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Injection molding is a manufacturing process for producing plastic parts from both thermoplastic and thermosetting plastic materials. Material is fed into a heated barrel, mixed, and forced into a mold cavity by a reciprocating screw or a ram injector, where it cools and hardens to the configuration of the mold cavity. After a part is designed, usually by an industrial designer or an engineer, molds are then manufactured by an injection mold company, where it is assigned to a mold maker (or toolmaker). The mold is usually constructed using 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 component to entire body panels of cars. Injection molding is the most common method of production, with some commonly made items including computer components to outdoor furniture.
Process Characteristics
- Utilizes screw-type plunger to force molten plastic material into a mold cavity
- Produces a solid or open-ended shape (typically a cavity and core shape) which has conformed to the contour of the mold
- Uses thermoplastic or thermoset materials
- Parting line, sprue, and gate marks are present
- Ejector pin marks are usually present
History
In 1868, billiard ball maker Phelan and Collander, John Wesley Hyatt invented a way to make billiard balls by injecting celluloid into a mould. Hyatt improved the celluloid so that it could be processed into a finished form. In 1872 John and his brother Isaiah, patented the first injection molding machine. This machine was relatively simple compared to the machines used by today’s injection molding companies. It contained a basic plunger to inject the plastic into a mold through a heated cylinder. The industry progressed slowly over the years producing products such as plastic collar stays, buttons, and hair combs until it the 1940s when it took off because World War 2 created a huge demand for inexpensive, mass-produced products. In 1946, James Hendry built the first screw injection molding machine, revolutionizing the plastics industry with an auger design to replace Hyatt's plunger. The auger is placed inside the cylinder and mixes the injection molding material before pushing forward and injecting the material into the mould. This allowed colored plastic or recycled plastic to be added to the virgin material and mixed thoroughly before being injected. Today screw injection molding machines account for 95% of all injection machines companies. The industry has evolved over the years from producing combs and buttons to producing a diverse array of custom products for many industries including, medical, aerospace, consumer, toys, plumbing, packaging, automotive and construction.
Applications
Plastic injection molding is the preferred process for manufacturing plastic parts. Injection molding is used to create many things such as electronic housings, containers, bottle caps, automotive interiors, pocket combs, and most other plastic products available today. It is ideal for producing high volumes of plastic parts, due to the ability of making multi-cavity injection molds, where multiple parts are made with one cycle. Some advantages of injection molding are high tolerances, repeatability, a wide range of material selection, low labor cost, minimal scrap losses, and little need to finish parts after molding. Some disadvantages of this process are expensive tooling investment, the need to prototype, as some custom complex parts may encounter problems during the injection molding process, such as warp, surface defects and so parts must be designed with molding consideration.
Examples of Polymers Best Suited for Injection Molding
Most polymers may be used, including all thermoplastics, some thermosets, and some elastomers. There are tens of thousands of different materials available for injection molding and that number was increasing every year. The available materials mixed with alloys or blends of previously developed materials means that product designers can choose from a vast selection of materials, one that has exactly the right properties. Materials are chosen based on the strength and function required for the final part but also each material has different parameters for molding that must be considered. Common polymers like Epoxy and phenolic are examples of thermosetting plastics while nylon, polyethylene, and polystyrene are thermoplastic.
Design guidelines for Custom Injection Molding
See our Design Guidelines for tips on designing a plastic injection moldable part.
Equipment
Injection Molding Machine
Injection molding machines, also known as presses, consist of a material hopper, an injection ram or screw-type plunger, and a heating unit. Molds are clamped to the platen of the molding machine, where plastic is injected into the mold through the sprue orifice. Presses are rated by tonnage, which is the calculation of the amount of clamping force that the machine can exert. This force keeps the mold closed during the injection molding process. Tonnage can vary from less than 5 tons to 6000 tons, with the higher figures used in comparatively few manufacturing operations. The total clamp force needed is determined by the projected area of the custom part being molded. This projected area is multiplied by a clamp force of from 2 to 8 tons for each square inch of the projected areas. As a rule of thumb, 4 or 5 tons/in can be used for most products. If the plastic material is very stiff, it will require more injection pressure to fill the mold, thus more clamp tonnage is needed to hold the mold closed. The required force can also be determined by the material used and the size of the part, larger plastic parts require higher clamping force.
Today electric presses are taking over the typical hydraulic injection molding machines, Injection molding companies prefere them as they offer 80% less energy consumption and nearly 100% repeatability, by utilizing electric servo motors. While the cost of an electric molding machine is typically 30% higher than a hydraulic press, higher demand is closing the gap on cost. It is estimated in the next 20 years that hydraulic molding machines will be a thing of the past as mor molding companies are making the switch to stay competitive..
Mold
Mold or Die are the common terms used to describe the tooling used to produce plastic parts in molding.
Traditionally injection molds have been expensive to manufacture. They were usually only used in mass production where thousands of parts were being produced. Molds are typically constructed from hardened steel, pre-hardened steel, aluminum, and/or beryllium-copper alloy. The choice of material to build a mold from is primarily one of economics, steel molds generally cost more to construct, but their longer lifespan will offset the higher initial cost over a higher number of parts made before wearing out. Pre-hardened steel molds are less wear resistant and are used for lower volume requirements or larger components. The steel hardness is typically 38-45 on the Rockwell-C scale. Hardened steel molds are heat treated after machining. These are by far the superior in terms of wear resistance and lifespan. Typical hardness ranges between 50 and 60 Rockwell-C (HRC).
Today aluminum molds cost substantially less, than steel molds and when higher grade aluminum such as QC-7 and QC-10 aircraft aluminum is used and machined with modern computerized equipment, they can be economical for molding hundreds of thousands of parts. Aluminum molds also offer quick turnaround and faster cycles because of better heat dissipation and can also be coated for wear resistance to fiberglass reinforced materials. Beryllium copper is used in areas of the mold which require fast heat removal or areas that see the most shear heat generated. Today’s Mold companies use CNC machining and Electrical Discharge Machining (EDM) in the manufacturing processes
Custom Mold Design
The mold consists of two primary halves, the injection mold (A plate) and the ejector mold (B plate). Plastic resin enters the mold through a sprue in the injection mold, the sprue bushing is to seal tightly against the nozzle of the injection barrel of the molding machine and to allow molten plastic to flow from the barrel into the mold, also known as cavity. The sprue bushing directs the molten plastic to the cavity images through channels that are machined into the faces of the A and B plates. These channels allow plastic to run along them, so they are referred to as runners. The molten plastic flows through the runner and enters one or more specialized gates and into the cavity geometry to form the desired part.
The amount of resin required to fill the sprue, runner and cavities of a mold is a shot. Trapped air in the mold can escape through air vents that are grinded into the parting line of the mold. If the trapped air is not allowed to escape, it is compressed by the pressure of the incoming material and is squeezed into the corners of the cavity, where it prevents filling and causes other defects as well. The air can become so compressed that it ignites and burns the surrounding plastic material. To allow for removal of the molded part from the mold, the mold features must not overhang one another in the direction that the mold opens, unless parts of the mold are designed to move from between such undercuts when the mold opens (utilizing components called Lifters or slides).
Sides of the part that appear parallel with the direction of draw (The axis of the cored position (hole) or insert is parallel to the up and down movement of the mold as it opens and closes) are typically angled slightly with (draft) to ease release of the part from the mold. Insufficient draft can cause deformation or damage to the mold. The draft required for mold release is primarily dependent on the depth of the cavity: the deeper the cavity, the more draft necessary. Shrinkage must also be taken into account when determining the draft required. If the skin is too thin, then the molded part will tend to shrink onto the cores that form them while cooling, and cling to those cores or part may warp, twist, blister or crack when the cavity is pulled away. The mold is usually designed so that the molded part reliably remains on the ejector (B) side of the mold when it opens, and draws the runner and the sprue out of the (A) side along with the parts. The part then falls freely when ejected from the (B) side. Tunnel gates, also known as submarine or mold gate, is located below the parting line or mold surface. The opening is machined into the surface of the mold on the parting line. The molded part is cut (by the mold) from the runner system on ejection from the mold. Ejector pins, also known as knockout pin, is a circular pin placed in either half of the mold (usually the ejector half) which pushes the finished molded product, or runner system out of a mold.
The standard method of cooling is passing a coolant (usually water) through a series of holes drilled through the mold plates and connected by hoses to form a continuous pathway. The coolant absorbs heat from the mold (which has absorbed heat from the hot plastic) and keeps the mold at a proper temperature to solidify the plastic at the most efficient rate.
To ease maintenance and venting, cavities and cores are divided into pieces, called inserts, and sub-assemblies, also called inserts or blocks. By substituting interchangeable inserts, one mold may make several variations of the same part.
More complex plastic parts are formed using more complex molds. These may have sections called slides, that move into a cavity perpendicular to the draw direction, to form overhanging or undercut part features. When the mold is opened, the slides are pulled away from the plastic part by using stationary “angle pins”or “horn pins” on the stationary mold half. These pins enter a slot in the slides and cause the slides to move backward when the moving half of the mold opens(like a cam). The part is then ejected and the mold closes. The closing action of the mold causes the slides to move forward along the angle pins.
Some molds allow previously molded parts to be reinserted to allow a new plastic layer to form around the first part. This is often referred to as overmolding. This system can allow for production of one-piece tires and wheels.
2-shot or multi-shot molds are designed to "overmold" within a single molding cycle and must be processed on specialized injection molding machines with two or more injection units. This process is actually an injection molding process performed twice. In the first step, the base color plastic material is molded into a basic shape. Then the second material is injection-molded into the remaining open spaces. That space is then filled during the second injection molding step with a material of a different color.
An injection mold can produce several copies of the same parts in a single "shot". The number of "impressions" in the mold of that part is often incorrectly referred to as cavitation. A tool with one impression will often be called a single impression (cavity) mold. A custom mold with 2 or more cavities of the same parts will likely be referred to as multiple impression (cavity) mold. Some extremely high production volume molds (like those for bottle caps) can have over 128 cavities.
In some cases multiple cavity tooling will mold a series of different parts in the same tool. Some toolmakers call these molds family molds as all the parts. Frequntly asked question on tooling.
Effects on the material properties
The mechanical properties of a part are usually little effected. Some parts can have internal stresses in them. This is one of the reasons why it's good to have uniform wall thickness when molding. One of the physical property changes is shrinkage. A permanent chemical property change is the material thermoset, which can't be remelted to be injected again. Material to Wall Thickness Guidline.
Tool Materials
Tool steel or Aluminum are often used. Mild steel, nickel or epoxy are only suitable for prototype or very short production runs.
High grade aluminum is fast becoming the material of choice, as it offers substantially lower tooling costs and better molding process conditions. Aluminu is used in both prototype and production molds. With high grade aluminum you can expect to get ten to hundreds of thousands of parts. It is recommended to shop injection molding companies that only use high grade aluminum such as QC-7 or QC-10.
Geometrical Possibilities
The most commonly used plastic molding process, injection molding, is used to create a large variety of products with different shapes and sizes. Most importantly, they can create products with complex geometry that many other processes cannot. There are a few precautions when designing something that will be made using this process to reduce the risk of weak spots. First, streamline your product or keep the thickness relatively uniform. Second, try not cramming to many details into one part may cause visual defects in show surfaces or the inability to fill some of the details without sacrificing others. It may be better to make multiple parts for your process. A good injection molding company can steer you in the right direction.
The size of a part will depend on a number of factors (material, wall thickness, shape,process ect). The initial raw material required may be measured in the form of granules, pellets or powders. Here are some ranges of the sizes.
| Method | Raw Materials | Maximum Size | Minimum Size |
|---|---|---|---|
| Injection Molding (thermo-plastic) | Granules, Pellets, Powders | 700 oz. | Less than 1 oz. |
| Injection Molding (thermo-setting) | Granules, Pellets, Powders | 200 oz. | Less Than 1 oz. |
Machining
Injection Mold Companies use two main methods to manufacture molds: standard machining and EDM. Standard Machining, in its conventional form, has historically been the method of building injection molds with a knee mill. With technological development, CNC machining became the predominant means of making more complex molds with more accurate mold details in less time than traditional methods.
The electrical discharge machining (EDM) spark erosion process has become widely used in mold making. Most injection mold companies have EDM in house, as it is essential to the mold build process of complex molds. EDM allows the formation of shapes which are difficult to machine, such as square corners or ribs. The process allows pre-hardened molds to be shaped so that no heat treatment is required. Changes to a hardened mold by conventional drilling and milling normally require annealing to soften the steel, followed by heat treatment to harden it again. EDM is a simple process in which a shaped electrode, usually made of copper or graphite, is very slowly lowered onto the mold surface (over a period of many hours), which is immersed in paraffin oil. A voltage applied between tool and mold causes spark erosion of the mold surface in the inverse shape of the electrode.
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 piece part cost is low, so with greater quantities the overall price decreases. With global competition, companies with an ISO-Quality system usually will have better pricing as they have streamed lined their process and produce less defects. Mold Cost Guidline.
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 called the barrel, where it is melted. As the plunger advances, the melted plastic is forced through a nozzle that seats against the mold sprue bushing, allowing it to enter the mold cavity through a gate and runner system. The mold remains at a set temperature so the plastic can solidify almost as soon as the mold is filled.
Injection Molding Cycle
The sequence of events during the injection molding 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. Molding companies typically use the formula below to determine a cycle time.
Time Function
|
The time it takes to make a product using injection molding can be calculated by adding: |
The total cycle time can be calculated using tcycle = tclosing + tcooling + tejection
- The closing and ejection times, can last from a fraction of a second to a few minutes, depending on the size of the mold and machine. The cooling times, which dominate the process, depend on the maximum thickness of the part.
Different types of Injection Molding Processes
Although most injection molding processes are covered by the conventional process description above, there are several important molding variations including:
- Co-injection(sandwich)molding
- Fusible(lost, soluble)core injection molding
- Gas-assisted injection molding
- In-mold decoration and in mold lamination
- Injection-compression molding
- Insert and outsert molding
- Lamellar (microlayer) injection molding
- Low-pressure injection molding
- Microinjection molding
- Microcellular molding
- Multicomponent injection molding(overmolding)
- Multiple live-feed injection molding
- Powder injection molding
- Push-Pull injection molding
- Reaction injection molding
- Resin transfer molding
- Rheomolding
- Structural foam injection molding
- Structural reaction injection molding
- Thin-wall molding
- Vibration gas injection molding
- Water assisted injection molding
- Rubber injection
- Injection molding of liquid silicone rubber
Process Troubleshooting
Optimal process settings are critical to influencing the cost, quality, and productivity of plastic injection molding. Process optimization is done using the following methods. Injection speeds are usually determined by performing viscosity curves. Process windows are performed varying the melt temperatures and holding pressures. Pressure drop studies are done to check if the machine has enough pressure to move the screw at the set rate. Gate seal or gate freeze studies are done to optimize the holding time. A cooling time study is done to optimize the cooling time.
Molding trial
When filling a new or unfamiliar mold for the first time, where shot size for that mold is unknown, an injection molding company technician/tool setter usually 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 solidification time is an important as it determines cycle time, 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. Once the parts are good enough and have passed any specific criteria, a setting sheet is produced for people to follow in the future.
Molding defects
Injection molding is a complex technology with possible production problems. They can either be caused by defects in the molds or more often by part processing (molding)
| Molding Defects | Alternative Name | Descriptions | Causes |
|---|---|---|---|
| Blister | Blistering | Raised or layered zone on surface of the Plastic 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 | Black or brown burnt areas on the plastic part located at furthest points from gate | Tool lacks venting, injection speed is too high |
| Color streaks (US) | Localized change of color | Plastic material and colorant isn't mixing properly, or the material has run out and it's starting to come through as natural only | |
| 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 | Tool damage, 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 must be set as fast as you can get away with at all times) |
| Jetting | Deformed part by turbulent flow of material | Poor tool design, gate position or runner. Injection speed set too high. | |
| Polymer degradation | polymer breakdown from oxidation etc | Excess water in the granules, excessive temperatures in barrel | |
| Sink marks | 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 | |
| Short shot | Non-fill / Short mold | Partial part | Lack of material, injection speed or pressure too low |
| Splay marks | Splash mark / Silver streaks | Circular pattern around gate caused by hot gas | Moisture in the material, usually when resins are dried improperly |
| Stringiness | Stringing | String like remain from previous shot transfer in new shot | Nozzle temperature too high. Gate hasn't frozen off |
| Voids | Empty space within part (Air pocket) | Lack of holding pressure (holding pressure is used to pack out the part during the holding time). Also mold may be out of registration (when the two halves don't center properly and part walls are not the same thickness). | |
| Weld line | Knit line / Meld 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) |
| Warping | Twisting | Distorted part | Cooling is too short, material is too hot, lack of cooling around the tool, incorrect water temperatures (the parts bow inwards towards the hot side of the tool) |
Tolerances and Surfaces
Molding tolerance is a specified allowance on the deviation in parameters such as dimensions, weights, shapes, or angles, ect. To maximize control in setting tolerances there is usually a minimum and maximum limit on thickness, based on the process used. Injection molding typically is capable of tolerances equivalent to an IT Grade of about 9–14. The possible tolerance of a thermoplastic or a thermoset is ±0.008 to ±0.002 inches. Surface finishes of two to four microinches or better are can be obtained. Rough or pebbled surfaces are also possible.
| Molding Type | Typical | Possible |
|---|---|---|
| Thermoplastic | ±0.008 | ±0.002 |
| Thermoset | ±0.008 | ±0.002 |
Lubrication and Cooling
The mold temperature must be maintained in order for the production to take place. Because of the heat capacity, inexpensiveness, and availability of water, water is used as the primary cooling agent. To maintain mold temperature, water can be channeled through the mold to account for quick cooling times. A consistent temperature mold is more efficient because this allows for faster cycle times. However, this is not always true because crystalline materials require the opposite of a warmer mold and lengthier cycle time.
Power Requirements
The power required for this process of injection molding depends on many things and varies between materials used. Manufacturing Processes Reference Guide states that the power requirements depend on "a material's specific gravity, melting point, thermal conductivity, part size, and molding rate. Below is a table from page 243 of the same reference as previously mentioned which best illustrates the characteristics relevant to the power required for the most commonly used materials.
| Material | Specific Gravity | Melting Point (°F) |
|---|---|---|
| Epoxy | 1.12 to 1.24 | 248 |
| Phenolic | 1.34 to 1.95 | 248 |
| Nylon | 1.01 to 1.15 | 381 to 509 |
| Polyethylene | 0.91 to 0.965 | 230 to 243 |
| Polystyrene | 1.04 to 1.07 | 338 |
Insert Molding 
Metal inserts can also be injection molded into the work piece. It is best to choose a company that has experience in insert molding as there are many variables to consider. For large volume parts the inserts are placed in the mold using automated machinery. An advantage of using automated components is that the smaller size of parts allows a mobile inspection system that can be used to examine multiple parts in a decreased amount of time. In addition to mounting inspection systems on automated components, multiple axial robots are also capable of removing parts from the mold and place them in latter systems that can be used to ensure quality of multiple parameters. The ability of automated components to decrease the cycle time of the processes allows for a greater output of quality parts. In low volumes a machine operator will remove the parts by hand
Specific instances of this increased efficiency include the removal of parts from the mold immediately after the parts are created and use in conjunction with vision systems. The removal of parts is achieved by using robots to grip the part once it has become free from the mold after in ejector pins have been raised. The robot then moves these parts into either a holding location or directly onto an inspection system, depending on the type of product and the general layout of the rest of the manufacturer's production facility. Visions systems mounted on robots are also an advancement that has greatly changed the way that quality control is performed in insert molded parts. A mobile robot is able to more precisely determine the accuracy of the metal component and inspect more locations in the same amount of time as a human inspector.
