Silicone Flow Marks | Common Defects Of Compression Molding

Flow marks/lines are irregular lines, traces, or spots in the silicone rubber / plastic parts, sometimes on surface, or both surface and internal of the products, causing the products with appearance defect, looks dirty. Commonly the flow lines defect easier to happen in silicone rubber compression molding and plastic injection molding, the defect called flow marks in silicone compression molding, but called weld lines in plastic injection molding, anyway, it is a kind of appearance problem and unavailable for products used for visible devices.

Silicone Rubber Flow Marks - Problem

During the compression molding process, both machine parameters setting (mold temperature, pressure, silicone rubber curing time) and silicone rubber raw material especially color pigment/paste performs significantly effects for the product forming. Silicone rubber flow marks always seen in compression molding process, not LSR injection molding, because the status of raw material is different. Solid silicone rubber for compression molding, liquid silicone rubber for injection molding. Therefore, the first step for compression molding is preheating solid silicone, making the solid silicone become liquid to flow into compression mold cavities (the shape of formed product), flow rate of silicone flowing causes flow lines in different status and different places in the formed products. Actually, the most important factor is the color. For silicone rubber products, rarely seen flow marks defects in the color of transparent, white, or black, however, the flow marks obviously happened in the color of red, orange, blue, yellow, green, especially in orange, red, yellow and green.

Silicone Rubber Flow Lines - Analyze And How To Improve?

Uneven mixing during the preparation of silicone products, that is to say, the mixing time after adding the color masterbatch is too short, resulting in the color masterbatch not being completely dispersed evenly, resulting in dark marks (ripples) on the silicone products when the silicone products are molded. This problem can be solved by recycling raw materials.

The vulcanization temperature is too high. Any kind of silica gel masterbatch has its own temperature resistance. When the vulcanization temperature exceeds the upper limit of the temperature resistance of the color masterbatch, it will cause the color masterbatch to change color at high temperature. The temperature resistance values of various color masterbatches are different, which may cause one color masterbatch to change color and another color masterbatch not to discolor, which will naturally cause dark marks (ripples) in silicone products. The problem can be solved by simply lowering the mold temperature. As for how much to lower it, the answer is to continue lowering it until dark marks (ripples) no longer appear.

When the mold temperature is continuously lowered, the dark marks (ripples) of the silicone products are solved, but the vulcanization of the silicone products is incomplete (immature). Basically, the temperature resistance of the silica gel masterbatch itself is too poor. It usually appears on some color masterbatches with brighter colors such as red, green, and yellow. The temperature resistance of such color masterbatches will be relatively poor, such as when used with other color masterbatches. When coloring, it is easy to have dark marks (ripples) in silicone products. At this time, you need to consider finding a color masterbatch with a similar color and remixing it to solve such defects.

A certain color pigment/paste has never had dark marks (ripples) on silicone products when used before, but when it is used for color mixing, however, dark marks (ripples) appeared. This is most likely because this color paste is incompatible with a certain color paste, causing dark marks (ripples) to appear on silicone products after molding. The solution is also relatively simple, just replace the color paste. If you know which color paste has poor compatibility, just replace it. If you don't know, you have to change and test one by one until you find out the reason.

Unreasonable locating of silicone rubber material when molding silicone products, which is the most likely factor to cause dark marks (ripples) in silicone products.

The silicone product rubber material is stored for too long after preparation, resulting in poor fluidity of the rubber material, which in turn causes the dark marks (ripples) of the silicone product. In addition, if too much vulcanizing agent is added during material preparation, such defects will also occur.