Why is my HDPE melting and going gummy?

The chip load is too low, so the cutter rubs and smears the plastic instead of shearing a chip. HDPE is counter-intuitive that way - the fix is to feed more aggressively with a single-flute bit so it cuts a real chip and clears it, rather than backing off. It is a forgiving material once the chip load is up.

How do I cut foam without it tearing?

Keep forces low: a sharp bit, light depth of cut, gentle workholding that doesn't crush the foam, and modest side load. Foam collapses under pressure rather than resisting it, so the goal is to shear cleanly rather than push through.

How to cut softwood, HDPE and foam on a CNC router

The three things that matter most:

Softwood is easy and rarely burns; the enemy is fuzzing, so keep the bit sharp and the chip load high enough to shear cleanly.
HDPE is gummy and melts if the chip load is too low - use a single-flute bit and feed aggressively so it cuts rather than smears.
Foam collapses under cutter pressure, so cut light with a sharp bit and minimal side load.

Feeds & speeds calculator, pre-filled for softwood

Set your cutter diameter, flutes and RPM and read the feed rate and chip load. This is the same tool as the softwood feed & speed page, which carries the full chip-load reference table.

Material

Bit type

Bit diameter

Flutes

Spindle RPM

Cut depth (mm)

Recommended feed rate

539.4IPM13,700 mm/min

Plunge rate

178.0 IPM

4,521 mm/min

Chip load

0.3425 mm

0.0135″ / tooth

Chip load bandIn range

0.279 mm0.406 mm

Your chip load is 0.3425 mm per tooth, which is within the recommended 0.279–0.406 mm range for softwood with a 6.35 mm upcut spiral.

Table range for softwood at 6.35 mm0.279–0.406 mm

Depth factor (cut is 0.47× diameter)×1.000

Geometry factor (upcut spiral)×1.00

Feed = 20,000 × 2 × 0.3425 mm13,700 mm/min

Softwood. Softwoods clear chips easily and tolerate an aggressive feed. Burning is rare; fuzzing on the cut edge is the usual sign the feed is too slow.

Upcut spiral. The baseline the published charts assume. Chips are lifted clear of the cut, which keeps heat out of the workpiece at the cost of tear-out on the top surface.

Which bit to use

Softwood cuts with almost any sharp bit; a downcut helps the top edge. HDPE and other soft plastics want a single-flute bit for chip clearance, like acrylic but more forgiving. Foam wants a sharp bit and light engagement so it shears rather than crushes - some use single-flute or specialised foam cutters.

Feeds, speeds & depth of cut

These materials are forgiving, but each has one rule. Softwood: keep the chip thick enough to avoid fuzzing. HDPE: feed aggressively - a low chip load melts and smears it gummy, so more feed, not less, is the fix. Foam: keep forces low. Start from the calculator's softwood window below and lean toward a healthy chip in the plastics.

Workholding

Softwood and HDPE hold with clamps or tape; HDPE sheet can be slippery, so tape-and-glue or a vacuum helps. Foam is light and crushes, so hold it gently - tape or a vacuum rather than hard clamps that dent it.

Reading your results

Softwood cutting clean throws chips and leaves a crisp edge; fuzz means a dull bit or too light a chip. HDPE cutting well curls off clean strings; a smeared, melted, gummy edge means the chip load is too low - feed faster. Foam should shear to a clean face; crushed or torn cells mean too much pressure or a dull bit.

Common mistakes

Feeding HDPE too gently - a low chip load melts and smears it. Feed aggressively with a single flute.
Using a dull bit in softwood - it fuzzes instead of shearing.
Clamping foam hard or cutting it heavy - it crushes; go light.

Tooling & parts for softwood

The consumables a fix like this usually calls for. Prices are live; we may earn a commission on a purchase, which never affects what we recommend.

A sharp bit (single-flute for HDPE)

Softwood needs a sharp edge to shear instead of fuzz; HDPE wants a single flute so an aggressive feed cuts a clean string rather than smearing gummy.

Genmitsu 10pc Nano Blue Flat Nose End Mills · or browse all options

$34.19

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Frequently asked

Why is my HDPE melting and going gummy?: The chip load is too low, so the cutter rubs and smears the plastic instead of shearing a chip. HDPE is counter-intuitive that way - the fix is to feed more aggressively with a single-flute bit so it cuts a real chip and clears it, rather than backing off. It is a forgiving material once the chip load is up.
How do I cut foam without it tearing?: Keep forces low: a sharp bit, light depth of cut, gentle workholding that doesn't crush the foam, and modest side load. Foam collapses under pressure rather than resisting it, so the goal is to shear cleanly rather than push through.

Feed & speed for softwood (the tool) →How to cut acrylic →Tearout & fuzzy edges →All material guides →

Tooling and technique are drawn from manufacturer documentation and established references; starting ranges are exactly that - the exact feed and chip load for your cutter come from the calculator above, never invented here.