How to Solve the Color Variation and Black Spots in Transparent Products?

Regarding the issue of color variation and black spots in transparent products, we will illustrate how to eliminate them from the root based on a customer’s case to ensure normal production in the future.

To protect our customer’s privacy, we will refer to this customer as Manager C below.

The Case:

Manager C has established an electrical assembly plant in Foshan, with production covering injection molding, painting, pad printing, assembly, and finished products.

Previously, there were fewer factories producing humidifiers, resulting in less competition. The assembly plant had good profits, and therefore, Manager C did not pay much attention to the production losses in the injection-molded components.

In recent years, this industry has become highly competitive, with more stringent product requirements. Despite the increased strictness in standards, unit prices have not seen a corresponding rise. However, labor and material costs have significantly increased.

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Afterward, Manager C approached us for assistance in providing consulting guidance at his factory. The production loss for their humidifier water box was significant, with 50 tons of material for the tank mouth accumulating in the warehouse over six months. The goal was to find a way to reduce the material loss in the injection molding production of the water tank by 30%.

Therefore, we sent our professional technical management personnel to the factory to understand their real production situation and provide guidance.

Our technical management personnel thoroughly investigated and analyzed the entire injection molding production of the humidifier water box at the factory. Subsequently, they conducted two sessions to educate the relevant personnel in the injection molding factory on how to prevent color deviation, black spots, and material defects.

The main production issues in their factory’s water box injection molding were concentrated on color deviation, black spots, and material defects. Humidifier water boxes are generally produced using SAN material, which is particularly prone to color deviation and the occurrence of black spots.

Methods to Solve Color Deviation and Black Spot Issues in Products

Through our efforts, we provided Manager C with the following methods to address color deviation and black spot issues in the products:

1. Enhance Training for Technical Staff

In the first step, on-site technical staff undergo training to deepen their understanding of the causes of color deviation.

(1) Mold-related factors causing color deviation in injection molded products

  • Trapped air in the mold causing localized high temperatures and resulting in yellowing.
  • Insufficient water flow in the gate, leading to yellowing due to shear heat during plastic flow.
  • Abnormal temperature control in the hot runner system causing yellowing.
  • Ineffective cooling in mold water channels leading to elevated mold temperatures and substandard color deviation in products.

(2) Setup Reasons for Color Deviation in Injection Molded Products

  • Improper setting of melting glue temperatureand hot runner temperatures, either too high or too low, can result in color deviation.
  • Impropermelting glue stroke setting, whether too long or too short, can lead to color deviation. The qualified range for the cushioning position is between 10 and 20.
  • Excessive back pressurecauses the melted plastic to yellow, resulting in color deviation.
  • Setting the mold temperature too high can cause the product to turn yellow, leading to color deviation.
  • Excessive screw rotation speed causes plastic shear heating, resulting in yellowing and color deviation.
  • Improperproduct storage, exposure to sunlight, can lead to yellowing or whitening.
  • Elevated temperatures in the storage space can cause yellowing or whitening.
  • Setting the cycle too slow allows the plastic to stay in the barrel for an extended period, causing yellowing and color deviation.
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(3) Design-Related Causes of Color Deviation in Injection Molded Products

  • Excessive differences in product thickness design result in color deviation, creating a contrast between light and dark shades.

(4) Machine-Related Causes of Color Deviation in Injection Molded Products

  • Issues with small machine components, damaged barrel, poor pre-plasticization quality leading to yellowing.
  • Unstable barrel temperature, high temperature causing yellowing.
  • Inaccurate control of the back pressure valve resulting in plastic yellowing.
  • Uncontrolled temperature in the material barrel causing overheating and yellowing.

(5) Material-Related Causes of Color Deviation in Injection Molded Products

  • Different batches of plastic compared to the mold result in a different base color, causing color deviation.
  • Abnormalities in color powder or masterbatch lead to color deviation.
  • Abnormal ratio of material in the nozzle causes color deviation.
  • Excessive variety in the material at the nozzle results in color deviation.
  • Color deviation due to not strictly following the operating instructions during mixing.
  • Prolonged drying time of the material leading to yellowing.

2. Curing Parameters

The second step involves curing parameters.

After the upper mold is initialized and the first piece is signed, no machine parameters, especially back pressure, mold temperature, and material baking temperature, are allowed to be adjusted. 

This is because color deviation and material flow marks (jetting, cold slug marks) in SAN plastic injection products are directly related to these conditions.

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3. Use Raw Materials from the Same Supplier

The third step is to gradually shift from mixing color powder and masterbatch in raw materials to using color-matched pellets from the same supplier’s factory, and curing SAN materials from the same supplier’s factory as the base.

Using color-matched pellets reduces the occurrence of defects such as black spots and color deviation caused by uneven dispersion of color powder and masterbatch. The SAN base material provides some flexibility, making the products less prone to breakage during injection molding.

4. Standardized Procedures for Machine Start-Up and Shutdown

The fourth step involves specific requirements for the shutdown and start-up of SAN material. Shutdowns that result in black spots must go through a specific procedure.

It is not permissible to use PP material during the startup process. This is because if PP material is used for startup when producing SAN transparent material, it may result in mixed white color during subsequent production. This is difficult to remove and will lead to material waste.

During shutdowns, ABS material must be used for machine cleaning. The machine should be purged with air injection until all plastic in the original barrel is completely expelled.

The screw travel should be extended by 50mm, back pressure set to 5 bar, and mid-pressure high-speed air injection should be performed no less than 10 times. When shutting down, the screw must be retracted by 60mm.

Those who neglect the proper shutdown procedures will be penalized with a deduction of 200 points for each occurrence.

5. Strict Compliance with Production Requirements at the Water Inlet

The fifth step involves stringent control of contamination at the water inlet. After the SAN material at the water inlet is crushed, the dust removal machine must be rigorously activated.

Each batch should have water inlet added in strict adherence to the 8% proportion. Violators will be assessed a penalty of 200 points per occurrence.

6. Improvement of Reward System

The sixth step involves implementing a reward system when the production loss of SAN material for the humidifier water tank decreases from the original 30%. Relevant personnel will be rewarded based on the factory’s specific conditions.

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