The so-called silver wire mainly refers to the striped filamentous defects that appear on the surface of the product when the plastic is interfered by gas (including water vapor, air, decomposition gas and solvent gas) during the mold filling process, and there are often problems such as gas trace that accompany each other.

Generally speaking, from the inspection of the molding process, there are some ideas as follows?

A. What kind of defects occur? When did it happen (at the start of injection molding or during production) and where? To what extent?

B. How often does the defect occur (every time, or by chance)? How many number of defectives are there?

C. What is the number of cavities? Do injection molding defects always occur in the same cavity?

D. Does the defect always occur in the same location during molding?

E. Is the defect at the gate, or away from the gate?

F. If the new unpacked raw material is replaced, will the defect still occur?

For another injection molding machine, whether the defect only occurs in one injection molding machine, or also occurs in the other injection molding machines?

Here, we will focus on three aspects: injection molding process, mold structure, and material end:

01 Injection molding process

No matter the size of the workpiece, this is the most common problem. The first is the molding temperature, when the injection molding temperature is too high, the material is easily decomposed by heat, the molecular chain is broken, and a series of small molecular substances are produced, forming appearance silver wires, carbonization and other defects. The maximum shear stress is concentrated near the gate, and when the injection temperature is too low, the viscosity of the entire material increases, resulting in an increase in shear stress. The local temperature near the gate increases, causing the molecular chain to break, so the defects of silver wire on the appearance of the product are mostly concentrated in the gate area.

02 Mold structure

●Poor mold exhaust

Similarly, gas is also the key cause of silver wires and gas trace, so in addition to reducing gas generation on the one hand, gas must be discharged from the mold in time on the other hand. For some materials that are easy to exhaust (such as ASA), in addition to ensuring the depth of the exhaust groove, the exhaust groove should be regularly maintained to prevent clogging. To be more specific, it can add some mold insert exhaust, ejector exhaust, exhaust steel exhaust, etc. where there are gas trace, and vacuum exhaust can also be used for some special molds. 

●Product gate design

Large parts are relatively more prone to silver wires than small parts. One of the big factors has to do with the flow-length ratio of the material. For some larger parts, such as grilles, multi-gate, sequential valves are often used for injection molding. On account of the fluidity of each material is different, the distance between the gates will also be different. When the distance between the gates exceeds its flow-length ratio, defects such as silver wires, weld bonding lines, and temperature difference lines will appear on the appearance of the product.

03 From the material side, we can use the following 3 tricks to avoid problems such as silver wires and gas trace!

● Strictly control the moisture content of materials:

[The moisture content of general raw materials < 0.05%, moisture-sensitive polyester raw materials (including PC, PBT, PET, PU, etc., which are easy to be hydrolyzed at high temperature) < 0.02%]. This method can easily improve the problem of silver wires and gas traces on the surface caused by excessive moisture content and the easy thermal decomposition of high-temperature melt in the barrel and strong shearing places such as nozzles and gates.

● Search for materials with good fluidity:

Reduce the molding injection temperature and adopt materials with good fluidity and easy processing. For example, ABS replaces PC/ABS alloy; or directly optimizes the flow processability of the material. However, ABS material is so expensive now, and it needs careful consideration and a higher cost to replace it.

● Improve the thermal stability of the material:

Actually, we can effectively reduce the probability of defects by changing the polymerization method of the polymer of a certain material without changing the process parameters. We selected the high thermal stability component to design the material formula (left side), the silver wire phenomenon disappeared, and the problem was solved.

Heat for 10 minutes, and then observe the condition of the silver wire.