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One Source. More Resourceful. A Guide to Polyolefin Injection Molding Equistar is one of the largest producers of ethylene, propylene and polyethylene in the world today. One of the largest, yet we pay atten- tion to even the smallest needs of our customers. Were a leading producer of polypropylene, oxygenated chemicals, performance polymers and resins and compounds for wire and cable. Were an industry leader with an unwavering commitment to being the premier petrochemicals and polymers company. Our commitment starts with each of our more than 5,000 employees. It stretches out from our headquarters in Houston across 16 manufac- turing facilities along the U.S. Gulf Coast and in the Midwest. It continues through our 1,400-mile ethylene/propylene distribution system that spans the Gulf Coast. We are the product of many minds coming together with the single focus of providing the right product for every customer. Thats what drives us to maintain an extended product line, enhanced operating efficiencies, greater geographic diversity and strong research and development capabilities. Thats what drives us to provide the resources that help us lead today and rise to the challenges of a changing industry tomorrow. One Source. More Resourceful. 1 A Guide to Polyolefin Injection Molding Table of Contents Introduction ..................................................................................................2 Polyolefins are derived from petrochemicals...................................................2 Molecular structure and composition affect properties and processability.......2 Chain branching......................................................................................3 Density ..................................................................................................3 Molecular weight ....................................................................................3 Molecular weight distribution..................................................................4 Copolymers .............................................................................................5 Modifiers and additives ...........................................................................5 Working closely with molders ........................................................................5 How polyolefins are made..............................................................................5 Low density polyethylene (LDPE)..............................................................6 High density polyethylene (HDPE) ............................................................6 Linear low density polyethylene (LLDPE)...................................................7 Polypropylene..........................................................................................7 Shipping and handling polyolefin resins .........................................................7 Material handling ....................................................................................8 How to solve material handling problems................................................9 Other material handling practices..........................................................10 The injection molding process......................................................................10 Injection units........................................................................................10 Plasticator specifications ........................................................................13 Screw designs........................................................................................13 Nozzles ................................................................................................14 Clamp mechanisms ...............................................................................14 Clamp specifications..............................................................................15 Injection molds......................................................................................16 Types of mold........................................................................................16 Sprues and runners ...............................................................................17 Mold venting.........................................................................................18 Gating ................................................................................................19 Mold cooling.........................................................................................20 Ejection devices .....................................................................................20 Spiral flow measurement.......................................................................20 General injection molding operating procedures ..........................................21 General safety .......................................................................................21 Heat ................................................................................................22 Electricity...............................................................................................22 Machinery motion .................................................................................22 The injection molding process and its effect on part performance................22 The molding cycle .................................................................................22 Shrinkage ..............................................................................................27 Warpage ...............................................................................................28 Color dispersion and air entrapment .....................................................29 Part ejection and mold release...............................................................29 Clarity ................................................................................................30 Gloss ................................................................................................30 Polypropylene integral hinges ................................................................30 Appendices 1: Injection Molding Terms ....................................................................31 2: Metric Conversion Guide...................................................................35 3: Abbreviations ....................................................................................37 4: ASTM test methods applicable to polyolefins ....................................38 5: Injection molding problems, causes and solutions..............................39 6: ASTM and ISO sample preparation and test procedures ....................43 7: Compression and injection molded sample preparation for HDPE......44 Introduction Polyolefins are the most widely used plastics for injection molding. This manual, A Guide to Polyolefin Injection Molding, contains general information concerning materials, methods and equipment for producing high quality, injection molded, polyolefin products at optimum production rates. Polyolefins that can be injection molded include: Low density polyethylene (LDPE) Linear low density polyethylene (LLDPE) High density polyethylene (HDPE) Ethylene copolymers, such as ethylene vinyl acetate (EVA) Polypropylene and propylene copolymers (PP) Thermoplastic olefins (TPO) In general, the advantages of injection molded polyolefins com- pared with other plastics are: Lightweight Outstanding chemical resistance Good toughness at lower temperatures Excellent dielectric properties Non hygroscopic The basic properties of polyolefins can be modified with a broad range of fillers, reinforcements and chemical modifiers. Furthermore, polyolefins are considered to be relatively easy to injection mold. Major application areas for poly- olefin injection molding are: Appliances Automotive products Consumer products Furniture Housewares Industrial containers Materials handling equipment Packaging Sporting goods Toys and novelties This manual contains extensive information on the injection mold- ing of polyolefins; however, it makes no specific recommendations for the processing of Equistar resins for specific applications. For more detailed information please contact your Equistar polyolefins sales or technical service representative. Polyolefins are derived from petrochemicals Polyolefins are plastic resins poly- merized from petroleum-based gases. The two principal gases are ethylene and propylene. Ethylene is the principal raw material for mak- ing polyethylene (PE) and ethylene copolymer resins; propylene is the main ingredient for making polypropylene (PP) and propylene copolymer resins. Polyolefin resins are classified as thermoplastics, which means that they can be melted, solidified and melted again. This contrasts with thermoset resins, such as phenolics, which, once solidified, can not be reprocessed. Most polyolefin resins for injection molding are used in pellet form. The pellets are about 1/8 inch long and 1/8 inch in diameter and usual- ly somewhat translucent to white in color. Many polyolefin resins con- tain additives, such as thermal stabi- lizers. They also can be compound- ed with colorants, flame retardants, blowing agents, fillers, reinforce- ments, and other functional addi- tives such as antistatic agents and lubricants. Molecular structure and composition affect properties and processability Four basic molecular properties affect most of the resin characteris- tics essential to injection molding high quality polyolefin parts. These molecular properties are: Chain branching Crystallinity or density Average molecular weight Molecular weight distribution The materials and processes used to produce the polyolefins determine these molecular properties. The basic building blocks for the gases from which polyolefins are derived are hydrogen and carbon atoms. For polyethylene, these atoms are combined to form the ethylene monomer, C 2 H 4 . HH || C=C || HH In the polymerization process, the double bond connecting the carbon atoms is broken. Under the right conditions, these bonds reform with other ethylene molecules to form long molecular chains. H H H H H H H H H H | | |||||||| C C C C C C C C C C | | |||||||| H H H H H H H H H H The resulting product is polyethyl- ene resin. 2 A Guide to Polyolefin Injection Molding For polypropylene, the hydrogen and carbon atoms are combined to form the propylene monomer, CH 3 CH:CH 2 . HH || HCC=C ||| HHH The third carbon atom forms a side branch which causes the backbone chain to take on a spiral shape. HHHHHH |||||| CCCCCC |||||| H HCH H HCH H HCH ||| HHH Ethylene copolymers, such as ethyl- ene vinyl acetate (EVA), are made by the polymerization of ethylene units with randomly distributed comonomer groups, such as vinyl acetate (VA). Chain branching Polymer chains may be fairly linear, as in high density polyethylene, or highly branched as in low density polyethylene. For every 100-ethylene units in the polyethylene molecular chain, there can be one to ten short or long branches that radiate three- dimensionally (Figure 1). The degree and type of branching are con- trolled by the process (reactor), cat- alyst, and/or any comonomers used. Chain branching affects many of the properties of polyethylenes including density, hardness, flexibili- ty and transparency, to name a few. Chain branches also become points in the molecular structure where oxidation may occur. If excessively high temperatures are reached during processing, oxidation can occur which may adversely affect the polymers properties. This oxida- tion or degradation may cause cross-linking in polyethylenes and chain scission in polypropylenes. Polypropylene, on the other hand, can be described as being linear (no branching) or very highly branched. Although the suspended carbon forms a short branch on every repeat unit, it is also responsi- ble for the unique spiral and linear configuration of the polypropylene molecule. Density Polyolefins are semi-crystalline poly- mers which means they are com- posed of molecules which are arranged in a very orderly (crystalline) structure and molecules which are randomly oriented (amorphous). This mixture of crystalline and amorphous regions (Figure 2) is essential in providing the desired properties to injection molded parts. A totally amorphous polyolefin would be grease-like and have poor physical properties. A totally crystalline poly- olefin would be very hard and brittle. HDPE resins have linear molecular chains with comparatively few side chain branches. Therefore, the chains are packed more closely together (Figure 3). The result is crystallinity up to 95 percent. LDPE resins generally have crystallinity from 60 percent to 75 percent. LLDPE resins have crystallinity from 60 percent to 85 percent. PP resins are highly crystalline, but they are not very dense. PP resins have a nominal specific gravity range of 0.895 to 0.905 g/cm 3 , which is the lowest for a commodity thermo- plastic and does not vary appreciably from manufacturer to manufacturer. For polyethylene, the density and crystallinity are directly related, the higher the degree of crystallinity, the higher the resin density. Higher density, in turn, influences numer- ous properties. As density increases, heat softening point, resistance to gas and moisture vapor permeation and stiffness increase. However, increased density generally results in a reduction of stress cracking resistance and low temperature toughness. LDPE resins have densities rang- ing from 0.910 to 0.930 grams per cubic centimeter (g/cm 3 ) LLDPE resins range from 0.915 to 0.940 g/cm 3 HDPE resins range from 0.940 to 0.960 g/cm 3 As can be seen, all natural poly- olefin resins, i.e, those without any fillers or reinforcements, have densities less than 1.00 g/cm 3 . This light weight is one of the key advantages for parts injection mold- ed from polyolefins. A general guide to the effects of density on the properties for various types of polyethylene resins is shown in Table 1. Molecular weight Atoms of different elements, such as carbon, hydrogen, etc., have differ- ent atomic weights. For carbon, the atomic weight is 12 and for hydro- gen it is one. Thus, the molecular weight of the ethylene unit is the sum of the weight of its six atoms (two carbon atoms x 12 + four hydrogen x 1) or 28. 3 Figure 1. Polyethylene chain with long side branches C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C Figure 2. Crystalline (A) and amor- phous (B) regions in polyolefin Figure 3. Linear polyethylene chain with short side branches C C C C C C C C C C C C C C C C C C C C C Unlike simple compounds, like ethylene or propylene, every poly- olefin resin consists of a mixture of large and small chains, i.e., chains of high and low molecular weights. The molecular weight of the polymer chain generally is in the thousands and may go up to over one million. The average of these is called, quite appropriately, the average molecular weight. As average molecular weight increases, resin toughness increases. The same holds true for tensile strength and environmental stress crack resistance (ESCR) cracking brought on when molded parts are subjected to stresses in the pres- ence of materials such as solvents, oils, detergents, etc. However, high- er molecular weight results in an increase in melt viscosity and greater resistance to flow making injection molding more difficult as the average molecular weight increases. Melt flow rate (MFR) is a simple measure of a polymers melt vis- cosity under standard conditions of temperature and static load (pressure). For polyethylenes, it is often referred to as melt index (MI). MFR is the weight in grams of a melted resin that flows through a standard-sized orifice in 10 minutes (g/10 min). Melt flow rate is inversely related to the resins average molecular weight: as the average molecular weight increases, MFR decreases and vice versa. Melt viscosity, or the resistance of a resin to flow, is an extremely important property since it affects the flow of the molten polymer filling a mold cavity. Polyolefins with higher melt flow rates require lower injection molding processing pressures, temperatures and shorter molding cycles (less time needed for part cooling prior to ejection from the mold). Resins with high viscosities and, therefore, lower melt indices, require the opposite conditions for injection molding. It should be remembered that pressure influences flow properties. Two resins may have the same melt index, but different high-pressure flow properties. Therefore, MFR or MI must be used in conjunction with other characteristics, such as molecular weight distribution, to measure the flow and other properties of resins. Generally, injection molding resins are char- acterized as having medium, high or very high flow. For injection molding grades, the MFR (MI) values for polyethylenes are generally determined at 190C (374F) using a static load of 2,160 g. MFR values for polypropy- lenes are determined at the same load but at a higher temperature 230C (446F). The MFR of other thermoplastics may be determined using different combinations of temperatures and static load. For this reason, the accurate prediction of the relative processability of different materials using MFR data is not possible. Molecular weight distribution During polymerization, a mixture of molecular chains of widely varying lengths is produced. Some may be short; others may be extremely long containing several thousand monomer units. The relative distribution of large, medium and small molecular chains in the polyolefin resin is important to its properties. When the distribu- tion is made up of chains close to the average length, the resin is said to have a “narrow molecular weight distribution.” Polyolefins with “broad molecular weight distribution” are resins with a wider variety of chain lengths. In general, resins with narrow molecular weight distributions have good low- temperature impact strength and low warpage. Resins with broad molecular weight distributions generally have greater stress crack- ing resistance and greater ease of processing (Figure 4). The type of catalyst and the polymerization process used to produce a polyolefin determines its molecular weight distribution. The molecular weight distribution (MWD) of PP resins can also be altered during production by con- trolled rheology additives that selec- tively fracture long PP molecular 4 AS MELT INDEX AS DENSITY INCREASES INCREASES Durometer hardness (surface) remains the same increases Gloss improves improves Heat resistance (softening point) remains the same improves Stress crack resistance decreases decreases Mechanical flex life decreases decreases Processability (less pressure to mold) improves remains the same Mold shrinkage decreases increases Molding speed (faster solidification) remains the same increases Permeability resistance remains the same improves Stiffness remains the same increases Toughness decreases decreases Transparency remains the same decreases Warpage decreases increases Table 1. General guide to the effects of polyethylene physical properties on properties and processing chains. This results in a narrower molecular weight distribution and a higher melt flow rate. Copolymers Polyolefins made with one basic type of monomer are called homopolymers. There are, however, many polyolefins, called copoly- mers, that are made of two or more monomers. Many injection molding grades of LLDPE, LDPE, HDPE and PP are made with comonomers that are used to provide specific property improvements. The comonomers most often used with LLDPE and HDPE are called alpha olefins. They include butene, hexene and octene. Other comonomers used with ethylene to make injection molding gra