Gauge, Modulus, and Reduction on Cast or Dipped Film
Executive summary
On film you dip, cast, or spread, fit is three knobs. Gauge is how thick the finished wall is. Modulus is how stiff the compound is at a given stretch. Reduction is how much smaller you cut the pattern than the body.
Fillers and graft modifiers raise modulus. A thicker wall of the same compound is a different move from a stiffer mix, and both are different from a smaller draft. Use this when a cast piece feels boardy, bags after wear, or a thicker wall is being treated as a tighter pattern.
Gauge: thickness of the film you formed
Gauge is thickness of the dry film, in millimetres or mils. You form that wall. How the film gets thicker or thinner is in Home-cast latex film and Coagulant dipping for wearable thickness.
A thicker wall of the same compound is harder to bend. On the body it asks for more force to reach the same stretch. That is geometry. It does not change the compound’s modulus number.
Modulus: stiffness you set in the liquid
Modulus is stress at a fixed stretch. Handbooks quote it as MPa at 300% or as psi at 500%. Higher modulus means a boardier film at the same gauge.
You set this in the compound, before you dip or pour.
Mineral clay stiffens and weakens in tension. In a handbook CR series, DIXIE clay from 0 to 50 phr dropped tensile from 3100 to 1500 psi while stress at 500% stretch rose from 500 to 1100 psi.
High-styrene SBR used as a polymeric filler raises modulus, tear, and permanent set together. At 20 phr in an NR film series, 300% modulus went from 0.90 to 2.76 MPa, tear to 1769 N/cm, and set from 6 to 30%. The same handbook calls that modulus rise a disadvantage for some latex uses.
MG graft latex (methyl methacrylate grafted onto NR) raises modulus, hardness, and tear in NR blends. Plants use it for tubing and glove puncture resistance. Moderate graft, blended with NR, is the industrial route to films thicker than about 0.5 mm without cracking.
Those figures are plant-film examples, not a home mixing card. Dispersion and loading rules sit in Pigment and filler compounding for liquid latex.
Elastomeric film is expected to stretch to at least twice its length and recover. Fit has to leave room inside that.
What is modulus?
Modulus here is how much force the film takes to stretch a given amount. It is not thickness. Two films can share a gauge and wear nothing alike if one is a high-modulus compound.
What does phr mean?
phr means parts per hundred rubber. Tables sometimes write pphr. 20 phr high-styrene SBR means 20 parts of that filler per 100 parts dry rubber.
What is permanent set?
Permanent set is leftover stretch after you let go. High set looks baggy after wear. That is less snap-back, not a thicker wall.
What is MG latex?
MG latex is natural rubber with methyl methacrylate grafted onto it. Industrial grades are talked about as MG 30 and MG 49 (graft level). Blended with ordinary NR, it is a modulus and tear tool, not a pigment.
Reduction: pattern shrink after you have film
Reduction is how much smaller the flat pattern is than the body. The garment has to stretch to close. Pattern makers also call this negative ease. It is geometry, not a phr number.
A stiffer compound makes the same reduction feel tighter, because the force to reach that stretch is higher. High set makes a snug draft slump after wear.
Decision cues
Build thickness with forming levers when the wall is too thin or too thick.
Raise or lower modulus in the compound when the film is the right thickness but the hand is wrong. Clay and high-styrene SBR stiffen. Watch set if the piece must snap back.
Change the pattern outline when you want a different negative ease.
Bagging after a few wears often traces to set or aging, not a wrong wall thickness. See Liquid latex aging, storage, and care.
Deep dives (optional)
Deep dive: High-styrene SBR polymeric filler
NR latex film series; high-styrene SBR load at 0 and 20 phr. Plant example, not a home mixing card.
| phr filler | Modulus 300% (MPa) | Tear (N/cm) | Set (%) |
|---|---|---|---|
| 0 | 0.90 | 1156 | 6 |
| 20 | 2.76 | 1769 | 30 |
Tear-first zones might tolerate the high load. A panel that must snap back should treat 20 phr as a stiffness and set warning.
Deep dive: Mineral clay direction
CR 750 compound with DIXIE clay at 0 / 25 / 50 phr (industrial cure 60 min at 127 °C): tensile 3100 / 1800 / 1500 psi; stress at 500% 500 / 600 / 1100 psi. The teaching point is the direction (stiffer, weaker in tension), not the absolute psi on a hobby-cured film.
Deep dive: MG graft latex
Methyl-methacrylate graft NR (MG 30 / MG 49) raises modulus, hardness, and tear in NR blends. Industrial uses include tubing and glove puncture resistance. Moderate graft blended with NR is the handbook route for films thicker than about 0.5 mm without cracking.