Splay Marks

Splay marks, also referred to as silver streaks, appear as a blister that radiates over the surface

of a part from the gate. If you were to take a small, sharp metal blade and scrape one of these

splay mark blisters, the blister will appear to delaminate away from the surface. These types of

defect cannot only look poor on the part surface, but if the part requires finishing, such as

painting or electroplating, this defect can be enhanced. For this discussion, splay is categorized

nto three types. The first is moisture splay, caused by moisture entering the material melt stream

thermal splay, caused by overheating the material; and shear splay, which results from exposing

he polymer material to high shear during processing. 

Causes:

1. Moisture from material that has not been dried enough or at all time is one of the prime causes of splay. Moisture in material is a source of gas, which can get into the melt stream and show up later in the cavity as splay

2. Excessive melt temperature can cause out gassing of the material, resulting in thermal shear. High melt-temperature settings, high back pressures, and high screw rotation are three causes of thermal splay

3. High injection speeds lead to shear splay since the material is exposed to high shear conditions

4. High nozzle temperatures can promote the formation of thermal splay marks from keeping the melt too hot going into the sprue bushing

5. Using decompression may force moisture into the melt, showing up as moisture splay in the part

6. Sharp transitions in the runner systems of both hot and cold runner systems can lead to shear splay of the material

7. Even in part design, sharp comers may cause shear splay

8. Condensation can form on the mold surface especially on humid days, forming moisture splay on the part surface

Solutions:

1. Plastic material, especially hygroscopic material must be dried

2. Reducing either melt temperature, reducing back pressure, or decreasing the screw rotation will eliminate thermal splay. Low back pressures can also be a source of splay since this is yet another source of trapping air in the

3. Using slower injection speeds reduces the risk of shear

4. Reducing the nozzle temperature

5. Dry material completely, and venting in the mold

6. Smooth transitions should be designed into those runner system

7. Radiused corners are recommended not only for reducing shear splay, but also to avoid part stresses

8. With materials that require running mold temperatures below75F(24℃)