How to Calculate and Save Costs of Injection Molding
(Includes Cavity and Molding Cost Calculator .xlsx)

This paper is for those who really want to understand why injection molding might be so expensive and how injection molding companies deduct their service pricing. The purpose of this piece is to take out the anxiety burden of sales engineers to tell the project cost when unprofessional RFQs are received, as well as an opposite side’s inconvenience to tell or not to tell the budget of the project.

Table of Contents:

What you will learn (an infographic overview):

Infographic

Starting with the basics

It is assumed that the reader is fully familiar with the common injection molding definitions and procedures. If not hover on these topics:

Basics

What is injection molding?

Injection molding is a serial production technology where molten thermoplastic pallets are injected in the molds. Cooled plastic solidifies and when the mold opens it is ejected as a brand new plastic part. Read more here.

What is mold and tooling?

Tooling is the process of mold machining. Mold is a 3D designed part ‘subtracted’ or carved out (milled) out of the block of metal (usually steel or aluminum). It is obvious that most of engineering know-how is condensed in this stage. Mold making is the core of injection molding technology and thus it has the biggest fraction of NRE cost.

What is cavity and core?

Since mold has two sides core and cavity take place in one or another. They are used to shape the injected plastic. In other words, the core forms the internal shape of the part and the cavity - external. It is worth to mention that by default cavity numbers can be distributed only in such a way: 1, 2, 4, 8, 16, 32, 48… This is because only than equal runner distances are possible for equal pressure distribution.

Disclaimer

This paper is used for understanding of the principle of injection molding pricing calculation. The examples given and calculations are simplified and may lack many factors which can change the results drastically as each project might have very unique parameters to include. Nevertheless, the paper is more than enough to familiarize customers with injection molding pricing method and is prepared for customization according individual needs.

Heading to the optimum

Just by a single look to the above infographic it becomes clear that estimating injection molding costs includes many variables. How not to get lost in this pile of varying factors and find the optimal solution for the buyer and seller – set an equilibrium cost/price correctly for both parties involved? As every mathematical problem or calculation should start with some reference point, this case is no other and the process starts with input variables setting.

Project and machine constraints

Disclaimer

Heading to the optimum

Project and machie constraints

In this particular case it is assumed that the client needs are the starting point. The customer provides the RFQ where the huge chain of processes starts to move and impact each other. Ideally, reputable customer should provide his sub-contractors with this information:

Production quantity interval
Lead time interval
Budget interval
Part CAD drawings
Material used specified
Tolerances specified
Surface finish specified

Budget. Really? (off-topic)

It seems that declaring budget interval might be the same as loosing bargaining leverage. However, experienced customers know that with complex projects, quotation procedure can be considerably prolonged just by molding company trying to provide budget A, B and C variants. For example, the budget can drastically decrease by eliminating automated inserts at the expense of a lead time or just by increasing molding cavities number (discussed further). So how the molding company should know what are the contractor’s preferences and future plans?

Moreover, experienced customers always know the ranges of pricing and can always head towards budgets with buffers for bargaining. The point here is that, ideally, both client and service provider should seek for professional and transparent negotiations where quote is just a good starting point. Transparency in pricing should guarantee that project shareholders would seek for most efficient, win-win manufacturing processes and goal completions but not capital-only targets.

Three main input values (back to-topic)

Project constraints (information from the RFQ) let us deduce 3 main input variables:

Production quantity interval (e.g. 20k-50k pcs.)
Lead time interval (e.g. 1-2 months)
Project cost interval (e.g. 1000-5000 EUR)

It becomes intuitively clear that this initial input data should initiate the whole injection molding cost/price calculation – the molder has all information he needs: quantity, time and price range. However, to provide customer with an optimal quote, there is still one more important factor to evaluate.

Fourth main input value (molding cavity calculation)

The other RFQ’s 3 input values (part drawing, material, tolerances) are bounded by machine constraints and all these intertwined dependencies contribute to the next very important input estimation – quantity of mold cavities:

budget

Three main inputs

Cavity number calculation

1. [Part drawings + material] -> [projected area + pressure] -> clamping force -> quantity of cavities

A1 – projection area of 1 cavity and runner;

Fn – required clamping force for n cavities;

Fm – machine clamping force;

f – coefficient (deduced empirically);

Qn – cavity quantity (Qn = 1, 2, 4, 8, 16, 32, 48);

Qmax – max. cavity number limit bounded by the size of machine.

Constraints:

Fn≤Fm;

1<Qn≤Qmax;

Qmax=16;

Solution:

Fn=PA1fQn => PA1fQn<Fm => Qn<Fm/PA1f;

Fm=60t;

1<Qn≤16;

Qn<60(t)/2(t/cm2)*3(cm2)*1;

1<Qn≤10;

Answer:

Qn=1, 2, 4, 8.

2. Part drawings -> part volume -> shot size -> quantity of cavities

V – part volume + runner volume

Vmax – max. shot volume

Qn – cavity quantity (Qn = 1, 2, 4, 8, 16, 32, 48)

Qmax – max. cavity number limit bounded by the size of machine

Constraints:

QnV≤Vmax

1<Qn≤Qmax

Solution:

Qn<Vs/V

Vs=80cm3

Qn<80cm3/10cm3

1<Qn≤8

Answer:

Qn=1, 2, 4, 8.

3. Material -> plasticity -> quantity of cavities (e.g. 1-16)

Determined experimentally.

4. Tolerances -> quantity of cavities (e.g. 1-16)

Determined experimentally.

The answer is 1-8 cavities

Project constraints combined with machine constraints determine the forth value interval - molding cavities number. It should be clear that due to machine constraints the maximum molding cavity number is the minimal limit bounded by machine constraints (example above: 1-8).

Why this is so important? Molding cavities number variation hugely affect budget of the project when manufacturing volumes increase, thus varying these total 4 input values together with molding cavities results in an optimal quote for the client.

Surface finish – last but not the least

Even though surface finish does not limit or affect cavity number calculation it becomes extremely important when tooling costs are calculated (discussed further). It is very important for the client to specify this as costs of tooling can be affected by 10-30%.

Sum up

By using customer’s provided data and machine constraints four main intervals of input variables were deduced:

Production quantity interval (e.g. 20k-50k pcs.)
Lead time interval (e.g. 1-2 months)
Project cost interval (e.g. 1000-5000EUR)
Molding cavities (e.g. 1-8)

As the clients’ RFQs might not always be so informative or have even wider ranges to choose from it is the service provider’s responsibility to find and suggest the optimal quotation for the customer. To propose an optimal quote within these ranges when so many variables take place is not an easy task for injection molder.

Surface finish

Sum up

Dependent, independent and control values

NB! For the simplicity reasons, the lead time interval will be not included in the calculations. It is assumed, that every project is done as efficient as possible and at a full capacity and thus the projects will be completed as fast as manufacturing capacity can allow but no faster or longer than the customer requires.

To put it simply, optimal in this case is that where minimum value is found of a function of several variables subject to a set of constraints. Yes, it’s going to be a graphical representation. As the molding cavity parameter is the only one which can vary it will be an independent variable (x-axis). As the most important result we are looking for is a project cost – it will be a dependent variable (y-axis) and for control values the production quantity will be set.

Finally, the molding cost calculation and its relation to production volume and cavity number will be revealed. To do that, one should first understand that price of injection molding itself contains several components:

Raw material costs
Mold manufacturing (tooling) costs
Fixed machine operating costs
Other manufacturing costs (e.g. mold set up, post production…)

These molding cost categories itself are also hugely affected by the primary input values:

part drawings -> part difficulty -> tooling costs
material choice -> raw material costs
clamping force -> machine operating costs
surface finish -> tooling costs
etc.

For every complex task ‘one bite at a time’ approach is always a good way to go. For better understanding please study and use this excel calculator of injection molding cost in parallel with this article:

Raw materials cost

Raw Material Cost vs. Cavities at Various Production Volumes

Dependent, independent

IM Cost

Download

Download Injection Molding Calculator .xlsx

Injection molding cost

Raw mterial costs

Raw material for injection molding calculation

Mold making cost

At first sight graph data looks intuitively misleading. More cavities mean more material used, thus it appears that curves should slightly slope upwards. But when looking closer to the process of injection molding it becomes clear that more cavities means less shots too. This data shows that even though more cavities use more material, less cycles use lesser material, so in overall result the material used decreases when cavity number increases. Now it is a matter of runners’ geometrical parameters – whether the curves will be flattened or steepened.

Mold making cost

To understand the relation better you can download the excel calculator with the graph above (sheet2):

Raw Material Cost vs. Cavities at Various Production Volumes

Mold Cost Calculator ^

This linear relation of cavity number and mold cost seems to be intuitively clear. The production volume does not affect mold making price since molding itself does not correlate with volume up to the mold life time. Tooling, thus, is the minimal NRE cost to initiate the production at all. To estimate the molding cost various input data is needed:

Fixed time to start (setting up the machine)
Raw material cost
Hourly machine and operators rate
Machine costs per hour (depreciation or/and leasing)
CAD design
Fixed time per cavity machined (empirical estimation)
Difficulty level (empirical estimation)
Surface finish

All these factors must be included to estimate the tooling price for injection molding. To play with input values and compare various prices use the excel calculator (sheet2).

Production costs (molding costs)

To understand the relation better you can download the excel calculator with the graph above (sheet3):

Molding Cost vs. Cavities at Various Production Volumes

Production Cost

Production Cost Calculator ^

From the graph above it is possible to observe how hugely cavity number impacts manufacturing costs. To get such evaluation such input values were used:

Cycle time for 1 cavity
Prolonged cycle time per cavity (due to increase of injected material volume)
Hourly machine op. rate
Machine cost per hour (depreciation or/and leasing)
Consumables and other costs per hour

It is worth to pay attention to the cycle time here. While other input values more or less stay the same and are fixed for any project, the cycle time plays the major role in manufacturing costs. Thus it is very important to optimize the manufacturing processes as well as the parts itself to minimize the cycle time. To see how sensitive, the manufacturing costs are to the cycle time use the downloaded excel file (sheet3).

Final injection molding project costs

To understand the relation better you can download the excel calculator above (sheet4):

Project Cost vs. Cavities at Various Production Volumes

Project cost

Project Cost Calculator ^

The total project cost very much resembles with the manufacturing cost graph and raw material graph which is obviously true as the cavity number affects those 3 values the same. However, positively sloped tooling cost curve in a long run forces the whole project cost to rise. It can be seen that for larger manufacturing volumes the flexion points of the curves shift to the right – the impact of rapid cost reduction due to increasing cavity number remains longer and this is because for small manufacturing volumes tooling costs puts a larger burden.

Getting a quote - optimal injection molding price

So in this case we had initial input values to start with (neglecting a lead time):

Production quantity interval (e.g. 20k-50k pcs.)
Project cost interval (e.g. 1000-5000 EUR)
Molding cavities (e.g. 1-8)

Optimal Project Cost at Various Production Volumes

Opimal cost

By using graph data and given input data, it is seen that optimal project cost can vary in region marked in red at various production quantities. However, it is also clear that project budget is underestimated by ~1000EUR by the client and the closest proposal to the given RFQ could be:

6000 EUR for 20000 pcs. with 4-8 cavities.

The range of 4-8 cavities is used because there is no need to completely trust the data received. The project should be evaluated holistically and the very small win with a cavity number might not be worth it. Machining complex part tools or unforeseen problems while machining them might in reality become huge loss, thus some safety factor should be used.

From the data above it can be seen that little does cost rise for more cavities at small production volumes. This is a perfect illustration of the power of injection molding in a mass production and economy of scale.

Micromolding wins at small quantities

From the discussion above it becomes clear that injection molding is for large manufacturing volumes only. Prototyping with injection molding becomes literally impossible. The main barrier to enter injection molding world is high tooling costs which disappear only when hundreads of thousands units are manufactured.

What if it would be possible to save hugely on tooling and machine costs?

Micromolding is a branch of conventional molding used for mass production of plastic parts. The main difference is that it is used to mold parts which are just fractions of a gram. However, micromolding technology can become useful when producing at low volumes or prototyping, since the molds used are small and made of aluminum. This reduces tooling costs and duration by 2x for small plastic part molding. Moreover, molding machines are small, thus operating costs are low too. In this way low-volume production and prototyping with injection molding becomes possible at low budgets. If the part is small enough to squeeze 4-8 cavities in the mold, competitiveness of micromolding at mass production scale matches the conventional molders.