How to Choose a Resin for Injection Molding: PP, PE, PET, PC and Reinforced Grades

Most injection moulding problems that surface in production are diagnosed as tooling problems or process problems. A surprising number of them are resin problems that were decided months earlier — in a material line on a drawing that nobody revisited after the first quote. The resin sets the shrinkage you have to design around, the wall thickness you can hold, the cycle time you can achieve, and the environment the part will survive. Change it late and you change all four.

This guide sets out how to approach resin selection using published guidance from a working injection moulding manufacturer, including the specific design rules that follow from each family of material. Where a figure appears below, it is one the manufacturer publishes on its own technical pages.

Why Resin Selection Is the First Decision

The published starting point is blunt: material properties — thermal expansion, flow characteristics and shrinkage — directly determine dimensional stability, and choosing the material is the first step in controlling it. That ordering matters. Shrinkage is compensated in the tool, which means the tool steel is cut to a specific resin’s behaviour. Selecting the resin after the mould is designed is not a revision; it is a retool.

There is also a cost asymmetry. Resin is a per-part cost and tooling is a fixed cost, so the temptation is to choose on material price per kilogram. But resin determines cycle time, and cycle time determines how many parts per hour the fixed tooling cost is divided across. On high-volume programmes, a resin that runs a faster cycle can be cheaper per finished part even at a higher price per kilogram.

The third reason to decide early is that resin selection is the point at which the part, the tool and the process stop being independent. The manufacturer publishing this guidance — DAYIN Plastic Products Co., Ltd., founded in 1989, focused on home appliance mould manufacturing with OEM and ODM services — treats material choice, part design and process control as one continuous specification. That is the right mental model for the buyer too: the resin is not an input you order, it is a constraint you design to.

The Resin Families Used in Production Today

The table below collects the material properties as published for packaging and technical moulding applications. It is deliberately limited to what the manufacturer actually states, because in resin selection an unverified property is worse than no property at all.

Two cautions on reading that table. First, “good dimensional stability” is a comparative statement, not a number — if your part has a tolerance that matters, ask the supplier for the shrinkage value for the specific grade and design to it. Second, the published guidance explicitly recommends avoiding materials with high shrinkage where dimensional stability is the priority, which is the single most useful filter available at concept stage.

Dimensional Stability: Shrinkage, Reinforcement and Additives

Dimensional stability is where resin choice becomes engineering rather than procurement. The manufacturer’s published approach has three levers:

1. Base resin selection — choose materials with good dimensional stability such as PC or PP, and avoid high-shrinkage grades.
2. Reinforcement — glass-fibre-reinforced plastics increase rigidity and strength and reduce deformation during moulding, which is usually the answer when a part is dimensionally unstable but already at its maximum wall thickness.
3. Additives — reinforcements, fillers and stabilizers enhance stability; heat stabilizers in particular lower thermal deformation during high-temperature moulding.

Additives also carry the environmental specification. In automotive interior work, the published requirement set is demanding: materials must withstand high temperatures, UV radiation from the sun and frequent physical contact, and they are compounded with additives to deliver UV stability, scratch resistance or flame retardancy. If your part lives in a car, a window or a kitchen, the additive package is part of the resin specification, not an optional extra.

Designing to the Resin You Chose

Every resin has a design envelope, and the published design rules for injection moulding define that envelope in numbers. These are the manufacturer’s stated guidelines:

Those numbers are the practical reason resin cannot be swapped late. Rib thickness is expressed as a percentage of wall, radius as a multiple of wall, and wall itself sits in a 1.5–3 mm band for standard thermoplastics. Change the flow behaviour of the material and every one of those ratios has to be revisited — which is why the design fundamentals and the resin decision belong in the same meeting.

Two Applications, Two Very Different Resin Logics

The clearest way to see how resin selection changes with application is to compare packaging with automotive interiors, both of which the manufacturer publishes on.

包裝 is a throughput problem — the manufacturer’s dedicated write-up on injection moulding for packaging solutions frames it almost entirely in terms of output and unit cost. Advanced injection moulding machines are published as achieving cycle times as low as 5–15 seconds per part, and the resin has to support that. The published enablers are thin-wall packaging that reduces material usage while maintaining strength, multi-cavity moulds that produce multiple units in one cycle, and features such as tamper-evident and child-resistant closures. Material versatility is framed as spanning thermoplastics, bioplastics and recycled resins, and the industry context published is a global injection-moulded plastic packaging market expected to reach 420 billion USD by 2030, driven by food, pharmaceutical and consumer goods demand. In that environment a resin that will not fill a thin wall fast is disqualified regardless of its other properties.

Automotive interiors are a durability problem, and the published case for injection moulding in automotive interior parts reads as a list of environmental stresses rather than a list of cycle-time targets. Dashboards, door panels, trim pieces and seating elements require intricate geometries, detailed textures and tight tolerances, and the material has to survive engine heat, UV exposure and repeated physical contact. Resin selection here is driven by strength, durability, heat resistance and aesthetic qualities, with additives specified for UV stability, scratch resistance or flame retardancy — and by the ability to hold a textured surface finish consistently across a production run.

Same process, opposite selection criteria. If a supplier recommends one resin for both, ask which of the two requirements they optimised for.

Process and Environment: Where Good Resin Choices Get Undone

A correct resin can still produce unstable parts if the process and the environment are not controlled. The published control points are:

• Injection temperature — excessive temperature degrades the material; too low a temperature causes incomplete filling. The correct setting ensures smooth filling and minimises dimensional deviations.
• Holding time and pressure — extending holding time reduces shrinkage and improves stability; holding pressure keeps the material under pressure as it cools, reducing deformation risk.
• Cooling time and temperature — insufficient cooling leaves the part incompletely solidified. Optimising the mould cooling system and ensuring uniform cooling minimises warping from temperature differences.
• Production environment — temperature and humidity are typically held at 20–25 °C and 40–60% relative humidity to prevent material property changes affecting dimensional stability.
• Mould construction — wear-resistant, heat-stable steel held to the appropriate precision, with parting lines placed away from critical product areas.
• Equipment maintenance — regular maintenance and calibration of injection machines and equipment to avoid dimensional deviations from parameter drift.

That environmental band is worth writing into a supplier quality agreement, because it is measurable and it is rarely specified. If a dimension drifts between a winter batch and a summer batch, humidity is the first suspect and the cheapest one to eliminate.

One more control is worth naming because it is decided before any of the others and is expensive to revisit: the mould itself. Wear-resistant, heat-stable steel held to the appropriate machining precision is the published requirement, and parting lines are placed away from critical product areas so that dimensional instability does not land on a functional surface. A resin change that alters shrinkage effectively asks the tool to do a job it was not cut for, which is why material substitution requests should always route back through tooling review.

經常問的問題

Which resin should I choose for dimensional stability?

The published recommendation is to select materials with good dimensional stability such as polycarbonate or polypropylene, and to avoid materials with high shrinkage. Where stiffness also needs to rise, glass-fibre-reinforced grades increase rigidity and strength while reducing deformation during moulding.

What wall thickness should I design to?

For standard thermoplastics the published guideline is 1.5–3 mm, with reinforcement or ribbing for thicker sections. Ribs should be 40–60% of the adjacent wall thickness, and inside radii should be at least 0.5 times the wall thickness.

How much draft angle do I need?

At least 1° per side, increasing with part depth. Texture depth also affects draft requirements, so finalise the surface finish before freezing the draft.

Can I use recycled or biodegradable resins?

Yes. Biodegradable and recycled resins are published as increasingly used to meet sustainability goals and regulatory requirements, particularly in packaging. Confirm that the grade’s flow behaviour supports your wall thickness and cycle time target before committing.

Why do dimensions drift between production batches?

The published causes are process and environment rather than resin: holding time and pressure, cooling uniformity, and production temperature and humidity, which is typically controlled at 20–25 °C and 40–60% relative humidity. Equipment calibration drift is the fourth cause and the easiest to overlook.

结论

Resin selection is the earliest decision in an injection moulding programme and the hardest to reverse, because shrinkage is compensated in steel and cycle time is priced into every part. The published guidance points the same way throughout: choose for dimensional stability first, use reinforcement and additives to close the gap, then design to the published envelope of wall, draft, radius and rib ratios.

The practical test is simple. If your drawing specifies a resin but not a shrinkage value, a wall thickness but not a tolerance, or an additive requirement but not a test method, the selection is not finished. Finishing it costs a week; discovering it in production costs the tool.