Welcome to CIK MOLD BLOG

Molds separate into two sides at a parting line, the A side, and the B side, to permit the part to be extracted. Plastic resin enters the mold through a sprue in the A plate, branches out between the two sides through channels called runners, and enters each part cavity through one or more specialized gates. Inside each cavity, the resin flows around protrusions (called cores) and conforms to the cavity geometry to form the desired part. The amount of resin required to fill the sprue, runner and cavities of a mold is a shot. When a core shuts off against an opposing mold cavity or core, a hole results in the part. Air in the cavities when the mold closes escapes through very slight gaps between the plates and pins, into shallow plenums called vents. To permit removal of the part, its features must not overhang one another in the direction that the mold opens, unless parts of the mold are designed to move from between such overhangs when the mold opens (utilizing components called Lifters). Sides of the part that appear parallel with the direction of draw (the direction in which the core and cavity separate from each other) are typically angled slightly with (draft) to ease release of the part from the mold, and examination of most plastic household objects will reveal this. Parts with bucket-like features tend to shrink onto the cores that form them while cooling, and cling to those cores when the cavity is pulled away. The mold is usually designed so that the molded part reliably remains on the ejector (B) side of the mold when it opens, and draws the runner and the sprue out of the (A) side along with the parts. The part then falls freely when ejected from the (B) side. Tunnel gates tunnel sharply below the parting surface of the B side at the tip of each runner so that the gate is sheared off of the part when both are ejected. Ejector pins are the most popular method for removing the part from the B side core(s), but air ejection, and stripper plates can also be used depending on the application. Most ejection plates are found on the moving half of the tool, but they can be placed on the fixed half if spring loaded. For thermoplastics, coolant, usually water with corrosion inhibitors, circulates through passageways bored through the main plates on both sides of the mold to enable temperature control and rapid part solidification.

To ease maintenance and venting, cavities and cores are divided into pieces, called inserts, and subassemblies, also called inserts, blocks, or chase blocks. By substituting interchangeable inserts, one mold may make several variations of the same part.

More complex parts are formed using more complex molds. These may have sections called slides, that move into a cavity perpendicular to the draw direction, to form overhanging part features. Slides are then withdrawn to allow the part to be released when the mold opens. Slides are typically guided and retained between rails called gibs, and are moved when the mold opens and closes by angled rods called horn pins and locked in place by locking blocks, both of which move cross the mold from the opposite side.

Some molds allow previously molded parts to be reinserted to allow a new plastic layer to form around the first part. This is often referred to as overmolding. This system can allow for production of one-piece tires and wheels.

2-shot or multi shot molds are designed to “overmold” within a single molding cycle and must be processed on specialized injection molding machines with two or more injection units. This can be achieved by having pairs of identical cores and pairs of different cavities within the mold. After injection of the first material, the component is rotated on the core from the one cavity to another. The second cavity differs from the first in that the detail for the second material is included. The second material is then injected into the additional cavity detail before the completed part is ejected from the mold. Common applications include “soft-grip” toothbrushes and freelander grab handles.

The core and cavity, along with injection and cooling hoses form the mold tool. While large tools are very heavy weighing hundreds and sometimes thousands of pounds, they usually require the use of a forklift or overhead crane, they can be hoisted into molding machines for production and removed when molding is complete or the tool needs repairing.

A mold can produce several copies of the same parts in a single “shot”. The number of “impressions” in the mold of that part is often incorrectly referred to as cavitation. A tool with one impression will often be called a single cavity (impression) tool. A mold with 2 or more cavities of the same parts will likely be referred to as multiple cavity tooling. Some extremely high production volume molds (like those for bottle caps) can have over 128 cavities.

In some cases multiple cavity tooling will mold a series of different parts in the same tool. Some toolmakers call these molds family molds as all the parts are not the same but often part of a family of parts (to be used in the same product for example).

[edit] Machining

Molds are built through two main methods: standard machining and EDM. Standard Machining, in its conventional form, has historically been the method of building injection molds. With technological development, CNC machining became the predominant means of making more complex molds with more accurate mold details in less time than traditional methods.

The electrical discharge machining (EDM) or spark erosion process has become widely used in mold making. As well as allowing the formation of shapes which are difficult to machine, the process allows pre-hardened molds to be shaped so that no heat treatment is required. Changes to a hardened mold by conventional drilling and milling normally require annealing to soften the steel, followed by heat treatment to harden it again. EDM is a simple process in which a shaped electrode, usually made of copper or graphite, is very slowly lowered onto the mold surface (over a period of many hours), which is immersed in paraffin oil. A voltage applied between tool and mold causes spark erosion of the mold surface in the inverse shape of the electrode.

[edit] Cost

The cost of manufacturing molds depends on a very large set of factors ranging from number of cavities, size of the parts (and therefore the mold), complexity of the pieces, expected tool longevity, surface finishes and many others. The initial cost is great, however the piece part cost is low, so with greater quantities the overall price decreases.

For the injection molding cycle to begin, four criteria must be met: mold open, ejector pins retracted, shot built, and carriage forward. When these criteria are met, the cycle begins with the mold closing. This is typically done as fast as possible with a slow down near the end of travel. Mold safety is low speed and low pressure mold closing. It usually begins just before the leader pins of the mold and must be set properly to prevent accidental mold damage. When the mold halves touch clamp tonnage is built. Next, molten plastic material is injected into the mold. The material travels into the mold via the sprue bushing, then the runner system delivers the material to the gate. The gate directs the material into the mold cavity to form the desired part. This injection usually occurs under velocity control. When the part is nearly full, injection control is switched from velocity control to pressure control. This is referred to as the pack/hold phase of the cycle. Pressure must be maintained on the material until the gate solidifies to prevent material from flowing back out of the cavity. Cooling time is dependent primarily on the wall thickness of the part. During the cooling portion of the cycle after the gate has solidified, plastication takes place. Plastication is the process of melting material and preparing the next shot. The material begins in the hopper and enters the barrel through the feed throat. The feed throat must be cooled to prevent plastic pellets from fusing together from the barrel heat. The barrel contains a screw that primarily uses shear to melt the pellets and consists of three sections. The first section is the feed section which conveys the pellets forward and allows barrel heat to soften the pellets. The flight depth is uniform and deepest in this section. The next section is the transition section and is responsible for melting the material through shear. The flight depth continuously decreases in this section, compressing the material. The final section is the metering section which features a shallow flight depth, improves the melt quality and color dispersion. At the front of the screw is the non-return valve which allows the screw to act as both an extruder and a plunger. When the screw is moving backwards to build a shot, the non-return assembly allows material to flow in front of the screw creating a melt pool or shot. During injection, the non-return assembly prevents the shot from flowing back into the screw sections. Once the shot has been built and the cooling time has timed out, the mold opens. Mold opening must occur slow-fast-slow. The mold must be opened slowly to release the vacuum that is caused by the injection molding process and prevent the part from staying on the stationary mold half. This is undesirable because the ejection system is on the moving mold half. Then the mold is opened as far as needed, if robots are not being used, the mold only has to open far enough for the part to be removed. A slowdown near the end of travel must be utilized to compensate for the momentum of the mold. Without slowing down the machine cannot maintain accurate positions and may slam to a stop damaging the machine. Once the mold is open, the ejector pins are moved forward, ejecting the part. When the ejector pins retract, all criteria for a molding cycle have been met and the next cycle can begin.^[1]

The basic injection cycle is as follows: Mold close – injection carriage forward – inject plastic – metering – carriage retract – mold open – eject part(s) Some machines are run by electric motors instead of hydraulics or a combination of both. The water-cooling channels that assist in cooling the mold and the heated plastic solidifies into the part. Improper cooling can result in distorted molding. The cycle is completed when the mold opens and the part is ejected with the assistance of ejector pins within the mold.

The resin, or raw material for injection molding, is most commonly supplied in pellet or granule form. Resin pellets are poured into the feed hopper, a large open bottomed container, which is attached to the back end of a cylindrical, horizontal barrel. A screw within this barrel is rotated by a motor, feeding pellets up the screw’s grooves. The depth of the screw flights decreases toward the end of the screw nearest the mold, compressing the heated plastic. As the screw rotates, the pellets are moved forward in the screw and they undergo extreme pressure and friction which generates most of the heat needed to melt the pellets. Electric heater bands attached to the outside of the barrel assist in the heating and temperature control during the melting process.

The channels through which the plastic flows toward the chamber will also solidify, forming an attached frame. This frame is composed of the sprue, which is the main channel from the reservoir of molten resin, parallel with the direction of draw, and runners, which are perpendicular to the direction of draw, and are used to convey molten resin to the gate(s), or point(s) of injection. The sprue and runner system can be cut or twisted off and recycled, sometimes being granulated next to the mold machine. Some molds are designed so that the part is automatically stripped through action of the mold.

Hawthorne Rubber Mfg. Corp. uses compression, transfer and injection molds. On our quotations, we may propose more than one type of mold. To clarify these different molding options, their advantages and disadvantages, we have put together three education/information web pages on molds. The following covers the most sophisticated type of mold, injection.

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Figure 1 is an example of a basic vertical type three plate multi-cavity injection mold, however, you are not limited to three plates. The mold does not require heater elements or temperature controllers. The molding temperature is fully controlled by the injection press it is running in.

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Molding Procedure: The press opens the mold. It remains in the press. Center plates are separated by the stripper bolts and hang from the top plate. (However, some core bars or plates may be designed to be removed to retrieve the finished molded parts.) When bonding to metal inserts, they are placed into the cavities at this time. The mold is closed by the press and the fully automated injection cycle begins. A large ram or screw forces preheated uncured rubber through the injection nozzle, through the mold’s runner system, down through the “sprues” and into the cavities. The uncured rubber is then forced into the shape of the cavity in the mold. A slight excess of material flows out of the cavity, along the gates and vents. The mold remains closed until the rubber is cured, completing the cycle.

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Figure 2 is an example of a basic horizontal type two plate multi-cavity injection mold. In general, you are limited to two plates. Again the mold does not require heater elements or temperature controllers. The molding temperature is fully controlled by the injection press it is running in.

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Molding Procedure: The molding procedures for horizontal and vertical molds are very similar. The main differences are:

• The lack of sprues in the two plate design. Cavities are normally filled from an injection point along the cavity parting line.
• Often, parts can be ejected and swept out of the mold by the press brushes.
• Utilizing the press brushes and ejector system, molds can complete the cure, eject the cull, runner and parts, brush off any remaining rubber and begin the next cycle without the intervention of the press operator.
• Less rubber lost to the injection runner system.

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General Comments: Injection molds share many of the same advantages and disadvantages that the transfer molds do over compression molds. Additional rubber is consumed in the “cull”, “runner” and “sprues” when filling the cavities. However, because the cavity plates start out closed, less rubber escapes the cavity, thereby limiting excess flash. This also makes it well suited for molding delicately shaped parts or securing inserts that are embedded in a product. The uncured rubber is automatically feed into the injection cylinder, preheated, and then accurately metered into the mold by monitoring and controlling the injection time, temperature and pressure. Because many different colored materials are run thru an injection cylinder, it will take several cycles to wipe out any prior pigments when molding light colored compounds while setting up the job. The transfer and compression mold are more suited for short runs of colored and translucent compounds. Injection molds have the shortest cycle and cure times. This is due to a greater level of automation, the preheating of the rubber in the injection cylinder, and a rapid transfer of heat to the rubber while being forced through the “runners” and “sprues”. Due to the more complex design of the injection mold, it is more expensive to purchase than a transfer or compression mold, but it may be better suited for your product design and size of the order.

CIK Mold Engineering specializes in moldmaking, toolmaking, and top tool design. Once mold design is complete, molds are then skillfully made by molders with specialized tools. Our work can be seen in automotive parts, houseware parts, and many other industries. We are experts in surfacing and modeling, and can also help with mold repair and tool repair.

Injection molding is a manufacturing technique for making parts from thermoplastic material in production. Molten plastic is injected at high pressure into a mold, which is the inverse of the product’s shape. After a product is designed by an Industrial Designer or an Engineer, molds are made by a moldmaker (or toolmaker) from metal, usually either steel or aluminum, and precision-machined to form the features of the desired part. Injection molding is widely used for manufacturing a variety of parts, from the smallest component to entire body panels of cars. Injection molding is the most common method of production, with some commonly made items including bottle caps and outdoor furniture.

Compression molding is a method of molding in which the molding material, generally preheated, is first placed in an open, heated mold cavity. The mold is closed with a top force or plug member, pressure is applied to force the material into contact with all mold areas, and heat and pressure are maintained until the molding material has cured. The process employs thermosetting resins in a partially cured stage, either in the form of granules, putty-like masses, or preforms. Compression molding is a high-volume, high-pressure method suitable for molding complex, high-strength fiberglass reinforcements. Advanced composite thermoplastics can also be compression molded with unidirectional tapes, woven fabrics, randomly orientated fiber mat or chopped strand. The advantage of compression molding is its ability to mold large, fairly intricate parts. Compression molding produces fewer knit lines and less fiber-length degradation than injection molding.

At least for the manufacturing segment of the economy, the first part of 2008 is clearly recessionary. Orders and shipments are down. Manufacturing output is down. This is a recession. But it is not a very deep one and will remain so unless policy makers and consumers panic.

Consumers are being hit hard with gas prices that are likely to stay very high and will clearly cut into disposable dollars, further reducing spending and thus orders. The housing troubles are unlikely to be alleviated until sometime in 2009.

So when will we see clear improvement in overall economic conditions and a return to growth for molders?

Our prediction is that the first four or five months of 2008 will have an overall negative growth rate for all of injection molding combined. But come late spring and early summer, conditions will improve and we will end this year with clear growth in the 2-2.5% range on an annualized basis. 2009 looks much better with growth well above 3% for molders.

We have adjusted our 2008 forecast in the index downward, and will possibly revise it even more in the weeks to come.

The core problems

The central issues that affect molders and consumers alike are steep energy prices (at this time oil hovers at about $108/bbl and may go up) and the sustained deep decline in housing starts. Unemployment has inched up, and so has inflation due to oil prices and higher costs for imported goods (thanks to the low value of the dollar, which, at the same time, boosts U.S. exports). The recent 0.75% rate cut by the Fed may alleviate some of the credit crunch.

So what causes us to be so optimistic that the economy will reassert strength this year?

We see several developments that will boost growth for molders and help maintain worker incomes, thus supporting the all-critical consumer spending that drives the economy.

The economic stimulus packages from Congress will affect consumer spending and, more importantly, confidence in late spring and early summer of 2008. Clear choices in the presidential contest and overall improvements in our two major wars—Iraq and Afghanistan—will remove some of the psychological pressures from the economy. But most importantly, we anticipate policy makers to exert international pressures to boost the value of the dollar, which recently hit a record low against the euro.

This will help reduce the run-up in oil prices while allowing continued growth in exports of manufactured goods—an area where many molders will benefit while the domestic economy is weak.

Core sectors of manufacturing apart from the housing-related segments remain in positive growth territory. Automotive parts (albeit mostly those made for foreign carmakers), medical disposables and components, and some consumer electronics now once again manufactured inside the United States are all projected to hold stable. We also expect sustained demand in markets for small appliances and other relatively low-cost consumer goods.

Orders and automotive tell the story

The U.S. economy’s manufacturing sector contracted in February, but somewhat less than feared, according to the reliable monthly index of the Tempe, AZ-based Institute for Supply Management (ISM). The ISM index showed a drop to 48.3 in February, down from 50.7 in January. (A reading above 50 indicates growth in the sector, while a reading below 50 indicates contraction.)

Other ISM index February figures compared to January: New orders fell to 49.1 from 49.5 in January; production dropped to 50.7 from 55.2; prices paid fell to 75.5 from 76.0; and inventories declined sharply to 45.4 from 49.1.

In past recessions, these drops were much steeper, and we anticipate the ISM index, as well as all other key economic indices, to move back into positive territory by May or June.

Domestic automotive companies remain in trouble. U.S. sales of the most profitable (as well as biggest consumers of molded parts) vehicles—trucks, sport utilities, and large sedans—have dropped sharply as consumers seek out more fuel-efficient alternatives. GM and Ford already announced second quarter production cuts.

In February, GM’s sales dropped 13% while Ford’s sales data showed a 7% decline. Chrysler dropped 14% and even Toyota saw sales drop 3%. At this point, we project March’s domestic auto sales data to be just as bad and perhaps a bit worse.

On an annualized basis, February sales stood at 15.4 million units for the United States, down sharply from the 16.6 million rate seen in February 2007.

What should automotive molders anticipate? The announced cuts in production are a guide. Ford, for instance, will cut production by about 10% for the second quarter.

GM’s production cuts have not yet been fully announced but will exceed 6% overall. Key Japanese carmakers—Toyota, Honda, and Nissan—will probably expand production of smaller vehicles by an overall rate of 7% for the year.

No big purchases for now

Fear of unemployment is another factor slowing major purchases. The U.S. Dept. of Labor released revised stats for December and January that paint a much bleaker picture than the original figures. They now say only 41,000 jobs were created in December, exactly half of the 82,000 new jobs reported earlier. Job losses from January have been revised up from 17,000 to 22,000. Another 63,000 jobs were reported lost in February, making it the highest month for declines in nearly five years and the first consecutive monthly declines since May and June 2003.

These figures have triggered reaction among economists and analysts that a recession is indeed in progress. The United States presently has the largest group of unmarried earners in history and the largest group of mothers working due to economic necessity rather than choice. These two large groups are less likely to invest in major purchases if they believe their job may be in jeopardy. Not surprisingly, new orders at U.S. factories fell 2.5% in January, the first decline since August. The Commerce Dept. said new orders for durable goods fell 5.1% in January.

The glut of existing homes at bargain prices doesn’t seem to be diminishing and new home sales are projected to stay dismal for the rest of the calendar year. Home inventories are at about 10 months, meaning it is currently taking an average of 10 months for a home on the market to be sold. Most residential builders start applying for permits when inventory in their area is at five months or less.

Why the glut in existing home sales? The subprime mortgage debacle has lenders requiring higher credit scores or at least 20% down on the purchase. This, in turn, makes many potential buyers have to wait longer either to repair their scores or to save for a larger down payment.

The National Assn. of Home Builders [link to http://www.nahb.org] is pessimistic about new home sales for this year. It is forecasting new-home sales to drop 22% in 2008, bringing sales 55% below the highs reached in late 2005. Housing starts are predicted to drop 31% this year, down 60% from 2005.