INJECTION MOLDING – HOW DOES THAT WORK AGAIN?

How does injection molding work? Of course, we’ve already covered the injection molding process on our website, but when we talk to visitors, we keep finding that there’s interest in the basics of the injection molding machine. Therefore, we would like to summarize some of the basic terms and the rough process of plastic injection molding.

THE INJECTION MOLDING MACHINE

According to the definition, an injection molding machine is a system for the discontinuous production of molded parts made of different types of plastic. These are usually delivered in granular form. The most commonly common plastics come from the group of thermoplastics.

How does injection molding work?  Thermoplastics become liquid when heated and solidify in the assumed form when cool. This process is repeatable. Thermosetting plastics and elastomers are common less often. These two types of plastic crosslink so much during the solidification process that they can no longer soften afterward.

There are major differences in injection molding in terms of pressure and temperature distribution. For the description of the injection molding machine, we shall use one of the simple elastomers.

Thermoplastic injection molding

How does injection molding work? In principle, plastics will be heated to such an extent that they liquefy. The thermoplastics are then filled into a die called a tool using a screw, pressed together, and cool. After that, the desired shape solidifies and can be removed.

The whole process closely time so that granulate for the next component is already being filled into the screw while the cooling process is still running. In this way, thousands of components can be produced in a relatively short period. The injection molding machine consists of two assemblies, namely the plasticizing unit and the closing unit.

THE INJECTION UNIT OR PLASTICIZING UNIT

How does injection molding work? In the injection unit or plasticizing unit, the plastics are usually fed in granulate form through a hopper and prepared for injection molding. To do this, they must be heated, liquefied, and homogenized.

The machine’s injection unit, which shapes like a cylinder heats from the outside by heating coils or bands. A screw is rotating inside sizes to fit snugly within the barrel.

While the granules are transported forwards by the screw towards the nozzle, they are also heated, grounded, and mixed or distribute by friction.

The plastic, which is evenly liquefied in this way, is prevented from flowing back when it reaches the nozzle by a non-return valve. This builds up a certain pressure with which the plastic compresses.

How does injection molding work? Then the plastic is pressed through the nozzle with another forward movement. At the same time, the nozzle also takes care of the dosing by closing precisely calculate and thus interrupting the injection.

The auger then retracts to process the next batch of granules. In this injection phase, the material is pressed into the clamping unit under very high pressure of up to 2000 bar, which can be up to 2000 bar. The filling speed must of course be calculated extremely precisely so that no cavities or cracks form in the workpiece.

THE LOCKING UNIT

How does injection molding work? The clamping unit contains, among other things, the shaping tool. This tool is mounted on two vertical plates. The tool forms the more or less complex cavity, i.e. the matrix, in which the new shape of the plastic creates. A stationary plate with one-half of the tool attached to the plasticizing unit with the screw.

The nozzle inserts through a hole in the middle of this so-call clamping plate. The second moving plate is usually brought up to the first plate on horizontal bars before the plastic injects. It carries the second half of the tool.

A third plate provides the pressure on the two clamping plates and connects to the moving plate by a hydraulic cylinder, electric motor, or mechanical toggle lever.

How does injection molding work?

The two clamping plates initially heat in such a way that it corresponds to the liquid mass. Additional pressure generates during injection, which gives the liquid plastic its shape in the tool. A certain holding pressure requires compensating for the volume loss of the plastic that occurs as a result of cooling or hardening.

Therefore, after the actual filling, some more liquid plastic still has to push in until the workpiece is perfectly shaped. After a short cooling phase, the moving plate moves away from the nozzle side to the ejector side.

How does injection moulding work? The injection mold part contained in it is then ejected. Which occurs as a result of cooling or hardening. Therefore, after the actual filling, some more liquid plastic still has to push in until the workpiece is perfectly shaped. After a short cooling phase, the moving plate moves away from the nozzle side to the ejector side.

The tool must be made of the best metal

How does injection molding work? As already mentioned, the tool represents the mold that gives the later injection-molded part its shape. Depending on the order, this can be a relatively simple Lego brick or a highly complex functional part, for example, a machine.

In any case, the tool must be made of the best metal in order to withstand the processes over the long term.

Hardened steel is usually common for this. Some manufacturers only use aluminum tools in exceptional cases and, for example, for small series. The tool manufactures with the greatest precision, nowadays mostly using 3D CAD specifications.

How does injection molding work?

It is the manufacture of this tooling that accounts for the lion’s share of the cost of injection molding. In plastic injection molding, molten plastic injects into the forming cavity (cavity) of a tool, compressed under pressure, cross-linked, or allows cooling, and then ejected as a mold part.

Conclusion

How does injection molding work? Injection molding is the most common process for the highly automatic production of plastic workpieces by processing powder or granules. Workpieces with complex shapes can produce in almost any size. The high level of accuracy and short cycle times predestine the process for economical series production.

INJECTION MOULDING PROCESS

What type of plastic is used for injection moulding? In principle, the injection molding process (also called injection molding process/injection molding) consists of 5 individual steps. This is a so-call forming process. The accuracy of the manufacture of injection molded parts is what makes it so popular.

• Plasticizing by heat

The granules are liquefied by heat, the mold is closed

• Injection

Plasticized (liquid) plastic is “injected” into the mold at high pressure.

• Repress

In this way, the shrinkage of the plastic (caused by cooling) compensates for the plastic that continues to flow.

• Cooling down

Furthermore, the mold cools with water from the inside. This cools the plastic & makes the component dimensionally stable.

• Demoulding

The tool opens and the plastic part is removed.

ALTERNATIVES TO INJECTION MOULDING

INJECTION PROCESS: WHICH MOULD IS THE RIGHT ONE?

The quality of the injection mold is crucial for the result. Aluminum tools are often cheaper to buy. Unfortunately, these can withstand significantly fewer injection cycles than molds made of steel.

STEELS OR ALUMINUM TOOLS

What type of plastic is used for injection moulding? Which of the two versions is the right choice can be easily calculated by taking into account the number of pieces to produce.

Due to the comparatively high acquisition costs of the mold, the injection molding process is only worthwhile for a certain number of items. However, large quantities can manufacture very economically thanks to the long service life of the injection molding tools. For small quantities, a choice of 3D printing can also be worthwhile.

A precise flow of the process

What type of plastic is used for injection moulding? Eventually, there are many entrepreneurs who are already very successful in marketing their own plastic products. Just imagine the ways you can use this process to create products that help make the world a better place.

Technical basics of plastic injection molding

Eventually, plastic injection molding is one of the primary manufacturing processes in which a defined shape produces from a shapeless mass by creating material cohesion. This process group also includes extrusion, which closely relates to plastic injection molding.

What type of plastic is used for injection moulding? In addition, injection molding is a cyclic process for producing workpieces with a specific geometry; extrusion is a continuous process for producing endless profiles. During extrusion, the plasticized plastic is not injected into a close mold but pressed through a die.

Moreover, in plastic injection molding, the raw material – usually in the form of granules – is first heated and plasticized in a screw cylinder. The plastic mass is then pressed into the tool with high pressure. This then cools in order to solidify the melt and finally remove the finished workpiece.

The most common methods for processing plastics include the following:

• Archetypes
  • Injection Molding
  • Extrude
• Reshape
  • Deep Drawing
  • Bending Forming
• Put
  • Welding
  • Adhere

Rubber and silicone parts

What type of plastic is used for injection moulding? Due to the high cost of manufacturing injection molds, the process can hardly use economically in the production of prototypes and small series.

Construction of an injection molding machine

The plastic injection molding process

In addition, the injection molding machine largely automatically coordinates the cyclical process sequence described using the process variables. The determining factors for the settings on the machine are:

• Plastic used
• Design of the mold (number and size of the cavities, wall thickness)
• Machine (locking force, stroke volume)

Special variants of plastic injection molding that require individual process management include:

• Injection molding of inserted parts
• Back injection molding of an inserted carrier part
• Creation of cavities in injection molded parts
• Injection molding of foam parts

Plastic injection molding machine

What type of plastic is used for injection moulding?  Different types of plastic place individual demands on the temperature control of the tool. The injection molding of a thermoplastic describes here as an example:

Plasticizing and Dosing

Subsequently, at the start of the injection molding cycle, the mold is open, and a dosed mass volume is present. The molding compound creates when the plastic granules draw in from the funnel by a screw and melted. 

As well as, the material plasticizes by the effect of heat in the heat screw cylinder. The screw transports the material into the front part of the cylinder, where it collects behind the nozzle. This creates a dynamic pressure that causes the screw to move backward.

Injection

What type of plastic is used for injection moulding? With the mold now closed and with a sufficient amount of melt between the nozzle and the tip of the screw to fill the mold, the injection can take place. To do this, the axially displaceable screw presses the plastic mass into the mould. Furthermore, the pressure increases to a maximum value during filling and then gradually decreases.

Press and Cooldown

What type of plastic is used for injection moulding? Moreover, the pressure maintains in order to be able to compensate for the shrinkage of the solidifying melt. The mass cushion remaining in front of the screw tip can thus flow into the mold. This is to avoid common mistakes in injection molding: sink marks and blowholes. The tool cools to speed up the cooling process of the molded part.

Demould and eject

What type of plastic is used for injection moulding? In addition, the tool for ejecting the solidified injection molding can now be opened. The workpiece falls out of the mold or is removed by a handling device. The sprue that does not belong to the molded part can then removes manually or automatically.

Which materials can be processed?

Plastic injection molding is suitable for processing all types of plastics:

• Thermoplastics
• Thermosets
• Elastomers
• Thermoplastic Elastomers

The different polymers require different temperature control of the tool. The injection molding of thermoplastic materials, especially polyethylene, is of the greatest economic importance.

Advantages of plastic injection molding

What type of plastic is used for injection moulding? The advantages of plastic injection molding are:

• High reproducibility of production
• Little or no post-processing of the molded part is necessary

Besides, the workpieces produced during injection molding can often use without further reworking. Accordingly, the direct route from the raw material to the finished part contributes to the attractiveness of the process.

WHAT ARE THE ADVANTAGES OF INJECTION MOLDING?

What type of plastic uses for injection moulding? Injection molding uses for the production of small series and offers a high level of efficiency with complex, small tools compare to lengthy, conventional production.

Furthermore, we offer short manufacturing times and consequently lower final manufacturing costs. Injection molding also produces series-identical precision parts.

Lastly, the injection-molded parts do not have to rework, only rarely. After the tooling for the injection molding and the installation of all components, the process runs automatically – so there is no need for constant monitoring.

OVERMOLDING UNDER LOW PRESSURE

Firstly, low-pressure overmolding has won a well-deserved place in the foundry industry. The essence of the method is that the melt under low pressure of air or gas fills the mold. 

Excessive pressure in the metal flow is necessary in order to overcome various resistances in its path. The over-molding mold according to this method is filled from the bottom up directly. 

Advantages of the low-pressure overmolding method:

• Possibility to automate and mechanize the mold filling process.
• Overmolding quality improves due to excess pressure in the liquid plastic. This makes it possible to obtain over moldings with very thin walls (3-4 mm) and a dense structure.
• The filling of the mold carries out with minimal heat loss on the path of the melt from the furnace to the mold
• The process of automatic filling of the mold cavity with plastic lasts from 1.5 to 6-8 seconds. It depends on the weight of the overmolding. This contributes to an increase in labor productivity.
• The output is overmolding with high mechanical properties, smooth surfaces, and precise geometric shapes. There is no need to machine them. 

Design and features of the overmolding

The overmolding has a retarding filter, which ensures smooth braking of the rising plastic flow, increasing the static pressure in it, and also avoiding deformation of the sand core. The filter freely passes the air displaced by the alloy from the mold cavity and does not pass the metal. The retarding filter is always made of all metal.

The braking element is a slots labyrinth formed by the outer surface of the central part and the inner surface of the detachable parts of the housing.

What is the speed of filling of overmolding?

Firstly, the mold steps, at first the melt enters slowly to the level of the collector, where it closes the electrical contact, turns on the additional pressure of the pneumatic system, and quickly fills the mold cavity. 

At the end of filling the mold cavity, the melt, closing the second contact of the electrical interlock, turns on a device that maintains the pressure in the crucible at the reached level until the overmolding is completely solidified.

How many types of molding machines?

In addition, when the overmolding completely solidifies, the so-call technological holding time switch activates, and the pressure in the crucible is automatically released. The mold is opened and the molded part is ejected.  It makes it possible to obtain thin-walled molded parts in various forms.

Currently, more than 20 varieties of thermoplastic for injection molding are known. Each type can be for certain types of products. Plastic can be used in universal or specially equipped injection molding machines, equipment, and technological methods.

Traditional injection molding

The most common type of injection molding carries out on universal injection molding machines (IMCs) and is used to manufacture parts with a wall thickness of 1.5 to 5 mm. The share of parts obtained by traditional overmolding exceeds 90% of the total number of molded thermoplastic parts.

Besides, injection molding (compression molding, press molding) involves the injection of a melt into a mold that is not completely closed. The final molding with increasing pressure in the mold occurs at the final closing of the mold. Injection molding produces simple-shaped parts of increased thickness.

Is the overmolding process at high speed?

Moreover, it has high-speed thin-walled overmolding for products with a wall thickness of 0.6 to 1.2 mm. It has its own characteristics in terms of overmolding units equipped with a hydraulic accumulator. Special equipment with a developed ventilation system, and overmolding modes that are fundamentally different from traditional overmolding.

Overmolding has thick-walled parts with a thickness of 10–30 mm. It carries out according to a technology close to traditional overmolding. It distinguishes by 2–4 ​​times longer holding time under pressure and cooling, and at the same time, the sprue diameter increases by 2–3 times.

How many producing stages of overmolding?

Furthermore, the overmolding process produces molded parts in two stages. In the first stage, the mold is partially filled with the melt during the rotation of the screw (similar to extrusion).

The final filling of the mold, holding under pressure occurs during the translational movement of the screw-piston forward without rotation. This method is used when the volume of the molded part exceeds the maximum injection volume of the injection molding machine.

How is Sandwich overmolding characterized?

Firstly, sandwich overmolding is by successive injection of two polymers through one sprue. The first polymer is creating a surface layer from a color primary polymer, and the second polymer, as if from the inside, inflates the surface layer. The inner layer can recycle or expandable polymer. The injection molding machine must equip with two injection units.

Two-component or two-color injection molding is characterized by the successive injection of different polymers using several mold cavities of different geometric shapes and volumes.

First, a plastic insert is a substrate, which moves with the mold half into another molding cavity of a larger volume. Where the insert fills with another polymer. The polymers must have good adhesion to each other. The machine equips with two injection units.

The molding machine uses high injections rates

In fact, foam molding of expandable thermoplastics carries out using high injection rates on hydraulic accumulator injection molding machines. The original thermoplastic contains a physical blowing agent (for example, isopentane) or a chemical blowing agent. There are super concentrates containing a blowing agent.

How does the molding machine work?

In addition, overmolding with gas begins with an incomplete injection of the melt. Then gas (usually nitrogen) supplies to the melt through the sprue or, directly, into the mold, bypassing the sprue. As well, gas under pressure of 50 – 120 kg/cm2 inflates the polymer shell, and presses the polymer from the inside to the inner surface of the cavity, eliminating sinks.

The structure of the overmolding has a pronounced gas cavity surrounded by a monolithic polymer. Gas injection is used for thick-walled products that do not require a monolithic polymer section.