The truth of injection mold plastics

The truth about plastic injection moulding.

Injection molding is a wide varied manufacturing process with much o learn about and understand. However, in principle it is a simple enough process however like most thing in life there is a right way of doing it and a wrong way. I create online courses explaining and coaching individuals/business on the principles of injection molding. I also provide consulting service to individuals/business on Specifying and sourcing requirements for equipment, tooling and materials. Aswell as offering advice on part design and tool design. 

The injection molding of plastics is a very interesting subject that covers a lot of engineering and science principles while also been very business-focused. If I can be of any assistance to you please contact:

Injection Mold  

The plastics industry was revolutionized in 1946, when James Hendry built the first screw injection molding machine with an auger design, replacing Hyatt’s plunger. ( An injection moulding process is a manufacturing process for producing parts by injecting molten material into a mould, or mold . (

After an injection mold cycle (typically only seconds long) the entirety of the molten plastic will cool leaving solid plastic in the sprue, runners, gates, part cavities themselves. A wide variety of products are manufactured using injection moulding, which varies greatly in their size, complexity, and application. ( The injection molding process requires the use of an injection molding machine, raw plastic material, and a mold. (


There are many cost Drivers for Injection molding however for example the tooling or mould can be a significant cost. A small, single cavity plastic injection mold usually costs between $1,000 and $5,000. ( However to some degree, asking what plastic injection molds cost is a lot like asking, “How much is a car?” Will your mold be a Pinto or a Porsche? ( Material selection is another consideration in determining what plastic injection molds cost. (

Plastic Injection  

To learn more about plastic injection mold materials and plastic injection molding in general, check out our free eBook, “ An Introduction to Plastic Injection Molding ,” or reach out to the team today for help in choosing a plastic injection molding material. (

So research your options carefully, and consult with an experienced plastic injection molder to help determine the ideal choice. (

Plastic Material  

Thermoplastic resin pellets for injection moulding Injection moulding uses a ram or screw-type plunger to force molten plastic material into a mould cavity; this solidifies into a shape that has conformed to the contour of the mould. ( With thermoplastic materials, the material can be recycled and used again. ( The plastic is melted in the injection molding machine and then injected into the mold, where it cools and solidifies into the final part. (


Plastic resin enters the mould through a sprue or gate in the injection mould; the sprue bushing is to seal tightly against the nozzle of the injection barrel of the moulding machine and to allow molten plastic to flow from the barrel into the mould, also known as the 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 specialised gates and into the cavity where the geometry to form the desired part.The amount of resin required to fill the sprue, runner and cavities of a mould comprises a “shot”. (

Cycle time.

Gate solidification time is important, as this determines cycle time and the quality and consistency of the product, which itself is an important issue in the economics of the production process. Holding pressure is increased until the parts are free of sinks and part weight has been achieved. ( thicker the walls the more material you will use, the longer the cycle time will be and the higher your cost per part will be. ( To reduce manufacturing costs it is advisable to try and minimize cycle time as much as it is possible. Small changes in part design can make a big difference and cutting a few seconds from your cycle time can translate into big savings when you’re producing millions of parts. ( The time that is required to open the mold and eject the part can be estimated from the dry cycle time of the machine and should include time for the part to fall free of the mold. (

Each material requires a different set of processing parameters in the injection molding process, including the injection temperature, injection pressure, mold temperature, ejection temperature, and cycle time . (

Materials used in injection mold plastics.

Polyethylene is a thermoplastic polymer with a variable crystalline structure and an extremely large range of applications depending on the specific type. ( two most common types of this plastic are high-density polyethylene (HDPE) and low-density polyethylene (LDPE). ( The advantages of polyethylene are high levels of ductility, tensile strength, impact resistance, resistance to moisture absorption, and recyclability. (


This is a very common plastic that is known for its flexibility. ( PP (polypropylene) is a very dynamic plastic and has been compounded for a wide range of properties. Polypropylene (PP) Polypropylene is used to make living hinges such as in this cap. ( Polypropylene can be washed in hot water without degrading, and it has high chemical and moisture resistance. ( Polypropylene (PP) is one of the most widely produced plastics in the world. ( Although flammable and vulnerable to UV radiation, polypropylene still offers good resistance to chemical solvents and electricity. ( A diverse range of applications makes use of polypropylene, as it can be easily customized by introducing special additives or altering the manufacturing process. (

High Impact Polystyrene 

High Impact Polystyrene (HIPS) is a popular and tough plastic that is in the Polystyrene family. ( Polystyrene is brittle and can be made more impact resistant if combined with other materials (in this form it is known as HIPS). ( Polystyrene (PS) is tough enough to be used for toolboxes. ( When it comes to injection molding, there are two types of polystyrene that are commonly used. ( High Impact Polystyrene (HIPS) and General Purpose Polystyrene (GPPS). (

High-impact polystyrene (HIPS) provides good impact resistance, excellent machinability, fine dimensional stability, outstanding aesthetic qualities, and highly customizable surfaces. ( High-impact polystyrene is used in many print applications, such as promotional signs, display materials, garden stakes and tags, posters, and retail packaging, as well as 3D lenticular or moving images. ( I can help you select the right material for your product application saving you time and money, but it also ensure optimal product performance and manufacturability. (

If you have any questions or concerns on what type of plastic you should be using, contact who can help you every step of the way. 

Type of plastic materials.  

 Thermoplastics also have an element of safety over thermosets; if a thermosetting polymer is not ejected from the injection barrel in a timely manner, chemical crosslinking may occur causing the screw and check valves to seize and potentially damaging the injection moulding machine.

When thermoplastics are moulded, typically pelletised raw material is fed through a hopper into a heated barrel with a reciprocating screw. ( Thermoplastic material can be recycled beside the press and then the material is feed back into the hopper that goes into the injection molding press. 

Part Design  

Design Considerations for plastic parts: You want to design the part from day one with injection molding in mind. ( To simplify your part design and reduce tooling costs, the part design should avoid any overhanging or undercut part features. A good product designer should have a deep understanding of all phases of tooling, including part design, tool design, and material sciences. (

Molded Part  

Before you can produce an injection molded part you first have to design and prototype a part (probably via CNC or 3D printing), then you have to design and prototype a mold tool that can produce replicas of the part in volume. ( Family molded parts In addition to a single plastic part being produced in a molding cycle, the mold can also be designed to produce multiple numbers of the same part in a single shot . ( Family molded parts are often part of an assembly, such as a mating top half and bottom half. (

Overmoulded parts are basically, a previously injection molded part of one material (the substrate) is re-inserted into an injection molding machine, and a different material (the overmold) is injected to form a new layer over the first part. (

injection unit 

This process reduces its viscosity , which enables the polymer to flow with the driving force of the injection unit. (

This typically means that the residence time and temperature of the chemical precursors are minimised in the injection unit. (

These factors have led to the use of a thermally isolated, cold injection unit that injects the reacting chemicals into a thermally isolated hot mould, which increases the rate of chemical reactions and results in shorter time required to achieve a solidified thermoset component. (

Injection – The raw plastic material, usually in the form of pellets, is fed into the injection molding machine, and advanced towards the mold by the injection unit. (

injection time  

The injection time is difficult to calculate accurately due to the complex and changing flow of the molten plastic into the mold. ( Often injection times are well under 1 second, however, the injection time can be estimated by the shot volume, injection pressure, and injection speed. (

Mold Cavity 

The front half of the mold, called the mold cavity, is mounted to a stationary platen and aligns with the nozzle of the injection unit. ( The two main components of the mold are the mold core and the mold cavity. (  In order to facilitate the ejection of the part, a mold release agent can be sprayed onto the surfaces of the mold cavity prior to injection of the material. (

 Plastic Mold 

Many injection molds are made from tool steel as opposed to aluminum for several reasons, including durability and the ability to injection plastic mold complex parts. (

There are various options for starting with injection moulding. You can do it your-self “in-house” injection plastic molds – those which you own your own injection moulding machines and run your own tools in them. Another option would be to contract your tools to a moulder and for them to do the moulding for you.  


Depending on your budget and quantity of parts you need to manufacture will determine whether you should get into the in-house moulding or contract out this work, likewise for the tool building. All in all plastic injection moulding is a very lucrative business where margins of 25% and more are achievable. However, the initial capital expenditure is high on machine, equipment, tooling and materials, not to mention labour costs. If you want any advice or training on injection moulding please contact