Injection Moulding: An Overview

Industries nowadays are looking for the freedom and flexibility to produce a wide range of parts quickly and competitively. Hence, injection moulding is one of the most used manufacturing processes today.

Take a look at your home, office, or car and you will undoubtedly find a myriad of products and parts that have been injection moulded. Let’s look more closely at what injection moulding is all about and how is it beneficial:

What is Injection Moulding and how does it work?

A semi-continuous process that involves injecting a polymer or ceramic in a molten state (or rubberized) in a mould closed under pressure and cold, through a small hole called a gate, is what happens in Injection Moulding. During this process, the materials first solidify in the mould, then begin to crystallize in semicrystalline polymers.

An injection plunger or piston rapidly moves back and forth to push the plastic softened by heat through the space between the walls of the cylinder and a reheated piece located in the centre of the cylinder. This central piece is used so that the heating surface of the cylinder is large and the thickness of the heated plastic layer is small. Under the combined action of heat and pressure exerted by the injection piston, the polymer is fluid enough to reach the cold mould. After a short time inside the closed mould, the plastic solidifies, the mould opens and the piece is removed.

The moulding principle

Injection moulding is one of the most famous plastic processing technologies, as it represents a relatively simple way of manufacturing components with highly complex geometric shapes. For this, you need an injection machine that includes a mould. In the latter, a cavity is made whose shape is identical to that of the piece that you want to obtain and for its size a contraction factor is applied which is added in the measurements of the cavity so that when the moulded piece is cooled, it is achieved the desired dimensions. The cavity is filled with molten plastic, which solidifies, maintaining the moulded shape.

The polymers retain their three-dimensional shape when cooled below their Tg (Glass transition temperature)- and, therefore, also their melting temperature for semicrystalline polymers. Amorphous polymers, whose useful temperature is lower than their Tg, are in a thermodynamic state of pseudo-equilibrium. In that state, the rotation and relaxation (disentanglement of the chains) of the polymer are highly impeded. It is for this reason that, in the absence of efforts, the three-dimensional shape is retained. The semi-crystalline polymers also have the characteristic of forming crystals. These crystals provide dimensional stability to the molecule, which is also -in the crystalline region- thermodynamically stable. The entropy of plastic molecules decreases drastically due to the order of the molecules in the crystals.

Historical background

The current design of the injection moulding machine has been influenced by the demand for products with different geometrical characteristics, with different involved polymers and colours. Also, its design has been modified so that the moulded parts have a lower production cost, which requires fast injection, low temperatures, and a short and precise moulding cycle.

The first mass-produced article in England was the fountain pen, produced during the 1930s by the Mentmore Manufacturing company. It used injection moulding machines from Eckert & Ziegler (Germany). These machines originally operated with compressed air (approximately 31 kg / cm2); the system of opening of mould and the extraction of the piece were made manually, and the controls included manual valves, without automatic control neither digital screens; Also, they lacked security systems.

At the end of the Second World War, the plastic injection industry experienced sustained economic growth. However, since the 1980s, improvements have focused on design efficiency, polymer flow, the use of CAD Software systems, the inclusion of faster robots for parts extraction, computer-assisted injection, efficiency in heating control and improvements in the control of product quality.

Creation and operation of injection moulding machines

The construction and operation of injection moulding machines or injection machines as they are known are also relatively simple. However, the benefits and results are superior to their complexity.

The injection moulding machines are composed of six necessary parts. The main components of the machine are the hopper, where the raw materials that are introduced in a barrel are concentrated, for the transport of the material to the heating unit, where it is melted to turn it into a fluid that, using a nozzle is pumped into the mould. Then it goes to an adjustment unit that solidifies it and using an ejector that ejects the finished product.

Then the liquid resin is poured into the hopper of the injection moulding machine to produce a moulded product. This is when dyes or inks are also discharged. As a result of gravity, the resin falls in a compartment, where the heating process fuses it until it reaches the liquid state.

An injection mechanism, by generating a screw or an alternative movement hammer gun pushes the liquid into the mould. The hammer gun is used when at least 20% of the quantity of the liquid must be transported to a mould, which indicates that, with the contents of the hopper, only five units would be produced.

Once the mould determines the shape of the finished product, and it cools, so that it becomes solid, an ejector separates it from the mould and ejects it, to have a finished product.

Performance problems of injection moulding machines and their solutions

Injection moulding machines have some problems with performance, but these are usually easy to solve. Sometimes an excess of raw material is observed, burned, but this can be prevented by reducing the temperature of the hopper or the processing time.

Some deformation problems are typically solved by adjusting the temperature on the mould surface or adjusting the mould. Check of the humidity levels, pressure, or temperature can correct certain imperfections in the surface of the finished product.

Control of pressure, temperature and time in detail

The quality of the products obtained in the injection moulding includes properties mechanical, surface quality, dimensions and density. To obtain a quality acceptable and reproducible, it is essential to keep the moulding process under precise control and so modern machines are controlled by means of microprocessors. The inputs to the control system are:

  • Temperatures in the jacket, nozzle and mould measured by thermocouples.
  • The pressure of the hydraulic fluid acting on the plunger arm.
  • Polymer pressure in the mould.
  • Position and speed of the plunger arm through a potentiometer type sensor

During the production process, the parameters determined optimum must be repeated from cycle to cycle in the most precise way possible.

Aspects of product quality

#1 Fundamentals of material response

The injection moulding process is about studying the behavior of the material during its treatment and its effects. It is about the quality of the product.

The main process control parameters are:

  • The temperature of the molten material
  • The temperature of the mould
  • The injection pressure and the holding pressure
  • The injection speed
  • The distribution of time for the various parts of the process cycle

Some difficulties can be avoided by good design of the product and the mould in the first place.

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#2 Fundamentals of design

Among the quality problems that can often be reduced by design optimization the following can be listed:

1) Welding lines: These are formed where the polymer flows are located, but with precautions in the design phase, they can be avoided or at least reduced to a certain level.

Once the design reduces the problem, process control can be applied to decrease the effect.

2) Sink and hollow marks: These moulding defects occur when the section of the product is too thick. The thick part retains heat and sinks by the forces of contraction, especially during the crystallization. If the outer layer hardens and, then, resists further subsidence, internal gaps are formed when the tensile strength of the molten material is driven through the solidification process.

This is mainly a design problem. When thick sections are required in a moulded part, for rigidity, it is better to use a modified procedure such as moulding in foam core, which avoids the problem of both sinking and gap formation. As an alternative, a thin-section rib pattern can be used.

3) The concentration of stress in corners that lead to product failure in service: The consequence of stress concentrations in parts moulded with sharp edges can result in fracture, especially if the product is made to support the load. Sometimes it is the distortion present when fibre reinforced polymers are used. This issue can also be taken care of with some intricate precision during the design process.

Advantages and disadvantages

The injection moulding presents a series of advantages and disadvantages that make it ideal concerning the other moulding processes for specific applications. Some of them are:

Advantages in Injection Moulding
  • High production levels and low costs
  • Moulding of pieces with very complicated geometries.
  • High or low automation, this depends on the value of the piece.
  • Once finished pieces require little finishing, they are finished with the desired roughness, colour and transparency.
  • Material handling is reduced because of the press hopper will usually contain enough material to mould pieces for an extended period.
  • The larger and smaller diameter core pins can be used because they can be held at both ends.
  • After the mould has been closed before any material is injected into it, parts containing metal inserts can be moulded without burrs of material in the inserts.
  • The relatively tighter tolerances through the separation lines are possible.
  • The flash in the separation line can be maintained at a minimum thickness if the mould is designed correctly and well maintained.
  • The injection moulding of thermoset materials is apt to automate the process, which can result in low prices per piece.
Disadvantages of injection moulding

Initial costs of production are generally very high due to the complexity in design, testing and other tool requirements. If you are planning to undergo large production volumes, you must ensure that the design is correct the first time. Matching the right design concepts includes:

  • Design and subsequent prototyping of the piece according to specification
  • The initial development of the prototype is usually done on a 3D printer and often on different material (such as ABS plastic) from which the final part will be manufactured.
  • Design of an injection moulding tool for the first round of production
  • Normally, the generation of 300-1000 prototypes moulded by injection in the production material requires the development of an injection moulding tool.
  • Refine every one of the details in the tool of the injection mould before the production in series in an injection mould manufacturing plant.

Cost elements of injection moulding

Injection moulding requires relatively expensive tooling. For this reason, it is usually applied in batches of production of large quantities of units. The production speed can be high, especially for small parts. Usually, multi-cavity moulds are used. It is also feasible to use single-cavity moulds made of less expensive materials for the production of prototypes in limited series.

The main elements of costs in the injection are those related to the materials, the amortization of the fuel transfer, the work cycle (from the feeding of the raw material to the expulsion of the finished piece) and the manufacture and thermal and superficial treatments of the metal moulds required.

Fixed Costs

The size of the injector is defined by the closing force, which in turn is related to the size of the pieces to be moulded and the number of cavities in the mould.

Mould Costs

The factors involved are the cost of the block of material and the costs of manufacturing the cavities. This cost can be approximated by a simplification of the methodology proposed by Geoffrey Boothroyd, Peter Dewhurst, Winston Anthony Knight in his book ‘Product Design for Manufacture and Assembly.’

The base cost corresponds to the material that will be machined to make the moulding matrix. This cost depends on the planned cross-sectional area and the required cavity depth, according to the design of the parts to be moulded. A guiding criterion is to take the outline or plan of the cross-section of the piece to be manufactured (or of the set of pieces to be made at each injection stroke if a multi-cavity mould is foreseen) and draw a square around this outline, leaving lesser than 50 mm between each piece (in the case of multi-cavity) and 50 mm between the perimeter of the cross-section and the edge of the external square. This margin of 50 mm will be used to design the attack channels through which the molten polymer will enter and, eventually, the forced cooling circuit of the mould if you want to reduce the working time for each injection stroke.

Variable Costs

The variable costs per unit produced are calculated as the sum of the raw material plus the operating time of the machine, which, as already mentioned, depends on the cooling time of the piece in the mould, which in turn is a function of the maximum thickness wall of the manufactured piece.

About raw material costs, the volume of polymer injected at each stroke is calculated (either to make a large piece or several small pieces in a multi-cavity mould), multiplied by its density and adding 3% extra material to take into account the waste (burrs and attack channels, fundamentally).

How to reduce the costs of injection moulding?

One of the simplest ways to reduce the unit cost of injection moulding is to increase the number of parts manufactured. This is because the initial cost of the design and machining of the mould are amortized according to the number of pieces.

However, it is possible that your moulding project requires only a few pieces. In such cases, here are some suggestions for the design intended for injection moulding that will allow you to adjust your budget:

1. Eliminate undercuts

Get rid of undercuts whenever you can, although in some cases it will not be possible. This happens, for example, when a lateral action or a removable insert is needed. An alternative could be the use of step cores, or the modification of the opening line and the demolding angles to facilitate the construction of the mould.

2. Get rid of unnecessary forms

The textured surfaces, the numbers of moulded parts and the company logos are very good, but you have to be willing to assume the extra cost in ways that are not necessary. However, many military and aerospace applications require permanent part numbers. Use an easy-to-mill font, such as Century Gothic Bold, Arial, or Verdana (unfinished fonts), always greater than 20 points and with a maximum depth between 0.25 mm to 0.38 mm. Also, prepare to increase the demoulding angle if the ejection of the piece is a problem.

3. Use a core-cavity method

If you need an electronic housing or a similar box-shaped part, you can drill the cavities of the walls in the mould, which requires long and narrow tools to machine grooves in the base of the mould, or machining the aluminium material downwards surrounding the core and mould the piece around it. This method, known as “core-cavity,” is much more affordable for moulding high walls and grooved surfaces. Moreover, what is more important, it facilitates the creation of uniform surface finishes, adequate ventilation, better ejection and can eliminate the need for very pronounced draft angles.

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4. Reduce the finishes and aesthetic aspects

The nice pieces are nice, but they usually need sandblasting techniques, or careful polishing of the mould to achieve an aesthetic appearance of great quality. This adds to mould development costs. Any finish higher than a PM-F0 (rough machining) requires a certain degree of manual work, up to an SPI-A2 gloss finish using a # 2 diamond polish. Avoid finishes as delicate as this one unless they are necessary.

5. Design self-assembled parts

Imagine you are designing a snap closure for medical components or two halves geared for a portable radio. Why build two pieces that fit when you can do just one? You can redesign the closures, so that the halves fit in any direction, and manufacture in this way, what is called “universal” piece. Only one mould is needed, so you save on the initial production costs. And you can mould the double of a single piece instead of half of two different pieces.

6. Pay attention to the design analysis for manufacturing (DFM)

DFM analysis identifies possible problem areas, as well as possibilities to improve the design. Insufficient angles, non-machinable shapes, impossible geometries, and more. These are just some examples of what can and should be done to improve the design of the piece.

7. Use a multi-cavity mould or a family mould

Are you interested in a larger volume of pieces? You can achieve a higher volume using aluminium moulds with two, four or eight mould cavities, depending on the geometry and size of the piece, which can reduce the unit price of the pieces, although they will increase the cost of mould development.

Do you have a family of pieces that fit together? Why not carry out multiple moulding projects at the same time? There is no reason to build a mould for each piece, provided that:

a) They are all made with the same plastic.

b) All have approximately the same size (for example, they have similar processing times).

c) Can be introduced in the same cavity without preventing the proper functioning of the mould.

Also, maybe you can join some of these pieces with a flexible hinge. This method is ideal, for example, for moulding the two halves of a shell-type container. If not, these parts would need a pin-type assembly to open and close. The only thing that must be taken into account is the use of flexible and robust material, such as polypropylene (PP).

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