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Ted the Kiwi said..
Quality of the blank over the glass job has more to do with the golf ball / caving effect I always thought. If the blank is soft it caves. If they take too much off the skin / top when shaping it it caves.
Beat me too it Ted...generally off the shelf boards are made with the lowest denisty blanks and the lightest glass they think they can get away with this means the board feels good in the shop but generally wont last as long
A brief overview of foam PU density from surf blanks below there are also some foam forluma's that work better on CNC and others that are easier if you work by hand
Density colour name/date of introdcution/Surfblanks original name.
Pink Foam 2008 - 1.82 lb cu ft developed into an impressive (very) core in 2010 when the Super Strong formula emerged. 2013 sees steady sales growth of this foam as makers realise its potential.
Surfblanks Yellow has never been matched for weight/strength anywhere in the world by any surfboard foam maker. It's easy to imagine how special Surfblanks Pink foam is. Primarily paddle speed is greatly enhanced, then manuevre response is found to be awesome. No EPS/epoxy hassles with this foam.
Yellow Foam 1998 (Elektrafoam) - 1.98 lb cu ft (approx minimum, without stringer) - Has developed substantially since 2002. Cell size is down and compression strength is up. As of 2006 Surfblanks has been making increasing numbers of blanks from this foam. Blanks made from this foam are always glued with yellow glue for identification purposes. In 2007, formulation changes made Yellow Foam easier to produce. The 25% surcharge has been removed.EPS/Epoxy users are turning to Yellow Foam since it avoids all the preparation required for an EPS blank. Finished weights are on par if not better. The buckle and crush strength of Yellow Foam dramatically exceeds that of EPS at a similar weight.
Green Foam 1990 (Hyperlight) - 2.14 lb cu ft (approx) - Widely used for team rider's shortboards, Green will produce what might be considered the lightest urethane foam surfboard on the planet. The popularity of this foam continues to grow even though the life of the finished surfboard may be short. It is rarely used in longboards. Always glued with green glue for identification.
Blue Foam 1985 (Megalight) - 2.31 lb cu ft (approx) - The 'all time' favourite foam for shortboards. High paddling buoyancy and lively surfing response on a wave make this foam the choice of hard core to recreational surfers. Needs a really good glass job so as not to dent.Red Foam 1972 (Ultralight) - 2.49 lb cu ft (approx) - This is the Median Density (base) foam when all blanks made by Surfblanks Australia are averaged out for density choice by all customers. It is desirable that blanks for shaping machines be made from this foam where cutting will significantly reduce the original blank volume.
Red Foam in shortboards is considered excellent for compression strength and once glassed is approximately equivalent in weight to, 1.Clark Supergreen, 2.Burford Green, 3.Bennett Green. This comparison is offered as a guide for first time Surfblanks users. Red Foam in longboards is considered very light and buoyant. However, Surfblanks encourages manufacturers to choose Black Foam instead for the reasons below.There are cutting edge manufacturers who make most of their shortboards from Blue Foam and their team rider boards from Green Foam. By comparison, more conservative manufacturers feel that dent free longboards should be made from either Black or Orange Foam. It is also true that these heavier foams require less glass and thus a saving is made.
Black Foam 1995 (Superlight) - 2.59 lb cu ft (approx) - Is an unique mid range density (not the full 8% heavier). This allows for a compromise density (not too heavy and not too soft) for longboards. Black Foam is often used in shortboards where over shaping may occur.
Orange Foam 1968 (Lightweight) - 2.69 lb cu ft (approx) - A strong, very even density foam used in longboards or thicker section shapes where distortion at the edge extremities must be avoided over long periods of temperature and time. This foam was very popular in the early 1970s and is now popular again for use in kite boards.
Purple Foam (Regularweight) - 2.91 lb cu ft (approx) - The original 1960's malibu (longboard) foam. This foam as a blank used to be glassed with two layers of ten ounce on the deck and one layer of ten on the bottom. Weight and strength.
Gold Foam 2004 (Classic) - 3.14 lb cu ft (approx) - Heavy and very solid. Great for that indestructible longboard blank. This foam is now popular for tow in blanks.
Silver Foam 2004 (Skimboard) - 3.39 lb cu ft (approx) - Very heavy and very solid. Silver Foam was specifically designed for skimboards, has been used in tow boards, and is basically the first of Surfblanks block foam densities.