Selection Of Thermoplastic For Plastic Part Injection Molding
Selection of thermoplastic will directly affect the performance of plastic parts, the molding process of plastic parts, the production cost of plastic parts and the quality of plastic parts. There are tens of thousands of synthetic resins thermoplastic on the market, thousand kinds of them have been industrialized, but there are only 20 to 30 most commonly used thermoplastic. In the actual selection process, some plastics are very similar in performance so that it is difficult to distinguish. It is a very complicated task for select the thermoplastic. We should considerate the following factors:
- Selecting thermoplastic must meet the functional requirements of manufacturing product:
① Mechanical properties of plastics: Strength, rigidity, toughness, elasticity, bending properties, impact properties.
② The physical properties of plastics: such as adaptability to changes in ambient temperature, optical properties, degree of thermal or electrical insulation, degree of precision work and appearance, etc., whether it can meet the requirements in actual application.
③ Chemical properties of plastics: such as if it have endurance when the plastic in water, solvents, oils, and drugs and whether it safety meets the use requirements.
- Plastic process performance must meet the requirements of the molding process
Process performance of Plastic is the key for if the molding process can be implemented well, confirmation of the mold structure and the quality of the injection molding parts, Process performance refers to the shrinkage rate of plastics, all the direction difference in shrinkage rates, fluidity , crystallinity, thermal sensitivity, etc., so as to correctly formulate the molding process and design the reasonably mold structure.
- Consider the cost of plastic
The cost of plastic parts mainly includes the price of raw materials, processing costs, service life, and maintenance fees.
- Plastics are generally used for structural parts with light weight and relatively high strength. General structural parts, such as covers, brackets, connectors, and handles, require lower strength and heat resistance, somealso require beautiful appearance. Such parts are in large quantities and require low costs. We can select modified PS, low-cost PE, PP, ABS, etc., PS,PE and PP can significantly improve mechanical strength and rigidity also increase heat deformation temperature once reinforced with glass fiber, ABS (acrylonitrile-butadiene-styrene terpolymer) is the most used plastic for good precision comprehensive performance requirements parts, and PA1010 (polyamide, aliphatic nylon) and PC (polycarbonate) are also the plastic for higher performance requirement parts.
- Wear-resistant plastics for transmission parts require high strength, rigidity, toughness, wear resistance and fatigue resistance, andwith high heat deformation temperature, such as various bearings, gears, cams, turbines, racks, sticks, couplings, Priority Plastic is MC PA, POM, PC. Secondly we can select Plastic: Polyphenol oxide resin, chlorinated polyether CPS (polychloroether), linear polyester (PET polyester, PTT, PBT), Among which MC PA can be quickly polymerized molding and used to make large plastic parts at normal pressure, PC can be used for various small modulus gear parts of small instruments, polyphenol oxide Phenoxyl resin is particularly suitable for precision parts and complex structural parts, while polychloroether CPS is suitable for corrosive media working bearings, gears, etc., as well as friction transmission parts and coatings.
- Anti-friction self-lubricating parts plastics. ThesePlastic parts are generally demand less force and lower mechanical strength, but require higher speed of movement with extremely low friction coefficient, such as piston rings, mechanical movement seals, bearings, and loading and unloading crates. These parts select plastic PTFE (polytetrafluoroethylene), or PTFE-filled POM (polyoxymethylene) and HDPE (high-density polyethylene, also known as low-pressure polyethylene).
- Corrosion-resistant parts plastic, it required to be resistant to strong acids, strong alkalis and strong oxidants, Plastic should be made of various fluorine plastics,such as PTFE (polytetrafluoroethylene), FEP (polyperfluoroethylene propylene), PCTFE (polychlorotrifluoroethylene), PVDF (polyvinylidene fluoride). Some chemical pipelines, containers and structural parts that require lubrication should be made of corrosion-resistant plastic materials.
- Plastics for high temperature resistant parts,Plastics used for general structural parts and wear-resistant transmission parts can only work at 80~150 ℃ and it can only work at 60~80℃ when suffered larger force. Plastics for high temperature resistant parts not only refer to fluorine plastics, but also PPE/PPO (polyphenylene ether), PI (polyimide) and Aromatic nylon such as PMIA PA1313 PATA PA1414, which can generally work above 150℃, and some can even work for a long time at 260℃~270℃.
- Optical plastics commonly used include PMMA (polymethyl methacrylate , also known as acrylic, organic glass), PC (polycarbonate), PS (polystyrene), PMP polymethylpentene-1, PE (polypropylene) , CR-39 (allyl diglycol carbonate) , styrene acrylate copolymers such as ASA acrylonitrile copolymer ABS, etc. And optional optical materials you can also select Epoxy resin, silicone resin, polysulfide, polyester, transparent polyamide, etc. are also. Optical plastics have been used in the military for night vision instruments, aircraft optical systems, all-plastic periscopes, and three-proof (nuclear, chemical, biological) protective glasses. In the civilian field, it has also been used in cameras, microscopes, telescopes, various glasses, copiers, fax machines, laser printers and other equipment.
The main basis for selecting optical materials is optical properties such as projection index, refractive index, scattering and light stability. Due to different use conditions, such as heat resistance, wear resistance, chemical corrosion resistance and electrical properties are also as the consideration.
- Food grade injection molding thermoplastic :
PET plastic (polyethylene terephthalate): commonly used to make mineral water bottles, carbonated beverage bottles, etc. PET plastic has good transparency, barrier properties and recyclability, but is not suitable for overheated food because toxic substances may be released at high temperatures .
PP plastic (polypropylene): widely used in food-specific plastic bags, plastic boxes, straws, etc. PP plastic is safe and non-toxic, resistant to low and high temperatures, and is the only plastic material that can be safely used for microwave heating.
HDPE plastic (high-density polyethylene): commonly used in plastic containers for food, with excellent operating temperature range, hardness and mechanical strength, non-toxic and safe .
LDPE plastic (low-density polyethylene): used for plastic parts for food, cling film, plastic packaging for medicines and drugs, etc. LDPE plastic is tasteless, odorless, non-toxic, and has a matte surface .
PC plastic (polycarbonate): commonly used in sports water cups, kettles, bottles, etc., with excellent transparency and resistance. It is considered a safe material. PS plastic (polystyrene: used for bowl instant noodle boxes, fast food boxes and disposable food packaging boxes. PS plastic has excellent cold resistance.
The shape of the plastic product should be good for mold filling, exhaust and shrinkage compensation, and at the same time meet the requirement of high efficiency cooling and hardening.
Plastic design should be referred to the overall structure of the mold design, especially the complexity of core pulling and product demolding. At the same time, the shape of the mold parts and the manufacturing process should be fully considered to the economical efficiency of product .
The main contents of plastic product design are the design of part shape, size, wall thickness, holes, fillets, ribs, threads, inserts, and surface roughness.
