Recycled Plastic for Injection Moulding: Grades, MFI and Trials

Injection moulders are among the largest users of recycled plastic in India. Crates, buckets, household items, furniture, automotive parts and appliance components are all routinely made with some share of recycled granules or regrind. The key is matching the recycled grade to the part, the mould and the machine, and then confirming the match with a proper trial.

This guide covers which polymers are commonly used, how to match flow behaviour, how much to blend, how to run a trial, and how to fix the defects that appear most often with recycled material.

Which polymers and grades work well

• PP: the most common recycled moulding polymer. Sources include PP injection scrap, crates, buckets and raffia. Used for crates, chairs, household goods and many automotive parts.
• HDPE: used for crates, bins, caps and closures. Moulding grades flow more easily than pipe or blow moulding grades.
• ABS and HIPS: common for appliance housings and electrical parts where stiffness and finish matter.
• Engineering plastics: PC, nylon and POM are recycled mainly from post-industrial sources, since they are harder to identify and sort in post-consumer streams.

Post-industrial scrap from a known process is usually more predictable than post-consumer material. For critical parts, start with known-source material.

Match the MFI to the part and mould

MFI tells you how easily the melt flows. Thin-walled parts and long flow paths need a higher MFI; thick, tough parts usually use a lower MFI. A recycled grade with too low an MFI will struggle to fill the mould, while one that is too high may give weaker parts and flash.

Compare the recycled grade's MFI with the virgin grade you use today, tested under the same condition: 230 C with 2.16 kg for PP and 190 C with 2.16 kg for PE. Ask for the value on each lot, and agree a target and tolerance rather than accepting a single figure.

Blending with virgin or in-house regrind

Many moulders do not run 100 percent recycled material. Blending with virgin resin or with their own clean regrind smooths out lot-to-lot variation and protects properties such as impact strength. The right share depends on the part, so begin conservatively in trials and increase only if the parts pass your checks.

• Blend materials of similar MFI so they melt and flow together evenly.
• Mix thoroughly; poor mixing shows up as streaks and patchy colour.
• Keep blend ratios recorded for each batch so you can trace problems.

How to run a useful trial

1. Get a sample large enough for a real production run, not just a few shots.
2. Record the settings: barrel temperatures, injection pressure and speed, hold time, cooling time and cycle time.
3. Run the trial on the actual mould and part you intend to produce.
4. Check the parts for dimensions, weight, appearance and the functional tests your customer requires, such as drop or load tests.
5. Keep retained samples of both granules and parts from the approved lot.

Our recycled plastic quality checklist lists the checks to run when a lot arrives.

Common defects and what they usually mean

Defect	Common cause with recycled material	What to check
Short shots	MFI lower than the mould needs, or a lot that is stiffer than the last one	Lot MFI, melt temperature, injection pressure, venting
Flow marks	Uneven melt, mixed MFI in the blend, or low mould temperature	Blend uniformity, injection speed, mould temperature
Black specks	Degraded material, burnt particles or dirt in the feed	Contamination, melt filtration at the recycler, barrel residence time
Silver streaks (splay)	Moisture in the granules, especially with hygroscopic polymers	Drying before moulding, storage conditions
Brittle parts	Polymer degraded by repeated processing, or contamination by another polymer	Impact test, polymer identification, MFI drift

Silver streaks deserve a special note. ABS, PC, nylon, PET and PMMA are hygroscopic and must be dried before moulding, whether virgin or recycled. Even PP and PE can carry surface moisture if they were washed and stored poorly. See our moisture and drying guide for practical steps.

Sourcing recycled moulding grades

When you post a requirement, include the polymer, the part you make, the MFI range you run today, colour, whether you need filled or unfilled material, and monthly quantity. PlasticSetu's sourcing desk collects quotations from verified suppliers on that basis, arranges trial samples and tracks dispatch once you place an order. Get quotes to post your requirement.