A good-looking prototype surface does not guarantee the finish of the injection-moulded part. Gloss, texture, flow marks and grain replication depend on the mould and the real process.
Key idea: a useful prototype does not simply prove that the part exists; it closes uncertainty that would be expensive in production tooling.
Ra does not explain everything
Ra roughness describes part of the surface profile, but it does not capture gloss, texture direction, material skin, flow lines or visual perception by itself.
There is process between mould and part
The part does not copy steel perfectly. Mould temperature, filling speed, pressure, material and venting affect how texture is replicated.
SPI, VDI and internal criteria
SPI or VDI references help specify finishes, but the project must translate them into measurable and acceptable criteria for the real application.
What the injection-moulded prototype validates
It compares real finish, texture, gloss, flow marks and visible defects with representative material and process before committing the production mould finish.
Frequently asked questions about surface finish
Is mould Ra the same as part Ra?
Not necessarily. The part replicates the cavity depending on material, temperature, speed, pressure and venting, so Ra must be verified on injected parts.
Why can gloss be misleading?
Gloss, texture and roughness are different. A part can look visually correct and still miss the specified roughness or texture criterion.
When should surface finish be measured?
In the prototype batch injected with representative material and process, before committing final production mould finish.