A Heated Ski Press

by Kelvin Wu
(forum login: kelvin)

October 24, 2005

Warning

This project requires the use and knowledge of high voltages, currents and heat. The possibility of injuring yourself is great, and if you are not comfortable working with electricity, you should find a professional to help you. Please read our disclaimer and terms of use before you begin reading this article, and please don’t electrocute yourself or burn your house down.

Overview

The following article outlines a design for a heated ski press.  By using heat (in addition to pressure), you can significantly improve the speed and quality of the skis you make.

Description

A heated press not only speeds up your cure time, but certain epoxies require a high temperature cure to reach full strength.  There are many ways of heating the skis while pressing, but after considering cost, efficiency, and ease of use, I decided that the best option is a silicone heat blanket.

There are hundreds of variations that can be done, and below I describe one way of doing it. Since each controller and components will be different, I won’t give a step by step description, but rather general guidelines and design considerations.

Design

Before ordering any parts, you will have to determine what voltages and current are available to you. Ideally you will have a 240 volt 20 amp circuit to work with.  Most household circuits are 120 V and 15 amps, which at max output would only give 1800 watts of power.  Once you know what voltages you are using, you have to make sure all the other parts run on the same voltage.

The critical part of this design is the heat blanket itself. I ordered mine through Michael’s enterprises in Idaho.  My blanket is 15”x72”,  3000 watts, with a built in type j thermocouple.  You will have to order the right blanket for the amount of power available and the size of your press. You are looking for 2-3 watts per square inch.

For the controller, I chose a digital auto-tuning PID controller, specifically an Omron EC5J.  There are hundreds of controllers out there and I suggest looking on ebay.  The main specifications to look for are: input voltage, input type, output, and control algorithms.  Output types are either, mechanical relay, DC pulse, AC pulse, or analog. Most of the controllers that I found on Ebay had analog output, which means it varies the current (or sometimes voltage) based on how much power is needed for the heater. While this will work, it requires a much more expensive SCR to power the heater.  I chose one that provides a DC pulse to control a solid state relay. A simple On/Off controller would also work, but would not provide the temperature stability of a PID controller.

Since the output of the controller is low power, you will need some way of supplying the heater with the high power it needs.  The easiest and cheapest way is a mechanical or mercury relay. These are fine for our purposes and have a rated life of hundreds of thousands of cycles. A solid state relay (SSR) is the next step up and is basically the same as a mechanical relay, but uses electronics instead of a mechanical contactor. Since there are no moving parts, there are fewer things to fail and it can be cycled much faster. Mechanical relays should not be cycled faster than 10 seconds, while a SSR can be cycled as fast as your controller can go. The faster your cycle time is, the better your controller can maintain the set temperature. When choosing an SSR, be sure to match the input of the SSR to the output of the controller, and also the output of the SSR must be able to handle the voltage and current of the heat blanket.

The other parts needed are fuses, switches, an enclosure and wires.  Make sure these parts are rated for the voltage and current that you are using. Most of these things can be found online inexpensively.

Basic material list:

• Heat blanket
• Controller
• SSR
• Switches
• Wire
• Enclosure
• Fuses holders and fuses

Schematic

The basic layout follows this schematic.  One switch controls the main power, while the other switch is a safety switch and allows me to turn off power to the heater in case something malfunctions. The DP contactor is also another safety device and is basically a giant relay. It allows you to use a low power switch to control a high power circuit. My intention was to use a thermal switch that would cut off power in case the press got too hot, but found that it really was not necessary, since I will be not leaving the press unattended. It would still be a nice safety feature to add in the future.

Building the Box

It is important that all the electrical components are put inside a sturdy box and everything is grounded properly. This not only protects the electronic components, it also protects you from deadly electrical shocks.

Heating the mold

Before putting the blanket on top of the mold, I made sure that all sharp edges and corners were removed so nothing punctures the blanket.  A sheet of 1/16” aluminum is placed on top and secured with a few bolts along the edges. Do not drill through the blanket and oversize the hole slightly to allow for thermal expansion. If everything is connected correctly, the press should heat up quickly to the set temperature and hold within a degree or two.