March 23, 2024

Solving Silver Streaks and Splay Issues in Injection Molding

The injection molding technique is a popular method of manufacturing plastic products. A plastic manufacturing factory uses injection molding machines to produce many plastic products. One such thing is a mobile phone case. It is one that we enjoy using in our daily lives. Silver streaks, often called splay marks, are one of the most common manufacturing defects in plastic injection molding. This type of imperfection mark can appear in several forms and with several causes.

This article will discuss common causes of silver streaks in injection molding and give many solutions to eliminate them.

What Is Splay In Injection Molding?

A splay mark, often called a silver streak, is a cosmetic imperfection that is commonly observed on smooth-surfaced plastic parts. Silver streaking meaning is the splatter markings on product surfaces generated by air and moisture volatilization or mixed dissimilar polymers decomposing and burning. The term “silver streak” is derived from the metallic scratches that appear on the surface of the injection-molded component. A water wave pattern may form after molding or shaping, which will be difficult to remove. It may also reduce the strength of the final product. Silver streaks in injection molding can be classified into five different types based on how they appear on the plastic surface. Analyzing the pattern of these lines, one can identify the root causes that led to this error.

  • Shear splay
  • The heat splay
  • Moisture splay
  • Decompression splay and air splay
  • Contamination splay

Splash marks are usually caused by a fast injection that prevents air release in the melting and cavity. Because of this, the air or gasses in the molding materials show up on the finished product’s surface as silver lines.

What Causes Splay In Injection Molding?

Material, heat, shear, moisture, and air are the most common causes of splay in injection molding. Examining these parts to determine what the main reason is may assist organizations in developing an effective action plan.

Each factor produces a different streak, making issues easier to identify and resolve.

1. Inadequate plasticization and melting of raw materials

When the material in the barrel is not completely melted and plasticized, it becomes visible on the plastic product as black specks and causes silver streak imperfections. These black specks resemble mica particles and are similar in size to material pellets.

Product specks may exhibit poor transparency due to light transmission, and convex raw material plastic pellets may be visible clearly.

2. Mold Temperature, Inadequate Mold Exhaust, or Material Temperature

Uncontrolled heating systems elevate temperatures, causing decomposition. Ensure that the heating elements, including the thermocouples and heating coils, are defect-free. Inadequately designed screws may also result in air leakage or deterioration.

Overheating can also decompose material. Too much heat in the feeding section can cause the plastic to melt too quickly, fill the groove, and block the air vent. To eliminate air between plastic particles, degassing is necessary.

More material flows, feeding buffers, material temperature, and mold temperature might affect melt fluidity and molding pressure. Mold temperature controllers are best for temperature stabilization.

3. Inappropriate or prolonged application of pressure

During pre-molding, low screw back pressure and rapid speed cause the screw to return too quickly, driving air and particles to the barrel’s front.

When the melt tube temperatures, back pressure, and melt velocity are excessive, or the forming cycle is prolonged, heat-sensitive polymers like PVC and PC are easily transformed into a gas at a high temperature.

4. Low Fluidity of Melting Material (FMI)

An excessive accumulation of material between the feeding throat and the drier source can result in silver streak defects because materials may absorb moisture in some areas within an hour.

When these combustible compounds are exposed to air while manufacturing plastics, they also transform into gasses at elevated temperatures in the solution. The procedure resembles inhaling water or oil in the inappropriate proportions or combinations of each component.

Preventive Measures for Silver Streaks and Splay Marks on Molded Parts

Silver marks reduce the aesthetic value of plastic components and also degrade their mechanical strength. To achieve the highest quality standards, processors and manufacturers must, therefore, comprehend how to prevent silver streak and splay in injection molding.

This kind of problem typically can be resolved in the following manner:

Remove any interference from gasses and other contaminants, and thoroughly dry the plastic. 

1) All items should be completely dry. Hygroscopic fillers may absorb moisture. 

2) Reduce material temperature, gradually adjust barrel temperature, and raise mold temperature. By lowering the temperature of the barrel, the plastic can remain molten and not burn or char. The molecules bind properly, eliminating splay.

3) Reduce the injection pace while increasing the injection pressure. To get rid of the volatiles, raise the back pressure setting. Higher back pressure distributes volatiles in the melt stream, preventing gas pockets.

4) Reduce the screw speed and adjust the pre-plastic back pressure. Slow down the screw’s rotation. This will heat and mix the material without degrading it. Start screw rotation at 120 rpm for a two-inch screw.

5) Make the flow line and cavity exhaust improved. Run the mold through a sized press to achieve the optimum shot-to-barrel size ratio. This reduces heat degradation risk.

6) Remove any obstructions from the runners, nozzles, and gates. Optimize the size and shape of the gate. Examine the boundary line and other regions around runners and gates. Eliminate obstructions if present.

7) Anneal after demolding to shorten the molding cycle and remove silver streaks in injection molding: elevate the temperature of the polycarbonate to 160 °C for a few minutes, and maintain the polystyrene at 78 °C for 15 minutes or 50 °C for 1 hour.

8) Set the machine to run on an automatic cycle. If something goes wrong, the user should interrupt in emergencies. Consider using a robot when an operator is needed.

Look for cracks, even if small, and fix them if you find any. This will keep water from getting into the cavity.

In conclusion, most issues with silver streaks in injection molding can be traced back to improper processing conditions, materials, or machine/mold maintenance. To manufacture high-quality plastic products, the entire production process must be controlled, from the injection mold, injection molding equipment, operators’ technical experience, the workshop environment, and raw materials’ high standards.

A brand of Freudenberg.
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