Properties of Transparent Plastics and Injection Molding Process

Properties of Transparent Plastics and Injection Molding Process

At present, the transparent plastics commonly used in industry include PMMA (polymethyl methacrylate, commonly known as acrylic or plexiglass), PC (polycarbonate), PET (polyethylene terephthalate), transparent nylon, SAN ( Styrene-acrylonitrile copolymer), PSF (polysulfone), etc. PMMA, PC, and PET are the most used plastics. The following three plastics are used as examples to analyze transparent plastics’ molding performance and injection molding process.

Transparent plastics must first have high transparency, and secondly, they must have certain strength and wear resistance, be able to resist impact, have better heat resistance and chemical corrosion resistance, and have lower water absorption. Only in this way can they meet the requirements of transparency in use. Unchanged for a long time. The following is a list of performance indicators for PMMA, PC, and PET three plastics.

Preparations before injection molding of transparent plastic

Due to high light transmittance requirements of transparent plastics, we must pay close attention to and even put forth strict or special requirements. The surface quality of plastic products must be strictly required, and there must be no defects such as streaks, holes, whitening, halos, black spots, discoloration, and poor gloss.

Transparent plastics have a high melting point and limited mobility, so to ensure the product’s surface quality, process variables like temperature, injection pressure, and injection speed are frequently explored and adjusted. This ensures that the mold can be filled during injection molding and won’t crack. Internal stress is produced to create product deformation and cracking.

The following are the details that need to be taken into consideration while preparing plastic raw materials when choosing equipment and molds, and when performing injection molding.

Drying of Raw Materials:

Pay close attention to the sealing of the raw materials throughout storage, shipping, and feeding to ensure that the raw materials are clean since any plastic impurities may influence the product’s transparency. The raw material will deteriorate after heating, particularly if it absorbs excessive water. 

Before using translucent plastic for injection molding, it must be dried. It should be noted that the input air should ideally be filtered and dehumidified during the drying process to prevent contamination of the raw material by airborne pollutants. The drying conditions of three popular transparent polymers are as follows:

Drying Process of Transparent Plastic

Cleaning of the Barrel, Screw, and Accessories:

There are old materials or impurities in the depressions of the screw and accessories of the injection molding machine, especially the residual plastics with poor thermal stability, which may pollute the raw materials of heat-transparent plastics. Before and after shutdown, the screw and its accessories should be cleaned with a screw-cleaning agent to prevent impurities from sticking. When there is no screw-cleaning agent, the screw can be cleaned by injecting PE, PS, and other plastics into the air.

When temporarily shutting down, to prevent degradation caused by raw materials staying at high temperatures for a long time, the temperature of the dryer and the barrel should be lowered. For example, the temperature of the barrel of PC, PMMA, etc. should be lowered to below 160°C (the temperature of the barrel should be lower than that of PC. down to below 100°C)

Problems that should be paid attention to in plastic parts and molds:

1. The wall thickness should be as uniform as possible, and the demoulding slope should be large enough.

2. The transition part should transition gradually or smoothly to prevent sharp corners, sharp edges, etc., especially PC products that must not have gaps.

3. The gate and runner should be as wide and short as possible, and the gate position should be set according to the shrinkage condensation process. If necessary, a cold material hole should be set.

4. The surface of the mold should be smooth and the roughness should be high (preferably less than 0.8um)

5. Vent holes, vent grooves, etc. must be sufficient to discharge air and gas in the melt in time.

6. Except for PET, the wall thickness should not be too thin, generally not less than 1mm.

Problems that should be paid attention to in the injection molding process:

1. An injection molding machine with a special screw and a separate temperature-controlled nozzle should be selected.

2. Under the premise that the plastic melt does not decompose, a higher injection temperature should be used.

3. Injection pressure is generally higher to overcome the defect of high melt viscosity, but too high pressure will generate internal stress, resulting in difficult demoulding and deformation.

4. The injection speed should be low when the mold filling is satisfied, and it is best to adopt a slow-fast-slow multi-stage injection process.

5. In the case of satisfying product filling without sag and bubbles, the holding time and molding cycle should be as short as possible to shorten the residence time of the melt in the barrel as much as possible.

6. Under the premise of satisfying the quality of plasticization, the screw speed and back pressure should be as low as possible to prevent plastic degradation.

7. The quality of the cooling of the product has a great impact on its surface quality, so the mold temperature must be accurately controlled. To reduce defects such as flow lines, the mold temperature of transparent plastic injection molding is generally high.

Other Issues: Mold release agents should not typically be utilized in the injection molding of transparent polymers to avoid contamination of the product’s surface. The addition of a receiving material must not exceed 20% of the raw material. Post-processing should be done after injection molding, except PET, to reduce internal stress. PMMA should be heated to between 70 and 80 degrees Celsius and dried there for at least four hours. In clean air, glycerin, liquid paraffin, etc., the PC should be heated to 110–135°C. The lengthiest time is greater than 10 hours and is based on the size of the product. PET requires two-way stretching to get appropriate mechanical characteristics.

Three translucent polymers that are frequently used in injection molding

PMMA injection molding procedure: Because PMMA has a high viscosity and a marginally low fluidity, injection molding requires a high material temperature and high injection pressure. Although injection temperature has a bigger effect on injection molding than injection pressure, raising injection pressure helps reduce product shrinkage. The material temperature can be varied across a wide range, the melting temperature is 160°C, the thermal decomposition temperature reaches 270°C, and the manufacturability is good. Start by increasing the injection temperature to increase the fluidity of PMMA.

PMMA products are easily scratched, fragile, and have low impact and wear resistance. To solve the aforementioned flaws, the mold temperature should be raised and the condensation process should be enhanced

PC injection molding process: PC has a high viscosity, high melt temperature, and poor fluidity, so it must be injected at a higher temperature (between 270 and 320°C). Relatively speaking, the material temperature adjustment range of PC is narrow, and the manufacturability is not as good as PMMA. Injection pressure has little effect on fluidity, but due to high viscosity, relatively high injection pressure is still required. Correspondingly, to prevent excessive internal stress on the product, the pressure holding time should be as short as possible.

PET injection molding process: The shrinkage rate of PC is large, the internal stress of the product is large, and it is easy to crack, so it is advisable to improve the fluidity by increasing the temperature instead of the pressure, and reduce the cracking from the perspective of increasing the mold temperature, improving the mold structure, and post-processing the product. possible. When the injection speed is low, defects such as ripples are likely to occur at the gate. The nozzle temperature of the injection molding machine should be controlled separately, the mold temperature should be high, and the resistance of the runner and gate should be small.