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Practical Aspects on Injection Molding of Arlen

作者:
蔡宏斌 國立宜蘭技術學院化工系教授
蔡瑞禧 大華技術學院化工系副教授
林建興 允拓材料科技股份有限公司

Abstract

Arlen is a series of modified nylon 6T resins with a high melting pointe -310°C), and the processing window is rather narrow.Some practical aspects about the injection molding of Arlen are described. In the development of high precision product such as connectors by Arlen, mold design is a very important key point. The design of appropriate air vent is often necessary to avoid short shot and scorching.The design of gate should be matched with the flow of the resin. The balance of runner and sprue should be always kept in mind in the mold design.In the injection molding of Arlen, the drying of the resin in necessity and mold temperature control are important. Especially, a suitable mold temperature(-120°C) is the key factor for Arlen to develop the desired properties. Sometimes, suitable injection molding machine is necessary for a special product based on the properties and cost effectiveness. Some practical cases of the injection molding of Arlen are described.

 

INTRODUCTION

Arlen is a series of modified nylon 6T resins developed by Mitsui Chemical, Inc. Arlen is derived from hexamethylenediamine, terephthalic acid and adipic acid, its melting point is around 310-320°C Arlen exhibits excellent heat resistance due to its high melting point, and this make Arlen show excellent reflow soldabiIity.Arlen’s flowability is excellent, it can be molded into thin-walled parts. Arlen shows good chemical resistance, moreover, the water absorption of Arlen is rather low among nylon resins, and its dimensional change due to water absorption is low enough. Arlen is capable of retaining its modulus a.t elevated temperature, it exhibits stable rigidity over a wide temperature range. Good balance in such performce characteristics make Arlen suitable for the. application of elootronic parts, in particular, for SMT connectors. Other applications of Arlen include automotive parts7 mechanical parts, etc.

If the processing temperature is over 340°C,the aliphatic chains in Arlen may decompose. Moreover, Arlen bas a high melting’point of about 310-320°C, thus the processing window is rather narrow.Some practical aspects about the injecti.on molding of Arlen are described in this article.

 

MOLD DESIGN

In the development of high precision parts:. three important factors including mold design, injection molding conditions and material selection may influence the performance of the made parts. By experience, the proportions in importance of mold design, molding conditions and material selection may be 70%~ 15% and 15%, respectively. Thus, mold design is a very impc)rtant key point for the development of high precision parts, such as connectors by Arlen, For example, good mold design roilY decrease the: number of mold trial increase the processing efficiency, and may sboneo the developing period and decrease the cost For Arle~ so~e practieal aspects on the mold design are given below.
1. Product precision

In the mold design of high precision parts by Arlen, the difference in . mold shrinkage in different flow directions should be oonsidered. For glass fiber reinforced grades, the mold shrinkage parallel to the flow direction is a:round 0.2-0.6%, which is lower than that transverse to the flow direction(0.5-0.9%).
2. Vent

The processing temperature of Arlen is rather high(>310 ), compressed air may cause scorching of the resin during injection molding, venting in mold is often necessary. The design of appropriate air vent can avoid not only scorching but also short shot.

It is advisable that the distribution of the positions of the vents is uniform. For convenience, the vents can get into a deeper venting channel with an open to contact outside as shown’ in Figure 1. The size of vent influences the perfOrmance significantly. If the dearance of the vent is too large, flash may occur. But too small clearance: may give poor venting. It is recommended for Arlen that the clearance of the vent is 10-15 um. Suitable size of the vent of Arlen mold is shown .in Figure 2, the width of the vent may be 3-10 mm, the length of the vent :rllay be 2-3 rom, and the depth of the venting channel may be 0.5 ‧. 1.0 nun. By experience, smaller vent is recotntnended to be used first, which can be enlarged upon demanding.


Figure 1. Vent and submarine gate.


Figure 2. Size of suitable vent (Urit::mm).

3. Gate

The design of gate should be matched with the flow of the resin from runner, through gate to cavity. The type of gate:. the size of gate and the position(s) of gate(s) are major important design factors .

Side gate is a simple design. The gate size depends on the wall thickness, shape and volume of molded product. The depth of gate can be 1/2 to 2/3 of wall thickness Qfthe product.

If the cavity is very smaIl, or multi-gated design is necessary to regulate the flow balance, pin point gate roay be used. The size with a gate diameter of 0.8-2.0 mm and a land length of 1.0-2.0 rom may be suitable for Arlen. Small gates may cause poor flow ability and plugging which may result sink marks. Once these problems are encountered, the gate diameter should be enlarged. Pin point gates rl~u1t weld lines which may affect the strength and cause warpage. The number gates and their positions may be decided after some trials. Submarine gates are often used to cut gates automatically. Common submarine gates are shown in Figure 1 and Figme 3(a). The feasible gate diameter is 0.8-2.0 mm, and the gate angle should be less than 45°. If smoother and easier cut is necessary, a modified submarine gate design is recommended as shown in Figure 3(b). The major point of this design is to decrease the gate diameter to 0.3-0.5 mm.

Figure 3.Submarine gate

4. Runner

The balance of runner should be always kept in mind in the mold design of high precision parts of Arlea The shape of runner also affect the flow of resin significantly as shown in Figure 4. As the molten’ resin flows through the runner an adiabatic skin layer close to the wall of runner forms which let the later coming maintain the molten state at high tempewure. The adiabatic skin layer is not uniform in a ditch shape runner as shown in Figure 4(c), this may cause not uniform flow, and is not favorabl for the high precision parts. Semi-eircular runner is fair as shown in. Figure. 4(b). Circular runner always gives better results as shown in Hgw-e 4(a). The runner size is preferably as smalle-s possible. Suitable rnrmer diameter is 3-5 mm for high precision pans of Arlen. Shorter runner length is better. In addition all comers should be smooth and round.


Figure 4.Shape of runner

5. Sprue

Short, round sprue is recommended for Arlen. The sprue diameter bad batter match the nozzlet runner and the volume per shot. For example, a diameter of 3.0 mm of the sprue in contact with a nozzle of a diwneter of 2.8 rom is suitable. The sprue taper angle is preferably 3-5 for good demolding of the parts.


Figure 5.Designning concept of full core nozzle

Table 1.Injection molding conditions of Arlen

6. Mold material

Common grades of Arlen are glass fiber reinforced and require a mold temperatur of more than 120C, thus~ the suitable mold material is high hardness and wear resistapt steel. Corrosion resistant stainless steel is often used for the flame retardant grades.


Figure 6.Effect of mold temperateure on the crystallinity and shrinkage of Arlen

 

INJECTION MOLDING MACHINES

Arlen can be processed by most screw injeetk)n molding machines.
1. Size of injection molding machine

The volume in the cylinder of the nyection rnolding machine should be an appropriate match for the volume of each shot. If the volume in the cylinder is much bigger, the residence time of the resin in the cylinder will be long which often cause drooling or brittleness of the molded parts. An economical and effective size is that the volume of each shot is 40-80% of the maximum injection cylinder volume.
2. Screw

A general purpose or nylon type screw of wear resistance may be used for Arlen. However, a corrosion resistant type screw is reoommended for the flame retardant grades of Arlen.
3. Nozzle

It is recommended that a standard nozzle is used. However Arlen resin may deoompose at the sing temperature if its residence time is unexpectedly long. Appropriate match of the nozzle to the cylinder to avoid the dead points may be necessary. A typical example is as follows. An injection molding rna.eh.ine was used to manufacture an SMT connector with an Arlen grade of natlU’e color. It was found that the made parts always exhibited many black spots. After some trials including the change of screw, cleaning and purging, that madun.e still failed. But other machine could work well. Finally, it was found that the nozzle part locked onto the cylinder formed dead corner, in which some resin bad been scorched. After change of a suitable nozzll;:, the injectiQn molding machine could work well. For convenience, it is recommended that a full cone nozzle as shown in Figme 5 is used for Arlen.
4. Cylinder

The material of cylinder follows similar requirements as those of the screw.
5. Clamping system

Common screw .injection molding machines can provide enough clamping force for the injection molding of Arlen. However, a mold temperature of more than 120″c is necessary for the development of enough crystallinty for Arlen, it is recommended that heat insulators are used to decrease the heat loss for maintaining the mold temperature.

 

lNJECTION MOLDlNG CONDITIONS

Arlen can be easily injection molded. Typical injection molding conditions are summarized in Table 1.

Table 1. Injection molding conditions of Arlen

Arlen is hygroscopic. As packaged it can be used immediately without dIying. If Arlen has been exposed to the atmosphere, it should be dried before using. Otherwise, drooling, decomposition of the resin, brittleness of the product may occur. The moisture content of the resin should be below 1000 ppm for practical injection molding operation. A drying condition at 110°C for 6 hours is practical.

Arlen is hygroscopic. As packaged it can be used immediately without dIying. If Arlen has been exposed to the atmosphere, it should be dried before using. Otherwise, drooling, decomposition of the resin, brittleness of the product may occur. The moisture content of the resin should be below 1000 ppm for practical injection molding operation. A drying condition at 110°C for 6 hours is practical.

The processing window is rather narrow. If the processing temperature is increased, the flowability of resin increased, but decomposition of the resin may occur. If the processing temperature is over 340°C the aliphatic chains in Arlen may decompose significantly. Thus, the residence time of resin in the cylinder is an important factor. It can be keep in mind that the residence time of resin at a processing temperature of 320°C; should be shorter than 15 minutes, and that at a processing temperature of 330°C should be shorter than 5 minutes.

The control of mold temperature control is very important in the . injection molding of Arlen. Figure 6 shows the effect of mold temperature on the crystallinity of Arlen and the slninkage after retlow of Arlen part. From the viewpoint of crystallinit; suitable mold temperature range is 120-150°C. But higher mold temperature requires longer cycle time, and a mold temperature of about 120°C may be an optimum one for Arlen.

Recycled Arlen resin exhibits poorer properties. It is recommended that less than 20% of recycled resin is used to mix with the virgin resin. Recycled resin should be dried before use.

 

CONCLUSION

Mold design is a key factor in the development of high precision parts by Arlen, Appropriate air vent is often necessary to avoid short shot and scorching. The design of gate should be matched with the flow of the resin. The balance of nmner is imponant in the mold design. In the injection molding of Arlen the drying of the resin is necessary if the resin has been exposed to the air. Mold temperatlm control is essential A suitable mold temperature(-120°C) is the key factor for Aden to develop the desired properties.