Injection Molding Overview

Injection molding is a manufacturing process for producing parts by injecting material into a mold. Injection molding can be performed with a host of materials, including metals, glasses, elastomers, confections, and most commonly thermoplastic and thermosetting polymers. Material for the part is fed into a heated barrel, mixed, and forced into a mold cavity where it cools and hardens to the configuration of the cavity. After a product is designed, usually by an industrial designer or an engineer, molds are made by a moldmaker (or toolmaker) from metal, usually either steel or aluminum, and precision-machined to form the features of the desired part. Injection molding is widely used for manufacturing a variety of parts, from the smallest components to entire body panels of cars.
Parts to be injection molded must be very carefully designed to facilitate the molding process; the material used for the part, the desired shape and features of the part, the material of the mold, and the properties of the molding machine must all be taken into account. The versatility of injection molding is facilitated by this breadth of design considerations and possibilities.

Injection Molding Cost

The cost of manufacturing molds depends on a very large set of factors ranging from number of cavities, size of the parts (and therefore the mold), complexity of the pieces, expected tool longevity, surface finishes and many others. The initial cost is great, however the per-piece cost is low, so with greater quantities the unit price decreases.

Injection Process

With injection molding, granular plastic is fed by gravity from a hopper into a heated barrel. As the granules are slowly moved forward by a screw-type plunger, the plastic is forced into a heated chamber, where it is melted. As the plunger advances, the melted plastic is forced through a nozzle that rests against the mold, allowing it to enter the mold cavity through a gate and runner system. The mold remains cold so the plastic solidifies almost as soon as the mold is filled.

Injection Molding Cycle

The sequence of events during the injection mold of a plastic part is called the injection molding cycle. The cycle begins when the mold closes, followed by the injection of the polymer into the mold cavity. Once the cavity is filled, a holding pressure is maintained to compensate for material shrinkage. In the next step, the screw turns, feeding the next shot to the front screw.This causes the screw to retract as the next shot is prepared. Once the part is sufficiently cool, the mold opens and the part is ejected.

Process Troubleshooting

Like all industrial processes, injection molding can produce flawed parts. In the field of injection molding, troubleshooting is often performed by examining defective parts for specific defects and addressing these defects with the design of the mold or the characteristics of the process itself. Trials are often performed before full production runs in an effort to predict defects and determine the appropriate specifications to use in the injection process
When filling a new or unfamiliar mold for the first time, where shot size for that mold is unknown, a technician/tool setter may perform a trial run before a full production run. He starts with a small shot weight and fills gradually until the mold is 95 to 99% full. Once this is achieved, a small amount of holding pressure will be applied and holding time increased until gate freeze off (solidification time) has occurred. Gate freeze off time can be determined by increasing the hold time, and then weighing the part. When the weight of the part does not change, we then know that the gate has frozen and no more material is injected into the part. Gate solidification time is important, as it determines cycle time and the quality and consistency of the product, which itself is an important issue in the economics of the production process. Holding pressure is increased until the parts are free of sinks and part weight has been achieved.

Tooling Materials Description Tensile Yield Flexural Strength Flexural Modulas Izod Impact Stength Heat Deflection Under Load Density
ABS Common thermoplastic with good impact resistance and toughness. 6,500 psi (45 MPa) 11,700 psi (80 MPa) 380,000 psi (2,620 MPa) 5.5 ft-lb/in (292 J/m) 190°F (88°C) 0.0379 lb/in3 (1.05 g/cc)
Polycarbonate / ABS Blend of PC and ABS that creates strong parts for a variety of applications. 8,000 psi (55 MPa) 13,000 psi (90 MPa) 370,000 psi (2,550 MPa) 13 ft-lb/in (689 J/m) 202°F (94°C) 0.0415 lb/in3 (1.15 g/cc)
Polycarbonate Thermoplastic material with good temperature resistance and impact strength. 9,000 psi (62 MPa) 18,000 psi (124 MPa) 340,000 psi (2,335 MPa) 15 ft-lb/in (795 J/m) 290°F (143°C) 0.0434 lb/in3 (1.20 g/cc)
Polyoxmethylene (POM) Dimensionally stable thermoplastic with high stiffness and low friction. 10,000 psi (70 MPa) 14,000 psi (100 MPa) 450,000 psi (3,100 MPa) 1.41 ft-lb/in (75 J/m) 216°F (102°C) 0.0513 lb/in3 (1.42 g/cc)
Polypropylene Thermoplastic polymer used for a wide number of applications. 4,900 psi (35 MPa) 26,100 psi (180 MPa) 210,000 psi (1,450 MPa) 0.6 ft-lb/in (32 J/m) 219°F (102°C) 0.0324 lb/in3 (0.90 g/cc)
PVC PVC is a polymer with good insulation properties, high hardness, and good mechanical properties. 4,500 psi (31 MPa) 7,150 psi (50 MPa) 275,000 psi (1,900 MPa) 15 ft-lb/in (795 J/m) 226°F (108°C) 0.0487 lb/in3 (1.35 g/cc)
Nylon Polymer material that is durable with high elongation and good abrasion resistance. 8,400 psi (58 MPa) 9,430 psi (65 MPa) 175,000 psi (1,200 MPa) 2.1 ft-lb/in (111 J/m) 190°F (88°C) 0.0411 lb/in3 (1.14 g/cc)
Nylon 32% Glass Fiber Polymer with excellent mechanical stiffness and elevated temperature resistance. 18,000 psi (125 MPa) 29,000 psi (200 MPa) 900,000 psi (6,200 MPa) 2.5 ft-lb/in (133 J/m) 380°F (193°C) 0.0498 lb/in3 (1.38 g/cc)
Polyether Imide (PEI) Thermoplastic with high heat resistance and excellent mechanical properties. 16,000 psi (110 MPa) 24,000 psi (165 MPa) 510,000 psi (3,500 MPa) 1.0 ft-lb/in (53 J/m) 400°F (204°C) 0.0549 lb/in3 (1.27 g/cc)
Styrene Light weight material popular for its high impact strength and toughness. 6,530 psi (45 MPa) 9,510 psi (65 MPa) 440,000 psi (3,030 MPa) 1.9 ft-lb/in (101 J/m) 174°F (79°C) 0.0379 lb/in3 (1.05 g/cc)
Acrylic (PMMA) Material with resistance to breakage often used for transparent applications. 9,400 psi (65 MPa) 8,500 psi (58 MPa) 250,000 psi (1,725 MPa) 1.0 ft-lb/in (53 J/m) 181°F (83°C) 0.0422 lb/in3 (1.17 g/cc)


Molding Defects

Injection molding is a complex technology with possible production problems. They can be caused either by defects in the molds, or more often by the molding process itself.

Molding Defects Alternative name Descriptions Causes
Blister Blistering Raised or layered zone on surface of the part Tool or material is too hot, often caused by a lack of cooling around the tool or a faulty heater
Burn marks Air burn/gas burn/dieseling Black or brown burnt areas on the part located at furthest points from gate or where air is trapped Tool lacks venting, injection speed is too high
Color streaks (US) Colour streaks (UK) Localized change of color/colour Masterbatch isn’t mixing properly, or the material has run out and it’s starting to come through as natural only. Previous colored material “dragging” in nozzle or check valve.
Delamination Thin mica like layers formed in part wall Contamination of the material e.g. PP mixed with ABS, very dangerous if the part is being used for a safety critical application as the material has very little strength when delaminated as the materials cannot bond
Flash Burrs Excess material in thin layer exceeding normal part geometry Mold is over packed or parting line on the tool is damaged, too much injection speed/material injected, clamping force too low. Can also be caused by dirt and contaminants around tooling surfaces.
Embedded contaminates Embedded particulates Foreign particle (burnt material or other) embedded in the part Particles on the tool surface, contaminated material or foreign debris in the barrel, or too much shear heat burning the material prior to injection
Flow marks Flow lines Directionally “off tone” wavy lines or patterns Injection speeds too slow (the plastic has cooled down too much during injection, injection speeds should be set as fast as is appropriate for the process and material used)
Jetting Part deformed by turbulent flow of material. Poor tool design, gate position or runner. Injection speed set too high. Poor design of gates which cause too little die swell and result jetting.
Knit lines Weld lines Small lines on the backside of core pins or windows in parts that look like just lines. Caused by the melt-front flowing around an object standing proud in a plastic part as well as at the end of fill where the melt-front comes together again. Can be minimized or eliminated with a mold-flow study when the mold is in design phase. Once the mold is made and the gate is placed, one can minimize this flaw only by changing the melt and the mold temperature.
Polymer degradation Polymer breakdown from hydrolysis, oxidation etc. Excess water in the granules, excessive temperatures in barrel, excessive screw speeds causing high shear heat, material being allowed to sit in the barrel for too long, too much regrind being used.
Sink marks [sinks] Localized depression (In thicker zones) Holding time/pressure too low, cooling time too short, with sprueless hot runners this can also be caused by the gate temperature being set too high. Excessive material or walls too thick.
Short shot Non-fill / Short mold Partial part Lack of material, injection speed or pressure too low, mold too cold, lack of gas vents
Splay marks Splash mark / Silver streaks Circular pattern around gate caused by hot gas Moisture in the material, usually when hygroscopic resins are dried improperly. Trapping of gas in “rib” areas due to excessive injection velocity in these areas. Material too hot.
Stringiness Stringing String like remnant from previous shot transfer in new shot Nozzle temperature too high. Gate hasn’t frozen off, no decompression of the screw, no sprue break, poor placement of the heater bands inside the tool.
Voids Empty space within part (Air pocket is commonly used) Lack of holding pressure (holding pressure is used to pack out the part during the holding time). Filling too fast, not allowing the edges of the part to set up. Also mold may be out of registration (when the two halves don’t center properly and part walls are not the same thickness). The provided information is the commond understanding, Correction: The Lack of pack (not holding) pressure (pack pressure is used to pack out even though is the part during the holding time). Filling too fast does not cause this condition, as a void is a sink that did not have a place to happen. In other words as the part shrink the resin separated from it self as there was not sufficient resin in the cavity. The void could happened at any area or the part is not limited by the thickness but by the resin flow and thermal conductivity, but it is more likely to happened at thicker areas like ribs or bosses. Additional root causes for voids are un-melt on the melt pool.
Weld line Knit line / Meld line / Transfer line Discolored line where two flow fronts meet Mold/material temperatures set too low (the material is cold when they meet, so they don’t bond). Time for transition between injection and transfer (to packing and holding) is too early.


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