Common Clear Injection Material Selection
Posted on : May16, 2022 By GREFEE
The priority of clear plastic is to ensure high transparency. Its high strength, impact and wear resistance, thermal resistance, chemical resistance, and low water absorption rate make it meet the needs for transparency.
For the clear material selection, PC is the ideal section but with high price and difficult injection molding technology. So, PMMA is the typical choice. however, a good mechanical performance of PPE should be obtained after stretching. This is the reason why many packages and containers have the PPE composition. GPPS is a kind of glossy material with transparent particles. It is applicable for making various instrument shell, lampshade, optical parts, instrument parts with the characteristics of light weight, cheap price, low water absorption rate, good coloring, dimensional stability, good electrical properties, transparent products, and easy processing.
Three typical clear plastic injection molding properties
PMMA has a high viscosity and poor flowability, so it must be processed under high material temperature and injection pressure. The influence of injection temperature outweighs its injection pressure. The increase of the injection pressure benefits the shrinkage of products.
It has a wide injection temperature range. The melting temperature is 160 degrees, and the degradation temperature is 270 degrees. Thus, the material temperature range widens with excellent processability. Meanwhile, flowability improvement can be achieved through the injection temperature.
If there are issues, like low impact-ability, wear resistance, easy to scratch and crack, increase the mold temperate and improve the condensation process to overcome these defects.
PC material is characterized by its high viscosity and melt temperature, and poor flowability. Therefore, must apply a high injection temperature (270~320 degrees). Compared to material temperature, the injection temperature range is limited, and its processability is lower than PMMA.
Injection pressure has less impact on flowability. However, due to its high viscosity, there is still a need for high injection pressure to prevent internal stress from being generated. What’s more, the time of pressure holding should be as less as possible.
PC material has a low contraction rate and stable size but high internal stress, so easy to crack. Thus, it is suitable to use high temperature rather than pressure to improve the flowability. What’s more, it is easy to enhance its mold temperature, improve the mold structure, and post-treatment reduce the possibility of cracking.
When the injection speed is low, the gate is likely to have ripples and other defects. The radiation nozzle temperature should be controlled separately, and the mold temperature should be high. Gate and nozzle forces are low.
3. Process characteristics of PET
The forming temperature of PET is high. Material temperature adjusting range is narrow (260-300 degrees). After melting, the flowability increases, so processability decreases. Moreover, a delay device should be installed inside the injection nozzles.
Mechanical strength and properties decrease after injection, which must be improved after stretching and properties alteration.
Mold temperature should be controlled accurately to prevent the happening of warpage. So, it is advised to use hot runner mold.
Clear material selection
1. Daily necessities material
① Transparent films: PE, PP, PS, PVC, GPPS and pet for packaging, PE, PVC and pet for agriculture
②、Transparent sheets and plates: PP, PVC, pet, PMMA and PC
③、Transparent pipes: PVC, PA, etc
④、Transparent bottles: PVC, pet, PP, PS and PC.
2. Lighting materials
Mainly used as lampshade, commonly used ones are PS, modified PS, AS, PMMA and PC, etc.
3. Optical equipment material
①、Hard mirror body: CR-39 and J.D
②、Contact lenses: HEMA.
4. Glass material
① Traffic glass: commonly used two types are PMMA and PC
② Building glass: PVF and pet are commonly used.
5 .solar energy material
PMMA、PC、GF-UP、FEP、PVF及SI are commonly used.
6. Firmer material
The core layer is PMMA or PC. The coating layer is fluoroolefin polymer and fluoromethyl methacrylate.
7 .CD material
PC、PMMA are commonly used.
8 .Transparent packaging material
Surface hardened PMMA、FEP、EVA、EMA、PVB, etc.
Considerations of clear plastic injection molding process
Due to the high light transmittance and quality requirements for clear plastic, defects like patterns, air holes, whitening, fog halo, black spots, discoloration, and poor gloss are not allowed on the product’s surface. Thus, in injection molding, we must have a specific requirement for raw materials, equipment, molds, and even the product design.
1. Preparation and drying of raw material
In the feeding process, the operator should pay attention to the sealing to ensure the cleanliness of raw material, especially the water in the raw material, which will cause the deterioration of raw materials. So, the raw material must be dried completely. In injection molding, the operator must use the drying hopper and use the air that has been filtrated and dehumidified to prevent material deterioration.
2. Cleaning of barrel, screw and its accessories
To prevent the raw material from being polluted and material residual in the screw bar and sink sites of accessories (especially resin with poor thermal stability). Thus, each part must be cleaned before utilization and after shutdown to remove all the impurities. If the screw bar detergent runs out, we can use PE, PS, and other resin to clean the screw bar.
When it comes to the sudden shutdown, to prevent the degradation of raw material under high temperature for long, the operation should lower the temperature of drying machining and machine barrel, such as PC, PMMA, of which the barrel temperature should be lower than 160 degrees.
3. Considerations for mold design (include product design)
To prevent situations like the poor molding, surface defects, and deterioration due to poor flowability and uneven cooling, there are several things we should be aware of, which are:
(1). wall thickness should be as even as possible， and the draft angle should be as but as possible
(2). smooth transition to prevent sharp corners and edges, especially PC products, must be without breaches.
(3). the gate should be as wide and short as possible. The position of the gate should according to the shrinkage and condensed progress. And if necessary, add charging well.
(4). The surface of the die should be smooth with low roughness (preferably less than 0.8)
(5). the vent holes and grooves must void the gas in the air and melt in time.
(6). besides PET，the wall thickness should not be too thin, not less than 1mm.
4. Problems needing attention in the injection molding process (including requirements for injection molding machine)
To reduce the internal stress and defects like surface quality, several aspects should be paid attention to in the injection molding:
(1). should select specific screw bars and injection molding machines with separate temperature control nozzles.
(2). increase the injection temperature without break down the plastic resin.
(3). the injection pressure is high to overcome high viscosity. However, high pressure will lead to mold releasing difficulty and deformation due to the generation of internal stress.
(4). the injection speed should be low to fulfill the mold filling conditions. It is preferable to apply [slow-quick-slow] multi-stage injection.
(5). the time for pressure holding and molding cycle should be short to reduce the time that melts stay in the barrels when fulfilling the mold filling without depressions, bubbles, and other defects.
(6). The screw speed and back pressure should be as low as possible for the plasticization quality to prevent the possibility of degradation.
(7). If it is possible, the mold temperature should be high. The Mold temperature has a great impact on the quality of products, so the mold temperature must be able to control the process accurately.
5. Other issues
(1). To prevent the deterioration of surface quality, it advises that reduce the use amount of mold release agents. When using recycled material, it should not be higher than 20%.
(2). Products must receive post treatment, except for PET, to reduce the internal stress.
(3). PMMA should be dried under 70-80 ℃ hot air circulation.
(4). PC should be heated to 110-135 degrees by the clean air, glycerol, and liquid paraffin. The time depends on the products, maximumly needs 10 hours.
6. Defects of clear plastic parts
(1). Craze Due to the effect of internal stress anisotropy in the molding filling and condense process, the stress generated in the vertical direction will make the resin flow-oriented, whereas form the flash silk lines with different refractive index from non-flow orientation. After the expansion, cracks may occur. Apart from the injection molding and molds, it advises that products should be annealed, such as the PC material can be heated above 160 degrees for 3-5 minutes and then cooled naturally.
(2). Bubbles as the moisture and other gases in the resin cannot be voided (in the condensation process), or resulting from the insufficient mold filling, the condensed surface forms some vacuum bubbles due to rapid condensation.
(3). Poor surface gloss Due to the high degree of roughness of the mold and premature condensation, the resin cannot copy the state of the mold surface. All these lead to a slightly uneven surface and poor luster.
(4). Vibration ripples refer to the dense ripples formed from the straight gate as the center. Due to its high melt viscosity, the material in front has condensed in the mold cavity. This condensed surface broke through by the materiel, resulting in vibration ripples on the surface.
(5). Whitening and fog halo aroused by high water content or dust in the raw material.
(6). white smoke、black spots plastic in barrels, since the break down and deterioration of resin in barrels due to the local overheat.
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