Introduction to the Injection Molding Process of Transparent Plastics

1. Characteristics of Commonly Used Transparent Raw Materials -Introduction to the Injection Molding Process of Transparent Plastics

Because transparent plastics require high light transmittance, the surface quality of the plastic products must be strictly controlled. Defects such as blemishes, pores, whitening, haze, black spots, discoloration, and poor gloss are unacceptable. Therefore, the entire injection molding process requires careful attention and stringent, even specialized, requirements for raw materials, equipment, molds, and even product design.

Secondly, because transparent plastics often have high melting points and poor fluidity, to ensure product surface quality, fine adjustments are often made to process parameters such as higher temperatures, injection pressures, and injection speeds. This ensures the plastic fills the mold completely without generating internal stress that could lead to product deformation and cracking.

2. Process Considerations

To reduce internal stress and surface quality defects, the following aspects should be considered in the injection molding process. • 1) Injection temperature: A higher injection temperature is preferable, provided the plastic resin does not decompose.

• 2) Injection pressure: Generally higher to overcome the high viscosity of the melt, but excessive pressure can create internal stress, leading to difficulty in demolding and deformation.
• 3) Injection speed: Generally lower, while ensuring mold filling, preferably using a slow-fast-slow multi-stage injection.
• 4) Holding time and molding cycle: As long as product filling is achieved without sink marks or bubbles, these should be as short as possible to minimize the residence time of the melt in the barrel.
• 5) Screw speed and back pressure: As long as plasticizing quality is met, these should be as low as possible to prevent potential degradation.
• 6) Mold temperature: The quality of product cooling is extremely important, so the mold temperature must be precisely controlled. If possible, a higher mold temperature is preferable. • 7) To prevent surface quality deterioration, mold release agents should be used sparingly during injection molding; when reclaimed material is used, it should not exceed 20%.

1. Injection Molding Processes for Commonly Used Transparent Raw Materials

Besides the common issues mentioned above, transparent plastics also have some unique processing characteristics, which are described below:

1. PMMA has high viscosity and slightly poor flowability, therefore, high material temperature and high injection pressure are required for injection molding. Injection temperature has a greater impact than injection pressure, but increasing injection pressure helps improve the shrinkage rate of the product.

PMMA has a wide injection temperature range, with a melting temperature of 160℃ and a decomposition temperature reaching 270℃, thus allowing for a wide temperature adjustment range and good processability. Therefore, improving flowability can be achieved by adjusting the injection temperature. It has poor impact resistance, poor wear resistance, is easily scratched, and is prone to cracking; therefore, increasing the mold temperature and improving the cooling process are necessary to overcome these defects. 2. PC has high viscosity and high melt temperature, resulting in poor fluidity. Therefore, it must be injection molded at a relatively high temperature (between 270-320℃). Relatively speaking, its temperature control range is narrower, and its processability is not as good as PMMA. Injection pressure has little impact on fluidity, but due to its high viscosity, a relatively high injection pressure is still required. Correspondingly, to prevent internal stress, the holding time should be as short as possible.

It has a high shrinkage rate and dimensional stability, but the product has high internal stress and is prone to cracking. Therefore, it is advisable to improve fluidity by increasing temperature rather than pressure, and to reduce the possibility of cracking by increasing mold temperature, improving mold structure, and post-processing. When the injection speed is low, defects such as ripples are prone to occur at the gate. The nozzle temperature must be controlled separately, the mold temperature must be high, and the runner and gate resistance must be low.

3. PET has a high molding temperature and a narrow temperature control range (260-300℃), but after melting, it has good fluidity, resulting in poor processability. Anti-spreading devices are often added to the nozzle. The mechanical strength and performance after injection molding are not high, and stretching and modification are necessary to improve performance. Accurate mold temperature control is crucial to prevent warping and deformation; therefore, hot runner molds are recommended. High mold temperatures will result in poor surface gloss and difficulty in demolding.
4. Defects and Solutions for Transparent Plastic Parts

• 1. Silver streaks: Caused by the anisotropy of internal stress during filling and cooling. Stress in the vertical direction causes the resin to flow and orient, while non-flowing or oriented resins produce different refractive indices, resulting in shimmering streaks. As these streaks expand, they may cause cracks in the product. Besides paying attention to the injection molding process and mold, it is best to anneal the product. For example, PC material can be heated to above 160℃ and held for 3-5 minutes, then allowed to cool naturally.
• 2. Bubbles: These are formed when water vapor and other gases within the resin cannot escape (during mold cooling) or due to insufficient filling and excessively rapid cooling of the surface, creating “vacuum bubbles.”
• 3. Poor surface gloss: Primarily due to high mold roughness and premature solidification, preventing the resin from replicating the mold surface. All these factors result in micro-unevenness on the surface, causing the product to lose its gloss.
• 4. Vibration marks: These are dense ripples forming around the sprue. The cause is excessive melt viscosity; the material at the front has already solidified in the cavity, and the subsequent material breaks through this solidified surface, resulting in vibration marks.
• 5. Whitening/haze: Mainly caused by dust falling into the raw material or excessive moisture content in the raw material.
• 6. White smoke/black spots: Mainly caused by localized overheating of the plastic inside the barrel, leading to resin decomposition or deterioration.https://solidcomould.com/product-category/injection-mould/household-mould/