THERMOFORMING: Determining the Right Temperature for Thermoforming
Determining the Right Temperature for Thermoforming
Improving your thermoforming process is not a question of one heating element versus another, but rather the optimum element for the given application. Early
thermoforming pioneers, namely Gaylord Brown and Robert Kostur, in 1954 concluded that it is the process, not the elements, that can make or break the successful (and cost-effective) forming of a quality part.
Sheet temperature is one of the most critical elements of the thermoforming process. It is sheet temperature that dictates the process, the end result being not just a better product, but also decreased cycle time, less scrap, less floor space and reduced energy and labor costs.
There are many types of heating systems that can be used with heavy-gage thermoforming machinery. These include electric ceramic, panel, tubular quartz, metal sheathed tubular, gas catalytic halogen and gas-fired ceramic.
But heat is heat is heat, and the type of heat that should be used is dictated by numerous factors. There is no one type of heating element that is across-the-board better than another for all material applications.
This, along with several other factors such as the type of machine, (rotary, shuttle, inline) the complexity of the part to be formed, sheet size, depth of draw, energy consumption, the thermoforming system’s process design and yes,
the savvy of the operator, are critical factors. Nearly all heating approaches are good but the optimum (and recommended) heating system depends on the material and its end use.
Know Your Temperatures
Knowing your temperatures is the key that unlocks the door to everything a thermoformer wants and needs to compete successfully in an increasingly competitive marketplace. Temperatures and differentials are critical to the
process. Therefore, temperature knowledge and control must be applied in the following areas:
◊ Uniform sheet (not oven) temperature
◊ Material core temperature versus sheet surface temperature
◊ Mold surface temperatures versus water temperature
◊ The ability to zone, shade and profile the oven to achieve the optimum end product.
Uniform Sheet Temperature
How does one achieve uniform temperatures across the sheet? When we speak of temperature, it is uniform sheet temperature that optimizes the process, not uniform oven temperature. Oven zones must be set at different intensities
to achieve uniform sheet temperatures.
What Is The Correct Temperature?
Each material has a specific orientation temperature. This is the temperature required at the core of the material, not at the material surfaces. The relationship
between core temperature and surface temperature will need to be determined. Sheet surface temperature will directly correspond to material gage.
For example, let’s use 0.250-in. high-density polyethylene (HDPE), which requires a core temperature of approximately 280°F for optimal forming. To achieve this core temperature, the former must calculate the temperature differential on both sheet surfaces. Fortunately, today’s technology has produced affordable, hand-held, non-contact infrared pyrometers that can be used to determine the sheet surface temperature.
The 10-10-5 Rule
The difference in temperature, or variance, must fall with what we call the 10-10-5 rule. The first 10 applies to the 10 locations on the sheet where temperature must be known: the four corners and center of both sides.
The next 10 refers to the fact that the temperature must not vary more than 10°F on any of the 10 sheet locations.
Each temperature at each location must not vary ±5°F. This ±5°F needs to be held throughout the heating, forming and cooling process. Thus, ±5°F is the last component of the 10-10-5 rule.
Why is this important? Many thermoformers think that simply increasing oven temperatures (to get the corners up to orientation temperature) is the answer to achieving an optimum part. But consider this: by increasing the temperature, you increase heating energy by as much as 40%. If you first figure out the optimum heat and heat balance to achieve proper core temperature you could achieve a 25% decrease in cycle time with a 35% reduction in energy costs. This results in more parts per hour with less floor space.
Contributed by MAAC Machinery Corporation, 590 Tower Blvd., Carol Stream, IL 60188, 800-588-MAAC, 630-665-1700, Fax: 630-665-7799, E-mail: email@example.com, Web; www.maaacmachinery.com.
COPYRIGHT 2000 BY MAAC MACHINERY CO., INC., CAROL STREAM, IL 60188. All rights reserved. No portion of this Publication, whether in whole or in part, can be reproduced without the express written consent of MAAC MACHINERY CO., INC.
|Plastic Processing Temperatures|
In Degrees F
|Material||Specific Gravity||Mold Surface & Demolds||Lower Processing||Orientation||Upper Temp|
| Polyethersulfone, glass filled 20%||1.51||410||535||560||720|
| Polyethelene, high density||0.99||180||260||270||430|
| Polypropylene, glass filled||1.05||195||265||280||450|
|Vinyl, rigid foam||0.70||160||240||260||350|
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