Running Delrin on Injection Moulding: Process Notes for a Branded Resin must account for acetal homopolymer’s semi-crystalline morphology, rapid solidification, and narrow thermal safety window. Delrin is a polyoxymethylene homopolymer historically supplied by DuPont; current grade datasheets identify density near 1.42 g/cm³ per ISO 1183-1:2019, water absorption of 0.2 % at 24 h per ISO 62, tensile yield strength near 70 MPa per ASTM D638-14, flexural modulus near 2900 MPa per ASTM D790-17, and notched Izod impact near 7.0 kJ/m² per ASTM D256-10e1. Melt volume-flow rate is measured at 190 °C under 2.16 kg load per ISO 1133-1:2022. A sharp melting point near 175 °C creates short-cycle potential and short-shot risk in thin-wall tools when injection velocity is insufficient. Process notes for this branded resin must also address formaldehyde generation above 240 °C, plate-out on cavity surfaces, vent clogging, and the influence of regrind on impact retention. The parameters that follow are written for production-scale injection moulding, not for extrusion or compression moulding, and are specific to unfilled general-purpose flows typical of branded homopolymer grades.

Drying, Hopper Residence, and Moisture Control Limits

In humid plants without sealed hopper loading, surface condensation absorbed during warehouse-to-plant transfer can generate splay even though polyoxymethylene is not extensively hygroscopic. Pre-drying at 80 °C for 2 h to 4 h is applied when hopper residence time exceeds 1 h or when ambient relative humidity remains above 60 %. Desiccant dryers with a delivery air dew point of -30 °C or lower and a hopper throat temperature of 70 °C to 80 °C prevent moisture-related melt surface distortion. Feed throat cooling is used to avoid bridging because acetal pellets soften sharply near the melt onset; hopper capacity for critical parts should not exceed 30 min of shot consumption. Final moisture content below 0.05 % by Karl Fischer titration is a conservative control limit for glossy, thin-wall parts. Over-drying at 100 °C or above can yellow the resin and should be avoided. Published guidance for acetal homopolymer does not require the same multi-hour pre-drying used for nylon, but humid environments change the acceptable hopper residence time.

Typically, melt temperature is established by air-shot pyrometry rather than by assuming barrel set-point equals melt condition. For a general-purpose screw with 18:1 to 24:1 L/D and 2.0:1 to 3.0:1 compression ratio, barrel zones are set as a reverse or flat profile: rear zone 180 °C to 190 °C, centre zone 200 °C to 210 °C, front zone 210 °C to 220 °C, and nozzle 210 °C to 215 °C. Melt temperature for medium-flow Delrin is maintained at 205 °C to 220 °C. Above 230 °C, homopolymer chain scission begins to release formaldehyde; above 240 °C, gas evolution becomes rapid enough to pressurize the barrel and create gas streaks. The degradation reaction is autocatalytic in the presence of oxygen or residual moisture and forms acidic by-products that attack tool steel over repeated cycles. Residence time at process temperature is kept below 20 min, and shot volume is held between 30 % and 70 % of barrel capacity to limit melt residence in the compression zone. When a machine is stopped for more than 10 min, the barrel temperature is reduced or the screw is purged with a stable purge compound. Nozzle tips with small internal volume and continuous temperature control reduce cold-slug formation and gate drool during long cycles.

Why Gate Bloom and Vent Clogging Appear in Extended Acetal Runs

When acetal melt passes through a restricted gate, shear heating can raise local temperature by 10 °C to 20 °C above the barrel set-point and initiate formaldehyde release. The deposit is observed as gate bloom or a white plate-out on cavity surfaces and vent channels. Extended runs therefore need venting that is not a static afterthought. Vent depth for unfilled acetal is typically 0.02 mm to 0.03 mm, with land length of 1.0 mm to 1.5 mm; vent width is distributed at the end of fill and around ejector pins. Vent channels should be cleaned every 2,000 to 5,000 cycles in high-cavitation tools because plate-out accumulates preferentially where gas velocity is highest. Gate geometry for unfilled Delrin is sized with a land length of 0.5 mm to 1.0 mm and a gate diameter or thickness 0.5 to 0.8 times the wall thickness; small gates can be used but only when high injection velocity is available. Hot-runner systems for Delrin must be fully heated and open-pipe in design; dead spots, unheated valve-gate gaps, or sharp bends create local stagnation and formaldehyde odour. Hot-runner manifolds are typically controlled at 205 °C to 215 °C, not above the melt degradation threshold. Purging between colours or after shutdown is performed with polyolefin or commercially available acetal purge compounds, not with PVC or acidic purge materials. Mould steel exposed to formaldehyde and moisture can corrode; hard chrome plating or corrosion-resistant vent inserts are specified for long production campaigns.

For wall thickness below 1.5 mm, fill time is controlled at 0.5 s to 1.0 s so that the flow front freezes shut only after the cavity is full. For wall thickness of 3.0 mm or greater, fill time is extended to 1.0 s to 2.0 s to avoid jetting and trapped air burns. Injection velocity is profiled: slow at the first 10 % of stroke to seat the check ring, high during flow, and reduced at the final 5 % to 10 % of stroke to prevent peak-pressure spikes at the gate. Hydraulic injection pressure on general-purpose machines commonly falls between 70 MPa and 120 MPa; hold pressure is set by gate seal time and is commonly 40 MPa to 80 MPa. Back pressure is kept low at 0.2 MPa to 0.7 MPa because acetal is shear-sensitive and high back pressure increases melt residence and formaldehyde formation. Screw decompression after recovery is set to 5 mm to 8 mm to prevent drool without pulling air into the cushion. Cushion length is maintained at 2 mm to 5 mm for consistent packing pressure transfer. On all-electric machines, injection velocity is set in mm/s rather than hydraulic pressure; a 40 mm screw may require fill velocities of 100 mm/s to 200 mm/s for thin-wall parts, while a 25 mm screw may use 60 mm/s to 120 mm/s for thick-section parts. The exact value is determined by short-shot progression so that a 90 % full short shot is obtained before hold pressure is applied.

Warpage in Thin-Wall POM Parts Responds to Packing Pressure More Than to Mould Heat

Before accepting a shrinkage value for tooling design, mould temperature and gate geometry must be measured together because unfilled acetal homopolymer exhibits anisotropic shrinkage. Typical unfilled Delrin grades show mould shrinkage from 1.8 % to 2.5 % in the flow direction and 1.5 % to 2.0 % transverse, depending on wall thickness, gate type, and mould temperature. Dimensional audits are performed on specimens moulded per ISO 294-3 or ASTM D955 after conditioning at 23 °C and 50 % RH for 48 h. A mould temperature increase from 60 °C to 110 °C typically raises flow-direction shrinkage by 0.1 % to 0.3 % while improving post-mould dimensional stability because cooling is slower and crystallinity is more uniform. Thin-wall parts with wall sections below 1.5 mm are more sensitive to packing pressure than to mould heat; gate seal time is determined by weight stabilisation tests rather than by a single generic table. Mould temperature zones with a difference greater than 10 °C between core and cavity create differential cooling and can increase warpage more than an absolute mould temperature change. The following representative ranges are for tooling direction only and must be verified on the actual production tool.

Crystallinity in POM homopolymer is influenced by melt temperature and mould cooling rate. Mould temperature at 80 °C to 100 °C provides a uniform crystalline skin and reduces post-mould shrinkage variation; rapid cooling to 40 °C may reduce cycle but increases internal stress and warpage in thick sections. Cooling circuit design should maintain a Reynolds number greater than 10,000 in turbulent flow and a core-cavity temperature difference below 10 °C.

When Regrind Levels Exceed 30 wt% in Continuous Production

Regrind streams drawn from hot-runner sprues and rejected parts introduce already-melted material that has undergone one or more heat histories. Up to 20 wt% clean, dry, uncontaminated regrind is common in non-critical parts; above 30 wt%, the effect on impact properties and formaldehyde re-emission should be revalidated on the production tool. Dried regrind must be blended uniformly; grinding must be controlled to avoid fines because fine particles melt faster, degrade more readily, and can increase MFR beyond the virgin grade specification. A property audit per ASTM D638-14, ASTM D256-10e1, and ISO 1133-1:2022 is performed when the regrind fraction changes. The representative ranges below are for unfilled medium-flow homopolymer; they are not a substitute for grade-specific qualification.

Regrind that has been stored in open bins should be re-dried at 80 °C before blending because surface moisture and absorbed formaldehyde volatiles may create splay or odour in the finished part. Closed-loop regrind conveying and a regrind-to-virgin ratio feeder with mass-flow metering prevent segregation. In-line vacuum receivers are preferred over open gaylords when the plant air contains oil mist or mould release sprays.

After ejection, parts are gauged for length and flatness after 24 h at ambient; acetal homopolymer can continue shrinkage for up to 48 h depending on mould temperature and part thickness. Draft angle for ejection is sized at 0.5 ° to 1.0 ° per side on textured surfaces, and ejector plates are balanced to avoid stress whitening. Specific clamp force is calculated from the projected area at a cavity pressure of 50 MPa to 70 MPa; flash-free filling is maintained without oversizing the machine. Gate seal time is determined by weight stabilisation, typically 0.5 s to 1.5 s per 1.0 mm wall thickness, after which hold pressure no longer controls part weight. POM parts are ejected after the surface temperature falls below 90 °C to avoid vacuum pulls and imprint marks. For regulatory compliance, unfilled food-contact grades may be described under FDA 21 CFR 177.2470 for repeat-use food-contact applications and under EU 10/2011, but the specific grade, colourant, and processing aids must be verified against the supplier’s regulatory documentation. REACH and RoHS declarations are grade-specific and are not automatically transferred when running Delrin on a line that has previously processed PVC or other halogenated resins.