Common Defects in Custom Injection Moulded Parts and How to Prevent Them

Most quality claims on custom injection moulded parts trace back to a small number of root causes. Learn to recognise them on the drawing, in the tool and on the finished part.

Use it as a working checklist when you write your next enquiry.

Product Overview

This category covers a family of products rather than a single item. In practice a buyer defines custom injection moulded parts by three things: the base resin, the format or dimension, and the performance or certification level it has to meet. Typical options in this category include shot weight 1 g to 3 kg; textured SPI finishes; assembly and ultrasonic welding. Materials most often specified are Polyamide, Thermoplastic Elastomer, Polyoxymethylene, Polycarbonate, and the choice between them changes stiffness, temperature resistance, chemical behaviour and price.

In the textile and garment sector the decision is usually driven by flexibility and abrasion resistance, with colour fastness as a secondary requirement. Typical end uses include fibre and filament, buttons and buckles, zipper components.

Key Material Properties

Typical values for a general purpose grade are listed below; filled, stabilised and speciality compounds will differ.

Polypropylene is frequently the default selection here: PP is the lightest mainstream packaging resin and keeps its shape under repeated flexing.

Failure Modes and Prevention

Each defect below has a root cause that can be removed at the design or tooling stage.

• warpage – usually caused by gate position; prevent it by controlling handling and storage.
• short shots – usually caused by gate position; prevent it by controlling incoming inspection.
• sink marks – usually caused by packing pressure; prevent it by controlling handling and storage.
• flash – usually caused by packing pressure; prevent it by controlling resin drying.
• weld lines – usually caused by tool wear; prevent it by controlling resin drying.

Common Grades and Variants

• Glass Filled Nylon based: two to three times the stiffness of unfilled nylon
• Polycarbonate based: outstanding impact strength (250x glass)
• PC/ABS Alloy based: balances PC toughness with ABS processability
• Acrylonitrile Butadiene Styrene based: high impact toughness

Beyond the base resin, most suppliers offer UV stabilised, food contact, flame retardant, anti-static and recycled content versions of the same product. Ask which additive package is included, because it is often the difference between a part that lasts three years and one that lasts ten.

Processing and Finishing Options

Each step below has its own control points, and skipping one of them is where most defects start.

• Pad printing
• Overmoulding
• Injection moulding
• Gas assist moulding

Secondary operations such as printing, welding, machining and assembly are often cheaper to do in the same factory, and they reduce the risk of mismatch between components.

How to Specify Custom injection moulded parts Correctly

Work through these points in order and most specification mistakes disappear.

1. specify the cosmetic acceptance standard
2. design uniform wall thickness
3. plan the tooling maintenance schedule
4. validate with a T1 sample report
5. agree the shrinkage allowance with your moulder

Send this list to your supplier with the drawing and you will receive comparable quotations from every factory you contact.

Frequently Asked Questions

Can you produce custom injection moulded parts in Polypropylene?

Yes. {mat1} is one of our regular materials for this item, and we can supply virgin, modified and certified grades with full traceability. Material certificates and test reports are issued per batch.

Which certifications can you provide?

We can provide ISO 9001 quality system documentation, material certificates, RoHS and REACH statements, and where applicable food contact, flame retardancy or drinking water approvals. Tell us your target market and we will match the document set.

How long does tooling and sampling take?

A new tool typically takes 25 to 45 days depending on cavity count and complexity, followed by T1 samples within one week. Existing tools can usually start production within 10 to 15 days of order confirmation.

How do you control colour consistency?

Colour is controlled with a masterbatch reference approved by the customer, and every production lot is checked against the approved sample under standard lighting before shipment.

What packing and shipping options are available?

Standard packing is export cartons on fumigation-free pallets, with custom labelling, barcode and retail packing available. We also handle consolidated containers and door to door delivery.

Conclusion

Our team supplies custom injection moulded parts to textile and garment customers worldwide, with samples, material certificates and full inspection reports available before mass production.