Plastic Injection Mould Making

Do you have a plastic component to bring into production and need a mould that truly works, with no nasty surprises at the first trial? FPM has been building custom plastic injection moulds for over 40 years, handling every stage in-house: from your initial idea through to a tested mould ready to run.

Want to find out whether we are the right mould maker for your project? Tell us about your mould.

A plastic injection mould is the tool in which the molten polymer is injected, held under pressure and cooled until the finished part is obtained. From the outside it looks like a simple block of steel, but inside it several systems work together: the cavity that shapes the part, the feed system that carries the material where it is needed, the cooling that governs its solidification and the ejection that releases the component at the end of the cycle.

The mould is mounted on an injection press and stays closed while the material enters under high pressure. The fixed half is clamped to the platen on the injection side; the moving half opens and closes on every cycle. This opening and closing, repeated even thousands of times a day, is what really puts the mould to the test: it must always return the same part, with the same dimensions and the same finish, from the first to the last batch.

To understand what makes a good mould you need to look at the process it will have to withstand. In injection moulding the thermoplastic polymer granules are heated until they become fluid, then injected into the mould, where they fill the cavity, cool, solidify and are ejected. We cover the transformation process itself on the page dedicated to plastic injection moulding; here we stay on the mould.

Every component of a mould has a precise job, and the care with which it is designed and machined determines the precision, finish and life of the tool. These are the main elements.

• Cavity (female). The recess that reproduces the external shape of the part in negative. It is usually in the fixed half of the mould.
• Core (male). Shapes the inside of the part and sits in the moving half. Cavity and core, mated together, define the wall thickness.
• Fixed half and moving half. The two halves of the mould. The fixed one stays clamped to the platen on the injection side; the moving one opens to release the part. The plane where they meet is the parting line.
• Feed system. The path that carries the molten material from the press nozzle to the cavity: sprue, runners and gates. It can be a cold runner or a hot runner (hot half).
• Gate. The opening through which the material enters the impression. Its position decides how the cavity fills and where the weld lines fall.
• Cooling system (thermal regulation). The channels, usually water-fed, machined into the mould to dissipate heat evenly and control the cycle time.
• Ejection system. Ejector pins, sleeves and plates that push the part out when the mould opens.
• Vents (gas relief). Small reliefs that let the air escape during filling and prevent burn marks and short shots.
• Draft angles. The tapers given to the walls so the part releases without scoring during ejection.
• Plates and mould base. The frame that supports and aligns all the elements and withstands the pressure of the material.

Machining each of these elements to tight tolerances, and making them work together, is the true craft of the mould maker. Poorly distributed cooling or a gate in the wrong place are paid for on every part produced.

Choosing the material we build the mould from is not a technicality that concerns only us: it decides how long the tool will last, how often you will have to stop it for maintenance and how much the first supply will cost you. We calibrate it on the material to be moulded and the production volumes.

• Aluminium has excellent thermal conductivity and is quick to machine: weighing about a third of steel, it is the choice for pilot moulds and very small series, where getting to the real part fast while keeping costs down is what matters.
• Pre-hardened steels (for example 1.2738) are the most widely used compromise for medium production runs: good machinability and adequate life without the time and cost of hardening.
• Hardened steels (for example 1.2343, or stainless mould steels such as 1.2083) withstand the wear of high volumes and filled polymers, where greater resistance is needed.
• Surface treatments such as nitriding or chrome plating increase surface hardness and abrasion resistance in the most heavily stressed areas.

There is no such thing as the right mould in absolute terms: it depends on how many parts you need to produce and how complex the component is. That is why we build three families of plastic injection moulds.

Pilot and small-series moulds

Do you need to validate a project or produce a few dozen or hundred parts? We make moulds in aluminium or pre-hardened steel: they cost less and get you to the real part quickly, before you invest in a high-volume mould. It is the right path when the product is still in development and you might have to change the geometry after the first tests. In many cases these moulds then become the basis for building the definitive production tooling.

Moulds for medium production runs

For annual consumption of around 50,000-100,000 parts we build 2-4 cavity moulds, the balance point between precision, part quality and mould cost. By increasing the number of cavities you produce more parts per cycle and lower the unit cost, but complexity grows: cavity filling has to stay balanced so that all the parts come out identical.

Multi-cavity moulds for high volumes

When the numbers go up, every detail counts. We build multi-cavity moulds with careful attention to materials, heat treatments and Hasco and Meusburger standard components, with interchangeable moulding inserts. On high volumes the real cost is not the mould, but machine downtime and scrap parts: we design the most heavily stressed areas so they can be replaced quickly, without dismantling the whole tool, and we take care of cooling and ejection to keep the cycle time low.

We guide you step by step, so you always know where your mould stands. Design, here, is not a separate service: it is one of the stages of the journey, the one where the success of everything else is decided.

01 Feasibility analysis and quote

We start from your drawing and the volumes you have in mind. We assess the plastic material, the most suitable number of cavities and the type of mould, and we give you a clear quote on costs and lead times. It is the moment when we put any critical issues with the part on the table, before they become a problem.

02 Co-design and industrialisation

We work the component together with you to make it truly mouldable: uniform wall thickness, draft angles, gate positions. Small adjustments here save you defects on the finished part and costly changes to the mould later on.

03 3D mould design

We design the tool in 3D, defining cavities, injection system, cooling and ejection. Where needed we simulate the filling to see in advance how the material flows into the cavities and where the weld lines will form.

04 Construction

We build the mould in our own department with CNC machining, EDM, grinding and assembly. Keeping construction in-house means controlling quality at every step and reacting quickly if something needs adjusting.

05 Mould trial and testing

We mount the mould on the press, mould the first samples and check the dimensions on the real part. We fine-tune the parameters until the result is right and we hand over a mould that is already run in.

At the end of the journey you have a mould ready to produce and already verified, not a gamble. And if in future the part changes or the mould needs overhauling, you know who to turn to: the project stays in our hands.

Most defects in a moulded part originate in design, not in production. That is why we work in Design for Manufacturing (DFM): we optimise the plastic component with you before building the tool. The points we focus on most:

• Wall thickness. It should be kept as uniform as possible, otherwise sink marks and warping appear during cooling.
• Draft angles. Without adequate draft the part does not come out cleanly and the surface is spoiled.
• Position and number of gates. They decide how the material fills the impression and where the weld lines fall.
• Vents. They let the air out and prevent burn marks and weak points.
• Fillet radii and ribs. They stiffen the part while reducing wall thickness, and therefore cycle times.

Seeing these problems in advance saves you costly changes to an already-built mould and leads to a compliant part right from the first sampling. It is the reason why we bring the customer to the table already at the drawing stage, instead of receiving a file and building with our heads down.

These are the two questions you are asked most often, and the honest answers are two “it depends” that are worth explaining.

The cost of a mould depends on the complexity of the part, the number of cavities, the tool material and the production volumes. A single-cavity aluminium pilot mould is in a very different order of magnitude from a multi-cavity hot runner mould in hardened steel. That is why we don’t publish price lists: when we quote we assess your case and give you a figure that brings together the cost of the mould and the cost per part over time. Sometimes it pays to spend more on the mould to pay less on every component produced; other times it is the opposite.

Lead times follow the same logic: from feasibility to testing they range from a few weeks for simple moulds to longer for complex multi-cavity ones. We define the exact timeline when we quote and we stick to it, because we do construction and mould trials in-house and don’t depend on third parties.

A mould that is standing still is lost production. That is why we also handle mould maintenance and modification: cleaning, rebuilding of worn parts and modifications, even on moulds you did not build with us. When you can’t afford a prolonged stoppage, we step in within 24-48 hours.

Have a mould to get back into production? Contact us with no obligation.