We design
Optimised molded product design
To see how we ensure high quality injection molded part design, read on.
High quality
Injection Molded Part Design
To discuss your project call us on 02030341334
Or email us on microworkshops@gmail.com.

What exactly is injection molding?

Injection molding is a manufacturing process in which solid resin pellets are heated and then injected into a metal mold. Then, it cools to a solid state and is ejected. There are 2 elements to a injection molding machine. Firstly,  the tool and secondly, the injection machine. The tool is a block of metal with a product cored out and cut in half. The machine heats the plastic and injects it into the tool.

Furthermore, Injection molding is the most widely used plastic production process. Nearly all consumer products are made this way for 3 main reasons: Its quick, its cheap (in volume) and high quality. Despite this, there is one caveat to all its pro’s, it can have a high upfront cost.

Injection molded part design

In the process of injection molded part design, one must give careful consideration to the manufacturing in respect to the design form. Design changes late in the process are harder to make so it costs more in cad fee’s. Manufacturability should be addressed as it is being developed, not after.

3D printing can create any geometry. However, injection molding has less degrees of freedom. Some shapes can be hard to mold for various reasons. Firstly, for example, If the design has multiple undercuts, it will be more complex and thus, expensive to build a tool to mold the product. We follow all the best design practises for injection molded part design. Therefore,  you can be sure when the development is finished there will be no surprises. See below for our design practises that ensure your design is optimised for injection molding.

The cost to produce a design is: tooling + material + machine hours. A common misconception is the material cost is the bulk of the cost, NOT TRUE! Initially, the cost of the tool, which is in the thousands of pounds. Secondly, the bulk is usually the cost of the machine per hour injecting the plastic into the steel mold (and the operator). For this reason, its of our mission to make designs that can be produced as quickly as possible without sacrificing quality.

How does injection molding work?

The molding machine is made of 3 elements: the injection unit, mold and clamp. Initially, starting in the injection unit plastic pellets are fed into a heated barrel, with a large screw. Then, the plastic softens as the screw forces the plastic to the nozzle. After that, the plastic flows through the nozzle and into the mold cavity. Furthermore, the plastic forces air out of the mold, through microscopic vents ad takes its place. Finally, The shape cools back into its original solid state in the mold and is ejected with ejector pins.

The machines has 3 fundamental parameters it can control to influence the quality and cost of the final part: pressure, temprature and time. Firstly, The pressure relates to how much force is being used to fill the cavity with plastic. Secondly, temprature concerns the melting point for the chosen plastic. Lastly, Time refers to how long it takes to inject the material and wait for it to cool before ejecting.

What plastics to select?

Plastic selection isn't as easy as it seems, there's over 10,000 types of polymers. To Choose the most appropriate material first one must ask, what are the mechanic, aesthetic, environmental and cost requirements of the product?

We leverage material databases like plastics prospector https://plastics.ulprospector.com

We specify the performance requirements and prospector outputs a list of plastics that can accomodate. It connects us to the plastic supplier and its datasheet which outlines all mechanical properties and relevant information that the molder needs to know for processing. There is some room for retrospective experimentation, mold trial allow us to produce small batches of your product in whatever material we target before final production.

Joining Plastics together

Plastics offer a variety of ways to connect multiple parts together. These include Snap fits, self tapping screws, glue, threaded inserts, ultrasonic welding. Each method should be chosed with consideration to the application. If it needs to reopenale then snapfits or screws would be appropriate. If it needed to be watertight then ultrasonic welding would be your choice.

Some methods have inherent cost saving due to less or no assembly labour. Screws you'll find are ubiqitous to most products as there robust, reopenable and in most cases easy to intergrate into the design. They do howevever create labour on the assembly line which would not be the case with snap fits.

What does an Injection molded part designer do exactly?

Injection mold part designers bridge the gap between customer/product & manufacturing. It is a part designers job to see the product from both a design and manufacturing perspective simultanouesly. They must balance a 3 way scale of beauty, function and manufacturability. Although the CAD designer doesn’t make the mold tool, he must have a proficient understanding of how mold tools are made and there limitations. The designer optermises the design to reduce cycle times for cheaper unit prices.

Its so important to have tooling knowledge present from the start because some forms are easy to mold and some are not. Its there job to save money but spotting these things in the moment. This helps mitigate creating objects that are simply not economic to mold. Imagine you’ve just spent a week developing a functional element of your design and the molders says it will add £4000 to the tooling cost. You wouldn’t be the first person to hear that either. Believe me this isnt fear selling, its just trueth.

Injection Molded Part Design Displines

To ensure a design is moldable and optermised we must be aware of engineering displines that govern the quality and cost of the part. Below is a checklist that applies to every molded part with information about why its important.

engineering-practises-for-injection-molding-illustration-of-2-boxes-with-uniform wall thickness
Uniform Thickness

Essential for the cooling process. Helps the part cool evenly throughout, mitigating the chance of cracking, warping and shrinkage. thick and thin sections cool at different rates which is the most common source of part distortion.

Draft Angles

Neccesary for the plastic part to leave the mold without being damaged. It can be optimised to make the mold cycle faster. Generally the higher the draft angle, the quicker and easier the mold and part are to seperate.


Features that prevent a plastic part from exiting a mold. Holes are added below to allow the plastic and mold seperate.. Parts should be optimised where possible for staight-pull molds to avoid heavy increase in tooling cost.

Corner Radius

Helps spread out stress on corners, thus making the part stronger. In addition to this, helps in terms of mold flow, because sharp corners create air pockets when the mold is shot  thus weakening the overall structure.


Happens in the cooling process, mostly in the mold, to denser parts that cool slower than the rest of the object. Can be mitigated by coring the part. Also, lowering the cooling time through the use of cooling rods in the mold.


Refers to the level of accuracy that something can be made. In injection molding, calculating shrinkage of your chosen polymer is paramount as the figure can range from 0.1 – 20%. This can be calculated with great accuracy.


The process of removing all unneccesary material from the molded part, whilst keeping true to the rules of uniform wall thickness and keeping any mechanical features as strong and secure as they can be.


Used to provide extra strength to a part. They efficiently compensate for plastic parts not being able to be one solid block. For larger flat surfaces they increase stiffness and reduce part disortion siginificantly. 


Used for fastenings such as thread cutting screws. The placement and form of bosses should be carefully considered and try to conform to as many of the injection mold rules as possible.

Living Hinges

Exclusive to injection molded parts, are the only moving mechanism that does not invlolve multiple moving parts. If done correctly can last millions of cycles, without the need for extra costly off the shelf or hinge mechanism.

Snap Fits

A solution to the aesthetics of a product being compromised by exposed screws and fastenings. Much consideration must be taken when designing a snap fit mechanism, therefore increasing the time/cost of the design process.

Knit Lines

Where moldflow that has been split or indeed two seperate flows meet in a mold. The lines that appear from this affect are refered to as knit or weld lines and are weak points. Commonly found around circular shapes in molds.

How much does injection molded Part Design cost?

The most common question is, how much does it cost? Unfortunatly, anything custom made always varies in price, so there is no one single answer.

Cost is also split into 3 elements: the tooling cost, injection machine cost and raw material. Firstly, The tooling cost is the initial/start-up cost to cut core cavity steel blocks (as seen below), which varies in price. However the price to inject the mold and raw materials are standardised costs.

Injection Molding Cost Breakdown

So the theoretical cost of molding a design is tooling + material + injection machine hours. These 3 elements are made up of many parameters but lets keep things simple for now. 

Here’s a simple example of a part we designed and tooled recently. The tool cost $700 and 1 unit costs $0.07 in machining and material cost. Its a tiling spacer made from ABS. 2.7cm in diameter, 1cm tall and weighs 4 gram (including the runners). its a multicavity mold so 4 spacers can be made at once. with a minimum order of 10,000

The Breakdown

So lets break this down to see how my supplier got to this figure. ABS pellets cost approximately $5/Kg so regarding material cost, 250 spacers can be made for $5 (5/1000 = 0.02). So 1 spacer costs just 2 cent in raw material. Wow super cheap! 

The remaining $0.05 comes from the machine hours. The machine costs $40/hour. After running a mold simulation we found that 4 units could be made with a total cycle time of 15 seconds. Therefore, approximately 12 hours to make 10,000 units. 12 hours x $40 = $480. So $480/10,000 units = approximately $0.05/unit. In addition if you factor in the tooling cost. In this example would be $700/10000 units = $0.07 per unit. Voila, the total unit cost comes to $0.12 per unit. The tooling cost accounted for 50%, the machine hours accounts for 36% and the raw material accounts for 14%.

Moreover, the tooling cost is factored into the unit cost by diving it by the number of units. Therefore the more units you make, the less siginificant the tooling cost becomes.