Press picked from memory
- Flash on thin-walled lids or large flat covers
- Parting line witness that grows as pack pressure rises
- Tool damage when operators compensate with higher pack
Glossary
What Is Clamping Force? Injection Molding Explained
Clamping force is what keeps the parting line shut while the cavity fills and packs.
Glossary
Clamping force keeps the mold shut while plastic pushes back
Why tonnage shows up on RFQs, and what flash really means for your parting line.
During fill and pack, molten plastic exerts pressure on every surface inside the mold. Clamping force is the closing force the press applies so the two halves stay sealed.
When available clamp is too low for projected area and resin pressure, the parting line opens slightly. You see flash, dimension drift, or oil contamination on the seam instead of a clean shutoff.
NetProto sizes production on hardened steel tools for real fill conditions. Upload CAD so projected area and material family enter the conversation before someone picks a press from habit.
Production
Clamp holds the parting line during pack
Photos from the floor rarely show tonnage labels, but flash tells the story.
- Flash at the parting line often traces to clamp margin, not only tool wear.
- Large projected area parts need honest machine window discussion early.
- Cosmetic parting lines need stable shutoff through pack.
Sizing
Undersized press versus matched clamp
Tonnage is not a badge. It is a ceiling on how hard the machine can hold the mold closed.
Clamp matched to geometry and resin
- Parting line stays shut through pack on qualified programs
- Cosmetic faces on the cavity half see stable shutoff
- Steel tool designed for the press window you will actually run
RFQ notes
Tell the molder before they pick a machine
These inputs change clamp math more than most buyers expect.
Largest projected area view with parting direction marked
Resin family or known fill pressure class, even if grade is TBD
Cosmetic class on the parting line versus hidden faces
Multi cavity layout if you already assume more than one impression
Go deeper
Where this term connects
Each link is a different angle on the same program, not a duplicate definition.
Tonnage
How shops name clamp capacity on the machine nameplate.
TonnageGate
Where plastic enters and how pack reaches the last fill region.
GateDesign guide hub
Wall, draft, and gate rules that change fill pressure.
Design guide hubDFM article
Long form guide for engineers freezing CAD.
DFM articleVisual reference
Photos from molding and tooling programs
Images are selected from the NetProto Forge media library for this topic.
Injection molding press on the production floor
Injection molding press on the production floor
Close up of a molded part parting line
Close up of a molded part parting line
Plastic enclosure components after molding
Plastic enclosure components after molding
FAQ
Clamping force questions
Is clamping force the same as tonnage?
Tonnage is how North American catalogs express available clamp on a press. The idea is the same: how hard the machine can hold the mold closed.
Can I request a minimum tonnage on the PO?
You can state constraints, but the molder still must validate projected area, flow length, and pack strategy against the tool they build.
Does more tonnage improve part quality?
Not by itself. Excess tonnage does not fix poor gate location, inadequate venting, or a parting line that telegraphs on a cosmetic face.
Next step
Match geometry to steel tooling
Upload CAD for a hardened steel tool quote with production piece price on the same program.