Ic trays: PMMA, PC and PET analysis of the characteristics and injection molding process of three types of

Because the IC tray has light weight, good toughness, and easy forming. Low cost and other advantages, so in modern industry and IC trays, more and more plastics are used instead of other materials, especially in optical instruments and packaging industries, and the development is particularly rapid.

However, due to the requirements of good plasticity, high wear-resistant parts, and good impact resistance, a lot of work must be done on the composition of the IC tray, the entire process of injection molding, equipment, molds, etc., to ensure that these are used instead The glass IC tray (hereinafter referred to as the plastic IC tray) has a good surface quality, so as to meet the requirements of use.

At present, the plastic IC trays generally used on the market include polymethyl methacrylate (commonly known as acrylic or plexiglass, code PMMA), polycarbonate (code PC), polyethylene terephthalate (code PET) ), plastic nylon. AS (acrylonitrile-styrene copolymer), polysulfone (code-named PSF), etc., among which we have the most contact with three types of IC trays: PMMA, PC and PET. Due to limited space, the following are examples of these three types of IC trays. , Discuss the characteristics and injection molding process of plastic IC trays.

 

1. The performance of plastic IC tray

Plastic IC trays must first have a high degree of plasticity, and secondly they must have certain strength and abrasion resistance, impact resistance, heat resistance, chemical resistance, and water absorption. Only in this way can they be used and can meet the requirements. The plasticity requirements remain unchanged for a long time. Table 1 is listed below to compare the performance of PMMA, PC and PET.

Plastic IC tray performance comparison

 

Note: (1) Due to the variety of varieties, this is just an average value, and the actual data of different varieties are different.

 

(2) PET data (mechanical aspect) is the data after stretching.

 

PC is an ideal choice, but mainly because of its expensive raw materials and difficult injection molding process, PMMA is still the main choice (for IC trays that are generally required), and PET needs to be stretched to obtain good mechanical properties. , So it is mostly used in packaging and containers.

2. Common issues that should be paid attention to during the injection molding process of plastic IC trays

 

Due to the high light transmittance of plastic IC trays, the surface quality of IC trays must be strict, and there should be no markings, pores, whitening, haze, black spots, discoloration, poor gloss and other defects, so the entire injection molding process For the design of raw materials, equipment, molds, and even IC trays, we must pay great attention to and put forward strict and even special requirements. Secondly, because plastic IC trays mostly have high melting points and poor fluidity, in order to ensure the surface quality of IC trays, process parameters such as higher temperature, injection pressure, and injection speed are often fine-tuned, so that the IC tray can be filled with molds. , And will not produce internal stress to cause deformation and cracking of the IC tray.

 

Let’s talk about the matters that should be paid attention to in terms of raw material preparation, equipment and mold requirements, injection molding process and raw material handling of IC trays.

(1) Preparation and drying of raw materials Since any impurities in the IC tray may affect the plasticity of the IC tray, it is necessary to pay attention to sealing and ensure that the raw materials are clean during storage, transportation, and feeding. In particular, the raw material contains water, which will cause the raw material to deteriorate after heating, so it must be dried, and when injection molding, a drying 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 the raw materials will not be polluted.

 

Plastic IC tray drying process

 

(2) Cleaning the barrel, screw and accessories

 

In order to prevent the contamination of raw materials and the presence of old materials or impurities in the recesses of the screw and accessories, especially the presence of resin 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 cleaner, PE, PS and other resins can be used to clean the screw. When temporarily shutting down, in order to prevent the raw material from staying at high temperature for a long time, causing the solution to drop, the temperature of the dryer and the barrel should be lowered, such as PC, PMMA and other barrels should be reduced to below 160 ℃.

 

(3) Issues that should be paid attention to in mold design (including IC tray design)

 

In order to prevent poor flow back or uneven cooling, resulting in poor molding of IC trays, surface defects and deterioration, generally the following points should be paid attention to when designing the mold.

 

A) The wall thickness should be as uniform as possible, and the draft angle should be large enough.

 

B) The transitional part should be gradual. Smooth transition to prevent sharp corners. Sharp edges are generated, especially PCIC trays must not have gaps.

 

C) Gate. The runner should be as wide as possible, thick and short, and the gate position should be set according to the shrinkage condensation process, and a cold well should be added if necessary.

 

d) The surface of the mold should be smooth and clean with low roughness (preferably lower than 0.8).

 

E) Vent holes. The tank must be sufficient to expel air and gases in the melt in a timely manner.

 

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

 

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

 

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

 

A) An injection molding machine with a dedicated screw and a separate temperature control nozzle should be used.

 

B) The injection temperature should be higher when the IC tray resin does not decompose.

 

C) Injection pressure: generally higher to overcome the defect of high melt viscosity, but too high pressure will cause internal stress and cause difficult demolding and deformation.

 

D) Injection speed: In the case of satisfying mold filling, generally it should be low, and it is best to use slow-fast-slow multi-stage injection.

 

E) Holding pressure time and molding cycle: Under the condition that the IC tray is filled with mold and does not produce dents or bubbles. It should be as short as possible to minimize the residence time of the melt in the barrel.

 

F) Screw speed and back pressure: Under the premise of meeting the plasticization quality, it should be as low as possible to prevent the possibility of degrading.

 

G) Mold temperature: The cooling of the IC tray 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.

 

(5) Other issues

 

In order to prevent the deterioration of the 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, IC trays should be post-processed to eliminate internal stress. PMMA should be dried in 70-80T hot air circulation for 4 hours. The PC should be in clean air and glycerin. Liquid paraffin, etc. are heated at 110-135°C, and the time depends on the IC tray, and it takes more than 10 hours at most. However, PET must undergo a biaxial stretching process to obtain good mechanical properties.

3. Injection molding process of plastic IC tray

 

(1) Process characteristics of plastic IC trays: In addition to the above common problems, plastic IC trays also have some process characteristics, which are described as follows:

 

1. Process characteristics of PMMA PMMA has high viscosity and poor fluidity, so high material temperature and high injection pressure must be used for injection molding. The influence of injection temperature is greater than injection pressure, but the increase in injection pressure will help improve the shrinkage rate of IC trays. . The injection temperature range is wide, the melting temperature is 160°C, and the decomposition temperature is 270°C, so the material temperature adjustment range is wide and the manufacturability is better. Therefore, to improve fluidity, start from the injection temperature. The impact resistance is poor, the wear resistance is not good, it is easy to scratch, and it is easy to be brittle. Therefore, it is necessary to increase the mold temperature and improve the condensation process to overcome these defects.

2. The process characteristics of PC 　　 PC has high viscosity, high melting temperature, and poor fluidity. It must be injection molded at a higher temperature (between 270-320T). Relatively speaking, the temperature adjustment range of the material is narrow, and the manufacturability is not as good as PMMA. The injection pressure has little effect on the fluidity, but due to the high viscosity, the injection pressure is still large. 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 size is stable, but the IC tray has high internal stress and is easy to crack. Therefore, it is advisable to increase the temperature instead of pressure to improve the fluidity, and increase the mold temperature, improve the mold structure and post-processing to reduce the possibility of cracking. When the injection speed is low, defects such as ripples are likely to occur at the gate, the temperature of the radiating nozzle must be controlled separately, the temperature of the mold must be high, and the resistance of the runner and gate should be small.

 

3. Process characteristics of PET　　PET has a high molding temperature and a narrow material temperature adjustment range (260-300℃), but after melting, it has good fluidity and poor manufacturability, and it is often necessary to add anti-spreading devices in the nozzle. The mechanical strength and performance are not high after injection, and the performance must be improved through the stretching process and modification. Accurate control of mold temperature is to prevent warpage. It is an important factor of deformation, so it is recommended to use hot runner molds. The mold temperature is high, otherwise it will cause poor surface gloss and difficulty in demolding.

 

Fourth, the defects and solutions of plastic IC trays

Due to space limitations, only the defects that affect the plasticity of the IC tray are discussed here. For other defects, please refer to the IC tray manual or other materials. The shortcomings are as follows:

 

(1) Silver streak: Affected by the internal stress anisotropy during the filling and condensation process, the stress generated in the vertical direction causes the resin to flow upward orientation, and the refractive index is different from the non-flow orientation to produce flashing silk stripes. After its expansion, cracks may appear in the IC tray. In addition to paying attention to the injection molding process and the mold (see Table 4, the IC tray is best to be annealed. For example, the PC material can be heated to above 160°C for 3-5 minutes, and then cooled naturally.

 

(2) Bubbles: 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 a “vacuum bubble”.

 

• Poor surface gloss: Mainly due to the large roughness of the mold, on the other hand, premature condensation makes the resin unable to copy the state of the mold surface, all of which make the surface produce small irregularities and make the IC tray lose its luster.

(4) Shock pattern: refers to the dense ripples formed from the straight gate as the center. The reason is that the melt viscosity is too large, and the front end material has condensed in the cavity, and then the material breaks through the condensation surface, causing the surface to appear shock lines .

 

(5) Whitening and haze: Mainly caused by dust falling into the raw materials in the air or the raw materials have too much water content.

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