Thermoforming Essays (785 words) - Packaging, Thermoforming

Thermoforming THERMOFORMING Thermoforming is one of many manufacturing processes that converts plastic resin into usable everyday products. Thermoforming is greatly relied on in today's society because of the tremendous need for high volume plastic products. Thermoforming is considered to be one of the most cost-effective processes in plastics manufacturing. Thermoforming is considerably economical because of the low molding costs and fast molding cycles. Speed and cost efficiency are the highlighted qualities that thermoforming offers which lead the way for the process becoming so important in industry today. The basic concept of thermoforming is quite simple. A premanufactored thermoplastic sheet is heated until it becomes soft and pliable. It is then forced against the contours of a mold until it cools to its original state. Once it has cooled it is removed from the mold while still obtaining the shape of the mold. Usually the product is then trimmed to produce the finished product. The operation seems simple and straight foreword but there are many different applications associated with the process. Thermoforming is a broad term; there are many different types of thermoforming processes. These processes all have similar traits but they do differ in certain aspects of the overall process. Trapped Sheet Forming is a specialized type of thermoforming. In trapped sheet forming a hot blow plate is used in both the heating and forming process. A plastic sheet is positioned between the hot blow plate and the female mold cavity. Air forced through the plate and pressure from the female mold combine to thrust the sheet onto the hot plate. The sheet is then heated and forced into the female mold by the use of air pressure. One advantage to trapped sheet forming that is such a simplified process that many products can be produced from this method. Also the process uses contact heating, which is easily used and is not subject to temperature fluctuation. The contact heating is a definite advantage but is also a slight disadvantage. The problem with contact heating is that heat can only be applied to the underside of the sheet. This slows down the process and restricts the use of the more durable heavier -gauge sheets. Plug-assist forming is one of the most widely used thermoforming processes today. In plug-assist forming a heated sheet is sealed over a female cavity. Once the plastic is sufficiently heated the plug-assist, which is shaped like the female cavity but slightly smaller in size, pushes the plastic sheet and forces it in the cavity. Vacuum force pulls the sheet onto the mold surface. The main advantage associated with plug-assist forming is that the walls of the container can be measured precisely with the assist. Wall thickness is uniformly the same throughout the container. Plug-assist forming works well with both heavy and light gauge materials and is commonly used to form deep-drawn containers. Pressure bubble plug-assist vacuum forming is similar to plug-assist forming. In this forming process a portion of the sheet is stretched to guarantee an even thickness of walls. The heated sheet is positioned over the female cavity. Air is then blown up through the base plate channel. This air causes the sheet to billow upward. The sheet is then pushed into the cavity by the plug-assist. Vacuum is then applied to transport the sheet to the mold. The advantage to this forming technique is that the wall thickness can be measured with great accuracy. This process, as with plug-assist forming, is used to create deep-drawn containers. Pressure bubble snapback forming is similar to bubble plug-assist forming except for one aspect. The plastic sheet is not formed by the female mold but rather the male mold. The sheet is snapped back to form against the male mold. This forming process improves material distribution because of its prestrecthing procedures. The snapback vacuum forming method is popular because of its simplified process. A sheet is clamped over a female cavity, air pressure through the base plate then stretches the plastic. The pressure is then turned off while the vacuum is turned on to pull the plastic into the mold. This process is widely used to produce auto parts and luggage due to its ability to create external deep draws. It works well with all medium

Understanding and Using Loops in Delphi Programming

Understanding and Using Loops in Delphi Programming The loop is a common element in all programming languages. Delphi has three control structures that execute blocks of code repeatedly: for, repeat ... until and while ... do. The FOR loop Suppose we need to repeat an operation a fixed number of times. // show 1,2,3,4,5 message boxesvar j: integer;beginfor j : 1 to 5 dobeginShowMessage(Box: IntToStr(j)) ;end;end; The value of a control variable (j), which is really just a counter, determines how many times a for statement runs. The keyword for sets up a counter. In the preceding example, the starting value for the counter is set to 1. The ending value is set to 5.When the for statement begins running the counter variable is set to the starting value. Delphi than checks whether the value for the counter is less than the ending value. If the value is greater, nothing is done (program execution jumps to the line of code immediately following the for loop code block). If the starting value is less than the ending value, the body of the loop is executed (here: the message box is displayed). Finally, Delphi adds 1 to the counter and starts the process again. Sometimes it is necessary to count backward. The downto keyword specifies that the value of a counter should be decremented by one each time the loop executes (it is not possible to specify an increment / decrement other than one). An example of a for loop that counts backward. var j: integer;beginfor j : 5 downto 1 dobeginShowMessage(T minus IntToStr(j) seconds) ;end;ShowMessage(For sequence executed!) ;end; Note: its important that you never change the value of the control variable in the middle of the loop. Doing so will cause errors. Nested FOR loops Writing a for loop within another for loop (nesting loops) is very useful when you want to fill / display data in a table or a grid. var k,j: integer;begin//this double loop is executed 4x416 timesfor k: 1 to 4 dofor j: 4 downto 1 doShowMessage(Box: IntToStr(k) , IntToStr(j)) ;end; The rule for nesting for-next loops is simple: the inner loop (j counter) must be completed before the next statement for the outer loop is encountered (k counter). We can have triply or quadruply nested loops, or even more. Note: Generally, the begin and end keywords are not strictly required, as you can see. If begin and end are not used, the statement immediately following the for statement is considered the body of the loop. The FOR-IN loop If you have Delphi 2005 or any newer version, you can use the new for-element-in-collection style iteration over containers. The following example demonstrates iteration over string expressions: for each char in string check if the character is either a or e or i. consts About Delphi Programming;varc : char;beginfor c in s dobeginif c in [a,e,i] thenbegin// do somethingend;end;end; The WHILE and REPEAT loops Sometimes we wont know exactly how many times a loop should cycle. What if we want to repeat an operation until we reach a specific goal? The most important difference between the while-do loop and the repeat-until loop is that the code of the repeat statement is always executed at least once. The general pattern when we write a repeat (and while) type of loop in Delphi is as follows: repeatbeginstatements;end;until condition true while condition true dobeginstatements;end; Here is the code to show 5 successive message boxes using repeat-until: varj: integer;beginj:0;repeatbeginj : j 1;ShowMessage(Box:IntToStr(j)) ;end;until j 5;end; As you can see, the repeat statement evaluates a condition at the end of the loop (therefore repeat loop is executed for sure at least once). The while statement, on the other hand, evaluates a condition at the beginning of the loop. Since the test is being done at the top, we will usually need to make sure that the condition makes sense before the loop is processed, if this is not true the compiler may decide to remove the loop from the code. var j: integer;beginj:0;while j 5 dobeginj:j1;ShowMessage(Box:IntToStr(j)) ;end;end; Break and Continue The Break and Continue procedures can be used to control the flow of repetitive statements: The Break procedure causes the flow of control to exit a for, while, or repeat statement and continue at the next statement following the loop statement. Continue allows the flow of control to proceed to the next iteration of repeating operation.