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The Very Best Tips for Investing Crate Mold with Less demanding

The Very Best Tips for Investing Injection Mold Less demanding

It could be hard to shop for a vegetable crate mold, as there could be a great deal of discussion between end user and seller before the completed vegetable crate mold is settled on. Nevertheless, the few suggestions here can assist save you a lot of time, and ensure the whole course somewhat less complicated.

Make an RFQ that incorporates a lot of specifics.As experienced as moldmakers have proven to be, they will not manage to guess what you is thinking when it concerns what you are in search of. Involve as much details as it is possible to at this stage, which involves the volume of cavities, the chemical substance, the most well-liked lifetime on your vegetable crate mold, as well as whatever promises which you may need. Once you are not too convinced on these concepts, then you should convey to your supplier, and they will make an effort to make it easier to settle on the points is befitting for your wants. The more exact you create the RFQ, the more correct a rate you will receive in return.
Be honest with regards to the reason you need a rate. If you solely want a general cost to pass away to some other unit, after that let the moldmaker understand- then they probably reply efficiently. Setting up an appropriate quotation can take a lot of time, and it is far from good to waste the moldmaker’s hours any time you don’t wish that extensive detail, or if you might not order from them.
Don not offend a provider’s original constructive idea. The information and advice available from your moldmaker remain their right- you can not only bring these suggestions to someone else to accomplish it for your company. If you determine a new moldmaker, therefore undertake their suggestions on board- not merely is choosing another company ideas not really understandable, nevertheless it might also baffle a final moldmaker, who exactly may not realize exactly why those options were prepared at the start.
Take into consideration forming a collaboration with your vendor. Through working tightly with your moldmaker on the subject of charges, plans, and part amount presumptions, you may perform the duties of a crew to reach better outcomes in the end.
Keep on straight dialogue with your dealer through the entire process. A lot of moldmakers will be glad to furnish consistent enhancement files, and keep you updated on the very latest advances for your mold. It is critical that you learn things are moving to timetable, so in case you require some info, make sure you seek so you can ease.
Make sure you forever pay in time. Many moldmakers get the job done to a little spending, and require fees to get paid out before they’re now able to go forward with your mod build. If you happen to postpone making payments, then you definately will not receive your mold on the dot- it is as simple as that. Different moldmakers have diverse settlement policies, therefore discuss with them to figure out an approach that works out for you both.

Modifying your item model will likely imply changing the mold on its own. Once you end up making changes to your piece design while the mold is now being constructed, you will probably be impossible to receive the mold at the charge quoted, or to the very first timeframe. Every changes indicates the mold ought to be adjusted consequently, which adds to both the cost and the time of the mold create.
Know in ahead of time when your mold will be .There are numerous classifications for a finalization date- they could differ from when the eventual settlement is completed, to any time you are given a trial piece, to shipment of the eventual product. In most cases, a mold may be known as completed after it is well prepared to create the part it is actually intended for. Nearly all moldmakers will be in a position to bring about small alterations to make a component based on print specifications. If these specifications modify late, then the mold should still be looked at as complete- any other alters will have to be paid out .
If a service is very low priced, there is frequently cause hiden. While there will be vegetable crate mold, makers these days who present you with a cheaper-than-average cost for the very good product or service, there will be lots of some others who supply reductions mainly because they cut costs . Eventually, it is better to pay out good money to obtain a high-quality item, instead of having a awful mold that doesn’t satisfy your demands.
When buying a mold, that medieval proverb is definitely correct- you get what you pay for. Every made items that you intend to manufacture is only as nice as the mold which you earlier develop them, which means you have to maintain your mold is flawlessly suited to your needs- prior to buying it.

Production Facilities

Production Facilities

Production Facilities

Mention has already been made concerning the absolute necessity of having a definite objective at the outset. Vague ideas on the subject are hazardous. Assuming then, that a plant layout has been determined and the floor space calculated for the presses to be used, the production problem must now be approached. If orders have not been actually taken or promised, requirements have at least been anticipated and estimates must have been made on the items to be sold. Knowledge as to whether the customer’s needs are five thou sand or a million units over a given period naturally affects the price he is quoted. This is true in the submission of any quotation due to the economy that can be realized in continuous production.

The settingup and dismantling of a plastic injection mold for small quantity requirements is expensive and a long run is usually the most desirable. Aside from these features, there is another factor which influences the newcomer in the plastic injection molds  making field, for with sufficient large quantity orders he is then in a better position to purchase his powder at a lower price. Buying molding plastic material, whether it be the phenol or urea type, is an expensive proposition when purchased in single drum lots. Even now, some of the smaller users are paying as high as twenty five percent more for molding powders than the larger plants. If this differential can be obliterated, one great disadvantage will have been overcome. But it will be impossible to enjoy the minimum price unless a sufficient amount of business is first obtained to warrant a large powder order.

plastic injection molds  making

So, rather than having one or two large accounts on the books, a much better condition is to gain the business of a half dozen or more. Before it is possible, however, to accept large orders it is essential that reasonable production is promised. Large quantity orders usually entail rapid deliveries and a definite schedule must be submitted to the customer. In order to live up to these promises, either a large multiple cavity die must be made or two smaller molds for producing the same article. In the past, general practice has leaned towards the larger mold, but for many reasons two dies are more practical. Such practice insures the molder, to a certain degree, against complete loss of production in the event that repairs have to be made on the die, or if the larger injection molding machines are unavailable. In other words, if something happens to one of two molds, repairs can be made while the other mold continues to run. Production is crippled, to be sure, but not completely stopped as would be the case with one large mold.

investment plastic stool molding

investment plastic stool moulding

plastic stool moulding

investment plastic stool molding, frequently referred to as precision molding, have a number of favorable characteristics which can be summarized by saying that they offer dimensional tolerances superior to the rougher shapes too complex for powder metal parts and of materials with melting points too high for die molding.

The stool parts to be made are poured in plastic stool mould which have themselves been cast in master molds. The plastic stool mould molding material is plastics .

Patterns for individual parts are made by forming wax, frozen mercury or plastics (usually polystyrene) in a master die. The patterns material which is allowed to harden. When the ceramic shell has set, the investment material (wax, mercury or plastics) is melted out to leave the cavity for pouring the casting.

Each of the investment materials has some advantage.

  1. Wax permits the greatest flexibility of design and is the least costly.
  2. Plastics permit a more rapid production of patterns and also allows a surplus of patterns to be made and stored for future use.
  3. The frozen mercury allows larger sizes to be cast and requires only a thin ceramic shell which in turn result in better metal quality.

plastic stool mould features

plastic stool mould featuresThe mechanical properties of alloys cast in shell molds are high and in some cases equal to wrought metal properties. Stool Parts to be made as investment molding can be quite complex. The chief restrictions to complexity are the need to remove the pattern from the master die. With the investment materials used, small elements of a pattern can be formed separately and then joined before covering with the refractory material to attain the highest degree

Dimensional tolerances possible with investment molding vary widely from foundry to foundry. Too, exceptionally close dimensional tolerances can be held if the customer is willing to pay for the extra work involved. The usual commercial tolerances are in the range of ±0.003 to 0.005 in. per in. of casting length, with closer dimensional control being possible on specific sections and very small parts.

Extremely small stool are well suited to production by investment plastic stool mould making. Production of such parts is speeded by molding larger clusters of parts at one pouring. Most investment molding are in the weight range of a few ounces up to a max of 26 lb. Some shops using frozen mercury patterns are making much larger molding, however. By far the greatest number of investment molding is in the smaller sizes, weighing from 1 oz up to 3 or 4 lb.

Massive metal areas are not desirable in investment molding. Good design limits sections to 1 in. thick or less. Extremely thin sections, too, are to be avoided where possible.


cap mould machining way-Shaper and planer

There are some plastic cap mould making ways as follows:



The metal-removing process of planing takes place when the cutting tool moves by a straight back-and-forth motion with respect to the work, or when the workpiece moves in a straight back-and-forth motion with respect to the tool, which is stationary.

Four types of machine tools operate according to one or the other or both of the above principles: the planer, the shaper, the slotter or vertical shaper, and the broaching machine.


In shaping, the tool is reciprocated and the feed of the steel for  cap mould manufacturing  is represented by the width of the cut. Shaping is particularly suited for small work in view of the design and construction of most shaping machines. (It is seldom used to machine work more than two feet square.) Shaping entails producing flat surfaces in horizontal, vertical, and angular planes. In addition, internal surfaces and odd-shaped surfaces can be shaped.

The work is usually clamped in a vise fastened to the table. The typical toolroom shaper has a universal table that can be tilted to 15° and swiveled through an arc of 180°.

Because of its flexibility, the shaper is considered a basic machine tool. It is widely used as a toolroom and die shop facility and, in view of the rate of metal removal, is of limited use in large production runs.


The vertical shaper, commonly known as a slotter, is similar to the shaper except that the ram is reciprocated in a vertical slide. The stroke range in vertical shapers is from 6 to 36 inches. For shaping clearance, the ram may be adjusted to move at an angle to the vertical.

Circular tables for holding the work are usually standard equipment with the vertical shaper. Round shapes can be generated by rotating the table by power feed; however, this is usually not the most economical technique for producing circular shapes.The vertical shaper is used primarily for slotting or key-seating operations.



The planer is used primarily in the machining of flat surfaces, where the magnitude of the work is such that it is impractical to machine on a shaper or milling machine.

The planer has a long horizontal bed upon which the work-holding table slides with a reciprocating motion. Above the work table, the tool head (or heads) is mounted on a horizontal crossrail. The tool head is mounted on a slide to permit vertical adjustment so as to set the cutting tool to the correct depth. The crossrail may be adjusted vertically in order to accommodate various sizes of work.


As mentioned previously, the shaper finds most application in the plastic cap mould shop. It is usually more economical to use a shaper rather than a planer for small work because:

  1.    It is faster and more simple to operate.
  2.    It is a less expensive piece of equipment and uses less power.

The planer is adapted for machining flat surfaces on large work. Similar to the shaper and the milling machine, vertical, angular, and horizontal surfaces can be cut. Work should be routed to a planer if:

  1.    Heavy cuts are required on large flat surfaces.
  2.    The material to be cut is relatively hard (large steel castings).
  3.    Accurate finish is required in such work as slides and guides.


Shaper and planer tools are similar except for size, the planer tools being considerably larger in order to accommodate the larger work.

Because of the intermittent cutting action of both the planer and shaper tool, toughness is an important criteria in tool-material selection. Consequently, high-speed steel is most commonly used; However, carbide-tipped tools are used for taking light or finishing cuts. It is important that the depth of cut, even when using carbides, be greater than 0.010 inch so as to get a cutting rather than a rubbing action.

The front clearance angle should be about 4° so as to prevent rubbing of the back of the tool on the work. A side clearance of 3° is usually considered adequate. In the cutting of mild steel, a 12 to 15° side rake is advocated. For cast iron where less shearing action is needed, 3° side rake is recommended. The back rake varies from 0° for roughing cuts to about 2° for finishing cuts. With these small rake and clearance angles, the tool is more able to withstand the force of impact at the beginning of each cutting stroke.

Counterboring,Spot-facing and Tapping

Counterboring,Spot-facing and Tapping are very important machining to form the holes on the crate mould making and they use different kinds of machines:



Counterboring is an operation intended to enlarge, for part of its depth, a hole previously drilled and produce a shoulder at the bottom of the enlarged portion. True seats for fillister head machine and cap screws are provided by counterboring, spot-facing, and countersinking operations

Counterboring is the same as flat-bottom drilling or end milling, except that there is a pilot in the center which fits a previously machined hole that is smaller in diameter than the counterbore diameter.

Spot-facing is the same operation as counterboring, except the cut is made only on the end to face a boss or to provide a seat on the top of a plane surface.

The pilots in the counterbore and spot-facer must fit the drilled hole, and are usually attached to the counterbore in such a manner that various sizes can be used on one or more sizes of the counterbores. Therefore, in order to avoid confusion in the shop, the designer should endeavor to standardize on counterbore and pilot diameters and their combinations. National counterbore standards have been established for the various machine screws and bolt sizes.


TAPPINGTapping is the cutting of internal threads within a hole that has been prepared by an operation such as drilling, boring, or coring. The preparation of the correct-sized inside diameter is very important in tapping operations. If the hole is too large, then only a portion of the desired thread will be produced and the resulting holding power of the mating member will be reduced.

If the hole is too small, then the tap will itself have to open the hole and this undue strain will result in excessive tap breakage.

Tap Selection. Selection of the most favorable tap must be based on the material being worked, the accuracy required, the length of thread, and the type of    tapped    hole (whether through hole    or blind    hole).

The following    general    information will assist in the    selection    of the best working taps:

  1.    Use cut-thread taps only where commercial accuracy is satisfactory. For class 2 fits, use commercial-ground taps; for class 3 fits, use precision-ground taps.
  2.    Two-fluted taps are best used for tapping deep holes where there is a tendency for    chips to    clog and break the taps.
  3.    Three-fluted    taps are    used for cutting softer materials. Used to    a large extent for cutting blind holes.
  4.    Four-fluted taps are used on materials such as cast iron where chips break up readily and are easily washed away. Used in hand tapping.

Design Factors. In order to minimize tapping costs, the following factors should be observed by the production design engineer:

  1.    Specify standard threads.
  2.    Do not specify closer tolerances than necessary. Class 2 thread tolerances are usually satisfactory for most work.
  3.    Select materials that can be easily tapped.
  4.    Provide adequate clearance on blind holes. This should be 2% to 3 * times the pitch of the thread.




Reamers are tools used for enlarging and finishing diameters of holes to accurate dimensions . A rose reamer cuts on the end and has a chamfer of about 45° on the edge to aid in entering the hole. The fluted chucking reamer, which does more accurate work, is tapered slightly on the end to aid in entering the hole and it cuts on this tapered surface. In general, the reamer follows the hole being reamed. It will change the direction of a hole only slightly. A reamer performs best as a sizing tool when driven by a floating holder that permits it to follow the hole as the reamer is driven through. There are many types of reamers, with straight or spiral flutes, expanding and adjustable blades. They are made to cut different materials, and designed to cut both taper and straight holes.

provision should be made for the reamer to pass through the hole. Blind holes are difficult to ream, and should be undercut at the bottom on the reamed surface.

When a designer specifies a reamed hole it means that:

  1. A drilled hole must be made accurate as to size.
  2. It requires a drill bushing which is removable (known as a slip bushing), and sometimes a reaming bushing added.
  3. The reamer must be available to provide the size in the particular material; and must be able to fit the machine tool.
  4. Gages (usually plug type) must be available to check the hole, for both operator and inspector.
  5. Duplicate sets of reamers should be available, because production must not be delayed when reamers wear and require sharpening.
  6. Wear on the flutes reduces the diameter of the reamer. When the diameter is below the tolerance required, the reamer may be either scrapped or salvaged by grinding for use on smaller diameter holes.

The designer should standardize reamed hole sizes. Hole sizes are based on mating part dimensions.

It is economical to make pins and shafts from cold-rolled and ground stock in order to reduce the amount of turning and grinding to size. Since this material comes in sizes which are to size or under, a series of reamers should be standardized which will ream holes ±0.002 inch or, according to the designers preference for clearance between shaft or pin and hole.

Whenever a surface must be turned or ground on a pin or shaft, the size should be specified so as to offer the proper fit for a standard-sized reamer, ±0.0005 inch. The oversize reamers cost more than standard-sized reamers, but when they wear they can be ground to a standard size, and thus have a double life.

Material removal processes-Drilling

Material removal processes include all those where, by the nature of the process, the material is cut in order to arrive at a predetermined size. There are five basic metal-cutting processes:

  1. Drilling
  2. Turning
  3. Planing
  4. Milling
  5. Grinding

All of the other metal-removing processes are closely related to or are modifications of these five basic processes. For example, the process of boring is internal turning; reaming, tapping, and counterboring modify drilled holes and, therefore, are related to drilling; hobbing is a milling operation; honing, lapping, superfinishing, polishing, and buffing are refined grinding operations; sawing can be either milling (if it takes a circular saw) or planing (if it is done by hacksawing or bandsawing); broaching is a form of planing.

The amount of material removal of the various cutting processes may be quite small,as in polishing and buffing operations, or it may be relatively large, as in milling and turning processes. It is the purpose of this chapter to present the various material removal processes that are available to the production design engineer so that he will be able to specify the most favorable manufacturing procedure.


Drilling is probably the widest used machining operation. There are only a few processes, such as punching, boring, and burning, which can be substituted economically for drilling operations. All of these, however, have decided limitations; and in many cases, although another process can be used, it is still more economical to drill. Good drilling practice will result in little variation in location of holes, and size and shape can be depended upon at a minor tool cost.

The principles of cutting metal apply to drills as well as to single-point tools. The surface of the drill flutes must be smooth so that friction will not retard the movement of the chips up and out of the drilled hole. The cutting angles must be ground to suit different materials, and adequate lubrication must be provided.

The most important factor to be controlled to assure satisfactory drill performance is the accurate grinding of the drill. If one lip is ground at a different angle from the other, the tool will feed off-center and will drill an oversized hole. Also if the angles are the same, but one side is longer than the other, a thicker chip will be cut on one side, causing the drill to cut oversize. In addition, improper grinding results in an unequal distribution of forces acting on the drill, which may cause drill breakage. Drills should be ground in a drill grinder rather than by hand so that an unskilled operator can provide drills that are ground properly and save the time of the skilled operator.

Drills for different kinds of material, such as plastics, nonferrous metals, cast iron, steel, and alloy steels should be stocked in the tool crib, available for use with the particular material. The use of proper feeds, speeds, jigs, and equipment with true-running spindles will increase the life of tools, result in the drilling of more holes per hour, and give greater accuracy. Drilling is a major operation, and a small percentage saved can amount to a considerable amount of money.

Improvement in drill performance has recently been made by the introduction of better tool material, polished flutes chromium-plated to reduce wear, and improvement of the shape of the cutting edge.


There are 20 different types of twist drills, as well as flat drills, straight-flute drills, core drills with three or four flutes, multicut drills, step drills, multiland drills, and combination drills and reamers. In order that the possibilities of the various types of available drills might be understood, the suppliers of drills have developed data which suggest the best shape of drills for each of the large variety of materials.


One design may require the use of several types of drilling equipment, jigs, and fixtures. A good design reduces the number of different machines and the sizes of drills, reamers, and taps used. Good designs endeavor to place the holes on a single plane and maintain constant depth of all holes drilled. Effective design and production will be made possible by a knowledge of drilling equipment, cutting tools, and auxiliary equipment.

When drilling is done, the work is brought to the fixed spindle of sensitive and upright drilling machines (both single spindle and gang drills) or the spindle of a radial machine is brought into position as the part is held stationary.

Parts are held in vises or jigs and moved under the drill spindle along the table. Often two, three, four, six, or eight spindles are mounted over one table so that the part can be drilled, reamed, tapped, or counterbored without removing the part from the jig. Quick-change chucks also enable additional drill sizes to be used on the same machine and spindles.

When radial drills are used, the part is stationary and usually in a jig. Often the jig is suspended in a trunion and can be drilled from any side parallel to the axis of the trunion. Quick-change collets enable the operator to change from one drill size to another, or from drill size to reamer, boring bar, or tap. Radial drills are designed so that feeds and speeds can be changed quickly to accommodate, use of the large variety of cutting tools.

Multiplc-spindle machines and single-spindle machines equipped with multiple-spindle drill heads may be fed simultaneously into the work. The length and time of feed are determined by the longest hole to be drilled and the drill having the lowest feed requirement. Rpm can be varied in some cases with special gearing,but rpm,s, in general, are the same for all drills mounted in the head. The drills are guided by bushings mounted on the drill press head or in the jig. Drilling, reaming,and tapping can be performed in multiple-spindle heads. Usually each operation is done in a separate machine. Wherever a number of holes can be drilled at the same time in a part of moderate activity, the multiple-spindle drill is economical. Multiple drilling is applied widely in mass production and can be economically adapted to job shop work. Bolt holes for cover plates, bearings, glands, and housings, can be standardized to use similar jigs and setups and promote the use of multiple-spindle drill presses.


The part, jig, and tool must be designed to withstand the pressure of the tool as it is cutting. The new cutting tool materials have raised speeds, feeds, and pressures until only the most modern equipment can take full advantage of the new features. The engineer should use the data furnished by equipment and tool suppliers as a guide, and experiment with feeds and speeds in order to remove the greatest amount of material consistent with an economical tool life. The cost of the operation and machine must be balanced against the cost of tool wear, and against sharpening and setup time.

A drill, on entering material, has a tendency to wobble until the entire drill is cutting. Thus the accuracy of the location of a drilled hole is increased by using a punch mark, a smaller diameter drill (which wobbles less), a stub or short drill rigidly held in position by a good spindle, and a guide bushing. If accurately located holes are required, it is necessary to make the part on a precision machine like a jig borer or a horizontal boring machine, or to use guide bushings mounted in a jig .

Numerically controlled turret-type drill presses with the table automatically positioned can drill, ream, tap, chamfer, and counterbore any quantity of parts without a drill jig. The table holding the work is accurately positioned, the turrets are rotated, proper speeds selected, proper advance and cutting feeds selected by a master tape or punched card. The operator merely places the tape in the control, installs the cutting tools in the turret, and locks the part on the table with vee jaws or clamps.



The most important considerations for a designer to observe are:

  1. Avoid deep holes. Any hole longer than five times its diameter is considered a deep hole and requires special procedures in the shop, such as withdrawing the tool at intervals to clear the chips, and forced lubrication.
  2. Start and finish holes on surfaces at right angles to the direction of the hole.
  3. Provide room for a bushing and its support in the jig or a fixture to guide the drill.
  4. Use standard-size drills for tapped holes and clearance holes for bolts, screws, rivets, and bushings, so that minimum stock of drills can be maintained.
  5. Use drilled holes instead of reamed holes wherever possible, provided the shop can produce good quality holes through proper grinding and tooling.
  6. Use the same size hole, or tap wherever possible, so that the minimum number of spindles and drills will be required.

Tool Finishing

Tool Finishing

Tool Finishing

The tool finishing section completes all tools produced, whether cast, laminated or both. Their work consists mainly of trimming, fitting, assembling and checking the entire tool for conformance to design specifications. Equipment required consists mainly of tool crib items, such as disk sanders, rotary files, and hand routers. All minor repairs and alterations to casts and laminates are made by this group. injection molders must be experienced, skilled tool engineers who know the entire plastics injection mold making operation and what is required of a tool in order that it function properly. They must be provided with work benches and adequate storage facilities for personal tools and equipment.

Tool Proofing.

This section is used to test or “proof” the tool in either actual production equipment or simulated equipment. Such equipment as draw presses, stretch presses, hydropresses, etc. are often involved. Personnel should consist of tool and die makers with broad enough experience to enable them to alter a given tool so that it will function properly, and co-ordinate with other tooling involved. Service Facilities. In practically all plastics tool shops a central tool and supply crib is most economical. Such cribs must be stocked with adequate precision cubes, knees, straight edges, height gages, vernier scales and the accessory equipment normally needed in conjunction with these tools. A stock of hand tools, cutting tools, etc., required to do the actual work must be maintained. The same section will also store and dispense shop supplies. It is also  a logical point to maintain inventory records of plastics materials, rein forcing material and all other major supply items required by the plastic moulding companies. Sanitary facilities must be provided. The most effective way to control industrial dermatitis is by practice of adequate personal hygiene. All personnel who come in contact with chemicals such as plaster, plastics resins, glass cloth, parting agents, etc., should be provided with individual steel lockers and adequate bathing facilities, including showers.

Integral Molds

Integral Molds

injection molds

There are many miscellaneous applications of plastics for injection molds. Some of the more important are covered in the following sections. Integral Molds Integral molds are those molds which actually become a part of the final assembly. Although somewhat different from the accepted concept of “plastics mold tooling,” some mention should be made of them. The mold maker technique is primarily being used in the electronics industry to encapsulate electronic assemblies. One of the most widely used techniques involves the use of the relatively new epoxy molding compounds. These are of particular benefit be cause of the extensive use of epoxy resins for potting and encapsulating. Thus, the encapsulating material is made of the same material as the mold. The epoxy compound is molded by high-production compression or transfer-molding techniques in the shape of a component case suitable to enclose the assembly. The electrical assembly is inserted in the case, and potting or encapsulating resin is poured to fill the case and enclose the assembly. Such operations can be carried out on a rapid assembly-line basis. After cure of the encapsulating material the assembly is complete. Obvious benefits result from elimination of mold preparation, and stripping of the potted assembly from the mold. Also, the integral mold provides a strong case around the complete assembly.


Nylon Mandrels

Nylon mandrels made from stock shapes are used to help bend aluminum extrusions by Lite Vent Industries, Inc. They are used both as mandrels to fit inside the extrusions and maintain original cross section during machine-bending, and as protective shoes to fit over pre-finished aluminum extrusions which must be hand-bent in the field. The mandrels are used to bend three tubes at a time in a machine. Aluminum tubes are slipped over the mandrels (supplied by National Polymer Corp. ) and clamped to the machine table. An air cylinder draws the forming roller and the mandrels around the bending plate. Mandrels slide smoothly, without galling, along the rough interior of the tubing. Nylon provides sufficient flexibility to conform to the bend, yet high enough compressive strength to keep the aluminum extrusion from crushing or buckling at the point of bend. Several types of mandrels are used. Cut grooves or slots, spaced about Y2 in. apart along the length of the nylon rod increase flexibility. Longitudinal grooves are cut in mandrels for clearance of internal flash in welded tubing. For hand bending, nylon shoes are placed over the straight end of an aluminum extrusion. As the forming roller is pulled across the face of the bending plate, the shoe pivots on the plate. The shoe does not mar the finished face of the extrusion.

Details of Mold Design

Details of Mold Design

After the basic plastic mold design, number of cavities, and machine have been chosen, numerous design engineering decisions still remain. a guide to the sequence of steps in design of an injection mold is following:

  • Given Part design
  • Order quantity – and schedule’s Target production costs
  • Select molding machine
  • Estimate cycle time
  • Calculate optimum number of cavities
  • Select type of mold Preliminary cavity layout Runner and gate layouts
  • Recheck runner and gate design Layout cooling channels for cavities and cores Finalize cavity placement
  • CAE
  • Fill analysis Shrinkage and cooling analysis
  • Mold layout and details
  • Select mold base Layout side core motions Layout ejection and venting Check mold parts for strength Final drawings and bill of materials
  • Plastic mold making
  • Trial run
  • Set starting operation conditions Optimize operation conditions

Sprue, Runner, and Gate Systems

The purpose of the sprue, runner, and gate systems is to conduct melt uniformly and with minimum pressure and temperature drops to each cavity, or to the furthest point in a large single cavity. Uniformity of flow means essentially equal flow rates through each gate. This implies equal pressure at the exit from each gate, assuming that all cavities are identical. In the case of a family plastic mold injection, the total flows may be different to each cavity, but, in a perfectly balanced design, the various cavities will be completely filled at about the same time. When a cavity pressure control system is used, switchover from injection to holding or packing is controlled by a pressure transducer in one of the cavities. This system works well when all cavities are filled at about the same time. However, if one cavity fills early and its gate freezes before the packing phase is initiated by the pressure sensor in another cavity, it may result in defects associated with insufficient fill and low density. Overpacking also may occur if flows are not balanced. Overpacking would be evidenced by ejection problems and by internal stresses in the moldings.

Therefore, the first step in multicavity design for plastic mold manufacturers is to lay out a balanced flow arrangement, i.e., one in which flow rates to each cavity are such that all cavities fill in about the same time. If all cavities are the same then this reduces to having equal pressure drops from the sprue to each gate exit. In simple cases, a naturally balanced layout has equal lengths and sizes for all flow channels; this also assumes that cavity plate and core temperatures are essentially uniform. Numbers of cavities for naturally balanced molds are 2, 4, 6, 8,16, 18, 24, 32, 48, 64, 72, 96, and so on, and any circular layout about a central point. Enough space should be provided between cavities to allow sufficient cooling for uniform mold temperatures. In more complex cases, the mold is either partially balanced or artificially balanced


Artificially balanced runner systems, designed using a computer or by experimentation, use runners with varying cross-sectional areas and equalized pressure drops. However, such systems have smaller processing windows because of complex interactions between rheology and heat losses from the runners. For very small parts, it is especially important to have natural balance for stability; the recommendation is to use small runners with high pressure drops. To minimize temperature fluctuations, such molds are preheated for startup and heat is supplied by the circulating coolant. There is not much heat input from the injected melt with small parts.

Sprue bushings are generally catalog items, but it is worthwhile to examine the different types available, in light of molding requirements. The main problem in the sprue area is to accommodate the large temperature difference between the nozzle and the sprue. The nozzle must be kept above melt temperature and the sprue, in a cold-runner system, must be close to mold temperature. Keeping the sprue as small as possible, providing a separate nozzle heating band, thermally isolating the sprue, or changing to a hot-runner design are all solutions to sprue area problems.

Considering other constraints on the mold design, runners should be as short and streamlined as possible. Runner sizes are determined by compromise. Large runners may fill cavities faster and more reproducibly from cycle to cycle and, hence, tend to produce higher quality parts. However, excessively large runners slow down the cycle by requiring longer cooling times, produce more regrind, and increase the required clamping force. (These objections to large runners do not necessarily apply to hot-runner molds.) Various shapes of runners are used, the most common being the trapezoidal or modified trapezoidal .

plastic mold gate

Runner and gate sizing for a multicavity mold is an iterative design procedure. First, a trial set of lengths and diameters of the runners is assumed. Then, shear rates, shear stresses, and pressure drops are calculated and compared with allowable or desirable pressure drops, as in the following example. Viscosity/shear-rate curves for the polymer at process temperatures are necessary data. Commercial software is available for these calculations.