RUG/Pennsylvania/State College/Electronics/Heated Bed
Introduction
Printing ABS or HDPE plastics using the mendel often results in warpage rendering the print useless. These deformations are due to moderate temperature gradients which develop in the plastic as a result of cooling. For example hot plastic from the extruder may leave the tip at 200 degrees Celsius while the base layer of the plastic part has cooled to room temperature. This temperature difference can be especially prevalent in larger prints causing noticeable distortions, lifting of the print off the bed, and interruption of the print. Experiments have shown that heating the print bed up to approximately 120 degrees Celsius has significantly reduced warp problems [1].
The purpose of this page is to present an engineering analysis for a preliminary design of a mendel heated bed. This evaluation takes into account a general heat transfer discussion, mechanical design considerations, as well as material costs and options. A mendel heated bed design is presented for consideration and later implementation at The Pennsylvania State University.
Discussion
The following topics cover a general approach to the design of a heated bed for a mendel. The mechanical and heat transfer calculations listed below are important for providing a reference to values such as the power needed to run the heating system, estimation of heat up time, geometrical tolerances etc.
General Heat Transfer Analysis
Abstract:
The attached document provides a general heat transfer analysis for a mendel printing bed being heated to 120 degrees Celsius. The objectives of the analysis are to provide estimations for: the rate of thermal energy loss from the bed at a steady state (i.e. once the bed has heated up to 120 degrees Celcius but has not started printing), the minimum heat flux and power input needed to maintain a steady state bed temperature (120 degrees Celcius both while stationary and while printing), and the time it takes for the bed to reach a steady state temperature (120 degrees Celsius) with various applied heat fluxes.
This analysis is accomplished by making several generalizations for the parameters of the mendel bed. These generalizations can be located by referring to the 'given' and 'assumptions' sections of the attached documents. These generalizations are imperative to arriving at the results listed in this analysis and should be duly noted. Parameters may be varied in order to use this general evaluation method for other designs, however it is important to take in consideration the fact that the results of these calculations are restricted to the listed assumptions. It must first be verified if a design actually meets these before an analysis like this is used. Otherwise values obtained through this method may result in significant errors. These generalizations include but are not limited to:
The results of this analysis show:
1. The heat flux and power needed to maintain the heated bed at a steady state (after it has initially heated up from room temperature) of 120 degrees Celsius may be minimal compared to transient heating.
2. The bulk consumption of power resulting from heating the bed takes place during initial heating from room temperature to 120 degrees Celsius.
3. The time it takes to heat up the bed is inversely related to the heat flux applied to the bed (as you increase the heating power it takes less time to reach the desired temperature).
4. The main source of cooling of the bed may be from natural convection rather than forced convection (the forced cooling resulting from the bed moving back and forth in the y direction) and the heat loss from radiation is minimal and may be negligible.
5. The use of insulation drastically helps reduce heat loss and is needed for an efficient design.
6. Times for the bed to heat up are tabulated for several power supplies.
- NOTE* - Once again this analysis is based on several assumptions which may or may not apply to a design. Data calculating the significance of forced convection compared to natural convection may need to be verified. These calculations are based off of information on heat transfer which can be found through [2]. There are several possible sources of error which are listed at the end of the analysis. Understand that correlations used in the analysis are generalizations based on experiments and typically result in at least a 6% error. The accuracy of these calculations need to be verified experimentally to determine their accuracy.
Design Proposal
The goal for a design for use at Penn State is to create a relatively low cost heated bed, which is not too difficult to install, and which does not drastically change the current structure of the Mendel.
Applying this design criteria a heated bed can be made using a flexible film heater, and insulated foam sheathing. The flexible film heater will be placed under the aluminum plate of the mendel bed and attached using adhesive. The insulated sheathing will then have to be cut and applied to the base of the bed to ensure that most of the heat from the flexible film heater makes it to the aluminum plate. The insulated sheathing should also be applied to make insulating washers to reduce heat transfer to bed fasteners. Kapton tape may be used to cover the aluminum bed and provide a surface which is adequate to print on. An independent power supply and thermocouple will also be needed.
Parts List and Options
Heated Bed Materials List
1. Insulation Materials – All of these materials are priced and available from places like Lowes and Home Depot.
a. Insulating Spray Foam Sealant ($7.00)
Cheap but if something goes wrong with wiring or heating there is no way to take the insulation off. Used on this reprap: http://reprap.org/wiki/Mendel_heated_bed
b. Insulating Fiber Glass ($10.00)
Pain to install. Not the best type of installation to use. Probably only available in larger quantities than needed for this project. Will require insulating washers to be bought as well.
c. Insulated Sheathing ($13.00)
Probably the easiest to install. Good insulator. Thin. Probably only available in larger quantities than needed for this project. Possibly can cut insulating washers out of this.
2.Flexible Heater – 7in x 8in flexible heater with 2.5W/in^2 power output. ($42.00)
Available from omega engineering (http://www.omega.com/ppt/getpricesc.asp) Used in this reprap design: https://shop.grrf.de/beheiztes-druckbett-p-279.html?language=en
Subtotal = insulated sheathing + Flexible Heater = $65 or less (flexible film and insulation may be available cheaper or in lesser quantities. Prices listed are for the median price range.)
References
[1]http://hydraraptor.blogspot.com/2010/01/will-it-stick.html
[2]Incropera, Frank P. Fundamentals of Heat and Mass Transfer / Frank P. Incropera ... [et Al.]. Hoboken, NJ: John Wiley, 2007. Print.
[3]
