Types of injection molding of polymers and plastics

Injection molding machine (illustration by Rutland Plastics)

When developing a mass-produced product for the electronics market, you will need a housing. And, most likely, it will be made of plastic. 3D printing is used for prototyping plastic parts and creating a housing prototype, and injection molding is used for mass production.

Injection molding technology is one of the most important points on the way of a product to the electronics market. Therefore, regardless of whether you have a technical background, you should understand the essence of this process at least at a basic level. Everyone knows that when developing a new device, the most expensive task is designing the electronics, but not everyone understands that when putting it into production, the plastic case will take up most of the budget.

This is primarily due to the high cost of tooling or so-called molds. In practice, the cost of molds for casting the body becomes one of the main cost items when introducing a new product to the market.

Types of injection molding

About this method of polymer processing

Plastic injection molding (PIM), along with extrusion, is the most common and studied method for processing plastics into finished or semi-finished products. Unlike extrusion, this method allows you to immediately obtain a part of specified dimensions and almost any geometry (with some limitations - see below). Casting is used mainly in the production of thermoplastic products, however, this processing method is also encountered from time to time for thermoset plastics. If the equipment for processing thermoplastics is called an injection molding machine (TPA), then thermosets are processed on thermoset machines, which are structurally different from TPA. In general terms, the equipment for this production method is often simply called an “injection molding machine.”

The casting pressure developed by injection molding machines is in the range of 80-140 MPa (800-1400 bar), however, leading companies and specialists in the manufacture of tooling (molds) do not recommend loading molds with pressure significantly higher than 100 MPa.

Plastic processing by injection molding is carried out on injection molding machines of the piston or screw (screw) type, and the first type of injection molding machine was until recently considered obsolete and out of use. However, after 2010, injection molding machine manufacturers returned to interest in piston plastic injection as the most accurate process. However, as a rule, modern equipment is screw-based, and the injection machine injection unit consists of a pair of screw-material cylinder.

Video 1. Operation of a modern injection molding machine

Main types of defects and methods for eliminating them

During the injection molding process, due to violations of the technological process, excess melt temperature and other factors, defects occur that reduce the quality of the product:

Incomplete filling of the mold (underfilling). Occurs due to insufficient volume of injected polymer. This situation is possible due to weak heating of the melt, low fluidity of the mass, clogging of the nozzle or distribution channels.
Exceeding the volume of the mold (overflow) during the plastic molding process, causing the formation of burrs at the joint (burst). The defect occurs due to a malfunction of the dispenser or exceeding the permissible temperature of the molten polymer. The release of excess mass also occurs when the mold is not closed tightly enough.
Joint seams are visible at the boundary of the soldering of individual parts of the part, which are characterized by low strength. Such joints are formed when the plasticization temperature is too low. The cause of the defect also lies in improper design or insufficient heating of the injection mold, which leads to premature cooling of the streams before they merge. As a result, complete welding cannot be achieved.
Bubbles form on the surface of the product, and there are voids inside. Such a defect is the result of the presence of volatile components in the melt, characterized by increased gas release when overheated. The soft mass swells.
Formation of shrinkage cavities (depressions) on the surface. The cause of the defect is increased shrinkage of the mass in case of overheating or insufficient volume of molten mass due to low pressure, as well as insufficient throughput of the nozzle. Such sinkholes appear when the injection mold is heated unevenly and has insufficient ventilation.
Warping of manufactured parts. Defects occur when excessive internal tension occurs due to temperature unevenness of parts of the mold. Poor-quality products are also obtained if the time required to hold the mass for complete hardening is not observed.
Cracks on the surface. They occur due to residual stress or in case of strong adhesion of the liquid mass to the walls.
Surface defects of parts, manifested by scratches and chips. Defects occur in the event of careless handling of finished products or poorly designed molds.
Patterns that resemble frosty patterns on glass appear due to excessive amounts of moisture in the injected molten mass. A deficiency occurs when the drying and ventilation process of the mold is disrupted.
If there is moisture in the polymer material in excess of the established norm, delamination of the structure occurs. The defect also occurs when there are frozen sprues that are not aligned with the main mass.
Uneven (different) tone color of the product. The reason is the use of a thermally unstable dye and overheating of the molten mass.
The received product does not meet the required dimensions, exceeding the standard tolerance. It occurs due to severe shrinkage of the plastic during hardening or a poorly made mold.

Applications of injection molding

Plastic molding has been used for more than half a century and allows for mass production of plastic parts weighing from hundredths of a gram to tens of kilograms. The smallest products can be, for example, microscopic lenses, components of small mechanisms, etc. The largest are various containers, including tanks and boxes with a volume of several cubic meters, plastic pallets, structural elements, etc.

Products obtained by the described processing method, in addition to their obvious advantages, have several limitations. In addition to the obvious limit on overall geometric dimensions due to the limited dimensions of the mold, there are several less noticeable ones. For example, the wall thickness of any product usually does not exceed a few millimeters. This is important for the economics of the process, because an increase in wall thickness leads to a sharp lengthening of the production cycle and a corresponding increase in cost and decrease in productivity. This limitation is removed when using a special method - gas casting (see below). On the other hand, the pressure of the injection molding equipment may not be enough to produce parts that are too thin-walled or very long. In addition, the product must be technologically advanced, that is, comply with the described method. Its design should assume a more or less equal-thickness structure, uniform filling with the polymer melt and simple, in most cases automatic, removal from the mold cavity.

Design for manufacturing (DFM)

The high cost of molds is just one of the disadvantages of injection molding.
The second disadvantage is the complexity and limitations at the development stage of the design and construction of plastic parts. Once you have a perfect working prototype made on a 3D printer, you have to spend significantly more time and money adapting it for injection molding.

The limitations of mass production should be taken into account already in the first stages of development. Some casting shape requirements, such as casting drafts, can be deferred at least until a second prototype is created.

Other requirements, such as uniform wall thickness and undercuts, must be implemented from the very beginning.

Casting bias

The main challenge when working with injection molded parts is to remove them from the mold correctly.
Once the plastic has cooled, the two halves of the mold open to reveal a new molded plastic part. Any 3D injection molding design must include a molding or process slope to fill the mold and release the finished product smoothly. Molding draft is essentially a small angle of inclination that is added to any vertical surfaces that coincide with the direction in which the product is removed from the mold. In most cases, 1-2 degrees is enough.

Examples of correct implementation of undercuts. Image courtesy of ICO Mold.

Some experts believe that undercuts should be implemented in the 3D model from the very beginning.

While accounting for undercuts early in design is important, it creates unnecessary complications when creating early prototypes. Therefore, it is better to add them to the project when you are completely confident in your prototype. Those. In most cases, undercuts should be added after the first or second version of the prototype.

Ejector pins

Ejector pins or pushers
are used to remove plastic parts from the mold. As the name suggests, these are small cylindrical pins that push the part out of the mold.

The pushers do not have a standard position, so you will have to think about where they will be located. Ideally, they should be located in the strongest part of the casting to prevent it from deforming when removed from the mold.

It is worth considering that ejector pins tend to leave small marks on the product. If you look closely at most plastic parts, you can see these tiny round marks that appear as the mold is ejected.

This is worth considering when developing a product. Try to ensure that the pushers contact the casting in places that are not critical to the appearance of the product. You can even try to hide the pusher marks under a label or logo.

Double push stroke

Some plastic parts cannot be removed from a simple two-part mold in one step; in such cases, inclined pushers and a double ejection mechanism are used.
The plunger is a component of the mold that is inserted before the casting begins and then removed before the main parts of the mold open. The inclined pusher moves perpendicular to the direction of movement of the two mold halves.

It is worth every effort not to use a double ejection mechanism, as it greatly increases the complexity and cost of the mold.

One of the main techniques that allows you to avoid double extrusion is to avoid using undercuts. An undercut is a protrusion or depression on the surface of a casting that prevents the product from being pushed out of the mold in one pushing stroke.

The situation with undercuts can often be corrected by adding a groove (slot) under the protrusion and using a single push instead of a double push.

In design 1, a double push stroke will be required due to the undercut. The groove in design 2 allows you to eliminate double ejection and remove the part from the mold in one stroke. Image courtesy of Proto Labs.

Uniform wall thickness

One of the important features of injection molding that has a huge impact on device design is the requirement for uniform wall thickness of the casting.
It is due to the fact that the plastic poured into the mold must cool at the same rate over the entire surface of the part. If cooling is uneven, the part may become deformed. Therefore, when designing an injection molded housing, ribs are used instead of thicker sections. Correctly designing a part with uniform wall thickness definitely requires experience.

Using a double push stroke and uneven wall thickness of the casting are two of the most common mistakes made by 3D designers who are not familiar with the technical limitations of injection molding.

It is worth making sure that the 3D modeling of your device is carried out by a specialist who is familiar with this technology.

Examples of structures with the same wall thickness. Image courtesy of ICO Mold.

Radius/corner rounding

Ideal corners and edges of parts are impractical for injection molding. The molten polymer will not be able to evenly and completely fill the entire sharp-edged mold, even under high pressure conditions. At least, you shouldn’t hope for it with large production volumes.

An example of a correct angle design. Image courtesy of ICO Mold.

All edges and corners should be rounded or beveled so that the polymer fills them evenly and completely.

Cold ducts vs. hot ducts

Cold runner/hot runner plastic feed
are variations of a gating system that directs molten polymer into mold cavities.

The wide gate allows the polymer to flow freely at lower pressures. However, wider channels require more time for the plastic to cool and create more production waste, both of which affect the cost of the part.

On the other hand, a narrow gate channel reduces cooling time and material waste, and ultimately minimizes casting costs. However, it has the disadvantage that a narrow channel requires higher pressure to force the molten polymer into the mold.

There is a solution that allows the use of narrow channels at low pressure - the hot runner system.

Heating elements are installed directly into the mold along the channels, which maintain the polymer in a more liquid state, thanks to which the plastic fills the mold at a lower pressure.

Unfortunately, you have to pay for everything, and hot channels also have their disadvantages: additional complexity in the manufacture of equipment, which always results in additional costs.

In most cases, at least initially, it is better to use channels without heating elements, i.e. cold runner gating system. It's always a good idea to start with the simplest and least expensive solution.

Form parting line

If you look closely at any plastic part, you will see what is called a parting line.
It will be located at the junction of the two parts of the mold. This junction of the two halves is never ideal; a little polymer always flows out along the contour. As the mold ages and wears, this leakage becomes more noticeable.

It is very important to choose the optimal location for the parting line. Ideally, it should be placed on an invisible part of the device.

Operating principle of injection molding machine

The injection molding machine loads granular (much less often powdered) polymer from the raw material loading hopper into the loading zone of the material cylinder. Then, by heating and plasticizing (mixing) the molten mass with a screw, it transforms into a viscous-flowing (close to liquid) state. After collecting the required dose of polymer, the injection molding machine, using the force created by a hydraulic cylinder, injects molten plastic into the mold. Then, in its cavity, the casting is kept under pressure and the cooling stage (for thermosets - hardening).

During the last stage of the production cycle, the machine opens the mold and pushes out the finished product; less often, the products are removed by the operator (semi-automatic mode). Modern production units include, in addition to injection molding machines, various automation equipment, usually called “robots”. Modern robots are involved in removing the casting from the mold area; they can also insert labels and embedded parts into the tooling cavity, and, in addition, participate in the “further fate” of the molded part, for example, in its post-processing, laying and packaging.

Characteristics of casting machines

To select the optimal production equipment, you should familiarize yourself with the following characteristics of the injection molding machine:

The force with which the material cylinder engages the casting mold. It is measured in kilonewtons.
Maximum injection volume. Despite the name, it is a mass, as it is measured in grams. The larger it is, the more forms the device can serve.
The pressure created by the plasticizer when the material is injected into the injection mold.
The maximum size of the mold used.

The given characteristics depend on each other; often the manufacturer indicates only the force of the material cylinder. This, of course, is not enough to fully compare the devices with each other.

Even without deep knowledge of economic theory, one can point out that the total cycle time is of great importance. The smaller it is, the more products can be produced in the same period of time. Time is made up of four components:

plasticization of material;
injection time;
time of exposure to pressure and cooling of the product.

The melting time of the raw material depends on its type and varies within a narrow range; it practically does not affect the change in the total cycle time. The duration of injection and pressure depends on the total volume of the mold and the design of the gating system. The largest contribution to the total production time is the cooling time. Unfortunately, it depends more on the characteristics of the form; the more complex the part, the longer it will give off heat.

Features of working with injection molding equipment

When processing thermoplastics, the temperature of the mold should not be higher than the glass transition temperature of the polymer or its crystallization temperature, therefore, mold cooling or thermostatting must be used. In thermoset processing, on the other hand, the mold is heated using various methods to a temperature above the curing point of the thermoset plastic.

Figure 2. Form installed on the injection molding machine

When making molds, it is important to remember the need to organize ventilation channels (evaporations), through which the molten mass, with its pressure, must displace air from the cavity of the technical equipment. The lack of fumes leads to numerous difficult-to-remove defects in finished plastic products.

Molds for plastic injection can be hot runner or cold runner. Hot runner molds are more modern, characterized by the absence or minimal amount of waste (gates), faster production cycle times, stable technological processes and less waste. The hot runner system transfers injection pressure to the mold area with minimal loss. However, hot runner molds are not recommended for processing some non-heat-resistant plastics, such as rigid PVC compositions.

Figure 3. Hot Channel Control Device

Where to work with molds?

Where is it better to produce plastic parts for your device: at home or in China?
In most cases, it is better to start working with a local manufacturer in your country (unless you live in a country where the industry is very weak). Then, when volumes exceed 10 thousand pieces, you can switch to a Chinese manufacturer to reduce costs.

China is simply the ideal choice for large-scale production. Just don’t start the initial development and debugging of the process there. With local manufacturers, any issues can be resolved much easier and faster.

Make the first launch and the first mistakes at the local level, and then move production to China.

Translator's note: it is important to consider that transporting a mold from one country to another (and even more so to China from Europe) is a complex and expensive undertaking. Therefore, we select local prototype manufacturers for our clients, and if the series is large in total, we immediately place it in China with the expectation that the mold will be amortized over several iterations of production. After all, if you start mass production from local manufacturers, then the mold will be local, and transporting it to China or creating a second mold in China will not justify itself.

Parameters of injection of polymer material for LPD

The pressure developed during injection of injection molding machines depends on several parameters:

viscosity of the molten polymer,
features of the gating system, in particular the presence of a cold or hot channel,
mold designs,
design of the plastic product and the melt inlet location.

The pressure in the mold during injection of the molten polymer mass increases as the mold cavity is filled and the casting is further exposed. In this case, as a rule, the value of the specified holding pressure reaches 30-50 percent of the specified parameter value. These parameters on modern injection molding machines are set in the control system and implemented using the hydraulic (less often the most modern - electric) system of the injection molding machine.

Features of choosing an injection molding machine

Read more in the article Selecting an injection molding machine

When choosing a injection molding machine for plastic injection molding, first of all, the volume of the dose is taken into account, that is, the amount of polymer melt required to produce each specific product. Also important is the clamping force of the injection molding machine, the compression force necessary to fix the mold during the injection and holding stages. If the closing force is selected incorrectly, the mold will open slightly. The third most important parameter is the geometry of the area where the equipment is attached to the injection molding machine, namely the size of the machine plates and the distance between the columns, as well as the “mold height”. These values determine the maximum and minimum size of the injection mold for installation on a particular injection molding machine.

In addition to the indicated most important basic parameters for selecting injection molding machines, several more special ones are used, which are described in detail in specialized industry literature. For example, the value of the maximum injection speed, the load capacity of the injection molding machine plates (primarily the movable plate), the ratio of the screw length to its diameter L/D, the presence of an intrusion mode, etc. It is also important to equip the injection molding machine with various components and options. For high-speed machines, hydraulic accumulators for injection and other movements are used. To connect robots and other auxiliary devices, the injection molding machine controller should be equipped with Euromap 12 or Euromap 67 connectors. Actual melt pressure sensors, molded part drop sensors, and others are used.

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Working on a vertical injection molding machine

This technology differs from the commonly used one in that a vertical type injection molding machine is used, and the mold also opens in the vertical direction. The method is good for small-scale production, because It is possible to use molds that are simpler and less expensive to manufacture. Vertical LPD is also widely used when using embedded elements (usually metal). The main disadvantage inherent in such casting is the complex automation of the process - products cannot fall out of vertical molds and they have to be removed manually or by robot.

Cascade casting (with shut-off valves)

This type of processing is gaining increasing popularity due to the fact that with relatively low investments it is possible to radically improve the quality of manufactured products. Cascade injection is possible only with the use of a special type of hot runner mold and differs from the standard one by the presence of a hot runner system with shut-off valves. The valves can be controlled pneumatically, hydraulically or, more recently, electrically and are carried out using special devices. The cascade allows you to control the injection of polymer into the mold at the request of the injection molding machine operator. In this way, it is possible to avoid adhesions, traces of flow of polymer material, burns and many other types of defects in LPD.

Injection pressing

This technological process differs from the standard one in that the polymer material is injected into a slightly open mold (in this case it is appropriate to use this name for the equipment) a short time before its final closure. The final compaction of the polymer and the formation of the finished product are carried out when the mold is completely closed. Various products are made from both thermoplastics and thermosets using injection molding. The method is applicable in the case of insufficient characteristics of the injection molding machine for a given casting, in particular the clamping force. Also, the quality of such pressing on an injection molding machine is less dependent on the orientation of macromolecules during injection (anisotropy), which can improve the quality of the product in terms of less shrinkage (if necessary), better mechanical properties and less warping.

Single and multiple molds

At a certain stage of production, it becomes possible to reduce casting time due to multiple molds (they are also called multi-cavity molds).
They are used to increase production speed and reduce the cost of workpieces. Multi-place molds, as the name suggests, allow you to create multiple copies of a single part with a single pour of polymer. Just don’t use these molds at the start until the process is debugged and ideal castings have not yet been created from single molds. It is advisable to produce at least several thousand units of products before switching to multi-piece molds.

As a rule, entrepreneurs on a tight budget make the most of their one-piece molds, unless the manufacturer itself finances the production of their molds.

Intrusion

Intrusion is the process of partial filling of the forming cavity in extrusion mode due to the rotational movement of the screw. It is usually used for the production of heavy, material-intensive plastic products. Thus, it is possible to use injection molding machines with an injection volume that is insufficient for a given product, because the mold is filled not only due to the translational, but also due to the rotational movement of the screw in the initial position. For intrusion, it is important that the polymer material is sufficiently fluid and that the gating channels have a sufficiently large cross-section. It is also important to note that not all injection machines are equipped with an intrusion mode; it is necessary to check its availability in the machine specification.

Family molds

In most cases, a separate mold is used for each individual plastic part within the device.
The body will need at least two parts: upper and lower. But most products will require more than two plastic parts. Molds are very expensive, and buying multiple molds at once is a major financial hurdle, so you need to aim for a minimum number of plastic parts.

An alternative option for minimizing the required molds is to use a special type of multiple molds, the so-called family molds. The family mold allows you to combine several different parts into one casting.

While the typical multi-slot form creates multiple copies of the same part, the family form creates different parts.

Sounds good, right? Unfortunately, not everything is so simple, you have to pay for everything. The main problem with family molds is that every piece has to be roughly the same size.

Otherwise, one of the mold cavities will be filled with molten polymer before the others. Family molds should be designed so that all cavities are filled with polymer at approximately the same rate. This clearly limits the possibilities of their application. It is unlikely that all body parts will be the same size.

Bicomponent and multicomponent injection

The essence of bicomponent and multicomponent injection molding is the use of two or more types of polymer, or the same plastic, but different colors, to produce one product on one injection molding machine. As a rule, in the case of this type of casting, injection molding machines with two or more plasticizing units are used (pairs of screw - material cylinder). First, the first component is injected into the mold, then additional cavities are opened in one way or another and the second component is injected, etc. In rare cases, components are supplied at the same time. When using bi- and multi-component (multi-color) plastic injection molding, technological equipment becomes much more complex. As a rule, molds with two or more separate hot runner systems are used. As for the possibilities of switching from one component to another, either rotary mechanisms are used directly in the mold, or the so-called “rotary table” as part of a bicomponent injection molding machine.

Heating elements for injection molding machine cylinder

To heat the working cylinder and melt the thermoplastic, ring heating elements of various designs are used. All types of ring heaters for injection molding machines are manufactured by Polimernagrev to order with individual characteristics.

Micanite ring heaters in a stainless steel case allow you to achieve heating temperatures of up to 350 degrees. They are efficient and reliable heating elements. To reduce the cost of ring heaters, you can use ring heating elements in a galvanized steel shell.

If you need to reach temperatures above 350 degrees, you need to use ceramic ring heaters, which allow you to heat the injection molding machine cylinder to 500 degrees. You can also use economical ring ceramic heaters, which, thanks to the outer insulated casing, can save up to 25% of electricity.

In cases where rapid cooling and increased uniformity of heating are very important during the molding process, it is necessary to use ring micanite or ceramic heaters in a cooling housing with a fan. Air and water cooling are also available in this type of ring heater for injection molding machines, such as aluminum cast ring heater.

Gas casting

For the injection molding production of very thick-walled polymer products, almost the only suitable technology is gas injection. Plastic processing in this way is carried out on standard machines, but using adapted molds and a special module for generating gas injection connected to an injection molding machine. The essence of the process in the general case comes down to the delivery of an incomplete dose of polymer material into the forming cavity, followed by the injection of gas compressed under a pressure of 5-20 MPa into the melt mass through special injectors. The gas compacts the plastic “from the inside” and presses it against the walls of the mold. This produces a fully molded part with voids inside. Gas injection can be used, in particular, to compensate for sink marks when there is a large variation in product thickness. In addition, it is important that LDP with gas is produced at a reduced pressure of the melt in the mold, which allows the use of injection molding machines with lower clamping forces than with the standard process.

Areas of application

Plastic injection molding has been used in mass production for over 50 years. Thanks to this, it has become possible to produce a variety of plastic products of different sizes and weights, structures and thicknesses. From the smallest, weighing up to several milligrams, to large ones, weighing tens of kilograms.

The main areas of use of plastic products produced by injection molding:

Electronics (device housings are made of plastic).
Automotive industry. Plastic parts for cars are also produced using this technology.
Medicine (polymers are used to make housings for medical devices).
Construction and repairs (interior finishing parts, tanks, boxes).