How to improve the transparency of plastic products

Since plastics require good transparency, high wear resistance, and good impact toughness, the ingredients of the plastic, the process of the entire injection molding process, and the equipment. A lot of work must be done to ensure that the plastics used to replace glass (hereinafter referred to as transparent plastics) have good surface quality and meet the requirements of use.

Currently, the transparent plastics commonly used on the market include polymethylmethacrylate (commonly known as acrylic or plexiglass, code-named PMMA) and polycarbonate (code-named PC). Polyethylene terephthalate (code name PET), transparent nylon. AS (acrylonitrile-styrene copolymer), polysulfone (code name PSF), etc. Among them, PMMA is the one that plastic parts manufacturers are most exposed to. PC and PET are three plastics. Due to limited space, these three plastics will be used as examples to discuss the characteristics and injection molding process of transparent plastics.

1. Performance of transparent plastic

Transparent plastics must first have high transparency. Secondly, they must have certain strength and wear resistance, be impact resistant, have good heat resistance, have excellent chemical resistance, and have low water absorption. Only in this way can they meet the requirements of transparency during use. requirements and remain unchanged for a long time.

PC is an ideal choice, but due to its expensive raw materials and difficult injection molding process, PMMA is still the main choice (for products with general requirements), while PPT needs to be stretched to obtain good mechanical properties. Therefore, it is often used in packaging and containers.

2. Common issues that should be paid attention to during the injection molding process of transparent plastics

Due to the high light transmittance of transparent plastic, the surface quality of plastic products must be strict, and there must be no markings, pores, or whitening. Defects such as halo, black spots, discoloration, poor gloss, etc., thus affecting raw materials and equipment during the entire injection molding process. The design of molds and even products requires great attention and strict or even special requirements.

Secondly, since most transparent plastics have high melting points and poor fluidity, in order to ensure the surface quality of the product, it is often necessary to make subtle adjustments to the barrel temperature, injection pressure, injection speed and other process parameters, so that the mold can be filled with the plastic without filling the mold. Internal stress will be generated, causing product deformation and cracking.

Let’s talk about the things that should be paid attention to in terms of equipment and mold requirements, injection molding process and raw material handling of products:

dry

Preparation and drying of raw materials Since any impurity contained in plastic may affect the transparency of the product, it must be stored and transported.

During the feeding process, attention must be paid to sealing to ensure that the raw materials are clean. In particular, the raw materials contain moisture, which will cause deterioration of the raw materials after heating, so they must be dry, and during injection molding, a dry hopper must be used for feeding. It should also be noted that during the drying process, the input air should preferably be filtered and dehumidified to ensure that it does not contaminate the raw materials.

Cleaning of barrel, screw and accessories

In order to prevent raw material contamination and the presence of old materials or impurities in the recesses of the screw and accessories, especially resins with poor thermal stability, screw cleaning agents should be used to clean all parts before use and after shutdown to prevent impurities from sticking. When there is no screw cleaning agent, PE, PS and other resins can be used to clean the screw.

When temporarily shutting down, in order to prevent raw materials from staying at high temperature for a long time and causing degradation, the temperature of the dryer and barrel should be lowered. For example, the barrel temperature of PC, PMMA, etc. should be lowered to below 160°C. (The hopper temperature should be reduced to below 100°C for PC) Issues that should be paid attention to in mold design (including product design) In order to prevent poor backflow or uneven cooling from causing poor plastic molding, surface defects and deterioration; generally When designing the mold, you should pay attention to the following points:

The wall thickness should be as uniform as possible, and the draft angle should be large enough;

Transitions should be gradual. Smooth transition to prevent sharp corners. Sharp edges are generated, especially PC products must not have gaps;

Gate. The flow channel should be as wide and short as possible, and the gate position should be set according to the shrinkage and condensation process. If necessary, a cold well should be added;

The mold surface should be smooth and clean, with low roughness (preferably less than 0.8);

Vent. The tank must be sufficient to discharge air and gas in the melt in a timely manner;

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

Issues that should be paid attention to in the injection molding process (including injection molding machine requirements)

In order to reduce internal stress and surface quality defects, the following aspects should be paid attention to in the injection molding process:

An injection molding machine with a special screw and a separate temperature-controlled injection nozzle should be used;

As long as the plastic resin does not decompose, a higher injection temperature should be used;

Injection pressure: Generally high to overcome the defect of high melt viscosity, but too high a pressure will produce internal stress, causing difficulty in demoulding and deformation;

Injection speed: When the mold filling is satisfied, it should generally be low, and it is best to use slow-fast-slow multi-stage injection;

The holding time and molding cycle should be as short as possible to minimize the residence time of the molten material in the barrel under the condition that the product is fully filled and no dents or bubbles are generated;

Screw speed and back pressure: On the premise of meeting the plasticizing quality, they should be as low as possible to prevent the possibility of degradation;

Mold temperature: The cooling quality of the product has a great impact on the quality, so the mold temperature must be able to accurately control the process. If possible, the mold temperature should be higher.

Other issues

In order to prevent the deterioration of the upper surface quality, generally use as little release agent as possible during injection molding; when using recycled materials, it should not exceed 20%.

Except for PET, products should be post-processed to eliminate internal stress. PMMA should be dried in hot air circulation at 70-80°C for 4 hours; PC should be dried in clean air and glycerin. Heating liquid paraffin to 110-135℃, the time depends on the product, it takes up to 10 hours. PET must undergo a biaxial stretching process to obtain good mechanical properties.

3. Injection molding process of transparent plastic

Process characteristics of transparent plastics

In addition to the above common problems, transparent plastics also have some process characteristics, which are described below:

1. Process characteristics of PMMA

PMMA has high viscosity and poor fluidity, so it requires high material temperature and high injection pressure for injection molding. The impact of injection temperature is greater than the injection pressure, but increasing the injection pressure will help improve the shrinkage rate of the product.

The injection temperature range is wide, the melting temperature is 160°C, and the decomposition temperature reaches 270°C, so the material temperature adjustment range is wide and the processability is good. Therefore, to improve fluidity, we can start with the injection temperature. It has poor impact resistance, poor wear resistance, easy scratching, and brittleness. Therefore, the mold temperature should be increased and the condensation process should be improved to overcome these defects.

2. PC process characteristics

PC has high viscosity, high melt temperature, and poor fluidity, so it must be injected at a higher temperature (between 270-320°C). Relatively speaking, the material temperature adjustment range is narrow, and the processability is not as good as PMMA. The injection pressure has little effect on the fluidity, but due to the high viscosity, the injection pressure still needs to be higher. Correspondingly, in order to prevent the generation of internal stress, the pressure holding time should be as short as possible.

The shrinkage rate is large and the dimensions are stable, but the internal stress of the product is large and it is easy to crack. Therefore, it is better to increase the temperature rather than the pressure to improve the fluidity, and to reduce the possibility of cracking by increasing the mold temperature, improving the mold structure and post-processing. When the injection speed is low, defects such as ripples are prone to occur at the gate. The temperature of the radiating nozzle must be controlled separately, the mold temperature must be high, and the resistance of the runner and gate must be small.

3. PET process characteristics

PET molding temperature is high, and the material temperature adjustment range is narrow (260-300℃), but after melting, the fluidity is good, so the processability is poor, and an anti-flow device is often added to the nozzle. The mechanical strength and performance are not high after injection, and the properties must be improved through the stretching process and modification.

Accurate control of mold temperature is to prevent warpage. Deformation is an important factor, so it is recommended to use a hot runner mold. The mold temperature must be high, otherwise it will cause poor surface gloss and difficulty in demoulding.

4. Defects and solutions of transparent plastic parts. The defects are probably as follows:

Silver streak

Due to the influence of internal stress anisotropy during the mold filling and condensation process, the stress generated in the vertical direction causes the resin to flow upward, and the refractive index is different from the non-flow orientation, resulting in glittering silk patterns. When they expand, they may cause The product has cracks. In addition to 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℃ for 3-5 minutes, and then allowed to cool naturally.

bubble

Mainly because the water vapor and other gases in the resin cannot be discharged (during the mold condensation process) or due to insufficient mold filling, the condensation surface condenses too quickly to form vacuum bubbles.

Poor surface gloss

Mainly due to the large roughness of the mold, on the other hand, premature condensation prevents the resin from copying the state of the mold surface. All of these cause slight unevenness on the surface, which makes the product lose its luster.

Seismic marks

It refers to the dense ripples formed from the sprue as the center. The reason is that due to the excessive melt viscosity, the front-end material has condensed in the mold cavity, and later the material breaks through this condensation surface, causing vibration marks on the surface.

White fog

It is mainly caused by dust in the air falling into the raw materials or the raw materials containing too much moisture.

white smoke black spots

It is mainly caused by the decomposition or deterioration of the barrel resin due to local overheating of plastic in the barrel.