Important Technical Points Of Injection Molding Manufacturing Transparent Products

plastic injection molding, product development

Due to their high transparency, transparent plastics require strict surface quality requirements for plastic products. They are should without any stripes, air bubbles, whitening, fogging, black spots, discoloration, poor gloss, and other defects. The entire injection molding manufacturing process, from raw materials to equipment and injection moulds, and even product design, requires strict and special requirements. Transparent plastics, in particular, have high melting points and poor fluidity, necessitating subtle adjustments to process parameters such as machine temperature, injection pressure, and injection speed to avoid internal stress and product deformation or cracking. It is essential to consider various factors when preparing raw materials, selecting equipment and molds, handling raw materials for transparent plastics, and executing injection molding processes.
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Raw Material Preparation and Drying

Any impurities in plastic can affect the transparency of the final product. Therefore, during storage, transportation, and feeding, the raw materials must be kept sealed and clean. In particular, if the raw materials contain moisture, they must be dried and fed using a dry hopper during injection molding. Additionally, during the drying process, it is best to filter and dehumidify the input air to prevent contamination of the raw materials.

Cleaning of Machine Barrels, Screws, and Attachments

To prevent contamination of the raw materials and old materials or impurities in the recesses of the screw and attachments, especially for resins with poor heat stability, they should be cleaned with a screw cleaning agent before and after use to ensure that they are free from impurities. To clean the screw when a screw cleaning agent is unavailable, a resin such as PE or PS can be used. To avoid material degradation from prolonged exposure to high temperatures, lower the drying machine and machine barrel temperatures when temporarily stopping the machine. For example, the machine barrel temperature for PC and PMMA should be lowered to below 160°C (the hopper temperature for PC should be lowered to below 100°C).

Mold Design (Including Product Design)

To avoid surface defects and degradation caused by uneven cooling or poor flow during plastic molding, it is essential to consider the following points in mold design, including product design.

  • 1. The mold’s wall thickness should be as consistent as possible, and the demolding angle should be sufficiently large.
  • 2. The transition section must have a smooth transition to prevent sharp corners and edges, particularly for PC products.
  • 3. It is recommended to make the gate and runner as short and wide as possible, and determine the gate position based on the shrinkage condensation process. If required, add a cooling well.
  • 4. The mold surface should be smooth, with a low roughness (preferably below 0.8).
  • 5. The vent slots must be sufficient to timely discharge air and gas from the melt.
  • 6. Generally, the wall thickness should not be too thin, typically a minimum of 1mm, except for PET.

Injection Molding Process and Injection Molding Machine

To reduce internal stress and surface quality defects, the following aspects should be noted in the injection molding process:

    • 1. A dedicated screw and injection molding machine with a separate temperature control nozzle should be used.
    • 2. Injection temperature should be relatively high, under the premise of not decomposing the plastic resin.
    • 3. Injection pressure should generally be higher to overcome the disadvantage of high melt viscosity, but too high pressure can cause internal stress and difficulty in demolding.
    • 4.Injection Speed: Generally, it is recommended to keep the injection speed low while ensuring full molding, and it is best to use a multi-stage injection process, such as slow-fast-slow injection.
    • 5. Holding time and molding cycle: The holding time and molding cycle should be as short as possible while ensuring that the product is fully molded without any dents or bubbles. This is to minimize the residence time of the molten material in the barrel.
    • 6. Screw speed and back pressure: The screw speed and back pressure should be as low as possible while ensuring good plasticization quality, to prevent the possibility of degradation.
    • 7. Mold temperature: The cooling of the product has a great impact on its quality. Therefore, the mold temperature must be precisely controlled throughout the process. If possible, it is recommended to keep the mold temperature relatively high.

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Other Issues

To prevent deterioration of the surface quality, it is generally recommended to use a minimal amount of release agent during injection molding. When using recycled materials, the amount used should not exceed 20%.

For products other than PET, post-treatment is necessary to eliminate internal stress. For example, PMMA should be hot air circulated at 70-80°C for 4 hours, while PC should be heated in clean air, glycerin, liquid paraffin, etc., at 110-135°C for a period of time, which varies depending on the product, and can take up to 10 hours. PET must undergo a biaxial stretching process to achieve good mechanical properties.

In general, due to the high light transmittance of transparent plastic parts, transparent plastic products manufactured by injection molding should ensure that there are no defects such as streaks, bubbles, whitening, fogging, black spots, discoloration, and poor gloss.

Sungplastic has rich manufacturing experience and Japanese TOSHIBA injection molding machine workshop. Guaranteeing the excellent quality of products and timely and effective communication are the cornerstones of our long-term cooperation with customers. If you want to design and manufacture a transparent product, such as LED lens, or transparent plastic accessories applied to other products, please contact us today for a free quote and project evaluation.
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About Sungplastic

About Sungplastic

Sungplastic is a plastic product manufacturer with rich experience in injection molding. According to the different product development requirements, we flexibly adjust the manufacturing process to achieve high quality, high efficiency and more economical.

We offer a variety of manufacturing services: Rapid Prototyping, Tool Making, Injection Molding, Product Design and Development, CNC Machining and Metal Stamping. You can choose from a variety of plastics, silicone rubber, or metal for your product. Regardless of mass production or small batch customization, Sungplastic has always been committed to providing assured, efficient and more economical one-stop processing services for your projects.

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