Sasol CRV648 PP Copolymer is a reactor-grade heterophasic polypropylene impact copolymer designed for injection moulding. The material is produced by sequential gas-phase polymerisation, generating a continuous propylene-rich matrix and a dispersed ethylene–propylene rubber phase. This reactor architecture distinguishes CRV648 from melt-compounded impact-modified polypropylene because the elastomer domain size and volume fraction are fixed before pelletisation rather than developed during screw mixing. The result is a medium-flow material with a controlled impact–stiffness balance and lower sensitivity to dispersive mixing history than masterbatch-based compounds.

The current manufacturer technical data sheet supplies the nominal values shown in Table 1. These values are laboratory characterisation data obtained under the stated standards and are not part-geometry-dependent design minima. Where the certificate of analysis reports a limiting value rather than a nominal value, that limiting value governs for lot acceptance.

Test specimens for Table 1 are prepared according to ISO 19069-2:2016. Tensile and impact bars are injection-moulded type 1A specimens with a thickness of 4 mm. Conditioning before testing is 40 h at 23 °C and 50 % relative humidity unless the individual standard specifies otherwise. The notched Izod values are sensitive to gate location and weld-line placement; the stated values are for single-gated laboratory bars without weld lines.

What processing constraints emerge from the heterophasic morphology?

Injection moulding of CRV648 differs from processing a homopolymer of the same melt flow rate because the dispersed rubber phase increases low-shear viscosity and melt elasticity. On a reciprocating-screw machine with an L/D ratio of 20:1 to 25:1 and a general-purpose polypropylene screw, melt temperature should be maintained between 200 °C and 230 °C. At melt temperatures above 230 °C with residence time beyond 5 min, oxidative gel formation and loss of stabiliser can produce black specks and a measurable drift in melt mass-flow rate. Nozzle set temperature is normally 220–235 °C, while back pressure is maintained below 1.0 MPa to limit shear heating of the rubber phase.

Drying is not mandatory for sealed pellets stored at ambient humidity below 40 %. When storage relative humidity exceeds 60 %, or when open storage of regrind is used, a desiccant-bed dryer at 80 °C for 2 h with a dew point of −20 °C or lower is recommended before moulding. Moisture levels above 0.10 % are associated with splay, silver streaks, and hydrolysis of processing stabilisers in hot-runner systems.

Lot-to-lot conversion monitoring on a production line should include a fixed-volume shot-weight check and a multi-cavity fill study at an injection speed of 120 mm/s and melt temperature of 210 °C. A fill imbalance exceeding 3 % across cavities or a shot-weight reduction greater than 2 % at constant stroke indicates a change in pellet densification or elastomer agglomeration rather than a simple melt-flow variation. In high-volume crate tools, raising back pressure above 1.2 MPa can increase melt temperature by 5–10 °C and should not be used as a substitute for a proper mixing screw if pigment dispersion is poor.

Through hot runners, CRV648 exhibits pronounced shear thinning. At runner shear rates from 100 s⁻¹ to 1000 s⁻¹, apparent viscosity decreases steeply; at the advancing melt front, low-shear viscosity remains higher than a homopolymer of equal melt mass-flow rate. This combination can improve surface gloss but may cause hesitation in ribs thinner than 1.5 mm. Sequential valve-gate opening is preferred over simultaneous opening in multi-drop tools where flow fronts meet behind weld lines.

Thick sections above 4 mm require packing pressure maintained until gate freeze. Cavity-pressure sensors should be used to control switchover and holding; timer-only holding is less reliable because gate freeze time shifts with melt temperature and part thickness. On a 2.5 mm cold-runner edge gate, gate freeze time typically falls between 1.5 s and 3 s at 220 °C.

Injection speed should be profiled rather than kept constant. A first-stage speed of 80–100 mm/s for the initial 10–20 % of stroke reduces jetting and air entrapment, followed by a second-stage speed of 150–200 mm/s to complete filling before the melt front freezes in thin ribs. On general-purpose machines, a melt cushion of 3–5 mm should be maintained to ensure consistent packing pressure transmission. Screw recovery should finish before cooling time expires; if recovery time exceeds 1.0–1.2 times the cooling time, barrel capacity or screw geometry is undersized for the shot.

Where CRV648 diverges from homopolymer and random copolymer PP in moulded parts

Table 2 summarises class-level property differences that distinguish CRV648 from homopolymer and random copolymer polypropylene. The values are typical rather than specification limits and must be confirmed against the individual grade datasheet and moulded-part tests. Published data for specific long-term property retention in some configurations is limited; end-use qualification under the relevant part test remains necessary.

The rubber phase in CRV648 causes stress whitening before yield. In a homopolymer PP part, a sharp notch at −20 °C often initiates brittle crack growth; in CRV648, the same feature can produce stable crack growth along a stress-whitened zone, provided the section is below 4 mm and the notch radius is not below 0.25 mm. This difference is operational in battery boxes and cold-chain crates, where low-temperature impact is a release criterion under ISO 180:2019. The stiffness penalty is visible in flexural modulus: CRV648 at 1200 MPa is below homopolymer injection-moulding grades but above many random copolymers. Designers replacing a homopolymer with CRV648 may need to increase rib thickness or select glass-filled material if the existing part operates near the deflection limit.

Compared with a random copolymer, CRV648 is opaque and has higher modulus, better dimensional stability above 50 °C, and improved low-temperature impact retention. Random copolymers are preferable for transparent or translucent housewares and sealing films, but they soften at lower temperatures and exhibit lower resistance to creep at elevated temperature. Compared with high-flow impact copolymers in the 20–30 g/10 min range, CRV648 retains greater impact at low temperature but fills thin-wall moulds less readily.

Chemical and environmental limitations are relevant. The ethylene–propylene rubber phase swells in aromatic and chlorinated solvents, so prolonged contact with such media at temperatures above 40 °C can produce surface whitening and stress cracking. Dilute acids, alkalis, and polar glycol-based fluids are less aggressive, but any stressed part should be tested under ISO 22088-3:2008 before production release. The base grade contains no high-dose UV stabiliser; parts intended for continuous outdoor service require an additive package validated under ISO 4892-2:2013.

In automotive 12 V battery boxes, CRV648 is used in tools running clamp forces from 1500 kN to 3000 kN; the part design typically uses wall thicknesses between 2 mm and 4 mm. The heat deflection temperature at 0.45 MPa of 95 °C supports under-bonnet exposure, but at 1.8 MPa the grade’s value is approximately 55 °C, so continuous structural load at elevated temperature is not recommended without ribbing or reinforcement. Cold impact testing after conditioning at −20 °C is routinely specified; the notched Izod value of 3 kJ/m² is a benchmark, not a guarantee for all weld-line locations. Weld-line impact in battery boxes can be 30–50 % lower than bulk impact unless moulding is tuned to place the weld line away from lugs and wall ribs.

Crates and tote bins made from CRV648 benefit from the material’s low-temperature impact retention during cold-storage handling. The material is appropriate for stackable containers with wall thicknesses of 2.5–4.0 mm. In such parts, shrinkage and warpage are controlled more effectively by uniform cooling than by melt temperature adjustments. Mould cooling circuits should maintain a temperature differential of less than 10 °C between the cavity and core sides; larger differentials produce warpage that cannot be corrected by holding pressure alone.

Mold filling, clamp force, and shrinkage compensation in multi-cavity tools

For a 2.5 mm wall, published spiral-flow results vary with gate geometry; no universal flow length is available for CRV648 because the heterophasic melt is more shear-thinning than a homopolymer of identical melt mass-flow rate. Tool-specific flow trials are required if the flow-to-wall ratio exceeds 150:1. In multi-cavity crates and tote bins, the pressure drop through the sprue should be kept below 25–30 MPa, and injection speed should fill the cavity within 0.5–1.5 s to minimise hesitation lines at rib bases. Clamp force should be calculated from peak cavity pressure. If cavity pressure at gate freeze is 35 MPa and the total projected area is 1000 cm², the required clamp force is 3500 kN before applying a safety factor. If peak cavity pressure reaches 60 MPa with sequential packing, the required clamp force rises to 6000 kN for the same area.

Mould shrinkage of CRV648 is anisotropic. Under ISO 294-4:2018, a 2 mm plaque measured after 48 h at 23 °C typically shows flow-direction shrinkage of 1.2–1.8 %. Transverse shrinkage is commonly 0.2–0.4 % lower. The presence of the rubber phase reduces the overall crystallinity and the transition-to-post-mould shrinkage ratio relative to homopolymer PP; however, ejection forces on textured surfaces can be higher because the material remains rubbery at the ejection temperature. Mould temperature should be controlled in the range 20–60 °C. At the upper end, surface gloss improves but cycle time lengthens and shrinkage anisotropy may increase.

Regrind use should be limited to 20 % by mass unless the moulder has verified impact retention and melt flow stability on three heat histories. Repeated extrusion at high shear can reduce the dispersed rubber particle size and shift the ductile-to-brittle transition upward, a failure mode that appears as unexpectedly brittle subzero impact after multiple recycling cycles. Amine-based purge compounds or copper-containing heat stabilisers should be excluded from contact with molten CRV648 because they may accelerate degradation of the elastomer phase and produce black specks or melt-pressure fluctuations during extended runs.

Weld-line weakness in CRV648 arises from the inability of the rubber phase to diffuse across the melt front. Flow-induced orientation at the weld line reduces impact by 30–50 % relative to un-welded material. Parts such as battery housings should not have weld lines at ribs or mounting lugs. If unavoidable, a local increase in wall thickness to 3 mm and a melt temperature near 230 °C can partially restore weld-line strength. Melt temperature should not exceed 235 °C during such adjustments because high-temperature residence can degrade the stabiliser package and reduce the very low-temperature ductility that the grade is selected to provide.