The problem
Symptom. A sheet is sampled along the machine direction only, but the customer samples across the transverse direction, so the shrinkage figures do not agree and the lot is questioned.
What it costs. Re-sampling both directions and reconciling the data costs an illustrative 6 h and holds the lot while the sampling rule is written.
This article quantifies the shrinkage mismatch between directions, sets the dual-direction sampling rule, and writes it into the material specification.
When MD and TD shrinkage do not agree
In one illustrative case a roll is sampled along the machine direction only and reports 0.8 % shrinkage, but the customer samples across the transverse direction and finds 2.1 %, so the lot is questioned on a sampling gap, not a material fault.
| Sampling trigger | What it misses | First figure to check |
|---|---|---|
| MD only sampled | TD never measured | Mismatch vs 1.5 % band |
| Customer samples TD | Data disagrees | Illustrative 6 h resample |
| Lot questioned | Held for reconcile | Direction stated |
| Batch not linked | Cannot trace | Batch on record |
Table 1 - Reading an MD-TD sampling gap before re-sampling
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Quantify the shrinkage mismatch between directions
Quantify the gap before reconciling it. Five sample groups return illustrative shrinkage mismatches of 0.8, 1.4, 2.1, 1.7 and 1.0 %; the worst exceeds the 1.5 % agreed band. These are illustrative worked values from stated assumptions, not a measured series or a quoted result.
A five-step diagnostic sequence
- Cut the MD sample.
Take the machine-direction coupon and label it clearly. - Cut the TD sample.
Take the transverse-direction coupon from the same roll. - Measure both.
Record the shrinkage of each direction against the band. - Compare the mismatch.
Flag any direction that exceeds the 1.5 % agreed band. - File the data.
Record both directions and the batch so the next sample matches.
The chart below is a typical result from that measurement step. Use it to separate a design limit from a setting that can still be recovered.
Set the dual-direction sampling rule
Set the rule at dual-direction sampling with the direction stated. With both directions reported, the mismatch is caught at 2.1 % and the data is reconciled; an illustrative 6 h of re-sampling is the cost of one-direction sampling. These are illustrative worked values from stated assumptions, not a market survey, a quoted price or a measured series.
| Clause or parameter | Agreed value | What goes wrong outside it |
|---|---|---|
| Sampling directions | MD and TD both | TD never measured |
| Direction stated | Labelled per coupon | Data ambiguous |
| Agreed band | 1.5 % mismatch | Lot questioned |
| Spec owner | Named per programme | Rule applied loosely |
Table 2 - Sampling clauses for the material specification
A controlled re-sampling sequence
Run the re-sample in a fixed order: cut MD, cut TD, measure both, then compare. The table ranks the actions by data-risk so the direction is never omitted.
| Control action | What it fixes | Cost or trade-off |
|---|---|---|
| Sample both directions | Data reconciles | Extra cut per roll |
| State the direction | No ambiguity | Label on sample |
| Link the batch | Traceable | Record to keep |
| Re-check on grade | No drift after change | One owner to maintain |
Table 3 - Sampling controls ranked by data-risk
Write the sampling direction into the specification
Put the rule in the specification: sample both directions, state which is which, and record the batch. A written rule turns a direction dispute into one comparable dataset.
Want the tray checked against your line?
Send us the tray drawing and the line conditions. We will flag the geometry and material choices that cause jams, and quote a tool with an acceptance test.
What to send us
- Tray drawing with draft and radii
- Sheet grade and gauge
- Stack depth and line speed
- Drawings of the magazine or denester
Keep the sampling rule current between grades
Re-check the rule at each grade change, record the 6 h actually avoided, and re-forecast the sampling needs. A written specification reviewed on dated evidence settles the direction question at sampling rather than at the customer.
Before you sign off this thermoformed tool
- The material specification samples both machine and transverse directions
- Each coupon is labelled with its sampling direction
- The shrinkage mismatch is compared to the 1.5 % agreed band
- The batch is linked to every sampled coupon
- A named owner re-checks the rule at each grade change
A tray that looks right in the carton can still stop a filling line. Test the stack, not a single part.
Why sample both directions?
Sheet shrinks differently along and across the roll. Sampling one direction only hides the other, so the customer's transverse reading disagrees and the lot is questioned.
How much does one-direction cost?
Using illustrative assumptions re-sampling both directions costs 6 h and holds the lot. These are worked example values, not a measured series.
What band applies?
A stated 1.5 % mismatch between directions. In this illustrative case the worst group reads 2.1 %, over the band.
Can I keep one direction?
Only if the specification names it as controlling. The default requirement is both directions with the direction stated.
Who owns the rule?
A named programme owner re-checks it at grade changes. Without one owner the direction is omitted.
Does the rule expire?
It is valid until the grade changes. Re-checking at each change keeps the sampling evidence current.
Get a tray that runs on your filling line
Send us the tray drawing or a sample stack. We will review draft, trim and material, then quote a production tool with a measurable line-acceptance test written into it.
To get an accurate quote, send:
- Tray drawing or sample stack
- Sheet material and gauge
- Line speed and denesting method
- Annual volume and target unit price
Request a tool and tray quotation
Why buyers work with Uplastech
ISO 9001, ISO 14001 and ISO 45001 certified. Founded in 2016, with injection moulding lines in Shanghai and an extrusion plant in Jiangsu. Moulding machines from 90T to 1100T and a maximum shot weight of 5100 g. Samples and a written quotation are available on request.
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