Pad printing on plastic and metal parts: when it makes sense and what determines the result

Pad printing solves small but demanding print areas

A plastic housing may need operating symbols; a metal component may need a logo, scale or identification mark. The print area may not be perfectly flat, may sit close to an edge or may be slightly curved. In situations like these, pad printing is often a highly effective solution.

It is not, however, a universal process for every product. The result depends on the combination of part geometry, material and surface finish, motif size, number of colours, required durability and production quantity. The more accurately these parameters are known in advance, the more reliably the ink, pad, fixture and inspection method can be specified.

How the motif reaches a three-dimensional part

Pad printing is an indirect gravure printing process. The motif is not transferred directly from the printing plate to the product, but by means of a flexible silicone pad.

1. The motif is prepared in a printing plate, often called a cliché. The printing areas form a finely recessed image.
2. Ink fills the recessed motif and the excess is removed from the plate surface.
3. During the cycle, part of the solvent evaporates from the very thin ink layer. This gives the ink the properties required for the silicone pad to pick it up from the plate.
4. The pad is pressed onto the product, conforms to its shape and transfers the motif.

The silicone pad makes it possible to print surfaces that would be difficult to reach with a direct process. Its flexibility does not mean that any deep recess or the full circumference of a cylindrical product can be printed without limitations. Pad shape, size and hardness are selected for the specific part; an unsuitable combination can distort the motif, cause incomplete transfer or produce an uneven ink layer.

When pad printing is the right choice

The process is typically suitable for:

• logos, symbols, scales, labels and other precise motifs on technical parts;
• plastic housings, controls, buttons, tools and appliance components;
• smaller print areas on flat, curved, convex or mildly structured surfaces;
• one or several spot colours where opacity and a consistent shade matter;
• repeat production runs of identical parts where stable positioning and appearance are important.

A separate printing plate is prepared for each motif and usually for each colour. Multi-colour printing therefore depends on accurate registration and repeatable part loading. During printing, the product is held in a fixture that restricts movement and helps maintain the position of the motif throughout the run.

Pad printing may not be the best choice for a large solid area, a photographic design with frequently changing data, personalisation of every item or printing around the full circumference of a container. Screen printing, a digital process, rotary printing or laser marking may then be more suitable. Our broader range of industrial printing services allows the process to be selected according to the product and its use, rather than according to a single available technology.

Knowing only that a part is “plastic” is not enough

Two plastic parts that look alike can behave very differently during printing. The exact polymer, additives, texture, coating, residues of mould-release agents and other surface treatments all affect the result.

Materials with lower surface energy, typically certain grades of polyethylene and polypropylene, often require pretreatment. Depending on the part, this may involve flame, corona or plasma treatment, or a suitable adhesion promoter. The purpose is to improve surface wetting and ink adhesion. A safe process cannot be specified from a photograph alone; we need the exact material designation or a physical sample.

For metals, it also matters whether the surface is bare, anodised, painted or powder-coated. An ink that adheres well to one finishing system may not provide the same adhesion on another. For an industrial order, the actual surface build-up is therefore more important than the general description “metal”.

Durability is not a property of the technology alone

The question “How long will the print last?” has no single correct answer without knowing the application. The result is affected by the part material, pretreatment, selected ink system, curing and the actual stresses placed on the product.

Important information includes:

• whether the part is touched frequently or exposed to mechanical abrasion;
• whether it is cleaned with alcohol, disinfectant, oil or another chemical;
• whether it will be exposed to higher temperatures, moisture or outdoor conditions;
• whether the print is decorative or a functional marking important to the operator;
• whether the customer has a test specification or requires sample approval.

Depending on the application, a one- or two-component ink and a suitable curing method are selected. For a new material-and-ink combination, we recommend a print trial. Adhesion, abrasion resistance and chemical resistance should then be verified by a method that reflects the product’s real operating conditions. Visual assessment of a freshly printed part alone is not sufficient for a technical production run.

Part geometry matters as much as the artwork

For a good transfer, the pad must be able to reach the print area and must not collide with surrounding ribs, edges or protrusions when compressed. Pronounced curvature can stretch the motif. A coarse texture can interrupt fine lines, while an unstable or thin-walled part may deform under pressure.

Our assessment therefore considers:

• the size and position of the print area;
• curvature, recesses, edges and nearby obstacles;
• part rigidity and a safe clamping location;
• dimensional variation between individual parts;
• the required tolerance of the motif position relative to design features.

A drawing or 3D model is useful for injection-moulded parts, but a physical product is usually the most valuable item for final verification. A trial demonstrates not only the motif transfer, but also how the part can be loaded into the fixture and what level of repeatability can be achieved.

What to send for a quick technical assessment

You can considerably speed up the initial assessment by sending the following with your enquiry:

• a photograph of the product and a close-up of the print area;
• a drawing showing the dimensions and exact position of the motif;
• the precise material and surface-finish specification;
• artwork, preferably in vector format;
• the required colours and number of colours in the motif;
• the quantity in the first run and expected repeat batches;
• a description of use, cleaning and required durability;
• the deadline and any requirements for a sample or approval.

If some details are not yet known, send at least a sample and a description of the product’s function. Tiskárna Jelínek has worked with pad printing since 1991 and also operates other printing and marking technologies. We can therefore assess not only whether the product can be printed, but also whether pad printing is genuinely the most suitable and economically sensible option for the required result.