Transparent plastics also have some different injection molding process characteristics, which are described below:
PMMA processability
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 product shrinkage. It has a wide injection temperature range, with a melt temperature of 160℃ and a decomposition temperature reaching 270℃, resulting in a wide temperature adjustment range and good processability. Therefore, improving flowability can be achieved by adjusting the injection temperature. However, 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.
Processing Characteristics of PC
PC has high viscosity, high melt temperature, and poor fluidity. Therefore, it must be injection molded at a relatively high temperature (between 270 and 320°C). 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. 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 increase temperature rather than pressure to improve fluidity, 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 should be high, and the runner and gate resistance should be low.
Processing characteristics of PET
PET molding requires high temperatures and has a narrow temperature adjustment range (260–300℃). However, after melting, it has good fluidity, resulting in poor processability, and often necessitates the addition of anti-drooling devices in the nozzle. Its mechanical strength and properties after injection molding are not high, requiring stretching and modification to improve performance. Accurate mold temperature control is crucial to preventing warpage; therefore, hot runner molds are recommended. High mold temperatures are essential; otherwise, poor surface gloss and difficulty in demolding will occur.
Common Defects and Solutions in Injection Molding of Transparent Plastics
The defects that affect product transparency are mainly as follows：
(1) Silver Stains
Silver streaks occur due to anisotropic stress within the melt during mold filling and solidification. Stress perpendicular to the melt flow direction often causes molecular orientation in the melt. The refractive indices of the oriented and non-oriented areas differ, resulting in silver streaks. As silver streaks expand, they can cause cracks in the product. Solutions include optimizing the mold structure and injection molding process, and annealing the molded part after injection molding. For example, for PC injection molded products, the part can be slowly heated to above 160℃ and held for 3-5 minutes, then allowed to cool naturally.
(2) Bubbles
Vacuum bubbles form because moisture and other gases within the resin cannot escape (during mold solidification) or due to insufficient mold filling and excessively rapid solidification of the surface.
(3) Poor Surface Gloss
High mold roughness and premature solidification prevent the resin from replicating the mold surface condition. All of these factors contribute to 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 mold cavity, and the subsequent material breaks through this solidified surface, resulting in vibration marks.
(5) Whitening
This is mainly caused by dust falling into the raw material or excessive moisture content in the raw material.
(6) White Smoke and Black Spots
These are mainly caused by localized overheating of the plastic inside the barrel, leading to decomposition or deterioration of the resin.