Properties of clear plastics and injection molding process

Posted on : Aug 12, 2022 By  GREFEE

Due to multiple advantages of plastic, such as light weight, great rigidity, simple molding, low cost and other aspects, clear plastic molding has been replacing the role of glass in the modern industry and daily used goods, especially the application in optical instruments and plastic bottle packaging industry, which is developed rapidly.  Since the clear plastic are required to be of high transparency and wear resistance, as well as a great impact toughness, the composition of the plastic and the whole injection molding process, equipments, and molds, are all to be well prepared to ensure the clear plastic to have a great surface quality to meet the requirements.

At present, the clear plastics commonly used in the market include Polymethylmethacrylate (PMMA, commonly known as acrylic or plexiglass), polycarbonate (PC), polyethylene terephthalate (PET), transparent nylon, acrylonitrile styrene copolymer (as), polysulfone (PSF), among which the ones we often use are the PMMA, PC, and PET. Next, take these three kinds of plastics as examples to discuss the characteristics of transparent plastics and injection molding process.

Properties of clear plastic

The priority of the clear plastic is a high grade of transparency. Second, it must have great strength and wear resistance, which is bearable to impact and with great thermal resistance and chemical resistance. Besides, the water absorption rate must be low so that it can ensure the requirement of transparency remains unchanged for a long time. We will compare the properties of PMMA, PC and PET under below.

Common issues in the process of clear plastic injection molding

Since there is a high demand for light penetrate rate for the clear plastic, its surface quality must be strictly controlled without any defects, such as patterns, air holes, whitening, foggy, black spots and gloomy.  Therefore, in terms of the raw materials, equipments, molds, and even the product design should be paid attention to or raising the requirements. On top of that, to obtain a high surface quality, the adjustment to the temperature, injection pressure and speed, as well as other coefficients should be made due to the high melting point and poor flowability of the clear plastic. By doing so, the Injection molded parts can not only be filled, but also produce no internal stress and cause deformation and cracking.

Considerations about the preparation and treatments of raw material, requirements of equipments and molds, process and products of injection molding.

Preparation and drying of raw material.Plastic with impurities will affect the transparency of the products, due to which the storage, transportation, and feeding should be paid attention to ensure the raw material is sealed and clean, especially for the raw material with water. The deterioration of raw materials occurs after being heated. Thus, the material must be fully dried. During the injection molding, the feeding must be performed with dried feeding hopper. Other than that, the air transmitted must be filtered, and dehumidificdated to prevent the material from being polluted.  

Cleaning of the barrel, screw and accessories

To prevent the pollution of the raw material and a possibility of the residue or impurities hidden inside the screw bar or depression areas, especially the resin with poor stability, each part should be cleaned with the screw cleaning agent before and after using to remove the impurities. When the screw bar cleaning agent run out, we can use PE, PS and resin to clean the screw bar. When the machine stops suddenly, to prevent the material from being exposed under high temperature for a long time and cause degradation, the temperature of the drying machines and barrels should be lowered. The temperature of barrel, such as PC and PMMA should be lowered below 160 degrees (the temperature for hopper to the PC should below 100 degrees).

Dos and Don’ts of mold designing (including product design)　

In case of the back flow, or the poor plastic molding and surface defects caused by uneven cooling,  the following points should be paid attention to. 　

(1).  The wall thickness should be uniform and the draft angle should be large enough.

(2). The transition should be gradual. Smooth transition to prevent sharp corners. Sharp edges, especially PC products, should not have gaps.

(3). Gate.The runner should be as wide as enough and as short as possible. The gate position is determined by the shrinkage and condensation process. The cold slug well should be added when necessary.

(4). The mold surface is required to be smooth with low roughness (at least less than 0.8).

(5).  Venting holes. The groove musth be enough to void the gas in the air and molten in time.

(6).  In addition to PET, the wall thickness is typically no less than 1mm.

Considerations of injection molding process (including requirements of injection molding machine)

To reduce the internal stress and surface quality defects, we should pay attention to the following aspects during the injection molding process

(1). Special screw and injection molding machine with separate temperature control nozzle are required.

(2). If the plastic resin does not decompose, higher injection humidity should be used for the injection temperature.

(3). Injection pressure: the injection pressure is generally high in order to overcome the defect of high melt viscosity, but too high pressure will generate internal stress and make the mold is hard to release or even deformation.

(4). Injection speed: if the mold is filled out, slow down the speed. It is better to adopt multi-stage injection as “slow -fast- slow”

(5). Pressure holding time and molding cycle: Under the condition that the product filling is met and there is no depression or bubbles, the time should be short to reduce the time that the molten stays inside the barrels.

(6). Rotating speed of screw bar and back pressure: If the plasticity quality is met, slow down the speed to prevent the possibility of degradation.

(7).Mold temperature: The result of the cooling effect has a huge impact to the quality. Thus, the mold temperature should be controlled accurately. If possible, the high mold temperature is better.

Other aspects 　

To prevent the deterioration of the surface quality, the use of the mold release agent should be as less as possible during the injection molding. The range is no less than 20% when using returns. Products should receive post-treatment to eliminate the internal stress, with an exception of PET. The PMMA should be dried for four hours under a 70-80t hot air circulation. PC should be heated to 110-135 degrees in clean air, glycerin. Heating of liquid paraffin, etc. The length of time is according to the products. The highest requires more than 10 hours. However, PET must goes through the biaxial stretching process to obtain a good mechanical property.

Injection molding process of clear plastic

Process characteristics of clear plastics: in addition to the above common issues, it also has some process characteristics, which are:

Process characteristics of PMMA

PMMA is of high viscosity, but poor flowability. Thus, it must be processed with high material temperature and high injection pressure. The injection temperature has a bigger influence than injection pressure. However, the increase of injection pressure is beneficial to improve the shrinkage of the products. The injection temperature has a wide range. The melting temperature of PMMA is 160 degrees, but the decomposition temperature is 270 degrees. Therefore, its processing performance is good due to the adjustable temperature range is wide. So, the flowability can be improved through changing the injection temperature. Other issues, like poor impact resistance, poor wear resistance, easy to scratch, or easy to crack can be modified by increasing the mold temperature to improve the condensation process to overcome these defects.

Process characteristics of PC

PC has high viscosity, high melting temperature and poor fluidity. Therefore, the parts must be processed at a higher temperature (between 270-320T). Compared to PMMA, its material temperature adjustment range is narrow, and the process-ability is not as good as PMMA. Although injection pressure has a small impact to the flowability, a high injection pressure is still required due to its high viscosity. In order to elimination the generation of internal stress, the pressure holding time should be as short as possible. High shrinkage and stable dimension, but large internal stress will still cause cracking. Therefore, the correct way to address this is by increasing the temperature rather than increasing the pressure to improve the flowability. Moreover, Through increasing the mold temperature and optimizing the mold structure and post-treatment, the possibility of cracking is reduced. When the injection speed is lowered, there will be flow marks appear around the gates. Thus, the temperature of hot nozzles should be controlled separately, the mold temperature should be high, and the resistance of gate and runners will be decreased.

Process characteristics of PET 　

The molding temperate of PET is high, and the material temperature adjustment range is narrow (260-300℃), but it has a great flowability after melting, so its processing performance is poor, and it is necessary to add an anti delay device in the nozzle. After injection, its mechanical strength and properties must be enhanced through the stretch procedures and modification. The mold temperature is an important element in preventing the warping and deformation. Thus, the hot runner molds is suitable because it can reduce the possibilities of poor surface gloss and difficulties of mold releasing due to the high mold temperature.

Defects of clear plastic parts

Defects of transparent products and solutions:

Sliver lining: during the mold filling and condensation process, effect of internal stress anisotropy, stress generated in the vertical direction makes the resin flow upward orientation, and produce flash silk lines with different refractive index from non flow orientation. When it expands, cracks may appear in the product. After the expansion, it many cause cracks in products. In addition to be aware of the external factors in injection molding process and mold (see table, it is better that the products are annealed. Such as the PC material can be heated to over 160 degrees and maintain for 3-5minutes, and then cooled naturally.

Bubbles: the water or other gases in resin cannot be voided (during the condensation process) or insufficient mold filling, the condensing surface condenses too fast so that the vacuum bubbles are formed.

Poor surface gloss:A higher level of surface roughness. Another one is early condensation that makes the resin cannot copy the state of the mold structure. All these make the surface uneven and make the products tarnish.

Chatter mark: It refers to the dense ripple formed from the straight gate as the center. It is caused by the high melt viscosity and the front material has condensed in the mold cavity. Later, the material broke through this condensing surface, causing the surface to vibrate.

Whitening & haze: The dust in the air falls into the raw material or too much water in the raw material.

White smoke & black spot: It is caused by the decomposition or deterioration of the barrel resin due to the high temperature of the local area.

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