Potato boxes are supplied and labeled in accordance with British Standard 7611 which determines the strength performance required for different stacking heights of box. Our box designs have been tested both in house and at external laboratories. Boxes for other purposes or other crops are manufactured and stamped to the same standard providing that the overall sizes and design fall within the size and performance requirements of the Standard.

MATERIALS USED:

The types of timber used in the manufacture of boxes is critical to their longevity. Below is an outline of the HK selection process.

The majority of components being the sides, ends and bases of the boxes are predominantly manufactured from European Redwood or Whitewood. Selection is made from Scandinavian, Russian or European supplies with particular emphasis on using reputable mills with  a proven  record of supplying consistently well graded goods. No timbers are used from the Lower Gulf areas of Scandinavia as these are found to be too fast grown. (ie weaker).

The principal qualities required of these timbers are that they

1. are out of slowly grown material, giving the required strengths in respect of bowing  stiffness and density.
2.  are kiln dried to a moisture content of  between 12% and 18% which in turn fully seasons the timber, kills any insect infestation and gives an increase in strength (see section explaining relationship between strength and moisture.)
3. are purchased on the basis that they are TOTALLY CHEMICAL FREE to prevent any possible contamination to produce stored in them. There are currently NO “food safe” timber treatments available.
4. Are supplied from sustainable sources only.
5. are full to size ie no less than 25mm in thickness (see section explaining effect of thickness on bowing / bend strengths) Note stenciled boards will be slightly thinner, but this does not adversely effect the strength of the box. However, this point is critical when considering planed boxes which can often lead to boards only 22mm or 23mm thick.

To summarize, selection of raw material is based on :

Knot content
Speed of growth
Square-ness
Accuracy of measure
Moisture content
Nail-holding properties

OTHER DETERMINING FACTORS:

CONNECTOR PLATES:

Two vertical steel connector plates are pressed one into each long side of the box to prevent racking or lozenging forces encountered during handling. These are hidden from the crop as they are sandwiched between the side boards and the vertical battens. Plates are classified as M20 x nominal 1.0mm thick galvanized plate with 8mm teeth. They comply with BS EN 10147. Teeth not pressed into timber (ie in gapped boxes) are crushed to remove sharp points.

END BRACES:

Into each of the two short end panels of a box, a diagonal brace is fitted to limit racking or lozenging forces encountered during handling. The profile of these is critical to facilitate the emptying of boxes and to prevent the accidental trapping of crop giving rise to unwanted debris left in the box.. We produce a brace with a 10mm radius on the top edge and more importantly a  25 degree chamfer on the underside.

Internal checks on head size and shank serration are carried out.
Regular batch testing of nails is carried out by an external laboratory to determine/confirm bend strengths using the Robiment test.

Effect of thickness on strength and stiffness of 25mm timber boards:

Effect on strength (likelihood of breaking)

Effect on stiffness  (outwards bow)

Results are applicable to all species, all lengths, eg a 10mm bow on sawn timber would give a 12.8mm bow with 23mm (10 x 100/77.9).

Examples of comparative breaking strengths of timber species, at different moisture contents, tested to BS 373 / ISO 3133

* Green = Timber freshly felled before air drying or kiln drying.