Injection moulding guide
This comprehensive guide helps you understand how plastic injection molding works, the different types of molds, design for manufacturing (DFM) rules, costs and lead times, as well as assembly and eco‑design solutions.
Plastic Injection Molding Guide
This design guideline for injection molding outlines the core principles of part design for injection molding and explains how to optimize plastic components for high‑volume production.
✓ Key points
- Understanding moulding tools
- Optimization Levers in Plastics Manufacturing
- Plastic Injection Molding Tolerances – ISO 20457
- Design for Manufacturing (DFM) in Injection Molding
- Plastic Part Wall Thickness
- Draft angles
- How to choose the right plastic?
- Plastic Injection Molding FAQ
L'plastic injection moulding is a polymer processing method thermoplastic"thermoplastics allowing the manufacture of plastic parts with high precision, excellent repeatability, and productivity that meets industrial requirements. Plastic injection molding constitutes a benchmark solution today for the production of medium and large runs in many sectors.
The purpose of this guide is Global and structured overview of the plastic injection process covering operating principles, technical choices, industrial constraints, economic considerations, and design best practices.
What is plastic injection moulding?
Plastic injection moulding is an industrial process that consists of molten plastic material is injected under pressure into a mold, under pressure, in order to obtain a final piece after Cooling.
This process is particularly well suited when:
- production volumes are high or recurring;
- the part geometry is complex;
- dimensional accuracy is critical;
- repeatability is a key requirement.
L'Plastic molding It relies on materials that can be reversibly melted and solidified, which distinguishes this process from other transformation techniques.
Principle of plastic injection moulding
The plastic injection moulding follows a cyclical process on industrial machines :
The main stages of injection moulding
- Plasticizing:The material is heated until molten.
- Injection:The material is injected into the tooling under high pressure.
- Cooling:The piece solidifies on contact with the mould.
- Ejection:The part is removed from the mould and the cycle restarts.
The mold is a central element of this cycle, but its detailed operation (cavities, ejection system, cooling, etc.) is covered in a dedicated guide: An injection mould is a tool used in the injection moulding process. It consists of two halves, a cavity and a core, that are precision-machined to form the desired shape of the plastic part. The mould is mounted in an injection moulding machine, which heats plastic pellets to a molten state and injects them under high pressure into the closed mould. Once the plastic has cooled and solidified, the mould opens and the finished part is ejected..
Mould Classes (SPE / SPI): A Useful Benchmark
The lifespan and quality level of an injection mould are often qualified according to the SPI (Society of the Plastics Industry) classes, sometimes called SPE classes. These classes generally range from 101 to 105.
| Class | Quality level | Typical volume |
|---|---|---|
| 101 | High-quality mould, maximum requirements | Very large series (several million cycles) |
| 102 / 103 | Standard industrial moulds | Established series production |
| 104 | Economy mould | Limited series |
| 105 | Simplified construction | Prototypes and pre-production |
This ranking allows for alignment Expected lifespan, manufacturing requirements, and budget.
Plastic injection application sectors
The Hybster Design Office operates across the entire technical plastic injection moulding cycle — from prototype to series production — in seven main sectors, each with its own specific material, tolerance, and finish constraints.
Automotive
Parts under the bonnet, engine mounts, cabin, on-board connectivity. Strict ISO 20457 tolerances, automotive material validations (PA6+GF, PBT, PC).
Electronics
Electronic card housings, component holders, precision connectors. High dimensional stability materials (PC, ABS, PC+ABS).
EV charging
High-power charging station enclosures, sockets and connectors, UV-resistant external parts. Flame-retardant materials (PC+ABS, PA66+GF).
Electric mobility
Electric scooters, electric bikes, urban micro-mobility. Lightness, shock resistance, and aesthetic finish. Reinforced ABS, PC, PP materials.
Mechatronics
Mechanical gears, sensor supports, mechanical-electronic integration. Metal-plastic overmoulding, fine mechanical tolerances.
LED lighting
Optical reflectors, diffusers, heat sinks, luminaire supports. Transparent materials (PMMA, PC) and technical (PA+GF).
Design
Technical machine parts, industrial equipment, assembly components. Controlled series production, ISO 9001 quality control.
Cycle time in plastic injection moulding: calculation and reduction levers
The cycle time is a key performance indicator in plastic injection moulding. It directly impacts productivity and the unit cost of parts.
Definition
The cycle time is the sum of several components:
T = Ti + Tc + Tm + Te
- Ti Injection times
- Tc Cooling time
- Tm mould opening / closing times
- Te ejaculation times
Industrial optimisation levers
Several actions can reduce the cycle time:
- choose materials with favourable cooling kinetics; ;
- design efficient cooling (covered in the mould article); ;
- automate opening, closing and ejection; ;
- Stabilise process settings.
This summary is based on the Hybster recommendations.
Types of moulds and injection technologies (overview)
Without going into detailed mechanical operation, it is useful to understand that different Types of moulds and injection moulding technologies existing, with a direct impact on cost, cadence and material waste.
These include in particular:
- single-impression or multi-impression moulds; ;
- moulds for hot channels or cold channels ;
- drawer slides, unscrewing or riser blocks; ;
- Sequential injection via hot spots.
Plastic injection gate: role and impact
The injection threshold is the point of entry for the molten material into the mould cavity. Its choice directly influences:
- the filling of the room; ;
- the surface appearance ;
- flow balancing ;
- sometimes the automation of deburring.
There are different types of thresholds:
| Type of threshold | Typical usage |
|---|---|
| Pin gate | Point threshold, automatic de-bottlenecking on ejection |
| Edge gate | Side threshold, simple and economical |
| Fan gate | Fan threshold, ideal for wide and narrow rooms |
| Tunnel gate | Submarine threshold, automatic degreasing with no visible trace |
Plastic injection moulding tolerances
Les thermoplastiques subissent un ramollissement et un durcissement réversibles lorsqu'ils sont chauffés et refroidis. Shrinkage on cooling. This withdrawal depends:
- of the chosen material; ;
- injection parameters ;
- of the overall thermal process.
the ISO 20457 standard structure the tolerances in plastics processing according to TG grills, taking into account:
- material ;
- Process ;
- expected quality level.
Defects on injection-moulded plastic parts: causes and solutions
The most common defects include:
- reassures ;
- sheer curtain ;
- weld lines ;
- Bubbles or surface defects.
They often appear at the interface between:
- the design of the part (thicknesses, ribs); ;
- Subject choice ;
- mould design ;
- process settings.
An approach Design for Manufacturability Combined with suitable tooling, this significantly limits these defects.
DFM for plastic injection moulding: design for manufacture
The DFM aims to simultaneously optimise:
- parts quality ;
- cycle time ;
- manufacturing cost ;
- Mould lifespan.
Key principles:
- carcasses sufficient to facilitate ejection; ;
- uniform thicknesses ;
- ribs to stiffen without adding thickness ;
- design of joint plan and of the’ejection.
These rules directly influence the overall performance of the process.
Choosing plastic for injection moulding
The commonly cited materials in plastic injection moulding include:
- PP, PE ;
- PS ;
- PC ;
- ABS.
The guide also recalls the importance of Recycling codes (1 to 7) as part of an eco-designed approach.
Plastic injection moulding material drying
Excessive damp can cause:
- bubbles ;
- stories ;
- surface defects ;
- Mechanical weakening.
The Drying material is therefore a standard in plastics processing, except for certain non-hygroscopic materials (e.g. polyolefins).
Plastics processing in France: quality, deadlines and industrialisation
Producing in France is often sought after for:
- smoother communication ;
- shorter deadlines ;
- a better controlled quality.
Hybster highlights a flexible approach, suitable for small and medium runs, with no prohibitive MOQs, and with production Fabriqué en France.
Plastic injection vs 3D printing: comparative table
Two processes complementary rather than competing. Here are 12 factual criteria for quickly identifying the right process according to your project — volume, budget, deadline, technical requirements.
| Criterion | Plastic injection | 3D Printing |
|---|---|---|
| Lead times & volumes | ||
| First piece deadline | 5 to 12 weeks (Mould fabrication included) | 1 to 3 days |
| Unit production cycle | 10 seconds to 1 minute | 2 to 48 hours per room |
| Ideal volume | 1,000 to several million parts | 1 to 500 pieces |
| Project Economy | ||
| Initial investment | €5,000 to €80,000 injection mould | No specific tooling |
| Piece cost — batch of 10,000 units | €0.50 to €5 / piece | 50 € to 500 € / piece |
| Break-even point | From 500-2,000 pieces depending on complexity | Still profitable for prototyping/units |
| Quality & precision | ||
| Dimensional tolerances | ±0.05 to ±0.3 mm (ISO 20457 grilles TG) | ±0.2 to ±0.5 mm depending on technology |
| Surface condition | Smooth, mirror possible, mould texturing | Visible layer lines, post-processing required |
| Dimensional repeatability | Excellent on millions of units | Varies depending on machine, orientation, batch |
| Mechanical properties | Complete isotropy, 100 % of the subject grade | Significant anisotropy, performance reduced by 20–50% % |
| Materials & design | ||
| Material choice | Over 1,000 grades Polyamide, Polycarbonate, Polypropylene, Acrylonitrile Butadiene Styrene, Polyether Ether Ketone, Polyamide + Glass Fibre, Polyoxymethylene, Polybutylene Terephthalate… | 20-30 dedicated 3D printing bays (PLA, ABS, PETG, resins) |
| Allowed geometries | Drafting limits (draft, undercuts via sliders) | Freeform geometries, Internal native counter-benefits |
Conclusion
L'plastic injection moulding is a structuring industrial process, combining material selection, intelligent design, process control and industrial strategy. When well mastered, it makes it possible to manufacture plastic parts Reliable, competitive and sustainable.
❓ Frequently Asked Questions
What is plastic injection moulding?
Plastic injection moulding is an industrial process that involves injecting molten plastic material into a closed mould, under high pressure, to obtain a solid part after cooling. It allows for high-volume production, at a very low unit cost, of technical or aesthetic parts with complex geometries.
From how many pieces does plastic injection become cost-effective?
There is no universal threshold: it all depends on the cost of the mould, the unit cost, the level of finish, and the volume. A frequently cited benchmark is that injection moulding becomes interesting “from a few hundred parts onwards”, but this varies depending on the part and the tooling.
What parameters affect the price of an injection mould?
In an injection moulding project, the mould represents the most expensive component. Its price depends on numerous factors: the complexity of the part, the number of cavities, the type of runners (hot or cold), specific mechanisms (sliders, unscrewing cores, lifters), and the expected lifespan. A well-designed mould ensures: consistent part quality, a reduction in rejects, dimensional stability, and high productivity over time. Conversely, an undersized or poorly adapted mould can lead to recurring defects, production stoppages, and significant maintenance costs. The choice of mould should therefore be considered as a long-term industrial investment, rather than a simple initial cost.
What is the lead time for an injection mould?
A common order of magnitude for a “standard” mould is 8 to 16 weeks, varying according to complexity, texturing, heat treatment, and assembly.
What are mould classes (SPE/SPI 101 to 105) used for?
They provide a framework for sizing tooling according to the targeted volumes/cycles (prototypes → very large series) and the associated design requirements.
Le temps de cycle en injection plastique se calcule en additionnant les temps de chaque étape du processus de moulage par injection : * **Temps de fermeture du moule :** Le temps nécessaire pour que les deux moitiés du moule se referment complètement. * **Temps d'injection :** Le temps nécessaire pour injecter le plastique fondu dans la cavité du moule. * **Temps de maintien (ou temps de plastification) :** Dans certains cas, une pression est maintenue pour compenser le retrait du plastique pendant qu'il refroidit et se solidifie. Ce temps est souvent désigné par "temps de maintien". Pendant ce temps, le plus souvent la vis tourne pour plastifier le prochain coup. * **Temps de refroidissement :** Le temps nécessaire pour que la pièce en plastique refroidisse suffisamment pour être éjectée du moule sans déformation. * **Temps d'ouverture du moule :** Le temps nécessaire pour que les deux moitiés du moule s'ouvrent complètement. * **Temps d'éjection et de retrait :** Le temps nécessaire pour éjecter la pièce du moule et la retirer. * **Temps de préparation pour le cycle suivant :** Ce temps peut inclure des opérations comme le nettoyage du moule ou l'ajustement de la pièce éjectée, et il est souvent inclus dans les autres étapes ou considéré comme un temps mort. **Formule simplifiée :** Temps de cycle = Temps de fermeture + Temps d'injection + Temps de maintien + Temps de refroidissement + Temps d'ouverture + Temps d'éjection/retrait **Exemple :** Si les temps sont les suivants : * Fermeture : 2 secondes * Injection : 3 secondes * Maintien : 8 secondes * Refroidissement : 15 secondes * Ouverture : 2 secondes * Éjection/Retrait : 3 secondes Le temps de cycle serait de 2 + 3 + 8 + 15 + 2 + 3 = **33 secondes**. Il est important de noter que certains de ces temps peuvent se chevaucher. Par exemple, le refroidissement peut commencer pendant que le maintien est encore appliqué, et l'ouverture du moule peut commencer avant que le retrait complet ne soit terminé. Un calcul précis nécessite une analyse détaillée du processus de chaque machine et de chaque moule.
It is made up of several phases: injection, cooling, mould opening/closing, ejection. A typical formula is: T = Ti + Tc + Tm + Te.
What levers allow for the reduction of cycle time and therefore the cost of an injected plastic part?
Examples of levers: part design (80% of the savings are directly linked to part design). Choice of material: directly impacts unit cost (quantity x price per kilo) and also impacts production cycle time (cooling of the part, energy required to melt the material) Mould optimisation Process efficiency (high-performance machine, optimised settings)
Plastic injection moulding involves temperatures ranging from 150°C to 300°C and pressures from 500 to 2000 bar.
Typical orders of magnitude: yielding around 200 to 250°C, and pressure that can reach 2,500 bar (depending on material/part/mould).
Les étapes d’un cycle d’injection plastique comprennent : * **Fermeture du moule :** Les deux moitiés du moule se ferment ensemble hermétiquement. * **Injection :** Le plastique fondu est injecté à haute pression dans la cavité du moule. * **Maintien :** Une pression est maintenue pour compenser le retrait du plastique lors du refroidissement. * **Refroidissement :** Le plastique refroidit et se solidifie dans la forme de la cavité du moule. * **Ouverture du moule :** Le moule s'ouvre pour libérer la pièce moulée. * **Éjection :** La pièce est éjectée du moule. Ces étapes se répètent ensuite pour chaque pièce produite.
The typical steps are: Mould closing: The mould is closed and locked (see injection moulding machine clamping force). Injection: The mould cavity is filled with molten thermoplastic at pressures and speeds set according to the material and mechanical or aesthetic requirements. Holding pressure: Pressure is maintained until the material solidifies. Cooling: The part becomes solid. Opening: The mould is opened to allow removal of the moulded part. Ejection: The part is pushed out of the mould by a mechanical or hydraulic system, and is then removed by an operator or a robot.
What is the difference between hot runner and cold runner systems?
Hot runners keep the melt molten and can reduce waste and improve quality, but with a higher initial cost. Cold runners solidify the material in the runners between cycles: cheaper to manufacture but often with more waste and lower efficiency in large-scale production.
What is sequential injection (sequential hot-plugging) and why use it?
Sequential injection injects material in stages: better flow control, reduction of certain defects (weld lines, internal stresses), better surface finish, and material/weight optimisation on complex parts.
What is the gate injection threshold?
The gate is the point where molten plastic enters the mould cavity. There are several types (pin gate, sprue gate, edge gate, fan gate, tunnel gate, cashew gate etc.), chosen according to appearance, automation of gate removal, and part geometry. Source
Why are plastic injection tolerances more complex than machining tolerances?
Shrinkage varies according to the material, direction (length/width), injection parameters (pressure/temperatures) and mould thermal conditions. The ISO 20457 standard proposes a structured approach via TG grids.
What is the difference between NFT 58000 and ISO 20457 regarding tolerances in plastics?
NFT 58000 is an older French standard; ISO 20457 is more recent and more widely used internationally, with a classification logic based on material/process/quality criteria.
Which plastic materials are mentioned as common in injection moulding?
The guide mentions PP, PE, PS, PC and ABS as materials used in injection moulding, with typical uses and different constraints depending on the requirements (mechanical, thermal, aesthetic).
What materials are often used for cosmetic parts (aesthetic)?
Examples cited: ABS, PC, PMMA, PP, PC/ABS, ASA — depending on requirements (surface, transparency, resistance, UV, etc.).
What techniques help achieve a flawless-looking workpiece?
Examples: overmolding, in-mould decoration (IMD), multi-shot, mould texturing, and various finishes (painting, polishing, etc.).
What quality checks are typical for cosmetic parts?
Checks cited: visual inspection (bubbles, scratches, colour variations), dimensional analysis (e.g. CMM), resistance tests, finish controls (paint/coating/texture uniformity).
What are the recycling codes (1 to 7) and what are they for?
The guide details codes 1 to 7 (PET, HDPE, PVC, LDPE, PP, PS, Other) to facilitate sorting and aid eco-design.
Plastic injection moulding vs 3D printing: how to choose?
Injection moulding is generally preferred for mass production, repeatability, and good robustness. 3D printing is often better suited for prototyping, small batches, and rapid iterations.
Manufacturing a mould in China or Europe: what points require attention?
Competitive costs and high capacity (China) versus high quality, proximity, easier communication, and environmental standards (Europe), with potential issues concerning logistics, communication, and intellectual property.
Why choose a plastics manufacturing plant in France for small/medium runs?
Hybster highlights French production with controlled quality, reduced lead times, fluid communication, and flexibility on volumes (small and medium runs, with no prohibitive MOQs depending on the project).
Les pièces plastiques techniques moulées par injection sont utilisées dans une multitude de secteurs, notamment : * Automobile * Médical * Électronique * Appareils ménagers * Industrie des biens de consommation * Aérospatiale * Emballage * Construction * Équipement industriel * Sports et loisirs
Injection-moulded plastic parts are all around you: Building, Automotive and transport, Electronics and electrical, Garden, DIY
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