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Notes on injection moulding: part design, tooling and cost

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What do UL 94 flame ratings like HB, V-0 and 5VA mean?

UL 94 ratings describe how a plastic material behaves in small-scale flame tests on moulded test bars. HB is the lowest of the common ratings: the bar burns horizontally below a set rate. V-2, V-1 and V-0 come from vertical tests and show how quickly the bar stops burning and whether flaming drips ignite cotton below. V-0 is the strictest of the three. 5VB and 5VA use a flame UL describes as about five times more severe. A rating applies to a material at a stated thickness, not to your finished product.

The rating groups

UL Solutions explains that UL 94 has 12 classifications. Six are for materials used in enclosures, structural parts and insulators: 5VA, 5VB, V-0, V-1, V-2 and HB. Three are for low-density foams: HF-1, HF-2 and HBF. Three are for thin films: VTM-0, VTM-1 and VTM-2. UL notes that VTM-0 is not equivalent to V-0, because the test methods differ.

Vertical burn criteria (V-0, V-1, V-2)

In the vertical test, as summarised by RTP Company, a 20 mm flame is applied to the bottom of a vertical bar for 10 seconds, removed, and applied again once burning stops. Dry cotton is placed 300 mm below. Two sets of five bars are tested at each thickness, one after 48 hours at 23 °C and 50 percent relative humidity, and one after 7 days at 70 °C.

CriterionV-0V-1V-2
Flaming after each flame application, per bar10 s or less30 s or less30 s or less
Total flaming, 10 applications on 5 bars50 s or less250 s or less250 s or less
Glowing after second application30 s or less60 s or less60 s or less
Burning up to the clampnot allowednot allowednot allowed
Flaming drips that ignite the cottonnot allowednot allowedallowed

HB and 5V

UL Solutions says an HB rating means the material was tested horizontally and burned slower than a specified maximum, with the rating calculated from burning rate and thickness. The 5V tests apply a larger flame up to five times. UL notes that 5V specimens may not drip flaming particles.

Why thickness matters

RTP lists typical test bar thicknesses of 0.7 mm, 1.5 mm and 3.0 mm, and each thickness is rated separately. As a hypothetical example, a grade could be rated V-0 at 1.5 mm but only V-2 or HB at a thinner section. Compare the rated thickness with the thinnest wall that matters in your part.

Checking a grade for a moulded part

  1. Find out from your product safety standard or test lab which rating the part needs, and at what thickness.
  2. Look up the exact grade in UL's listings and note the rated thickness and colours covered.
  3. Ask the resin supplier whether colour changes or masterbatch affect the rating.
  4. Agree a regrind policy. UL Solutions explains that, as defined by UL 746D, in-house regrind of the same grade can make up to 25 percent of the mix; ask how the moulder controls it.
  5. Recheck any time the grade, colour or supplier changes.

Summary of public UL Solutions and RTP Company material for education. Product certification requirements come from the relevant end-product standard and test lab.

Questions buyers ask

Does a V-0 material make my product "UL listed"?

No. Material ratings are one input; the finished product is evaluated against its own standard. RTP notes that using recognised materials can remove the need for some product tests.

Is V-0 always better than HB?

For flame behaviour, yes. But flame-retardant grades can cost more and may differ in toughness or flow, so use the rating the product needs.

Can I add flame retardant to a standard grade myself?

Not without new testing. The rating belongs to the tested formulation.

Does mould design affect it?

Wall thickness does. A part thinned to save material may fall below the rated thickness.

Sources