Looking for calculations I could apply to a vacuum-forming machine

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Discussion Overview

The discussion revolves around calculations and considerations for a vacuum-forming machine, focusing on aspects such as evacuation time, heat transfer, and the application of heat to the plastic sheet. Participants explore theoretical and practical elements related to the design and operation of the machine.

Discussion Character

  • Exploratory
  • Technical explanation
  • Homework-related

Main Points Raised

  • Some participants have calculated evacuation time and basic heat transfer but express uncertainty about their correctness and seek further advice.
  • There is a request for clarification on the variables in the relevant equations, with assumptions made about temperature and time.
  • Participants inquire about the specifics of the mold and workpiece, including dimensions and materials, to better understand the context of the calculations.
  • One participant suggests using a hot air gun for heating, while another proposes using a 220V rod element, indicating that heat would be applied via radiation.
  • Questions arise regarding the capacity of the vacuum pump and whether a vacuum reservoir is necessary, with a preference for a smaller pump for desktop applications.
  • There is a discussion about the wattage of the heating element, with one participant noting it is rated at 0.3 kW.
  • A later reply highlights the complexities of heat transfer by radiation, mentioning factors such as the temperature of the emitter, view factor, and emissivity of the absorber, suggesting a differential equation for modeling the heating process.

Areas of Agreement / Disagreement

Participants express various viewpoints and uncertainties regarding the calculations and methods for heating the plastic, indicating that multiple competing views remain without a consensus on the best approach.

Contextual Notes

Participants note limitations in their calculations and assumptions, particularly regarding heat transfer and the specifics of the vacuum system design.

SaldanaKS
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Homework Statement
Designing a vacuum forming machine and stuck on creating calculations to supplement the project and aid in proof of design and feasibility. I have done some basic vacuum calculations such as determining the evacuation time as well as a heat transfer calc however I'm not fully confident in their correctness and was hoping to get some advice on any calculations that could be done relating to the vacuum system, heating element, and thermodynamics or the machine in general?

The machine is a simple vacuum forming machine where an aplastic sheet is heated and then drawn over a mold, the vacuum is applied through a platen, and the sheet forms around the mold. Im creating the CAD model however I need to illustrate some calculations based off the design and considering the critical parts.

Any input, suggestions or advice would be much appreciated.
Relevant Equations
Forming area = 300 mm^2 = size of the sheet

Using a diaphragm vacuum pump

volume to be evacuated = 0.015 m^3

Using a steel rod heating element:

Power = mass x specific heat capacity x change in temp x time
P= 300 W

Rate of heat transfer, Q = 8 J/s
So far I have determined the evacuation time, a basic heat transfer between the heating element and plastic sheet, and a hold down force of the forming bed.
 
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Welcome to PF. :smile:

Could you please define the variables in your "Relevant Equations"? I'm guessing that T is Temperature (in K) and t is time? :wink:

Also, could you please provide a lot more details on your question, including sketches or pictures of your mold and workpiece (including dimensions) and the material that you are forming? Can you post links to the reading that you've been doing so far about vacuum forming?

Finally, just wanting to verify that this is for a schoolwork question and not for your work? I can move it to the ME forum if it is for your work. Thanks.
 
berkeman said:
Welcome to PF. :smile:

Could you please define the variables in your "Relevant Equations"? I'm guessing that T is Temperature (in K) and t is time? :wink:

Also, could you please provide a lot more details on your question, including sketches or pictures of your mold and workpiece (including dimensions) and the material that you are forming? Can you post links to the reading that you've been doing so far about vacuum forming?

Finally, just wanting to verify that this is for a schoolwork question and not for your work? I can move it to the ME forum if it is for your work. Thanks.
Thank you, I will update those and try to provide more information and some of my work completed thus far. Its for a university mechanical design project, I wasn't sure which topic that would fall under?
 
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You are posting in the correct forum here at PF if it is for a school project. If we need to page in some ME experts at some point who have seen your thread, we can certainly do that. :smile:
 
SaldanaKS said:
I have done some basic vacuum calculations such as determining the evacuation time as well as a heat transfer calc however I'm not fully confident in their correctness and was hoping to get some advice on any calculations that could be done relating to the vacuum system, heating element, and thermodynamics or the machine in general?
How do you apply the heat from the rod to the plastic ?
Maybe use a hot air gun ?
Does the pump need a big capacity, or do you have a vacuum reservoir ?
 
Baluncore said:
How do you apply the heat from the rod to the plastic ?
Maybe use a hot air gun ?
Does the pump need a big capacity, or do you have a vacuum reservoir ?
Using a 220V rod element to heat the plastic, therefore heat would be applied via radiation.

Not sure if a hot air gun would be as effective as a heating element, i have considered quartz and ceramic however.

lastly, the pump is to be as small as possible while being able to have enough suction for the vacuum. Its for a desktop application. So there won't be a reservoir.
 
SaldanaKS said:
Using a 220V rod element to heat the plastic, therefore heat would be applied via radiation.
Usually the heating elements are rated with wattage? Is it like a ##10 \, \rm{kW}## or a ##1 \, \rm{kW}## heater?
 
SaldanaKS said:
Its for a desktop application.
erobz said:
Is it like a ##10 \, \rm{kW}## heater?
Toasty!
 
pbuk said:
Toasty!
🥵
 
  • #10
After reading the OP more carefully. Apparently its rated as ##0.3 \, \rm{kW}##.
 
Last edited:
  • #11
Have you taken a course in Heat Transfer? I ask because heat transfer by radiation is a bit more involved than what you have seemingly computed. There is (at least) the temperature of the emitter to consider (simplest to assume the emitter is a blackbody), the View Factor between the emitter and the absorber (which depends on a number of characteristics pertaining to the physical setup), the emissivity of the absorber (simplest to assume total absorption, not necessarily most accurate), and the thermal characteristics of the absorber to consider. With some (perhaps arguably) reasonable assumptions, I think you are going to end up solving something like following differential equation for the time it takes to heat the plastic sheet to its forming temperature.

$$ \alpha \frac{dT}{dt} = -\beta T^4 + \kappa $$

If you are interested, the details can be filled in.
 
Last edited:
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