The principal method by which polymer or plastics material is converted into useful products is by moulding, usually under heat and pressure, in a hollow mould or die. The material takes the shape of the mould, from which it is then removed. The relationship between the final product and its mould is of critical importance, both dimensionally and with regard to material properties.
Any successful design requires comprehensive information on a wide range of parameters. This Engineering Design Guide adopts a practical approach in considering those parameters that are of primary importance in the design of plastics mouldings. Because of their complementary nature, thermoplastic and thermosetting materials are dealt with jointly.
Although processing techniques and their effects on the physical and chemical structure of the product vary，all the materials in the range under consideration soften under heat and flow under pressure. It is important to remember that any one class of material may consist of between six and sixty variations accounted for by changes in molecular weight, by chemical combination with other materials, or by the use of fillers. Examples of these variations include modifiers, lubricants，and co- polymers (such as ethylene-propylene copolymer).
It must be emphasized that good design cannot result from a knowledge of material parameters only. The effect of processing on the material is extremely important, and all aspects of the design process must be covered including the technical characteristics of the materials and the correct design and temperature control of machines and tools.
Other related titles in the Engineering Design Guide series are The engineering properties of plastics and The selection and use of thermoplastics.
At some point in every design, it is essential for the eye to see and to comprehend the actual size of an object. For example, it is possible to represent, on a 12-in. CRT, the scale representation of a 10 ft mold. This means that a plate of 12-in. thickness will appear on the CRT approximately 3/4 in. in dimension. Try looking at a picture drawn at 3/4 in. and then try to convince your mind it is actually 12 in. We do not wish to belabor this point except to recite an engineer’s observation: That injection machine (manufacturer nameless) was the most overdesigned machine he had ever seen. No one had bothered to lay out on a piece of paper the actual thickness of the platens. They were almost twice as thick as they needed to be.By the same token, a .005 in. section can be “zoomed，’ to fill the 12-in. CRT. The deception to the eye and mind is still there, but in the opposite manner.
At the present time, the technology for modeling nongeometrical (meaning irregular) three-dimensional shapes has not reached the point where all such shapes can be accurately described. Those areas on a model which are now blended by the skilled pattern or moldmaker present the greatest challenge to manufacturers of CAD/CAM systems. To overcome this limitation, some CAD/CAM manufacturers offer a digitizing option to generate the data required to establish a tool path for the NC machine. This digitizing option requires “scanning”a model with a stylus similar to that used on the automatic duplicator. The stylus movement is then translated to “digital code” which the NC machine understands and can translate into movement of a cutter or movement of a table, or both simultaneously.
If the makers of NC equipment have their way, all hand duplicators or automatic duplicators will become obsolete. It is true that usage of the less sophisticated machinery will decline in the future. However, bear in mind the areas of the world in which the “industrial revolution” is still to come. The authors believe that this text can educate our successors, even while we address our contemporaries.
With the completion of the mold cavities, they are fully machined on those surfaces which must mate with inserted parts and are sized to fit the Particular machine for which the mold is intended. The mold halves (for a two-part mold) then have dowel pins and bushings added to assure proper alignment of the cavity.
Simultaneously with the machining of the cavity halves, inserts to form the neck and base ends of the cavity are being completed. Inserts serve two purposes. First, they include areas that are likely to wear and can therefore be replaced without reworking or scrapping the entire mold. Second, this provides an opening to the cavity that permits the mold maker to match up accurately the cavity halves at the parting line.
Before assembly of inserts, cooling lines are added to the mold. The use of drilled lines and milled channels is usually employed to carry the coolant. It is desirable to maintain a large area through which the coolant flows and at the same time keep the cross section of the cooling stream at a minimum. The latter requirement keeps the amount of cooling medium to a minimum and assures a rapid change of coolant over the surfaces that are to be cooled. The consistency of cross-sectional area through which the coolant flows is also important. With an increase in the area the coolant velocity decreases, developing a hot spot in the mold because the heat is not being conducted away as rapidly as from other areas in the mold.
Drilled lines are usually added to all inserts making up the mold assembly. All parts of the mold are normally cooled independently to afford the blow molder greater flexibility to his molding cycle through varying coolant temperatures.
With the assembly of inserts and matching of halves mentioned above, relief is milled in the mating mold faces to permit displacement of the excess plastics material. The depth of the relief is critical and dependent on the weight of the article molded. If the relief is too shallow, the mold will not close completely and poor seam lines will appear on the molded article. If the relief is too deep the cold mold will not be in intimate contact with the excess plastics material and remove its heat. When this happens the article is removed from the mold, retaining hot pieces of attached excess material. These, in turn, can adhere to and spoil other articles.
Because it is so difficult to accurately determine the depth of pinch relief prior to molding, the use of a blown pinch has been employed. By cutting the relief relatively deep the excess plastic can be blown by either bleeding off some of the major blowing air or providing a separate air source. The heat in the excess material is now rapidly removed by touching the cold mold face on the outside and by the cooling air on the inside. The final operations in making a blow mold consist of attaching back-plates, water testing all cooling systems for leakage, and blasting the cavity surface with an appropriate abrasive material. The mold cavity is usually given a rough finish except for those items requiring a high surface gloss. This rough surface vents the air trapped between the plastics and the cavity surface during the blowing cycle. Without this rough mold surface or other venting means, the resultant surface on the blow article would be rough and pock marked from air entrapments.
Nearly all molds in use today consist of two halves matching on a single flat surface. There are, however, molds having irregular parting lines and 伊 more than two movable pieces. This type of mold becomes necessary when the part to be molded has reentrant surfaces. While multiple-action molds increase the design possibilities of blown articles, the cost of tooling can be considerably higher. Only the economics of a specific job can dictate the type of tooling.
While the hand duplicator or copy milling machine is still the backbone of the mold-making industry because of its ability to cut just about any shape with a high degree of accuracy, it is slow and requires a highly skilled operator in order to obtain the optimum quality and accuracy of cut. This hand duplicator has given way to the automatic duplicator. Even more recently, it has given way to the numerically-controlled (NC) or computer numerically-controlled (CNC) machine, because automatic duplication or computer control will increase productivity as well as provide more accurate contours needing less handwork to finish to final surface and dimension.
The automatic duplicator with its multiple heads can cut as any duplicates as there are cutting heads. The path of the cut is controlled by a stylus which traces over a pattern of the desired shape. The stylus is connected, in turn, to a sensing device which will transmit to the hydraulic °r electronic control units the information they need to direct the machine to cut the correct shape of the reproduction. In all instances, some movement of the stylus is required to transmit a change in direction to the tool# path. This fact, together with the necessity for the stylus diameter to be larger than the cutter diameter, eliminates this method of duplication when fine detail is required.
Numerically controlled machinery has been available for a number of years for two and two-and-one-half axis machining. Now these machines have added controls with their own computer to enhance their flexibility of operations. The most recent technological improvement has been the use of computer graphics to program or direct the operations of the NC machines. This recent technology is the so-called CAD/CAM system (CAD is the acronym for Computer Aided Design; CAM is the acronym for Computer aided Manufacturing). In reality, the mold designer uses a CRT (cathode-ray tube) on which to draw what would normally be drawn with pencil and paper. Figure 11.33 illustrates this CRT technique. That which is drawn on the CRT can be reproduced at any desirable size or scale to make the familiar “tracing” associated with drafting. In the case of CAD, this reproduction is called “plotting.” The important point here is this: whatever shape can be drawn or modeled on the CRT can be reproduced by the NC or CNC machine within the normal limitations of tool geometry. It should be obvious that the potential for the CAD/CAM system of mold-making is enormous. The overall cycle time to make a mold can be greatly reduced by eliminating the need for a three-dimensional pattern, by increasing, the handwork on a mold, and by increasing the accuracy of the duplication. Many NC machines performing multiple tool operations (using automatic tool changers) are capable of unattended operations on a 24-hour basis. However, someone who knows the “shut-down procedure，，should e nearby，for that rare occasion when “automatic” is not good enough to Prevent catastrophe.
The stretch-blow process is used to manufacture plastic containers which generally are lighter in weight and have greater clarity and improved physical Properties，compared to containers manufactured by other blow-molding Processes. In stretch-blow molding, the container material is biaxially ented by stretching and blowing the parisons at a critical temperature, which is different for each of the materials commonly used PET, also PETP), PVC, and PP. The parisons, after being formed, are temperature condtioned，stretched longitudinally up to twice their original length by mechanical means, and then blown in a blow mold to the container shape. Stretch-blow machines and molds are more complex and more costly than those used in other blow-molding processes and, as a consequence, the stretch-blow process tends to be limited to high-volume production.
The machines commercially available vary in the means used to form, temperature-condition, stretch, and move the parisons from station to station in the machine, some of which are shown below.
Single Stage Machine. In a single-stage machine, all of the steps involved in producing a fully formed, biaxially oriented container occur in continuous sequence in a single machine.
In one kind of machine, the parisons are precision-formed in a fashion similar to the injection-blow process. After the parisons have cooled to a point at or below the temperature at which orientation occurs, they are removed from the parison tooling and transferred to a conditioning station where they are heated and/or allowed to equilibrate to their critical orientation temperature. The parisons then are moved to the blow station, where they are held by the neck finish, stretched mechanically by rods inserted into the open necks, and immediately blown to the shape of the container. In one type of machine, the sequence of operations occurs in a rotary fashion, and in another type the operations are carried out in linear movements. Such single-stage machines are being used to produce clear, light-weight PET containers in a size range of 1/2 to 2 liters for packaging carbonated beverages.
Another kind of single-stage machine is similar to a continuous-extrusion shuttle machine except that a preform station and mold have been interposed between the parison extruding and blowing stations. This type of machine is used primarily to produce containers from PVC or other heat-sensitive materials, in a size range of V3 to 2 liters.
Two-Stage Machine. In two-stage processing, parisons are formed in a first-stage machine which is normally off-line from the second-stage machine where the containers are blown. Parisons are formed either by injection molding，using specially designed runnerless molds, or by extrusion molding in a fashion similar to pipe extrusion. Injection-molded parisons are precise by nature with fully formed necks, whereas extruded parisons are essentially short pieces of pipe which require secondary forming at both ends. The fact that parison forming is independent from parison blowing allows for the possibility of molding parisons in one location and transporting them to another location for blowing.
The parisons are normally at room temperature when they are fed by automatic unscramblers into the second-stage blowing machine. The parisons pass through a chamber where they are gradually heated to the orientation temperature required for the material being processed. The parisons generally rotate as they pass through the chamber, and the chamber is usually equipped with multiple heat zones to optimize the temperature of the parisons.
In one kind of second-stage machine, which uses extruded parisons, the heated parisons are transferred from the heat chamber to the blow station, where one end of the parison is reformed by neck rings which close around it, and by a core (swage) which compression-forms the neck finish; the other end of the parison is gripped by a picker which mechanically stretches it to a length sufficient for the blow-mold halves to close around it and pinch the end shut. This type of machine is being used to produce relatively clear PP containers in a size range of 4 to 48 ounces for packaging sterile medical solutions, household chemicals, and food products.
In another version of the second-stage machine, which uses injection molded parisons, the parisons are restrained at one end by a bead at the base of the neck finish, stretched mechanically by rods inserted into the open necks，and then immediately blown to the shape of the container. This version of the second-stage machine is widely used to produce clear, light-weight PET containers in a size range of 1/2 to 2 liters for packaging carbonated beverages.
The injection-blow process combines the function of blowing with the precision of injection molding; it requires a parison-forming mold with integral neck ring, multiple core rods (blow pins) and a blow mold. A core rod is moved into the parison mold and material is injected into the mold to form a Precision parison with a fully formed neck. Within certain limits, the parison can be shaped internally by the core rod，and externally by the parison to control the uniformity of the wall thickness of the blown product, lie the body of the parison is still hot and plastic, the core rod with the Parison on it is lifted from the parison mold and then transferred to the blow mold. Air is introduced through the core rod to blow the parison to the final form of the product. After cooling, the product is removed from the core rod as a completely finished article without the need for secondary trimming operations.
The injection-blow process is used to manufacture products requiring close control of dimensions, weight, capacity, and material distribution. There are three main types of injection-blow machines commercially available. They are discussed below.
Three and Four-Station Rotary Machines. The parison mold and blow mold are mounted in a fixed radial position with respect to each other. The core rods are mounted in a head which indexes intermittently to transfer the core rods from the parison station to the blow station and then to an ejection station. The four-station machine has an extra station, after the ejection station, for conditioning (heating or cooling) the core rod before it is transferred into the parison mold.
The rotary machines are the most widely used of the injection-blow machines because of their versatility in producing precision containers in a size range from a fractional ounce to 64 ounces.
Two-Station Machine. In comparison with the four-station rotary machine, the ejection and conditioning stations have been eliminated. Ejection of the finished product occurs as the blow mold is opened. Conditioning in a two- station machine is normally accomplished by the circulation of temperature-controlled fluid through the core rods.The two-station machines are used primarily for producing specialized containers for paint or food products.
Adaptive Tooling. Various kinds of systems have been designed and built to adapt standard injection molding machines for blow molding. Such systems generally find application in injection molding companies that want the flexibility to occasionally use their standard machines as blow molders.
Formed Neck Process
In this process, the neck of the container is precision-formed by pressing the neck ring integral blow pin against the extrusion die and extruding the plastic material into the interior space. After the neck is formed, the neck ring and blow pin move away from the extrusion die while additional material is extruded at a controlled speed for the section of the parison which form the body and base of the container. In one variation of the process, a short blow pin is used and the parison is pinched off by the mold halves which close on it. In another variation, a long blow Pin is used which supports the entire length of the parison; when the desired parison length has been extruded, it is cut off and the parison is rotated 180° by the blow pin to the blow mold. Commercially available machines are not yet in wide use, but proprietary machines employing the first variation of the process have been used for many years.