Generally black Specks or gloss difference on plastic molded parts can tracked back to three sources : the mold itself ,the processing of the material in the mold and the material being used to make the part.
The surface of the plastic part is the reproduction of the cavity surface of the plastic mold.Therefore, if there are scratches, corrosion,small pit and other defects on the surface of cavity steel, all will appear on the surface of plastic molded parts as defects. When grease mark, excessive moisture, or improper use of mold release agent, the surface of the plastic part will show different color gloss. So the cavity surface should be maintained in a gloss condition through mirror polishing or chromium plating. Meanwhile cavity surface must be kept clean with oil and water stains being removed in a timely manner.The type and amount of release agent should be appropriate.
In addition, the other mold failure, such as the molding draft being too small, the abrupt thickness change in the cutting section, thick ribs, small gate and runner sections ,over shearing force of the injection system,the molten material flows in a turbulent state, poor mold venting and so on, all these may lead to color streak defect.
- the processing of the material in the mold
a. injection speed is too fast or too slow
b.injection pressure is too low
- the hold time is too short
- the supercharger pressure is not enough
- cushion is too large.
- Machine & Material
- Nozzle is too small or the temperature is too low
- Dispersion property of the fiber reinforced plastic is too poor, and the filler is exposed or the aluminum foil filler will be distributed in random direction.
- The barrel temperature is too low
- The molten plastic has poor property of plasticizationor the supply of plastic is insufficient.
All above reasons may lead to poor gloss on the surface of plastic parts.In this regard, adjustments should be made according to the specific situation.
The mold temperature also has a great impact on the surface quality of the plastic parts. Generally, different types of plastics show different color gloss under different mold temperature conditions. High or low mold temperature may result in poor gloss .If the mold temperature is too low, the molten plastic will solidify immediately after contact with the mold cavity, which will reduce the reproducibility of the mold cavity surface. In order to increase the gloss, the mold temperature can be appropriately increased. It is preferable to use introducing warm water into the mold cooling circuit to transfer heat in the cavity in a faster way and shorten molding cycle time. This method also helps to reduce residual stress during molding. Please be aware that if the mold temperature is too high,the surface of the plastic parts might get dark as well.
The material being used to make the part.
- If color streak defect appears near the gate or at the cross sectionand there is a thin layer of milky whiteness on the surface of the plastic part, reduce the injection rate, change the gate position, enlarge the gate area and increase the arc transition at the variable section to solve this.
- If the black speck defect is caused by poor dispersion of the filling plastic, replace the resin with better flow properties or replace the screw with strong mixing ability.
Failure to meet the requirements of the raw materials will also result in poor gloss on the surface of the plastic parts.The causes and solutions are as follows:
1.The content of moisture or other volatile substances in the molding raw material is too high, the volatile components will get solidified between the cavity wall of the mold and the molten material during the process of injection molding, resulting in poor gloss on the surface of the plastic part.The raw materials should be pre-dry.
- Degraded material or contamination enter with the resin feedstock, and poorly dispersed pigments in the color masterbatch.3.The flowability of the raw materials is too poor, and the surface of the plastic parts is not dense, thus gloss is poor.Replace the resin with better flow properties or add appropriate amount of lubricant and increase the processing temperature.
4.The raw materials are mixed with foreign materials or incompatible materials.Use new materials.
5.The raw material particle size is not uniform.Raw materials with too different particle sizes should be picked out.
6. The crystalline resin is poor in gloss due to uneven cooling.The mold temperature and processing temperature should be reasonably controlled. For thick-walled plastic parts, if the cooling is insufficient, the surface of the plastic part will be hairy and the gloss will be dark. The solution is to remove the plastic parts from the mold and immediately put them into fixtures soaked in the cold water.
- The proportion of recycled materials in the raw materials is too high, which affects the uniform plasticization of the molten plastic .The amount should be reduced.
At as a injection molder with over 20 years of experience, HS Mold knows how to help you avoid different defects in the injection molding process. For more information, please feel free to contact us at firstname.lastname@example.org