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Abstract.

The hot runner technology is an advanced & applied technology in process of forming of an injection mold. As soon as the hot runner technology is applied in injection molds, it will obtain the aim for optimization of product quality, increasing efficiency of production, saving raw materials, saving energy, decreasing cost of products and so on. The article will mainly discusses the applied study of hot runner technology in injection mold, and has a detailed explanation for the design instances of hot runner technology in injection mold, the suitability of hot runner system and the technological & economical analyses of hot runner molds.

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INTRODUCTION TO INJECTION MOLDING

Injection moulding

Injection moulding has always been one of the most common processing methods for plastics. Nowadays countless parts in many electrical appliances, automobiles and office equipment are injection molded. The most common injection moulding machinery is the reciprocating screw machine, whose process can be divided into several stages as seen in Figure 6. At the plastication stage, the feed unit operates as an extruder, melting and homogenizing the material in the screw/barrel system. The screw, however, is allowed to retract in order to make room for the molten material in a space at the cylinder head, called material reservoir, between the screw tip and a closed valve or an obstruction of solidified material from the previous shot. At the injection stage, the screw is used as a ram (piston) for rapid transfer of the molten material from the reservoir to the cavity between the two halves of the closed mould. Since the mould is kept at a temperature below the solidification temperature of the material, it is essential to inject the molten material rapidly enough to ensure complete filling of the cavity. A high holding or packing pressure is normally exerted, to partially compensate for the thermal contraction of the material upon cooling. The cooling of the material in the mould often limits the production time because of the low thermal conductivity of polymers. The mould, after being cooled, can be opened and the solid product ejected. Although the screw machine is by far the most popular, plunger injection machines are also used to give products some unique features. There is no shearing or mixing action, as a plunger does not rotate. The resulting moulded part can take on a marbled appearance with swirls of two or more colours. This may be the desired finish for certain products. Regardless of different machines, injection moulding yields a high productivity and allows the products to have many fine details such as bosses, location pins, mounting holes, bushings, ribs, flanges, etc. All these features can eliminate many

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subsequent assembly and finishing operations. A large variety of products can be injection molded. These include (a) micro-products, molded in multiple cavity moulds on small precision machines, such as components for watches and microelectronics; (b) medium size products molded continuously in very large numbers in dedicated machines or in relatively small runs; and (c) large products, molded by large machines, such as car dashboard frames, TV cabinets, garden furniture, and small boat hulls. Many of these large plastic parts have a solid skin and a cellular inner structure, hence the process is also known as structural foam moulding.

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Reaction injection molding

Reaction injection moulding is a relatively new process, which involves

the rapid mixing, in precise proportions, of two or more highly reactive

liquid components and the immediate injection of the mixture in a

closed mould. Polymerization takes place in the mould in a very short

period of time, yielding a solid product. The process is particularly

suited to the production of large and relatively thin parts, with less

capital investment and operating costs than in thermoplastic injection

moulding. The process is also energy efficient, but requires good control

of complex reactions. By and large, each moulding process mentioned

above has its pros and cons in terms of the materials, products and cost.

Appendix 2 presents a summary of the characteristics of each moulding process and its applications.

MOLD PARTS

The main parts of an injection mold are :• COLD RUNNER SYSTEM • CORE• CAVITY• EJECTOR ASSEMBLY

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COLD RUNNER SYSTEM:

The runner system mainly comprises of:

• Register Ring: This ensures the complete butting of the machine nozzle to the mould.

• Sprue Bush: The processed plastic material is transferred to the impression from the machine through a passage called sprue. The bush in which the passage is machined is called sprue bush.

• Runner and Gate: The material reaches the impression through the runner and gate system.

Cavity: This is the half of the mold that forms the outer surfaces of the part. It is characterized by the negative impression of the part carved into the cavity block.

Core: This is the half of the mold that forms the backside of a part. In general, the core material rises up from the core block and almost fills in the back of the cavity. The resulting space between the core and cavity is the wall thickness of the part that will be molded.

Parting line: This is the interface where two parts of the mold, such core and cavity or slide and mold, come together. This term refers both to the interface and the resulting witness line that is molded into the part.

Shutoff: This is a place where two parts of the mold shut against each other and prevent plastic from passing through. Technically speaking, the main parting line interface is a shutoff, but the term is seldom used in this instance. Usually, the term applies to a situation where one part of the mold closes against another to form a slot or hole, or it is sometimes used to refer to the interface where a slide shuts against a core or cavity. Sometimes, the shutoff surfaces are parallel to the direction that the mold opens. When this happens, draft has to be added to avoid grinding the mould parts of the mold against each other. This type of shutoff is

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sometimes referred to as a sliding shutoff or a slide-by. An example of this will be seen on the vent mold.

SPRUE BUSH

COLD RUNNER SYSTEM

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MOLDING DEFECTS AND REMEDIES

The different molding defects and its causes and remedies are given below:

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DISADVANTAGES OF C.R.S

Cold runner system has some disadvantages such as

• High cost of energy: In cold runner system , the component undergoes larger cooling time. This results in longer running of the machine to attain the production rate. Hence energy consumption rate also increases.

• Workmanship: Since the component is ejected along with the runner and gate , extra workforce is required for the process of degating. Which is the separation of components from the runner and gate manually.

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• High scrap ratio: Since the solidified runner and gate is rejected , it increases the scrap ratio.

• Low product quality of surface appearance: Results in flash and other molding defects.

• Requirement of high Injection pressure

HOT RUNNER SYSTEM

Introduction

Hot runner technology is widely used in all kinds of products of the injection molding. It is the applied research and development with bright future to develop hot runner injection mould on the economic and technological basis depending on different category, different kinds and different performance of thermoplastic materials. In recent years, with the gradual extension of hot runner system, the injection products inEuropean and American countries are depending more on hot runner system. Currently product of molds not used with the hot runner technology in molding products is difficult to meet the requirements of the customers, and this has been prompted by many mold manufacturers of hot runner system on the transformation of consciousness. But because the price of many imported hot runner device is quite high, many domestic manufacturers can not accept it, which leads to producing it by some of them. Relatively speaking, most of production enterprise in China adopt relatively low cost of hot runner system with generally inner-heated or out-heated hot runner device. Some production enterprises adopt the world's advanced level of more sophisticated needle-valve type of hot runner device with high technology and high cost. The type of the runner system is one of the most influential factors on injection molding process and the properties of the injected parts.

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Conventional runner systems (CRS) have some disadvantages such as high cost of energy and workmanship, high scrap ratio, low product quality of surface appearance and requirement of high injection pressure.Therefore, the mould designers are attracted to hot runner system (HRS) which is able to provide precisely adjustable process temperature, uniform filling in multi cavity moulds, even heat distribution within the mould, improvement on mechanical properties of injected part, and reduction in injection pressure. In addition, HRS allows significant cuts in production costs by saving material as a result of eliminating sprue, shorter mould opening distance because of the absence of sprue, and shorter cycle time.

Applicability of Hot Runner System

Generally speaking, the hot runner technology applies only to the injection molding, and also not each injection mold is suitable. At the same time, it should be clearly known that none of hot runner system cannot be applied to all of plastic materials and all kinds of injection products. Hot runner system for certain thermoplastics is applicable, but not applicable to another kind of plastics. Which is because the use of hot runner system still depends on many other factors, such as: injection quantity, injection rate, length of flow, shape of mold cavity and color plastic etc. Table 1 lists some of the common plastics relative to the applicability of hot runner system.

Material and type PP PE PC ABS PVC PMMA

Needle Nozzle VG VG POB VG POB VG

Universal Nozzle VG VG _ POB _ POB

VG – Very Good, POB – Possible

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Comparison of the Cost Analysis of Hot Runner System

Analysis of production cost is one of the aspects of being reasonable or not to use the hot runner system. To use hot runner system is the important condition of production of products. In comparison with other production technology, the purpose of cost analysis is to determine whether the adoption of hot runner system has good economic benefit. Cost analysis must contain all the expense, the producer is used to face the choice of either hot runner mold or the traditional mold of cold runner. Only when the cost difference is less, would the advantages and disadvantages of hot runner system become an important factor. Practical data shows that for the cold runner multi-cavity mould, only there are a considerable quantity of products, can it save materials and benefit it.Saving time and materials should be priority in the production for hot runner system. For instance: a multi-cavity mould needs injection forming from the front gate, such as: plastic bottle cap, spray gas cap and cup. Below are the some examples of traditional mold design to use hot runner system, Fig. 1 (a) shows a three-plates mold, the sprue to use hot runner plate and side pouring nozzle, injection cycle is reduced by 35% and product cost is reduced by 40%. Fig. 1 (b), mold in the cold sprue cup is replaced by hot runner switch nozzle, injection cycle reduces 18% and product cost is cut down 8%.

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Experimental setup

A box-shaped part that consists of some details such as ribs, holes, large flat surface, conical surface and rounded edge was adopted, in order to observe the effects of runner systems on its dimensions and shape. The main dimensions of the part are 112 · 78 · 36 mm and the thickness of the walls is 2 mm. The drawing and the general view of the part are presented in Fig. 1.

In order to examine and to precisely compare the effects of HRS andCRS, it is necessary to perform the experiments at the same conditions.Thus, a two-cavity mould which can be used with both HRS and CRS was designed and produced. The diameter of the nozzles was determined as 1.9 mm. Six resistances of 400W for heating the nozzles and manifold, and four thermocouples for controlling the melt temperature were fitted to the mould. The determination of gate location was carried

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out by utilizing the Mold Flow analysis software for both HRS and CRS, as shown in Fig. 1. For each mould cavity, four cooling channels with parallel connection were generated. The core and cavity halves of the mould are assumed to have the same temperature to minimize warpage. After completion of the experiments with HRS, the mould was modified for CRS by assembling some appropriate components such as sprue pulling pin and plugs for nozzle nests. Thus, the possibility of doing all experiments with both runner systems on the same mould and comparison of the results precisely was provided.

When using CRS, the flow of molten plastic becomes more difficult because of the heat dissipating in the runner channels. In addition, increase in flow length increases the frictional pressure losses. Using HRS provides significant pressure gain by eliminating the disadvantages of CRS. PP material was injected at temperatures of 170, 200 and 260deg C, and at pressures shown on the diagram in Fig. 4.

It was observed that significant pressure gain was provided when using HRS. At mean process temperature of 200 deg C, e.g., the required average injection pressure is 70 MPa for HRS, instead of 85 MPa for CRS which means a pressure gain of 17.64%. The pressure gains at other process temperatures of 170 degC and 260 degC were determined as 7.5 and 20%, respectively. When considering the low peaks instead of the average values of the injection pressure, it was determined that the pressure gains rose up to 18.75%, 42.85% and 33.33% at 170, 200 and 260 deg C, respectively. This reduction on injection pressure allows considerable saving in production costs and increases the lifetime of the mould and injection machine. Pressure gains obtained when using HRS for ABS and PP materials are presented in Table 3.

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2) Shrinkage evaluation

It has been reported that the primary influential factors on shrinkage of an injected part are both the magnitude and duration of exerted packing pressure. The excess shrinkage on the remote regions of the part from the sprue is largely attributed to the reduced effect of the packing pressure at the outer regions. There is a direct relationship between orientation and shrinkage due to the fact that molecular chains are oriented in line with the flow direction under the effect of friction and elongation. This effect is more pronounced at the outer zones of the part where the material sets relatively faster. Therefore, the sample produced by injection moulding experiences more shrinkage in the direction of flow.The calculated shrinkage rates in length and width for both runner systems vs. injection pressure at the process temperature of 225 deg C for ABS and 170 deg C for PP are presented in Figs. 6 and 7, respectively. According to these figures, it was observed that the

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shrinkage rate decreased with increasing injection pressure for both runner systems. This point is also stated by previous researches in the literature. Liao et. al. reported that the packing pressure is the most important process parameter for shrinkage, because it becomes effective during cooling down period whereby the material starts. Under the condition that the packing pressure is high, the polymer can be squeezed into the cavity to reduce and even the shrinkage. For HRS and CRS, the average shrinkage rates in length and width for ABS and PP polymers are presented in Table 4. These results showed that using HRS decreases the shrinkage rates for both of the polymers in comparison with CRS. It is interpreted that this shrinkage-decreasing effect of HRS is resulted from more influential packing stage due to late solidification of the gates, lower heat losses and better fluidity of the molten plastic. In addition,using HRS makes the adaptation of central gate location possible in multi-cavity moulds. This shortens flow length, decreases pressure loss and contributes to achieving more influential packing stage. In the case of using CRS, reduction in shrinkage rates requires impractically high working pressures. For example, CRS results in low shrinkage rate at 170 deg C for PP polymer, but it requires 120 MPa injection pressure. Same shrinkage rate can be provided at much lower injection pressures when compared with HRS.

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BENEFITS OF H.R.S

• Materials cost savings - no runner to regrind or reprocessLeast expensive cost / piece.

• Reduction of energy costsShorter, faster cycle times - no runners to cool.

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• Smaller machines - reduced shot volume into runners.

• Automated processing – runners do not need to be separated from the parts

• Gates at the best position for economical design.

• No runners to remove or regrind, thus no secondary work needed.

• Lower injection pressures.

• Lower clamping pressure.

• Shorter cooling time.

• Shot size reduced.

• Cleaner molding process.

• Consistent heat within the cavity

Lower Cycle Time, Increase Output.

The cycle time of any mold is largely influenced by the cooling cycle how fast the resin can be sufficiently cooled so that the part can be ejected without permanent deformation. In any given mold, the areas that take longest to cool are those with the thickest wall section. injection time is another component that differs between comparable hot and cold runner equipped molds. The injection time difference will be the extra time required to fill the cold runner. Close and open stroke of the press is extended with cold runner equipped molds. The travel must be increased to accommodate safe ejection of the cold runner. Parts molded with hot runners better lend themselves to automated part removal. With no runner to interfere with part removal, secondary mold

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processing times involving manual labor, including part/runner separation, part trimming and packaging, are reduced or eliminated entirely.

Significantly Reduce Production Costs.

Although a more expensive capital investment upfront, a hot runner system is a significantly more cost-efficient means to keep production costs to a minimum over the long run.

Resin Savings.

Since there is no cold runner to discard or recycle, resin consumption is reduced. Depending upon the molding application (i.e., medical components or parts requiring FDA approval), the product may require 100 percent virgin material—increasing overall consumption.

Energy Savings

Energy is wasted plasticizing, cooling and regrinding each cold runner that is produced. Increased energy consumption also is a direct result of extended cycle times.

Labor Savings

Secondary operations—such as manual part de-gating and trimming—are eliminated entirely with a hot runner system.

Mold Cost Savings

A smaller cavitation hot runner equipped mold may be able to satisfy production quotas using a smaller number of cavities since it runs at a

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faster cycle. The smaller mold frame size may enable installation into a smaller press.

Injection Press Costs

Hot runners allow reduced injection pressures during packing, as the system does not have to deal with injecting resin through a cooled runner. Melt in the cold runner may lose heat en route to the gate, possibly requiring higher heats and/or pressures from the injection molding machine. By reducing the injection pressure and clamp tonnage required, it is often possible to run the same part in a smaller tonnage machine as the clamp tonnage required is not as great.

DISADVANTAGES OF H.R.S

• There are, however, a few disadvantages to hot runner systems that need to be considered.

• Hot runner molds are more complex and expensive to build than cold runner molds.

• Higher initial start-up costs than for cold runner systems.

• Risk of thermal damage to sensitive materials.

• Elaborate temperature control required.

• Higher maintenance costs – more susceptible to: o Breakdowns o Heating element failure