(3) receivables (4) inventories (5) other intangible assets (6) Property, Plant, and equipment Amortization expense related to intangible assets was approximately $393,000, $478,000 and $164,000, respectively, for the years  ended December 31, 2014, 2013 and 2012. Future amortization for the years ending December 31 will be approximately (in thousands): property, plant, and equipment consist of the following (in thousands): Depreciation and amortization expense for the years ended December 31, 2014, 2013 and 2012, were approximately $4.0 million, $3.6  million and $2.8 million, respectively. the carrying values of the Company’s definite-lived intangible assets as of December 31, 2014 and 2013, are as follows (in thousands):  Receivables are written off against these reserves in the period they are determined to be uncollectible, and payments  subsequently received on previously written-off receivables are recorded as a reversal of the bad debt provision. the Company  performs credit evaluations on its customers and obtains credit insurance on a large percentage of its accounts, but does not  generally require collateral. the Company recorded a provision for doubtful accounts of approximately $171,000 and $32,000  for the years ended December 31, 2014 and 2013, respectively. Inventories consist of the following (in thousands): december 31 2014 2013 Accounts receivable—trade  $ 16,972 $ 17,544  Less allowance for doubtful receivables   (502)  (512)  $ 16,470 $ 17,032  Receivables consist of the following (in thousands): december 31 2014 2013 Raw materials  $ 7,145  $ 6,627 Work in process  1,142  1,056 Finished goods  4,606  3,365 $ 12,893  $ 11,048 Patents  non-compete  customer List  total estimated useful life  14 years  5 years  5 years Gross amount at December 31, 2014  $ 429 $ 512  $ 2,046 $ 2,987  Accumulated amortization at December 31, 2014   (429)  (325)  (1,280)  (2,034) net balance at december 31, 2014  $ —  $ 187 $ 766 $ 953 Gross amount at December 31, 2013  $ 429  $ 512 $ 2,046 $ 2,987  Accumulated amortization at December 31, 2013   (429)  (249)  (963)  (1,641) net balance at december 31, 2013  $ — $ 263 $ 1,083 $ 1,346 2015  $ 318 2016 318 2017 317 total  $ 953  december 31  2014 2013 Land and improvements  $ 1,613 $ 840 Buildings and improvements  15,988 12,576 Leasehold improvements  2,897 2,918 Machinery & equipment  47,756 41,964 Furniture, fixtures, computers & software   5,291 4,903 Construction in progress–equipment  2,278 1,373 $ 75,823 $ 64,574 25