Vertical Injection Molding Process

Vertical Injection Molding Process

What distinguishes a micro injection process from a “normal” Vertical Injection Molding
process? Except for parts or structure sizes, the process is almost identical. Oen,
even the same procedures are applied, both in the actual injection molding process as
well as in the manufacture of the molds. Several studies (NEXUS Task Force Market
Analysis) also point out the current potential as well as future developments. The
entire process is very important in microinjection molding. Therefore, the process
is also oen referred to microsystems technology (MST).

The coordination of the process includes:

  • Molded part,
  • Injection mold,
  • Injection molding machine,
  • Peripherals (e.g., clean room, handling facilities), and
  • Quality assurance and further steps

should be an integral element of the MST.
This presents a new challenge for designers, project engineers, as well as development
Molded Part Design
In micro injection molding, two major groups of components can be differentiated:
Injection molded parts (macro-injection molded parts) with microstructures or
areas with microstructures . This mostly affects the surfaces.
Micro injection molding parts, in other words, small parts with low weight (mass
in mg) and the smallest dimensions and structures .
In either case, different tolerance, surface features, and accuracy may be required.
For the micro injection molded parts themselves, the same rules apply (e.g., avoidance
of sink points, etc.) as for “normal” plastic parts. Experience in micro-mold
making and preliminary testing is mostly necessary. Here, the collaboration with
the customer is a priority, much more so than in the area of standard shapes.

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How to decide plastic injection mold factory location


plastic injection mold factory location

Locality decision as to just where the plastic injection molds plant will be located should be governed by many factors. In the first place, it is well to consider the relation of plant location with that of the sales field. In other words, it would be better by far to be situated in the midst of activity rather than hundreds of miles away. If for instance, the greater part of the business were to be obtained in New England territory, a plant in California would be at quite a disadvantage.

Centralized location with respect to marketing is of utmost importance. Not only is there a great saving in freight charges on finished goods but in addition, there is no loss of valuable time in communication and delivery of merchandise.

Heavy freight charges must necessarily be reflected in the cost of the goods and any small increase in selling price is a draw back to the producer, inasmuch as there is just that much more sales resistance to be overcome.

plastic injection molds plant

Second in importance, is the proximity of the plastic  injection molding plant, to its source of raw material supply. Unfortunately, sufficient stress on this point has not been made in many industries and even though lack of consideration of this factor would not necessarily cause failure to a business, yet it would have a decided effect on the stability of a new organization. In the plastic molding industry this point deserves particular consideration inasmuch as the raw material constitutes a great part of the finished article so far as the cost is concerned. Once again freight costs must be taken into account and remoteness from a definite supply of molding powder tends not only to boost this price but also necessitates the carrying of large inventories to insure constant production. In many industries, the building up of large raw material supplies is advantageous but such is not the case in the plastic molding business.

Drums of powder are often exposed to atmospheric conditions and consequently become perishable. Moisture affects the condition and texture of the powder and in many instances makes it almost impossible to use after a definite length of time. Then too, floor space is sometimes at a premium and seldom is any provision made for an overstock of raw material like steel for plastic injection molds making.

Railroad and waterway facilities have a certain bearing on the location selected and some forethought of future legislation with respect to prospective new inland water routes or any contemplated express highways for trucking must be given due thought. These factors should be considered not only in the light of incoming raw material advantages but also outgoing transportation conveniences. One or another definite means of fast and efficient transportation should be available.


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Milling is the use of several single-point cutting tools grouped around the periphery or face of the cutter or hob. The principles of single-point tool cutting apply to milling. In milling, the tool rotates and the movement of the table, the spindle, or the arbor feeds the work into it.

The face of the cutter is usually straight and at right angles to the axis of rotation. The periphery is usually straight and parallel to the axis of rotation. The cutting edges are ground so as to maintain the correct cutting angles and chip space. The outside diameter and height of teeth decrease with grinding. The cutters may be solid or they can be made with inserted blades; both may be tipped with cast alloys or cemented carbides. If inserted blades are used, the size of matched or gang cutters can be maintained more economically.

Odd shapes or contours are incorporated in milling cutters known as form cutters and hobs. The back-off angle is continued in such a way that the contour is maintained by grinding only the face of the cutter. As the cutter is ground, its diameter is reduced. Cast alloys and cemented carbide tips also can be used in this type of cutter.

Economies in manufacture can be made by milling more than one surface at a time, or by cutting the separate surfaces without removing the part from the machine. More than one surface may be cut by group-milling cutters on an arbor. Different diameters in various in mill surfaces parallel or vertical to the axis of the milling . When slots or broad, flat surfaces are milled, interlocking blade cutters can be used to assure a continuously milled surface or the correct width of a slot The interlocked cutters are spaced apart by washer spacers at the hub of the cutter to compensate for the grinding of the cutter.

Parts are often designed with milled surfaces at an angle to other milled surfaces, or on a plane higher or lower than another. These surfaces can often be located in the same plane by altering the design of the part attached to the milled surface. In this way they can be milled in one operation .


With the use of cast alloy and cemented-carbide cutters, speeds and feeds have been increased so much that only the modern machines have the power, strength, and rigidity to use these cutters effectively. Chips are red-hot when parts are milled at these high rates; therefore, the engineer must again make certain that his part is rigid enough to withstand the cutting pressures, and that the part can be held firmly in the fixture or on the table. Vibration in spindle, arbor, table, fixture, or part is injurious to the tool and gives a poor surface finish. The high speeds used in cutting cast iron, brass, aluminum, and magnesium also require strong fixtures and machines to prevent vibration. Spindle speeds and feeds are so high that machines are now designed especially for the aviation industry to efficiently machine the light alloys.

contributed by John,a China plastic stool mould making manufacturer

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thermoplastic and thermosetting

 thermoplastic and thermosetting

All plastic articles are initially derived from molding compounds. These molding compounds consist of a resin or binder and one or more of the following components: fillers, plasticizers, dyes and pigments, and lubricants. The “resin,” as the principal component, gives the compound its name and classification, and imparts the primary properties to it It is the cohesive and adhesive agent which provides rigidity and binds together the filler particles. The “filler” is usually an inert, fibrous material which modifies the properties of the resin or imparts special properties to it. “Plasticizers” are added to the compound if the flow or softness of the compound must be regulated, while “dyes and pigments” are added to impart color to the molded part. Lubricants of wax or stearates are added occasionally to a molding compound to facilitate its removal from the mold. (Its degree of granulation also has a major bearing on molding qualities.)

Natural and synthetic rubber are plastics which are very important to industry. These incompressible elastic plastics present problems very similar to those encountered in the manufacture and application of other types of plastics. Some specialized equipment,such as callenders, is required for the preparation of raw material. Rubber-like plastics may be extruded into shapes, filaments, or sheets, and formed by the extrusion, transfer, and compression types of molds.

Some thermoplastic and thermosetting plastics are used as adhesives. They are tough, strong, and reliable, and can be applied between almost any combination of materials. One of the first and most successful applications was the use of transparent plastic sheets between two plates of glass to form our present-day safety glass. The tough plastic adheres to the glass and prevents splinters from flying. Plywood made with plastic adhesives withstands weathering and water and now can be used for concrete forms and outside sheathing for homes. Large wooden columns and thick panels can be built by curing the adhesive by induction heating. The combination of plastic and wood makes a strong structural member. Wood furniture and metal cabinets are held together by adhesives which simplify their design and reduce cost of manufacture.

Fabrics are made of many plastics in pleasing colors. They are durable, tough, and easy to clean. Natural fabrics such as cotton and wool are facing stiff competition from these new plastic filaments.

Reinforcing of plastics by metal and glass fibers has produced strong, flexible, and light materials, such as that used in bullet-proof vests for the armed services.

Plastics are manufactured under controlled conditions to give uniform raw material and finished products. Color, surface, strength, and size variations are minor. Failures usually are due to misapplication by the engineer. There is no such thing as a bad plastic; all plastics are good if compounded properly for a particular use. Sufficient information for making a proper choice is available from material suppliers and plastic-molding companies.

The cost of plastics is being reduced constantly as improvements are made and demand increases. (See Table 8-1 for relative costs of one type of plastic.) In general they are more expensive than metals on a “per-pound” basis. It will probably be some time before automobile bodies, kitchen cabinets, or refrigerator housings are predominantly made of plastics. By coating metals with plastics the benefits of both materials can be obtained. Plastics must do the same job as another material at less cost, or a better job for the same money, or be in such a position that a plastic is the only material that will do the job, before they are used more extensively.

The cost of metal molds for plastics is about the same as that of die-cast molds. A superior polish in a mold for plastic is transferred authentically to the molded piece. The molding operation in some cases may be slower than metal-die and permanent-mold castings; however, the trimming of flash associated with die casting is not usually as time consuming with plastics. (See Factors Affecting Costs under “Molding.”)

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The term “plastics” has been applied to those synthetic non-metallic materials that can be made sufficiently fluid to be shaped readily by casting, molding, or extruding, and which may be hardened subsequently to preserve the desired shape.

Plastics as engineering materials are constantly proving themselves useful in present-day designs. In spite of the continued introduction of new materials and processes on an extensive scale, the industry has achieved maturity. The use of plastics is following the same course of development and application that was characteristic of the introduction of light metals and stainless steel. Costs have been reduced, uniformity and reliability of materials have been improved, and possible applications have been determined. Plastics are attractive materials and offer advantages in weight, cost, moisture and chemical resistance, toughness, abrasive resistance, strength, appearance, insulation (both thermal and electrical), formability, and machinability.

One of the earliest and largest users of plastics was the electrical industry, which is a leader in the use of thermosetting and laminated plastics. Electrical companies entered the field of plastics to capitalize on benefits to electrical products. Their products spurred, in turn, the development and application of plastics for many parts that had no electrical function.

By virtue of their thermal characteristics, plastics usually are divided into two groups, thermoplastic or thermosetting. Those that undergo no chemical change in the molding operation may be softened again by heating to the temperature at which they originally became plastic, and therefore are termed “thermoplastic.” Since they become increasingly softer with increase in temperature, certain members of the thermoplastic family are liable to permanent distortion under mechanical strain at relatively low temperature (140°F.). They may flow to an appreciable extent under load at room temperatures. Plastics that are hardened permanently by a fundamental chemical change in the molding operation are termed “thermosetting plastics.” The chemical change is called “polymerization,” which is defined as “the reaction by which single molecules are linked to form large molecules without change in fundamental chemical composition.” These materials, once molded, will distort under stress at approximately 250°F., but they will not become soft or fusible. Thermosetting materials will char and burn at high temperatures. They are inclined to have greater tensile strength and hardness, and, in some cases, are lower in raw material cost than thermoplastics. Thermoplastics, on the other hand, have generally higher impact strength, pleasing appearance, and can be converted into a finished product at lower manufacturing cost.

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Lettering or numbers on injection molding

Lettering or numbers on injection molding

Lettering or numbers on a molded piece may be either raised or depressed. As a general rule, the raised letters are less expensive due chiefly to the fact that they can be stamped in the cavity. The exception to this rule is found when a hob is used to produce each impression. Furthermore, any depressed lines are usually filled with some contrasting color and this finishing operation is quite expensive. The height of raised letters does not have to be more than 1/32″ to give the necessary effect of relief and depressed letters also can be shallow as long as they are not too wide. The method of wiping in the color pigment necessitates a very narrow width letter of sufficient depth to hold the pigment when the operator removes the excess paint from the surface.

injection molding design

It is of utmost importance that no thin edges or sharp points appear anywhere in either the plastic injection mold or the piece itself, for constant wear soon dulls any sharpness and hence the dimensions are not held within the required tolerances. Tolerances vary on molded parts in accordance with the design. The vertical tolerances should be about ±.008″ but can be held to ±.005〃 if necessary. On the horizontal dimensions, that is, the areas perpendicular to the line of ram action, tolerances of ~.002″ can be maintained, but it is customary to have ±.005″. The reason for the difference in the vertical and horizontal limits is that the closing of the press may vary a few thousandths while the horizontal areas remain constant and are affected solely by the shrinkage of the material itself. Molds are built to allow for shrinkage and hence very close tolerances can he maintained on the horizontal dimensions.

Where inserts are placed in corners the wall section around them should blend in with the interior of the piece itself and particular attention must be given to having sufficient thickness so that the exterior will not crack.

Sometimes a piece must have its parting line on a curved surface, but wherever possible this should be avoided. It is a difficult task to remove the fin under such conditions. Very often a buffing operation has to be resorted to in order to completely eradicate the flash line. Wherever possible the parting line should appear on the sharp section of the piece so that filing or grinding will tend to improve rather than destroy the desired appearance.

injection molding design

There are innumerable rules of a definite nature which govern correct die design for plastic molding, and a thorough understanding of all these fundamentals is highly essential to the competent manager, sales engineer, draftsman, and estimator. Every item is different and every new proposition presents a problem in itself which requires utilization of a general knowledge of die design. The sales engineer should know at a glance whether or not a proposition presented to him is practical from a molding standpoint and should also be prepared to suggest any changes in design which might facilitate the molding operation. The estimator should carefully check the prints for possible changes while he is preparing his quotations, and the tool room should also be on the alert for any errors which might have inadvertently passed the attention of the engineering department. With a thorough checking of the work it becomes quite improbable that anything but the best design will result, and therefore the most efficient mold will undoubtedly be constructed.

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draft and hole design

Unless otherwise specified ,a slight draft is ordinarily given to the vertical surfaces of the piece. The reason for this is obvious when consideration is given to the fact that the injection molding plastic piece has to be released with the least amount of trouble. About a 3° taper is sufficient to accomplish the desired result and even less is sometimes permissible. Again

it should be explained that innumerable pieces are molded with the vertical sides at 90° with the horizontal, but it is a much more difficult molding proposition due to the tendency to adhere to either the top force or bottom cavity.

Very often it is desirable to have the piece cling to either the top or bottom depending upon the knockout mechanism. Small dints are made on the force or in the cavity which, of course, act as very moderate undercuts, and hence accomplish the desired result. This procedure is resorted to in instances where the part fails to adhere to the section of the mold, as planned in designing the die.

When small holes are part of the design of the finished piece these are made with steel pins while holes of large diameter are obtained by the use of plugs. Care must be exercised in designing the depth of a hole inasmuch as long pins in the mold will tend to bend or break off during the closing of the injection molding machine under heavy pressure. A good rule to observe in this respect is never to have the depth of the hole over three and one half times its diameter. One method of molding long and narrow holes is to have two pins meet half way through the hole. This results in gaining a depth of seven times the length of the diameter, but is possible only where the hole goes through the entire piece. A liberal draft must also be provided for any long pins.

No holes should ever enter a piece on a slant, for as can be seen, any such injection mold design would prevent straight draw of the top force. Of course, die construction of this type is possible inasmuch as slanted pins could be removed prior to the opening of the press, but seldom if ever are inclined holes placed in a piece. Side holes can be made by entering retractable pins in the side of the mold, but their length is limited even more than when the holes are placed vertically. Down pressure on a pin supported only at one end prohibits its extension very far toward the interior.

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injection molding cost

injection molding cost

injection molding cost

We have investigated in detail the presses, how to make plastic molds , finishing machinery and auxiliary equipment incidental to the molding process and are now ready to turn to other aspects of the business, With the foundation attained in manufacturing knowledge we are fully prepared to deal with such problems as production methods, costs,estimates and many other advanced elements of successful business management, therefore, first thoroughly investigate estimates and costs and analyze the intricacies of their makeup, for it is upon a definite understanding of these two fundamentals that a successful business ultimately depends.

In the first place, it should be pointed out that the relationship existing between an estimate and a cost is that the former is based on a prognostication of the latter. Furthermore, a cost is constantly changing as a result of varying conditions in the plastic injection molding company, and therefore sufficient latitude should be provided for in the estimate. In preparing figures for the estimated cost of a molded part,the estimator is usually furnished with a print or model and the necessary information pertaining to the quantity required over a definite period and the type of material desired in the finished part. If the model, or sample, is made of some material other than the plastic in which it is desired the net weight is determined by applying the specific gravity ratio. When a blue print or sketch is submitted however it becomes necessary to ascertain the volume as accurately as possible and then convert this figure into the equivalent number of ounces or pounds. It is common practice to estimate selling prices on the basis of a thousand pieces and hence any weights obtained must be expressed in the number of pounds per thousand.

injection moldingThe cost of producing molded parts, like any other commodity, is based on the three fundamental factors; material, labor, and overhead, and these elements are dealt with in the order given. Whether a print or sample is used for the basis of figuring is of little importance although the latter will permit closer figuring of the material weight. As previously explained, the cost of material is comparatively easy to obtain inasmuch as it requires only the price per pound multiplied by the gross weight. Gross weight is that which includes the amount of waste and overflow, the net weight being that of the finished piece. Where net weight only is available a certain percentage is added in order to obtain the gross weight. This varies from 10% to as much as 50% depending on the design and size of the piece and the type of die in which the part is to be molded.

Determination of total labor cost is much more complicated. In the first place, the size of die must be ascertained, after which the cycle time has to be prognosticated. From this the molding cost per thousand is obtained and further thought must then be given to the many other operations necessary. The experienced molder is able to determine by examination of the print, or model’ just what finishing operations a piece requires and he estimates the length of time required in performing these various operations. The figures are then converted into cost per thousand so that all the calculations will be expressed on a uniform basis. Allowance must be made for rejected parts and this is based on either past performance or arbitrarily set at an estimated percentage. Use of lighter shades such as ivory or white increases the rejection figure due to the darker marks being so much more in evidence than in the black or brown colors. After the rejection costs have been appended, and, of course, the overhead included, the three major elements are added in order to obtain the estimated prime factory cost. Consideration of overhead has been purposely omitted at this point due to the fact that we shall give the subject detailed study later on.

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What Plastics Are

What Plastics Are

Plastics are giant molecules, or high polymers. They consist of basic simple repeating chemical structural units called mers or monomers. When these monomers are joined together to form a long chain or giant molecule (by a polymerization process), the resulting “plastic” is called a polymer. Actually, the differences between high-molecular-weight polymeric materials and low-molecular-weight materials are largely physical. For example, ethylene with a molecular weight of 28 is a gas; polyethylene produced by a high-pressure polymerization process to yield a material of high molecular weight of about 20,000 is a soft, flexible plastic, or high polymer.

Molecular size, shape and structure rather than chemical analysis separate the gas from the plastic. There is a large number of high polymers or plastics, each plastics family being identified by the chemical type of repeating unit within the polymer’s long-chain molecules; e.g., polyethylenes are identified by repeating ethylene mers, polypropylenes by repeating propylene mers, polyesters by repeating ester linkages, epoxies by recurring epoxide groups, etc. Another factor that plays a strong part in determining properties of a plastic material is the geometric relationship of the mer units in the chain. For example, in linear polymers, mers are hooked end-to-end in long, relatively straight chains. Linear branched polymers are long, relatively straight chains with side chains of atom groups or radicals extending from the straight chain. (The well-publicized “ordered” or “isotactic” polymers— polypropylene, for one—are linear branched polymers in which the side chains are located at specific predetermined points along and around the chain equidistant from each other.)

Linear copolymers are long, straight chains in which two different chemical types of mers are hooked up in the chain. Graft copolymers are those in which the basic long chain is of one type of mer, with longchain “grafts” of the other type of mer extending from the base chain. Block copolymers are those in which “blocks” of repeating mers are hooked end-to-end and inserted in a long chain of different types of mers. All these geometric types of polymers discussed above are essentially thermoplastic. The chains are packed into the material, either in random configurations such as amorphous polystyrene, or in varying degrees of orientation. The strength of amorphous thermoplastic materials is primarily dependent on the secondary bond forces ( van der Waals ) between the chains.

Crystalline thermoplastics, e.g., polyethylene, nylon, are those materials in which areas of each polymer chain become closely aligned with areas of other polymer chains, these areas of alignment forming crystals. Portions of chains which extend into crystalline areas are very close to other chains in the crystallite; consequently, the secondary forces are high. In most crystalline polymers, crystallinity is partial. Evidence indicates that in such materials, individual polymer molecules pass successively through several crystalline and amorphous regions. This is believed to account for the superior strength properties of crystalline, as opposed to amorphous, polymers. In all these thermoplastics, since the forces holding the material in shape are secondary forces, the materials repeatedly soften when heated and harden on cooling. Theoretically this softening and hardening can be repeated indefinitely if the temperature does not become high enough to degrade or break the polymer chains.

On the other hand, a thermosetting plastic will not soften appreciably up to its decomposition temperature. Such a plastic has a linear, relatively low-molecular-weight thermoplastic polymer chain with “cross links” which bond the long chains together with primary valence bonds ( which are orders of magnitude stronger than secondary bonds ) . Such a three-dimensional, highly cross-linked polymer is of very high and indeterminant molecular weight, and generally rigid. Actually these mate rials can be produced with properties ranging from rubber-like to highly rigid depending on the number and lengths of the cross-linking chains, and the size and nature of the so-called main chain. Tooling plastics such as epoxies, phenolics and polyesters are all thermo setting materials. When purchased in liquid form they are mixed with curing agents, hardeners or catalysts, which cause polymerization and cross-linking to form strong, infusible solids.

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