5 critical costing considerations for plastic injection moulding

There are many considerations when it comes to plastic injection moulding parts, so it is complex to work out overall production costs.

Some costs involve a one-off investment, including design and mould creation. Some grow linearly, including assembly and shipping. And others are variable and depend on the type of plastic used and the tooling choice.

Original Equipment Manufacturers (OEMs) often receive quotes from different Electronics Manufacturing Services (EMS) that vary widely—making it complicated to choose a partner.

For the most comprehensive quote that does not include hidden costs, OEMs and EMS providers should have technical discussions from the beginning. The following five considerations help demonstrate why it is essential to ask the right questions from the start. 

1. Which steel grade should be used to manufacture the injection moulding tool?

Focusing on the type of plastic used for the project seems logical as this will dictate the durability and quality of the part. However, the steel grade used to manufacture the injection moulding tool also has an important impact on the project’s outcome—think overall cost, tool maintenance, and production volume. 

Choosing lower-grade steel, or not discussing the required steel grade for the project with your EMS partner, can prove costly in the long run. There are many problems relating to using the wrong steel, including increased waste and a reduced shot warranty (the number of times the tool is opened and closed)—both of which increase long-term production costs.

If OEMs do not specify the required steel grade, some EMS providers will supply the cheapest steel. Alternatively, they might reduce the steel grade for a high-specification product to lower the overall project price. However, any initial saving could mean future headaches and increased costs.

Discussing the project specifications and considering all the necessary variables will clarify what steel grade should be used and how many individual parts can be produced by the tool.

Here are the most common steel grades:

H 13 tool steel 

This is a robust steel used for producing high-volumes of plastics containing abrasive materials. It is versatile, resistant to cracking, and strong. However, it will corrode over time when exposed to chemically enhanced plastics and cooling water contamination. 

P 20 tool steel 

This is a general-purpose tool steel for injection moulding. It is versatile and generally does not require further heat treatment. It is most commonly used with plastics that do not contain abrasives, such as glass fibres, but it is generally not used for high-volume production.

S 7 tool steel 

This is used for producing high volumes and tight tolerances. In addition, S 7 resists softening at high temperatures and has excellent wear resistance.

420 stainless steel

This is mainly used for custom injection moulding as it provides the highest hardness of all stainless steel grades, excellent wear resistance, and maximum corrosion resistance.

Bespoke tools

For certain projects, and to reduce production costs, you may want to use tools that have high thermal conductivities. These tools are often made from speciality materials, including aluminium or tungsten carbide.

2. What is the tooling cost vs part price?

Every EMS provider attributes a different weight to the tooling cost and part price, which can be confusing and misleading. 

Some sell the tool at a low cost, which initially looks attractive, but to compensate, they increase the part price. Others offer the part price at a very low cost but compensate by inflating the cost of the tool. There are also instances when EMS providers increase the cost of the part over time, which OEMs rarely consider when pricing their product.

To get the best long-term price, the key is to calculate the total cost of the service. This means taking into consideration the tooling/NRE unit pricing and how many piece parts the tool can produce over a set period of time, also known as ‘shot warranty’. 

Solely focusing on the tool cost can lead to a false economy. It is essential to calculate how much the tool costs and how much it will be used as well as the part price to give a complete picture of the project's cost. It is often the case that an increased tool price but a reduced part price will lead to long-term savings due to waste reduction. 

Once again, effective communication between the OEM and the EMS provider is crucial. Knowing how many units will be produced from the beginning allows the right number of machines to be calculated. OEMs should also clarify if they want the product made in stages or at once. This will have an impact on costs as, if the product is manufactured in stages, there will be a floating investment as parts will be replaced over time. 

Understanding why EMS providers’ quotes vary and choosing the best solution for the project in terms of cost, quality, and timescales will ensure long-term success.

3. What information does the EMS provider need to give an accurate quote?

The principal reason quotes vary significantly is due to different interpretations of the OEM’s request. An unspecific request leads to suppliers quoting based on what materials they already have or the ones that are easiest or cheapest for them to obtain. But these are not necessarily the right materials for the project, which is often only discovered when there is a post-production product defect. 

Specifying project requirements from the beginning will allow the EMS provider to make the right choices. The OEM should provide (at least) the following information to receive the most accurate quote:

  • Projected timeframe
  • Tooling files
  • Projection of how many units per year will be required
  • Grade of plastic required for the part

Moreover, to provide an accurate quote, the EMS provider will need both a 3D CAD model and 2D files. The 2D files allow the supplier to see critical dimensions and tolerances and also any special design features or requirements. This information allows the supplier to see if there are any ‘undercut’ issues: negative angles causing the part to get stuck in the tool or more difficult to remove. This will result in a longer and more expensive process and an increased risk of damage. 

Only providing one data set means the quote is subject to change, which is not useful for an OEM trying to choose their EMS partner. The more complete information provided, the more accurate the quote will be, and the more chance the project has of being successful.

4. What is the correct resin for the part?

The cost of a part differs greatly depending on the required grade of plastic; therefore, choosing a suitable resin is fundamental. There are many different types of plastic with different flexibility and durability properties, and resistance to heat, cold, and chemicals. 

Different resins are suitable for diverse types of products; for example, some resins are suitable for electronic products, some for mechanical, and others are resistant to chemicals. 

Thermoplastics comprise the majority of manufactured polymer resins and are used in the injection moulding process. There are three main categories of thermoplastics: commodity resins, engineering resins, and speciality resins. 

  • Commodity resins are easy to process and cheap, which is why they are generally used in everyday, mass-produced items such as packaging.  
  • Engineering resins are more expensive; however, they are more resistant to chemicals and environmental exposure and stronger.
  • The high-performance resins are for high-end parts and are also expensive.

Choosing the correct resin for the end product will ensure it meets the market needs.

When OEMs are communicating their design to the EMS provider, there are five requirements they should consider, which will help determine the type of resin used in the project:

  1. Final part appearance, including surface texture and colour
  2. The part’s strength, flexibility, or rigidity
  3. Chemical or environmental resistance
  4. Any regulatory requirements 
  5. The part’s life expectancy

Technical discussions from the start will help define these requirements and establish common expectations. 

5. What is a suitable injection moulding machine to use?

Injection moulding machines have a range of capabilities and are defined by three main characteristics: type of injection moulding, injection moulding process, and size of the machine. Having an understanding of these characteristics can help choose the correct machine for the part.

1K and 2K injection moulding

The majority of moulded parts are produced using 1K injection moulding. The process produces plastic parts made from one material in one injection process with one colour. 

2K injection moulding produces two colours in one solid part. This moulding process is complex but fast, efficient, and of high quality.

1K and 2K injection moulding are used in several sectors, including the automotive industry, the medical and pharmaceutical industries, and the electrical industry.

Injection moulding processes

There are two types of injection moulding processes: vertical moulding and horizontal moulding. 

In vertical injection moulding, the two halves of the mould move vertically. Injection can be vertical or part line injection (horizontal). As open clamps and rotary tables are used in vertical injection moulding, multiple moulds can be worked on, and simultaneous operations can be carried out. This leads to higher efficiency, increased productivity, and reduced costs.

In horizontal injection moulding, the mould opens and closes horizontally, which means a consistent, correct injection pressure must fill the mould cavities and help ensure proper packing and cooling. Generally, horizontal moulds are made with more cavities than vertical moulds, meaning they can produce more parts per cycle.

Injection moulding machine tonnage 

Although often overlooked, injection moulders must be set to the correct tonnage or the parts produced may be faulty. Tonnage refers to the amount of force needed to keep the tool closed during the injection process. 

Knowing what force is required to select a suitable injection moulding machine is essential. A small machine could be 25 tons, and the largest could be around 4,000 tons. An average machine will be approximately 460 tonnes.

Conclusion 

OEMs searching for an EMS partner are understandably concerned about the cost of plastic injection moulding, assuming of course this capability is available in-house in the first place. Quotes can vary widely, which can be confusing and misleading. But to create a successful product, OEMs must understand the logic behind a quote and be sure the EMS provider will meet their requirements and that any competitive pricing received will meet their unique needs.

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