a gas and this can cause a Splay problem. You then have to decide the optimal way to lower the melt temperature. Using too high a percentage of regrind is another possible cause of Splay.

Finally, Splay is not always just a cosmetic problem. It can also weaken the molded part. This is particularly true for Polycarbonate.  Every molder should check to see if the plastic they are molding is hygroscopic and make sure that the material is thoroughly dried. Even for plastics you are molding that are not hygroscopic, make sure the plastic granules reach room temperature before you mold them.

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