Injection Moulded and Overmoulded Knife Handles: Tooling, Materials and Bonding

📅 Aug 10, 2026 👁 views

Moulded handles make the cheapest high-volume kitchen knives possible and the worst cheap kitchen knives possible. The difference is entirely in the tooling, the material choice and the process control — none of which is visible in the finished product until it fails.

This article covers injection moulding and overmoulding in cutlery, the tooling economics, and what to specify.

Injection moulding versus overmoulding

Process	What happens	Result	Tooling	Unit cost
Single-shot injection moulding	One material injected into a cavity around an insert tang	Uniform handle in one material	One mould	Lowest
Two-shot injection moulding	Two materials injected sequentially in the same machine, without removing the part	Hard core with a soft grip, chemically or mechanically bonded	One complex mould with two cavities and a rotating or sliding element	Low at volume, high tooling
Overmoulding	A pre-moulded hard part is placed in a second mould and a soft material injected over it	Hard substrate with a soft grip layer	Two moulds plus a placement step	Low to moderate
Insert moulding	The metal tang is placed in the mould as an insert	Handle bonded mechanically to the tang	One mould plus loading fixtures	Low
Co-injection or sandwich moulding	Two materials injected as a core and skin	Recycled core with a virgin skin	Special machine and mould	Moderate

Materials

Material	Character	Dishwasher	Grip when wet	Cost	Notes
Polypropylene (PP)	Tough, slightly waxy, flexible	Excellent	Moderate	Low	Highly recyclable, common
ABS	Rigid, good surface finish, can be glossy	Good	Poor	Low	Good appearance, can look plasticky
Acetal (POM)	Hard, low friction, dimensionally stable	Good	Poor	Moderate	Used for hard components and some handles
Nylon (PA)	Tough, absorbs moisture, dimensionally variable	Moderate	Moderate	Moderate	Moisture absorption can change dimensions
TPE / TPR (soft grip)	Rubbery, soft	Good to excellent depending on grade	Very good	Moderate	The standard soft-grip overmould material
TPU	Tough rubbery, abrasion resistant	Good	Very good	Moderate to high	Better wear than TPE
Silicone	Very soft, heat resistant	Excellent	Very good	High	Rarely used as a handle body; sometimes as a grip element
Glass-filled PA or PP	Stiffer, harder, less ductile	Good	Poor	Moderate	Stiffness where needed
Recycled PP or bio-based grades	Varies widely	Depends	Depends	Low to moderate	Requires material evidence to support a claim

Two material decisions matter most. First, whether the handle is dishwasher-safe at all: this eliminates some grades immediately. Second, whether a soft grip is used, because a soft grip adds wear, peel and cleaning concerns in exchange for a better wet hold. For professional and dishwasher-heavy channels, a well-textured single-material handle is often the better engineering answer than a soft grip.

Mould design in cutlery

Mould element	Effect on the product	Cost driver
Number of cavities	Throughput, and consistency across cavities	Roughly proportional to cavity count
Runner system	Waste and fill quality; a hot runner reduces scrap	Hot runners are significantly more expensive
Cooling channels	Cycle time and shrinkage control	Conformal cooling costs more, moulds faster
Parting line location	Whether a visible seam falls on a grip surface	Design effort
Ejection	Whether the part is marked or distorted on removal	More complex ejection, more cost
Insert location features	Whether the tang sits centrally and repeatably	Fixture design
Surface texture	Grip and appearance, from a polished cavity or a textured one	Texture is added by etching the cavity
Gate location	Weld lines and cosmetic defects	Simulation and iteration

Parting line and gate location are where cheap handles announce themselves. A part line running across the finger contact area, or a visible gate scar on the side of the handle, reads as low quality to a buyer even if they cannot name the cause. Both are avoidable at the design stage and expensive to fix afterwards, because a mould has to be modified or replaced.

Insert moulding around a tang

Requirement	Why	Typical specification
Tang section sufficiently large	Load bearing and grip against rotation	A defined minimum width and thickness
Mechanical interlock features on the tang	Prevents the handle pulling off or rotating	Holes, notches or a widened section
Tang surface condition	Adhesion to the polymer	Clean, degreased, and either roughened or coated as specified
Tang centring in the mould	Symmetry and wall thickness consistency	Fixture design; a stated positional tolerance
Tang temperature at moulding	Affects shrinkage and bond	Operator instruction or oven preheat
Wall thickness around the tang	Shrinkage voids and sink marks	A minimum wall thickness, typically several millimetres

The mechanical interlock is the point where cheap handles fail. A tang that is a smooth, straight rod relies on adhesion alone, which is why rat-tail handles rotate and detach. Holes or notches in the tang let the polymer flow through and form a mechanical key. This is free to design in and impossible to add later. See tang and attachment methods.

Defects and their causes

Defect	Cause	Effect	Control
Short shot	Insufficient fill pressure, low temperature, poor venting	Incomplete handle, scrap	Process window control, venting design
Sink marks	Thick sections cooling unevenly	Visible depression, poor appearance	Uniform wall thickness, adequate packing
Weld lines	Two flow fronts meeting	A visible weakness line and cosmetic defect	Gate location and flow simulation
Flash	Mould parting faces not tight, excess pressure	Finishing labour or a visible seam	Mould maintenance
Burn marks	Trapped gas igniting from compression	Discoloured patches	Venting
Delamination of soft overmould	Poor material compatibility, contamination, cold joint	Peeling grip, a hygiene trap	Material pairing validated, cleanliness, mould temperature control
Colour variation between batches	Masterbatch dosing, regrind content variation	Range that does not match	Regrind percentage limit, colour measured against a standard
Warpage	Uneven cooling, fibre orientation in filled grades	Handle does not sit straight	Cooling design, gate location

Regrind content is the hidden variable that causes most colour and property variation. Most factories use a proportion of regrind, which is normal and reasonable, but the percentage should be capped and stated. A handle that varies visibly in colour from one shipment to the next is almost always a regrind content issue or a masterbatch dosing issue, and it is entirely controllable.

Tooling economics

Mould type	Indicative tooling cost tier	Economic volume	Notes
Simple single-cavity prototype mould	Low	Tens to hundreds	Aluminium or soft steel, short life; useful for form and feel trials
Production single-cavity, hardened steel	Low to moderate	Thousands to tens of thousands	Long life, lower throughput
Multi-cavity production mould	Moderate to high	Tens of thousands and up	Throughput per cycle scales with cavity count
Two-shot mould	High	High volume	Complex, expensive, only justified for large programmes
Overmould tooling set	Moderate to high	Moderate to high	Two moulds plus a placement process

For a first private label order of a few thousand pieces, a simple hardened single-cavity mould plus a modest cycle time is usually the right answer. A multi-cavity mould only pays back when the volume is large or when the handle will be shared across several SKUs — which is exactly why handle standardisation is the biggest cost lever in range planning. See range planning and tooling amortisation.

Specification points

Material for each component, with a named grade and the colour reference.
Regrind percentage limit.
Whether the handle is single-material or overmoulded, and the soft material grade.
Tang interlock features, with dimensions on a drawing.
Surface texture or finish on the grip area.
Parting line location, and the requirement that it not fall on a primary grip surface.
Wall thickness specification and sink mark acceptance.
Dishwasher exposure expectation, and a qualification test if the product claims dishwasher safety.
Adhesion test for any overmoulded layer, with a pass criterion.
Golden sample retained by both parties.