The short answer
Often, yes, if the new resin shrinks within a few tenths of a percent of the old one, runs at a mold temperature the tool can hold, and is not corrosive or much more abrasive than what the steel was built for. Check shrinkage first, then fill, mold temperature, venting, gates, ejection, drying and steel. Then measure first parts and get customer approval. A swap that fits on paper still needs a trial.
The tool was cut for one resin's shrinkage and processing window. A different resin can mold parts that look good but are out of tolerance, or that measure fine and then crack or warp in service.
1. Shrinkage and the dimensional math
A cavity is cut larger than the part by the shrinkage: cavity = part ÷ (1 − s). Some toolmakers use part × (1 + s); at normal shrink levels the difference is negligible.
Example: a 100.00 mm part in ABS at 0.5% shrink needs a cavity of 100.00 ÷ 0.995 = 100.50 mm. Run a resin that shrinks 0.8% in that cavity and the part comes out at 100.50 × 0.992 = 99.70 mm, about 0.3 mm small. A 0.3% shrink difference moves a 100 mm dimension about 0.3 mm. If the drawing allows ±0.1 mm, the part fails.
Glass-filled resins shrink differently along and across the flow, so check both. Going from PA66 GF30 (about 0.5% along, 1.1% across) to PA6 GF30 (about 0.3% along, 0.75% across) moves a 100 mm dimension about 0.2 mm along the flow and 0.35 mm across it, and the warp pattern changes too. Run your own numbers in the shrinkage calculator, using the shrink values from each grade's data sheet, not family averages.
Common swaps at a glance
| Swap | Shrink, current | Shrink, new | Main checks | Typical verdict |
|---|---|---|---|---|
| ABS → PC/ABS | 0.4–0.7% | 0.5–0.9% | Hotter melt (240–280°C / 464–536°F), drying is critical, heat resistance goes up | Often drops in with process changes. PC vs. PC/ABS |
| PC/ABS → PC | 0.5–0.9% | 0.5–0.75% | Stiffer flow in thin walls, melt 280–345°C / 536–653°F, mold 70–120°C / 158–248°F, small gates may limit packing | Usually needs process changes. PC vs. PC/ABS |
| PA66 → PA6, unfilled | ~1.5 ∥ / 1.8 ⊥% | ~0.9% | Parts come out larger; lower melt temperature; HDT drops from 70–90°C to 55–65°C | Usually needs process changes; tight parts may need steel. PA6 vs. PA66 |
| PA66 GF30 → PA6 GF30 | 0.5 ∥ / 1.1 ⊥% | 0.3 ∥ / 0.75 ⊥% | Different warp pattern; HDT drops from 240–255°C to 200–210°C | Usually needs process changes. PA6 vs. PA66 |
| PA66 GF30 → PPA GF30–33 | 0.5 ∥ / 1.1 ⊥% | Grade-specific; check the data sheet | Mold 135–165°C / 275–329°F for most grades, melt 320–345°C / 608–653°F, more gas, lower toughness at weld lines | Often needs mold changes (heating, sometimes steel). PPA vs. PA66 |
| POM homopolymer ↔ copolymer | 1.7–2.2% | 1.2–2.4% | Small shrink shift; homopolymer has a narrower melt window (about 215°C / 419°F); both give off formaldehyde if overheated | Often drops in; measure first parts. Homo vs. copolymer |
| PP homopolymer ↔ impact copolymer | 0.8–2.5% | 1.2–2.0% | Copolymer is less stiff (flex modulus 0.9–1.4 vs. 1.2–1.8 GPa), so check ejection and part stiffness | Often drops in |
| PP → HDPE | 0.8–2.5% | 1.7–2.9% | More shrink and warp, lower stiffness, poor living hinges | Usually needs process changes; tight parts may need steel. PP vs. HDPE |
| HIPS → ABS | 0.4–0.8% | 0.4–0.7% | ABS needs drying, a hotter melt and more pressure | Often drops in. ABS vs. HIPS |
| PBT GF30 → PET GF30 | 0.3 ∥ / 1.1 ⊥% (one producer) | 0.1–0.3% along flow | PET crystallizes slowly: mold 95–121°C / 203–250°F vs. 60–100°C / 140–212°F; stricter drying | Usually needs process changes, often a hotter mold. PBT vs. PET |
| GF30 → GF33, same base polymer | e.g. PA66 GF30: 0.5 ∥ / 1.1 ⊥% | Slightly lower | Stiffer, a little more abrasive; small dimensional shift | Often drops in |
| ABS → ASA | 0.4–0.7% | 0.5 ∥ / 0.9 ⊥% | Similar processing; ASA melt 240–280°C / 464–536°F | Often drops in. ABS vs. ASA |
| PC → PMMA (clear) | 0.5–0.75% | ~0.4% typical (0.2–0.7%) | Fits the tool, but impact falls from 600–900 to 15–25 J/m and HDT from 124–132°C to 85–100°C | Dimensions often close; the part may not survive. PC vs. acrylic |
Shrinkage values are typical family figures from our processing chart (∥ along flow, ⊥ across flow); individual grades vary, and part geometry, wall thickness and packing move real shrinkage. Verdicts are general starting points, not a guarantee for your tool.
2. Melt flow and fill
Compare melt flow at the same test conditions, or better, the spiral flow or viscosity data on the data sheet. A stiffer-flowing resin can short-shot thin walls or long flow lengths that filled easily before, and it needs more pressure, which can push the press toward its clamp limit. An easier-flowing resin fills fine but may flash at parting lines and slides. See processing parameters by resin for melt and mold temperature ranges.
3. Mold temperature range
Check that your temperature controller and water lines can hold the new resin's recommended mold temperature. Unpressurized water units top out a little below boiling. Resins such as PET GF (95–121°C / 203–250°F) and most PPA grades (135–165°C / 275–329°F) usually need pressurized water or oil. Running a semi-crystalline resin too cold gives parts that look fine but keep shrinking and lose properties later.
4. Venting
Vents sized for ABS or PC (often around 0.025–0.05 mm / 0.001–0.002 in deep) can flash with low-viscosity resins such as nylon, PP or acetal, which typically want 0.013–0.025 mm / 0.0005–0.001 in. Gas-heavy resins (acetal, flame-retardant grades, PVC) also leave deposits that block vents, so plan for more vent area and more frequent cleaning.
5. Gate size and gate freeze
A gate that worked for one resin can freeze too early on another, so the part cannot be packed and shows sinks, voids or short dimensions. Semi-crystalline resins (nylon, acetal, PBT) need the gate open long enough to pack until it freezes; viscous resins such as PC need larger gates than PC/ABS; glass-filled grades need generous gates to limit fiber breakage. Run a gate-freeze study with the new resin rather than reusing old hold times.
6. Ejection and draft
A higher-shrink resin grips cores harder; a lower-shrink resin can hang in the cavity. Undercuts that a soft PP or HDPE part could be bumped off will crack a stiff PC, PMMA or glass-filled part. About 1° of draft per side is a common starting point (0.5° is often treated as the minimum), and textured walls need more; a tool with minimal draft is less forgiving of a swap. See parts sticking in the mold.
7. Drying
Moving from a resin that needs no drying (HIPS, PP, PE) to one that does (ABS, PC/ABS, PC, nylon, PBT, PET) means you need a desiccant dryer sized for the throughput, for example about 80–95°C / 176–203°F for ABS and 120°C / 248°F for PC. Check the resin drying guide.
8. Steel and corrosion
PVC gives off hydrogen chloride, and many halogenated flame-retardant grades release corrosive gases, so their tools are usually stainless or plated. Glass and mineral fillers are abrasive: a prehardened tool steel (often around 28–32 HRC) wears quickly at gates and high-flow areas with glass-filled resin, where hardened steel or hardened gate inserts last much longer. Never run acetal and PVC through the same barrel; the combination can release corrosive gas.
9. Customer approval
A different resin is a material change. If the drawing names a material or grade, or the part is UL-listed, food-contact or automotive, get the customer's approval before shipping parts; automotive parts normally need a new PPAP. Flame ratings and FDA compliance attach to a specific grade, not to the family. See how to find an equivalent resin grade for matching and trial steps.
How to run the trial
- Compare data sheets for shrink (both directions), melt flow, mold temperature and drying.
- Run the numbers on your tightest dimensions with the shrink difference.
- Mold a sample, starting from the new grade's recommended settings, not the old process.
- Measure conditioned parts (nylon parts grow as they absorb moisture) against the drawing and against parts from the current resin.
- Test function: fit, snap fits, drop, heat and chemical exposure.
- Submit for approval where required, before production orders.
What to send us
- The resin and grade the tool runs now, or its data sheet
- The resin you want to switch to, and why (cost, supply, heat, appearance)
- Your tightest tolerances and the mold temperature your tool can hold
- Quantity and ship-to city
A polymer chemist reviews every request and can compare shrink and processing data before you trial. Samples of about 50 lb are available upon credit approval, every lot ships with a COA, and orders start at one gaylord. We aim to send your quote within one to three business days.
Common questions
Can I run a different plastic in my existing injection mold?
Often, if the new resin shrinks about the same, runs at a mold temperature your tool can hold, and needs no different steel. Close swaps such as ABS to ASA or HIPS to ABS usually need only process changes; swaps with a large shrinkage gap, a much hotter mold or a corrosive resin can need steel work.
How much does a shrinkage difference change part size?
Roughly the shrinkage difference times the dimension. A 0.3% difference on a 100 mm feature moves it about 0.3 mm, which is already outside a ±0.1 mm tolerance.
How is a mold cavity sized for shrinkage?
The cavity is cut larger than the part: cavity = part ÷ (1 − shrinkage). A 100.00 mm part in a resin that shrinks 0.5% needs a cavity of about 100.50 mm.
Can I run PA6 in a mold built for PA66?
Often, with process changes. PA6 runs cooler and usually shrinks less, so parts come out slightly larger, and its heat deflection is lower, so check the part's service temperature and measure first parts.
Can I run PPA in a nylon 66 mold?
Sometimes, but plan for changes. Many PPA grades need oil-heated molds around 135–165°C / 275–329°F to crystallize fully, plus hotter melt and good venting; a few grades are made to mold at water-heated temperatures.
Do I need customer approval to change resin in an existing part?
Usually yes. A different resin is a material change even if the parts measure good, so approved, UL-listed, food-contact and automotive parts normally need the customer's sign-off, and automotive parts usually a new PPAP, before production.
Is running PVC in a standard steel mold a problem?
Yes. PVC releases hydrogen chloride at processing temperature, which corrodes ordinary mold steel, so PVC tools are usually built in corrosion-resistant stainless or plated, and a standard steel tool will pit if it runs PVC for long.
Tell us the resin your tool runs now
A polymer chemist reviews every request, then matches your grade or finds a qualified equivalent and prices it.
What to send for an accurate quote
- The grade name, from any supplier, or a data sheet
- Color (natural, black or a custom match)
- Fill type and percentage, if any
- Melt flow, if you know it
- Quantity for this order and what you use in a year
- Ship-to city and state or province