Sim arrived in Australia from his native Dumbarton (Scotland) in 1842, working as a ship's steward on the voyage out, and on arrival he worked as a woodcutter and wheelwright.

In 1849 or the early 1850s he established his own business at 107-09 Swan St, Morpeth, where years later at the company's peak about 60 men were employed in fashioning a wide range of iron products.

Farm equipment bulked large in the business, including hand- or horse-driven lucerne presses, wine presses, mowing machines, rakes, corn shellers, millet hacklers, harrows, ploughs and broom-making machines. There were also domestic appliances like baths, laundry coppers and cast-iron kitchen stoves that burned wood.

Railway rolling stock was constructed for the carriage of hay and other farm products, and much decorative iron lacework destined for use on Sydney and Hunter Valley houses was produced.

Iron work for several Hunter bridges came from Sim's works as did decorative iron columns, and the cast iron 'Wallaroo stove' manufactured at Morpeth became a feature of many New South Wales homes during the 19th century before gas and electricity were used for cooking.

D Sim and Sons produced a very wide range of products over its roughly 70-year existence.

Periodically the foundry exhibited its wares at the regular shows run by the Hunter River Agricultural and Horticultural Association in Maitland. A mowing machine won an award at the Sydney International Exhibition in 1879. Such exhibitions were important in publicising a firm's products.

Some of the equipment made at the foundry is still in use, on John Wright's Phoenix Park farm.

The foundry business thrived and Sim became comfortably well off. He and his wife Sarah (nee Ingall) had nine children (five sons and four daughters) who between them produced 18 grandchildren.

Like others with significant business interests in the area's early years, Sim was active in civic affairs He was an alderman on the inaugural Morpeth Borough Council, established in 1865, and he served four year-long terms as Morpeth's mayor during the 1870s. He supported the Morpeth School of Arts and was a member of the Maitland Hospital Committee and a Justice of the Peace.

After his death the foundry business was taken over by his sons Robert and Ernest. It was sold in 1919 but continued to operate until its closure in 1926. Sim and his wife were buried in the Morpeth Cemetery.

The Maitland Mercury and Hunter River General Advertiser in its obituary called him "an excellent man of business" "a trustworthy tradesman" and "altogether a worthy member of the community".

His foundry was a central element of the Hunter economy at a time when the area had a strong industrial base, and it helped Morpeth to survive even as the shipping trade fell away and its branch railway line to East Maitland failed to attract the business hoped for it. Sim had been instrumental in the establishment of the line.

Duncan Sim was inducted into the City of Maitland's Hall of Fame in 2017 for his contribution to business and public affairs in the area.