Foam Workshop: Cutting and Bonding Inserts
A foam shop ships what its flow allows, not what its machines allow. Reorder two benches, let work in progress sit an extra two days, or lose ten points of humidity in the rainy season, and the same drawing starts producing a different part each morning. The clip follows material across the floor and the card that travels with the batch. Five sections cover station order, WIP limits, climate, scrap and rework, and shift output. Treat the figures as bands, then correct them against your own log.
01Station Order: Cut First, Bond Second, Check Last
Wrong order shows up as rework. The workable flow runs six steps one way — lay-up, cut, bond, trim, verify, bag — with no return leg. Cut parts go straight to bonding without touching the floor, and trimmed scrap never reaches the inspection bench. The common mistake is placing bonding upstream of cutting, so finished parts travel 15–20 m backwards, lose corners on the way, and get rescued at the trim station.
Give every station its own tote and colour, and keep totes inside their zone. A batch gets one card at lay-up listing material, layer count, size, quantity, operator and date, and the card rides to bagging. Its purpose is not inspection: it tells the next station what the last one did, so a fault can be traced to a lot, a shift or a machine adjustment instead of sorting the whole batch.
02WIP Under Two Days: Inventory Is Also Risk
Hold work in progress under two days. Beyond that, two things happen: dimensions follow the weather, and batches blend. A fifty-set batch normally clears cutting to bagging in one or two days. Leave it five and the early pieces have already taken up moisture while the late ones are dry, so sets from one box fit differently, some loose and some tight. A hundred sets in process occupy roughly 8–12 m² plus a matching fleet of totes.
Size the queue at the bottleneck, not at the idlest bench. If bonding manages 60 sets a day, cutting 200 is manufacturing inventory. The floor-level version of kanban is a painted box for one or two totes per station: fill it and stop upstream. It reads faster than any report. Stack totes more than three high and the bottom layer takes print-through, which thin-wall inserts never recover from.
0320–26 ℃ and 45–65% RH: EVA Moves 0.2–0.5%
EVA is closed-cell, not sealed. Take relative humidity from 45% to 65% and a sheet resting seven days can change 0.2–0.5% in dimension, which on a 300 mm insert is 0.6–1.5 mm — enough to turn a comfortable 0.5 mm side clearance into interference. Rainy seasons swell the foam; heated northern rooms dry it and shrink it. Log temperature and humidity twice a day, because the trend explains a size complaint that a single reading cannot.
The answer is not a cleanroom, it is letting material and shop reach the same condition. Rest incoming foam 24 hours on the floor before lay-up rather than feeding it off a truck or out of cold storage. Move cut parts to the next step the same day, never overnight in the open. Dehumidify above 65%, shorten storage below 40%. Keep the log readable: when a dimension drifts, the curve usually dates the cause before a blade change does.
04Scrap Recovery 55–75%, Rework 2–6%
Foam yield is a nesting question. Regular shapes reach 75–85%; multi-pocket and irregular parts often sit at 55–65%, with the rest leaving as offcuts. Those offcuts are stock, not waste: pieces over 100×100 mm become small parts, spacers and test blanks, and anything smaller is baled by material type. Recovery of 55–75% is a normal band; pushing past it means better nesting and fewer single-material small orders.
Rework lands between 2% and 6%, concentrated in three causes: out-of-tolerance size, layer misalignment, and surface print. Size usually traces to machine condition or weak hold-down, misalignment to pressing before layers are registered, print to totes stacked too high. Record the cause or the rate means nothing. Review the Pareto monthly — if those three hold seventy percent, fix them before touching anything else.
05120–400 Sets a Shift: Find the Station That Sets It
An eight-hour shift gives 6.5–7 productive hours once changeover, cleaning and paperwork come out. Simple flat inserts run 300–400 sets with three people; multi-layer, multi-pocket work needing piece-by-piece checks lands at 120–200 with two to four. Per-set time spreads from 60 to 180 seconds, and the spread tracks step count and inspection method far more than hand speed.
Find the constraint the dull way: count one hour of output at every station. The lowest number is the bottleneck; everything upstream of it is just inventory. If cutting outruns bonding, add a person there or split large laminations into pre-bonded sections. A 20% gain rarely needs new equipment — it needs waiting and carrying removed. Only past 400 sets does the constraint move to floor space and climate control.
| Shop temperature | 20–26 ℃ |
|---|---|
| Relative humidity | 45–65% RH |
| Moisture dimension change | 0.2%–0.5% |
| Output per shift | 120–400 sets |
| WIP dwell time | ≤2 days |
| Offcut recovery | 55%–75% |
| Rework rate | 2%–6% |
| Batch card | 1 per 50 sets |
FAQ
Does the shop need full climate control?
Not full. A 20–26 ℃ and 45–65% band is enough if material is conditioned with the room: rest incoming foam 24 hours, move cut parts the same day. Dehumidify above 65%, shorten storage below 40% — cheaper than a chiller plant.
How much work in progress is too much?
Anything past two days at the bottleneck. If bonding clears 60 sets a day, cutting 200 is building stock. Paint one or two tote positions per station, stop upstream when they fill, and the floor shows the answer without a report.
Is a 5% rework rate acceptable?
It sits inside the normal 2–6% band, but the mix matters more. When size, alignment and print account for seventy percent, fix those three first. A rate logged without causes cannot point anywhere.
How do you estimate shift output?
Start from 6.5–7 productive hours. Flat inserts take three people to 300–400 sets; layered work with checks takes two to four people to 120–200, at 60–180 seconds each. Count one hour per station and the constraint appears.