Injection moldings

Injection mold-grade PLA can be injection molded on most conventional equipment, but there could be some torque limitations if the screw design has a high compression ratio. Compression ratios of 2.5 to 3 should be adequate and the recommended melting temperature is 200 to 205°C. Since PLA has a lower glass transition temperature (about 58°C) than PS or PET, it might take a little longer time to set up in the mold [30]. 

The cast film process involves extruding a molten polymer through a slit die and drawing it around two or more highly polished 

The orientation in PLLA films depends on the draw rate, temperature, and ratio. High strain rate, low temperature, and high stretch ratio favor strain-induced crystallization during orientation. PLA has excellent optical properties and high modulus. However, it has low elongation, tear, and burst strengths. To overcome these shortcomings, PLA is often coextruded with other polymers to form multilayer structures to enhance its properties [27]. 

Injection molded parts also exhibit blend morphology variations associated with melt processing. These tend to be more complex than those described earlier. As a hot blend melt moves through a cold mold, the dispersed phase in regions near the mold 

Hancock, T. (1857) Personal Narrative of the Origin and Pn ressqftheCaoutekous or India Rubber Mani actwre in En and, 

Killheffer, D.H. (1962) Banbury the Master Mixer, Pahnerton, New York. 

(1968) The History of Baker Perkins, W. HefFer Sons, 

Herrmann, H. (1985) in Kunststoj ein Werkstqffe MadU Karrier (ed. W. Glenz) Carl Hanser, Mimich. 

Injection molding is a common process that we use to convert polymer granules to solid objects. Unlike the products made by continuous extrusion processes, discussed in Chapter 11, products made by injection molding are discrete objects, produced in individual mold cavities. We encounter injection molded products of all sorts in our daily lives, ranging from combs, bottle caps, and ballpoint pens to car steering wheels, camera bodies, and the keys on our computers. 

Injection molding is a major polymer processing operation for producing identical articles from a hollow mold. It is an intermittent cyclic process with the following steps i) filling stage a polymer melt is injected into the cold-walled cavity where it spreads under the action of high pressures and fills the mold  

The filling stage is the most complex and important step in the sequence and has attracted much attention in the literature. The implications and the problems related to the mathematical modeling of mold filling have been presented in a review by Mavridis et al. [173-175]. Earlier reviews have also been presented (see. Ref. [176]), while the latest review can be found in a most recent thesis [20]. 

Other people have also made contributions, too numerous to include in this chapter, so the interested reader may refer to Kennedy s thesis [20]. 

The focus of this chapter was based on the work done so far both in academia and in commercial software companies dedicated to polymer processing. Examples from some interesting and important polymer flows have been reviewed, such as those that arise from polymer processing operations with viscous (inelastic) and viscoelastic polymer melts. The topic is vast and this chapter deals exclusively with the continuum approach of flow problems. Although experimental evidence has accumulated over the years from various researchers and for various processes, the theory and predictions have lagged behind, mainly due to the complexity of the subject. 

On the basis of what has been laid out in this chapter, an attempt is made in Table 4.3 to show the impact of computations on various polymer processes outlined 

Use of an injection molding machine is the most common method for thermoplastic material. PE or other polymer samples are preheated in a cylindrical chamber to a temperature at which it will flow and then is forced into a relatively cold, closed mold cavity by means of quite high pressure applied hydraulically through a ram or screw type plunger. The screw is moved forward to force a fixed volume of the molten polymer into the closed mold, where the polyethylene is soHdified. Finally, the screw rotates and moves backward to charge the polymer for the next cycle. 

Gompared with injection molding, control of the anisotropy by orientation of wood flour is difficult in extrusion molding. Anisotropy has many demerits, except for some properties such as use in a sound component In order to overcome these demerits, cross-sectional design of the composite is elaborated in many cases rather than controlling the orientation by molding conditions. 

The products obtained by extrusion molding are mainly used in present day WPGs. Since WPG has been developed as alternatives for wood products, extrusion 

In most cases of injection molding, the raw material for molding is in the form of a solid which has to be melted internally or externally. Many materials used for injection molding contain plasticizers to control the processing viscosity of melt but also to modify properties of the polymer and to obtain elastic products. 

Requirements of special performance of plasticizers used in the injection molding process are seldom. In optical disk molding, the material requites mote plasticization during processing than ditring the product use. For this reason carbon dioxide was used as an additional plasticizer. Primary plasticizers were selected in such a manner that they will facilitate dissolution of carbon dioxide in melt. Figitre 14.17 shows that the more carbon dioxide dissolves, the lower is the glass transition tern-process, melt viscosity is decreased and 

Besides acrylics or PMMA, injection molding was also carried out on regular and optical-quality PC chips [187]. Furthermore, three PS plastic plates were injection molded to create the microfluidic channels, which were employed for a biomolecular interaction study. The upper plastic plate with ridge patterns was ultrasonically welded to the middle plate. The middle plate was sealed to the bottom plate with a molded silicone rubber layer [188], 

LDPE homopolymers and copolymers for injection molding are characterized by their softness and flexibility. Due to the polymer structure, these resins show excellent processability and flowability at low melt temperatures, resulting in world-class organoleptic properties. 

Typically, injection molding grades are used for masterbatches. One of the main masterbatches used in the polyolefins industry is LDPE-based silica masterbatches, since Si02 is used in film applications as an antiblocking additive. 

This process is more popular in polymers. For the injection molding of ceramics, a thermoplastic polymer is required to the extent of 40 vol.%. This is called the binder. For example, to injection mold silicon carbide, ethyl cellulose is one such binder that is used. During the firing process, a binder removal soak, called binder burnout, is given to eliminate the polymer from the final product. 

The initial investment on the machine and the mold is rather high. However, the production is rapid, and the molds are reusable and permanent. The machine operates at low temperatures, just above the melting temperatures of the binders. 

Cross-sectional side view of a screw-type injection-molding machine. 

PCT forms the basis of a family of reinforced, crystalline plastics for injection molding. As mentioned above, the high melting point of the polymer is a key property, as this results in high heat deflection temperatures (HDTs) in glass-fiber-reinforced formulations. Good toughness, flow into the mold, and rapid crystallization are also important in these applications. 

Flame-retarded grades are widely used for various computer connectors and circuit board components. Representative applications are edge card connectors, grid arrays and memory modules. Non-flame-retarded grades find use in automotive under-the-hood applications, typically connectors and related parts. As higher temperature soldering techniques become more common in the automotive industry, the use of high-temperature plastics such as PCT is expected to increase. It is also possible to formulate unreinforced PCT with crystallization aids and 

A well-established application for extruded unreinforced PCT (copolymerized with isophthalate) is crystallized, thermoformed trays for foods. Crystallized PET is widely used for this application, but where higher-temperature performance is needed the PCT-based polymer may be chosen. Such trays are formed from extruded sheet, using a hot mold to promote crystallization. Isophthalate-modified PCT polymers are approved by the Food and Drug Administration for high-temperature food contact use. 

The good hydrolytic stability of PCT-based polymers leads to applications for monofilament in paper machine belts. Monofilament is extruded from high-molecular-weight polymer, drawn and crystallized, and then woven into a screen. Such belts are found in the drying sections of paper machines, where there is a combination of high moisture and high temperature. Because of their hydrolytic stability, PCT-based polymers provide much longer service life in this application than PET-based materials. 

FIGURE 14.5 Maj or components of an inj ection molding machine showing the extruder (reciprocal screw) unit and injection molds. Top the screw rotates and slides backward to allow polymer melt to accumulate in front of the screw. Bottom the mold clamps up followed by moving the screw forward to inject polymer melt into the hot runner and fill the mold cavities. 

FIGURE 14.6 Typical cycle for an injection molding process. 

FIGURE 14.7 PVT plots for PLA polymers. Top 7000D (NatureWorks LLC, 4.25% D-isomer) [53] bottom Lacea H-IOOE (Mitsui Chemicals, Inc., % D-isomer unknown) [54]. The continuous lines represent the fitted results based on the two-domain-modified Tait model (Equation 14.1). 

The PVT relationship can be modeled mathematically using the modified two-domain Tait model [55-57, 112]  

The constants have some physical meanings. Constants and Z 2m are related to the intercept and slope of melt-state V-riine, respectively, when P = 0. On the other hand, bi and Z 2s are related to the intercept and slope for the solid-state V-P lines when P = 0. Constants b, b, b, and Z 4s affect the shape of the V-T plots when P O. Constant b is the 

FIGURE 14.21 Schematic cross section of typical plunger (or ram or piston) injection-molding machine. (Data from U. S. 1. Chemicals/Quantum, Petrothene Polyolefins A Processing Guide, 5th edn., National Distillers and Chemical Corporation, New York, 1986.) 

The mold is closed and clamp pressure is applied to keep the mold closed during injection. 

The screw acts as a plunger and the molten polymer flows into the cavity (filling stage). 

When the cavity is full, the pressure on the molten plastic is increased to add more material into the cavity in compensation for the shrinkage caused by cooling (packing stage). 

When power ultrasound oscillation was introduced to polymer melts, the mobility of the polymer molecules was increased such that, many of the common problems encountered in polymer processing could be resolved. For example, in the case of injection molding, ultrasonic osdllations were applied to improve molecular diffusion across the weld line so as to enhance the weld line strength. 

This method, like extrusion, requires a molten polymer. The equipment is schematically illustrated in Fig. 3.5a. In this method, polymer granules are fed into a hopper and a screw transports the polymer through a barrel where the polymer is heated to make it flow, and the polymer melt is forced through a nozzle into a closed mold. The mold is cooled after it is filled, and the molded part ejected, and the equipment readied for the next shot. Injection molding is thus a semi-continuous batch process and is usually used for large-scale production, where high throughput and reproducibility are important. The process is economical. The method is commonly used for thermoplastics. E.g. PLA, PGA and copolymers. Some 

Injection molding subjects the polymer to greater shear mechanical and thermal stresses than extrusion. Thus, a biodegradable polymer that can be extruded may be difficult to shape by injection molding. 

In closing, significant development in the theoretical analysis of the extrusion process has contributed to a better understanding and design of new equipment, an increase of output in existing machinery, and development of concise control via computers. 

The processing of thermoplastics by injection molding comprises a major portion of the industry, representing around 50% of processing machinery. The first injection machine was built in 1872 (Hyatt) for the processing of cellulose nitrate. With time, the process imderwent improvements and sophistication, so that it may also be used for the processing of thermosets. The distinct difference between injection and extrasion lies in the fact that injection is a batchwise-cyclical process, where the final shape is obtained in a cooled mold, made of two parts that alternatively open and close. 

The major utility is for the production of multi-dimensional objects (not longitudinal) with complicated precise shapes including metallic elements, in various sizes starting from minute ones. The common thickness is 4 to 6 mm. In most cases the product injected has the mass of 300 to 3,000 g, but 

The first part resembles in principle an extrasion process with a motor, feed hopper, and a heated barrel containing a screw that melts and positively drives the material. However, operation of the screw is cyclical. It moves forward as a plunger in order to convey a batch of melt into the mold and then retreats through rotation while mixing material and melting a new batch which is then pushed to the front. Actually, prior to use of the screw in injection, a plunger (ram) pushed pellets into a melting zone, in which an 

FIGURE 5-13 The layout of an injection molding apparatus (a) driven by a ram (with torpedo), (b) driven by a screw 

Processing technique Plastic resin Typical products Base technology 

Film blowing (extrusion HOPE, LDPE, and Shrink film, stretch film, bag film or container liners. Melt extrusion and air 

Blow molding HOPE, LLDPE, PP, PET, PVC Bottles and containers, fuel tanks, air ducts, garden supplies Melt extrusion/molding 

Injection molding ABS, nylon (PA), PP, and PS Polycarbonate (PC) Power-tool housing, telephone handsets, TV cabinets, computer housing, DVDs, automotive bumpers, dashboards, syringes, kitchenware, cutlery, crates, bottle lids/closures Melt injection Variations include gas-assisted injection molding and stretch injection molding 

Extrusion is the process where a solid plastic, usually in the form of beads or pallets, is continuously fed to a heated chamber and carried along by a feed screw within. As the solid resin is conveyed, it is compressed, melted and forced out of the chamber through a die. Through the cooling process the melt part results in a resolidification of the resin 

In the case of biocomposites, the basic extruder equipment works in a similar way or has few supported modifications. Generally, the bead or pallets of the thermoplastic resin are mixed together with 30 to 40% of short/long natural fibers, compressed, melted (wherein the natural fibers are impregnated with resin) and forced out through a die. The equipment which had supported modifications is intended to create raw materials for the injector. 

Very complex shapes and intricate detail possible. 

inserts, threads, lettering, color, bosses and minor undercuts possibie. Uniform section thickness should be maintained. 

Marked section changes should be tapered sufficiently. 

Living hinges and snap features allow part consolidation. 

Placing of parting line important, i.e. avoid placement across critical dimensions. 

Heating chamber 300-700°F Hardened shell of cool polymer 

FIGURE 10.7 Schematic diagram of injection molding machine. 

Level I comprises the exterior base plates and clamps that never change for a family of parts with the same overall maximum dimensions. 

Level II comprises the basic mold sections, core and die cavity main bodies that may or may not change for a subfamily of parts. 

Level III comprises the core and die inserts that are specific to an individual manufactured part. 

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Soil, S.K. and Chang, C. J., 1986. Boundary conditions in the modeling of injection mold-filling of thin cavities. Polym. Eng. Sci. 26, 393-399. 

Properties Polybutylene extrusion Homopolymer Copolymer Impact copolymer Polyallomer Injection molding ... 

Foam-injection molding Foam plastic sheathing Foam products Foam regulators Foam rheology... 

Polyurethane is pulverized to iacrease its bulk density, mixed with 30—80% of a thermoplastic mol ding material, gelled, and then granulated to give coated urethane foam particles 0.1 to 0.15 mm in size (48). The particle bulk density is three times that of the polyurethane, while the volume is 15% less. This material may be injection molded or extmsion molded into products (49). Other technologies for recycling polyurethanes have also been reported. 

The economics of recycling PET are more favorable than recycling HDPE. To iacrease the recycling of HDPE, the separation of bottles made of these two plastics could be omitted and a mixture processed. Coarse, light-colored powders of the two polymers have been prepared by an experimental soHd state shear extmsion pulverization process (55). The powder has been successfully injection molded without pelletization. 

Fabrication. Acetal resins are most commonly fabricated by injection mol ding. A homogeneous melt is essential for optimum appearance and for performance of injection molded parts. A screw compression ratio of no less than 3 1 is advised and the size of the injection molded shot should be 50 to 75% of the rated capacity (based on polystyrene) of the barrel. 

Table 1. Properties of Injection-Molded Commercial SAN Resins ...

Processing. SAN copolymers may be processed using the conventional fabrication methods of extmsion, blow mol ding, injection molding, thermoforming, and casting. Small amounts of additives, such as antioxidants, lubricants, and colorants, may also be used. Typical temperature profiles for injection mol ding and extmsion of predried SAN resins are as follows (101). 

R. E. Nuim, Injection Molding Handbook, VanNostrand Reinhold Co., New York, 1986, Chapt. 3. 

Cjcolac Brand ABS Resin Injection Molding, Technical PubHcation CYC-400, GE Plastics, Pittsfield, Mass., 1990. 

Hexafluoropropylene—tetrafluoroethylene copolymers are available in low melt viscosity, extmsion grade, intermediate viscosity, high melt viscosity, and as dispersions. The low melt viscosity (MV) resin can be injection molded by conventional thermoplastic molding techniques. It is more suitable for injection molding than other FEP resins (51). 

Teflon-FTP Fluorocarbon Resin, Techniquesfor Injection Molding, information bulletin 95d, E. I. du Pont de Nemours Co., Inc., Wilmington, Del., 1969. 

Injection Molding. Any standard design plunger or reciprocating screw injection machine can be used for PEA 340, although a reciprocating screw machine is preferred (32). Slow injection into mold cavities avoids surface or internal melt fracture, and control of ram speed is important at low... 

The sandwich-type stmcture of polyurethanes with a smooth integral skin produced by the reaction injection molding process provides a high degree of stiffness as weU as exceUent thermal and acoustical properties necessary for its use in housing and load-bearing stmctural components for the automotive, business machine, electrical, furniture, and materials-handling industry. 

There are five basic types of polystyrene foams produced in a wide range of densities and employed in a wide variety of apphcations (/) extmded polystyrene board (2) extmded polystyrene sheet (2) expanded bead mol ding (4) injection molded stmctural foam and (5) expanded polystyrene loose-fiU packaging. 

Injection Molding. Matched metal molds are used in the fabrication of plastic closures, specialty packages, and botde preforms. In conventional injection mol ding the plastic resin is melted in an extmder which forces a measured quantity or shot into a precision-machined chilled mold after which the nozzle of the extmder is withdrawn. 

Injection molds are constmcted of metal precision-machined with internal cooling, multiple cavities, and multifaces, and with devices for extraction... 

Injection-molded articles can be decorated by in-mold labeling or by post-mold decoration. In the former method, printed film is inserted into the mold cavity before injection. The plastic forms an intimate contact with the graphic material. Post-mold decoration includes hot stamping, dry offset printing, and decal printing. 

To enhance water-vapor- or gas-barrier properties, layers of different plastics may be injected together or sequentially. Multilayer injection-molded pieces may be prepared as packaging or for blowing into bottle or jar shapes. 

Mold Coolers for Plastic Injection Molding. Heat pipes are used for local temperature control in the injection molding of plastics (see Polymerprocessing). A heat pipe is often used to force local cooling within a mold to speed operation, control viscosity, retention of material in a difficult mold area, or to reduce thermal stresses on cooling. 

At the same time, a decrease in orientation- and stress-birefringence for the modified BPA-PC could be achieved by optimizing the processing conditions (184,187,189,190,193) this lowers birefringence below 20 nm/mm (single path) for injection-printed injection molded CDs and substrate disks (Fig. 24) (191). The typical characteristics of a modified BPA-PC especially developed for CD production are Hsted in Table 6 (187,193). 

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