EUROLINIA infrared tunnel ovens: basic information
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Thermoforming is a process of changing the shape of flat blanks (sheets or films) made of thermoplastic polymer material under high temperatures into three-dimensional molded products.
During the thermoforming process, a number of the following errors could happen:
• improper product design;
• wrong choice of material;
• wrong machine for a specific application;
• improper installation/arrangement of the machine;
• improper thermoforming tool;
• starting a new thermoforming tool;
• errors when producing samples;
• improper or inadequate heating;
• insufficient pressing force during vacuum and pneumatic forming.
Some of them are reviewed in the presented material.
When choosing the most suitable machine, you have to consider the following factors:
• product dimensions (length L, width W, height H);
• type of polymer material;
• type of material (sheets or rolled);
• product's geometry;
• accuracy specifications for components;
• number of molds produced;
• existing thermoforming machines;
• tool costs;
• material consumption and amount of waste;
• personnel costs;
• costs for machine.
If the produced product is subject to thermoforming on several machines or there are different versions of the product, you have to calculate the manufacturing costs of the product and compare them between all these options. For small batches of products, tool costs are of great importance, and material costs become more important with larger batches. When purchasing new machines, the costs of maintenance, service, technical and consulting services must be taken into account.
The proper installation of the thermoforming machine is very important to manufacture products that require high accuracy in reproducibility and variation of wall thickness. Machinery should not be located near driveways, open ventilation, or similar passages with high air velocity. In case the inconvenient location cannot be avoided, the machine must be protected from air leaks by means of an efficient screen or wall.
You have to provide sufficient space for transporting tools, sheet materials, and finished products. The requirements for the premises must consider the passage of available industrial vehicles.
You should provide sufficient space between the machine and the walls to ensure maintenance of the thermoforming machine and, for example, to replace large vacuum pumps.
When providing compressed air, vacuum, and cooling water, requirements established by machine and equipment manufacturers for the flowrate and pressure must be met completely. The compressed air must be dry and oil-free as it passes through the machine. The cross-section size of the supply pipelines should not be smaller than the connecting inlets on the machine. The supply pressure shall not be lower than the pressure set on the inlet valve of the machine. If several machines are combined in a common process line, the capacity of this line should be checked with each new connection.
The choice of material for the thermoforming tool depends on the volume of the batch produced and the requirements for product quality. Potential materials are listed here in ascending order (by cost): wood, polymer resin, and aluminum (without thermal conditioning and with thermal conditioning).
An "inexpensive" tool does not always guarantee low production costs for thermoformed products. It is more cost-effective to manufacture large batches of products using thermal-conditioned aluminum molds. The calculation of the manufacturing costs for the thermoformed products ensures the correct choice of the most suitable material for the tool. When ordering a thermoforming tool, you have to find out - does it include the fine-tuning of the tool in the field.
Polymer resin thermoforming tools are rarely subject to therm conditioning because poor thermal conductivity does not allow to use thermal equalizing in full. A short cooling time can only be obtained by using thermally conditioned metal molds with excellent thermal conductivity, such as aluminum. Long channels for heating/cooling of a long forming tool with direct thermal conditioning causes high-pressure losses if the cross-sections of the channels are too small. A sharp reduction in the flow rate of the cooling medium impairs thermal conditioning or even makes it inefficient. The temperature of the tool rises in an uncontrolled manner. For the forming tool with indirect cooling, you have to ensure that the contact or heat transfer area through the cooling plate is as large as possible. The forming tool should have, if possible, several contact planes, and should not reduce the dissipation of heat due to the transfer medium inside the plate.
The temperature or temperature distribution along the surface of the thermoforming tool determines the quality of the molded products. If the only option is to adjust the temperature of the water in the thermal conditioning device, it is absolutely necessary to control the temperature of the tool. The target is to adjust the water flow and/or water temperature in the mold using sensors so that the temperature of the tool is as constant as possible. This is determined by the level of craftsmanship for manufacturing the forming and blanking tools. It is important to install thermocouples in the right places. The experience of the manufacturer, as well as the tool maker, is of particular value here.
The thermoforming tool must be rigid enough to withstand the pressure of molding.
If small bubbles (air inclusions) appear on the surfaces of the product, this is usually a sign of lack or poor air suction in the tool. The air remaining between the product and the forming tool cannot be removed. Therefore, the surface of the forming tool should be sandblasted or roughened, if at all possible. Polished molds are an exception; only molds for completely transparent products or only the corners of male molds need to be polished.
If the cavities in the multiple nested molds are all identical, you have to ensure that not only the outlet channels or slotted openings of each mold element, but also the vacuum channels are similar. Each vacuum channel must be approximately the same size up to the point where the vacuum is connected to the machine table. In any case, they should not be smaller, so the vacuuming should be identical in all zones. The outlets on the surface of the forming tool not only serve to remove air during the molding process but also to supply air for removal. In particular, for high male molds, the cross-sections of the channels must be large enough to supply the necessary amount of air for the quick removal of the product. We recommend using drilled nozzles or valves here.
The detection of defects immediately upon installation of the forming tool in the machine saves costs, as long as they are installation defects and they could be rectified! It is very important to check the outlet channels and cross-sections on the vacuum-forming molds. Once the tool has been installed, you have to check the air suction. The scale of the vacuum gauge (from 0 to -1) should show about -0.2 with free air removal for large molds without material and about -0.3 for small ones. This only applies to molds with fully sealed bases. During thermoforming, i.e. with the material inside, the vacuum line reading should be checked again, it should be at least -0.9 if the machine is operating at an elevation of no more than 500 m above the sea level. If the pressure gauge shows a lower ultimate vacuum value, the machine and tool must be checked for leaks, as the ultimate value affects the imprint, and the heating/cooling time and therefore it directly affects the molding cycle.
To check the uniformity of heating, it is advisable to start with the installation of a "common heating panel". For modern sheet thermoforming machines and automatic thermoforming machines using rolled materials, such heating panels are designed using the appropriate software.
The data on the setting up of the thermoforming machine, obtained during the trial runs, are recorded on the lay sheet or loaded into the database (hard disk or external drive). The practice has shown that it also makes sense to print copies of the setup data - at least for the most important molded products - and place them in a file cabinet.
All conditions when manufacturing product samples must be identical to the conditions of subsequent mass production. Every worker, from the toolmaker to the machine operator and personnel for mechanical processing, should be familiar with the dimensions and tolerances for the thermoforming product under production.
In case the operator of the thermoforming machine is aware of the cutting contour of the molding, he will be able to make his own decision regarding the accuracy of the parts in the cut area, contributing to the reduction of waste and cost savings. To avoid problems, the material used must be identical to the sheet material required for manufacturing in terms of quality, polymer grade, color, shrinkage, and surface texture. When cutting and installing blanks, you have to take into account the "position" of the material (direction of extrusion). The formation of wrinkles and properties of material during the forming process depends on the direction of extrusion. To ensure optimal use of the material, you have to accurately determine the shape of the tool and the size of the clamping frame. The thermoforming tool should be designed so that the distance from the cutting line to the clamping edge or the clamping plane is as small as possible (waste reduction). The dimensions of the molding area must be identical to those for the manufacturing tool. If possible, the making of product samples and full-scale production should be carried out on the same machine model. If, due to cost-saving, a male mold is used to manufacture the samples, and full-scale production will be based on the female molds, the review of the design can only be completed in general terms, and it will not be possible to conclude on the stability, variation of wall thickness and accuracy of the product during the manufacturing cycle.
To get an accurate geometric configuration of products, it is important to choose the correct molding pressure.
Too low molding pressure occurs:
• due to poor rubber sealing on the tool and the clamping frame;
• on single-position machines when the base of the mold is too high;
• when unused connections of the table are not sealed;
• if the clamping frames at the molding station have been deformed due to too weak or too strong tightening;
• if the base plates are uneven;
• if air-permeable layers of wood are used as a base plate;
• if pipelines have leaks;
• due to errors made during the installation of the machine, for example, poor compressed air supply because of too small cross-sections of holes and pipelines.
When several machines are combined into a closed process line, and all machines are running at full load at the same time, the vacuum or molding pressure values will be unstable if the line cannot cope with the demand.
The reasons for the wrinkles encountered in thermoforming can be divided as follows.
Wrinkles due to the material:
• surface wrinkles because of too much sagging of the material during heating;
• orientation wrinkles caused by excessive alignment stresses during material extrusion.
Wrinkles caused by tool design:
• angular wrinkles on male molds;
• wrinkles on vertical walls in the corners, if the radius or degree of taper is too small;
• wrinkles caused by the insufficient spacing between individual cavities in multiple nested male molds;
• wrinkles in deep female molds with a molding ratio greater than 1:0.4, and a very large ratio of length L to width W.
Specific conditions of the forming process that cause wrinkles:
• preliminary blow molding during pre-molding is too high;
• too high molding speed due to the suction through a vacuum hole with too large cross-section;
• the molding speed is too slow, so the thin material is being colled too much before molding.
Wrinkles are never generated when the material is being extruded, but only when it is shrinking, if the molding temperature is too low, and/or the shrinkage rate is too high.
How to avoid wrinkles:
• Ensure less heating for materials with too much sag.
• Heat the materials more if a preliminary blow molding is not applied or applied at a slight degree, and the sagging of the material is not too high yet; increase the temperature of the forming tool.
• When doing preliminary blow molding, reduce it slightly, apply less heat, and slower the suction, if possible. Increase the temperature of the forming tool.
• Reduce the molding area.
• Install male molds, make sure small wrinkles come into contact with the auxiliary mold, or support the wrinkles from below with a mold of the appropriate configuration during the preliminary blow molding. One auxiliary mold is used to arrange the preliminary extrusion of the polymer material in the area where wrinkles appear during the molding process.
• Change the configuration of the forming tool by creating additional surfaces: increase the size of the radius, degree of taper, or volume of the tool below the wrinkles.
Good luck in thermoforming! Let us know your thoughts at info@infra-heater.com