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An experimental study of the water-assisted injection molding of glass ?ber ?lled poly-butylene-terephthalate (PBT) composites
Abstract
The purpose of this report was to experimentally study the water-assisted injection molding process of poly-butylene-terephthalate (PBT) composites. Experiments were carried out on an 80-ton injection-molding machine equipped with a lab scale water injection system, which included a water pump, a pressure accumulator, a water injection pin, a water tank equipped with a temperature regulator, and a control circuit. The materials included virgin PBT and a 15% glass ?ber ?lled PBT composite, and a plate cavity with a rib across center was used. Various processing variables were examined in terms of their in?uence on the length of water penetration in molded parts, and mechanical property tests were performed on these parts. X-ray di?raction (XRD) was also used to identify the material and structural parameters. Finally, a comparison was made between water-assisted and gas-assisted injection molded parts. It was found that the melt ?ll pressure, melt temperature, and short shot size were the dominant parameters a?ecting water penetration behavior. Material at the mold-side exhibited a higher degree of crystallinity than that at the water-side. Parts molded by gas also showed a higher degree of crystallinity than those molded by water. Furthermore, the glass ?bers near the surface of molded parts were found to be oriented mostly in the ?ow direction, but oriented substantially more perpendicular to the ?ow direction with increasing distance from the skin surface.
2006 Elsevier Ltd. All rights reserved.
Keywords: Water assisted injection molding; Glass ?ber reinforced poly-butylene-terephthalate (PBT) composites; Processing parameters; B. Mechanical properties; Crystallinity; A. Polymer matrix composites; Processing
1. Introduction
Water-assisted injection molding technology [1] has proved itself a breakthrough in the manufacture of plastic parts due to its light weight, faster cycle time, and relatively lower resin cost per part. In the water-assisted injection molding process, the mold cavity is partially ?lled with the polymer melt followed by the injection of water into the core of the polymer melt. A schematic diagram of the water-assisted injection molding process is illustrated in Fig. 1. Water-assisted injection molding can produce parts incorporating both thick and thin sections with less shrinkage and warpage and with a better surface ?nish, but with a shorter cycle time. The water-assisted injection molding process can also enable greater freedom of design, material savings, weight reduction, and cost savings in terms of tooling and press capacity requirements [2–4]. Typical applications include rods and tubes, and large sheet-like structural parts with a built-in water channel network. On the other hand, despite the advantages associated with the process, the molding window and process control are more critical and di?cult since additional processing parameters are involved. Water may also corrode the steel mold, and some materials including thermoplastic composites are di?cult to mold successfully. The removal of water after molding is also a challenge for this novel technology. Table 1 lists the advantages and limitations of water-assisted injection molding technology.
Water assisted injection molding has advantages over its better known competitor process, gas assisted injection molding [5], because it incorporates a shorter cycle time to successfully mold a part due to the higher cooling capacity of water during the molding process. The incompressibility, low cost, and ease of recycling the water makes it an ideal medium for the process. Since water does not dissolve and di?use into the polymer melts during the molding process, the internal foaming phenomenon [6] that usually occurs in gas-assisted injection molded parts can be eliminated. In addition, water assisted injection molding provides a better capability of molding larger parts with a small residual wall thickness. Table 2 lists a comparison of water and gas assisted injection molding.
With increasing demands for materials with improved performance, which may be characterized by the criteria of lower weight, higher strength, and a faster and cheape production cycle time, the engineering of plastics is a process that cannot be ignored. These plastics include thermoplastic and thermoset polymers. In general, thermoplastic polymers have an advantage over thermoset polymers in terms of higher impact strength, fracture resistance and strains-to-failure. This makes thermoplastic polymers very popular materials in structural applications.
Poly-butylene-terephthalate (PBT) is one of the most frequently used engineering thermoplastic materials, which is formed by polymerizing 1.4 butylene glycol and DMT together. Fiber-reinforced composite materials have been adapted to improve the mechanical properties of neat plastic materials. Today, short glass ?ber reinforced PBT is widely used in electronic, communication and automobile applications. Therefore, the investigation of the processing of ?ber-reinforced PBT is becoming increasingly important [7–10].
This report was made to experimentally study the waterassisted injection molding process of poly-butylene-tere-phthalate (PBT) materials. Experiments were carried out on an 80-ton injection-molding machine equipped with a lab scale water injection system, which included a water pump, a pressure accumulator, a water injection pin, a water tank equipped with a temperature regulator, and a control circuit. The materials included a virgin PBT and a 15% glass ?ber ?lled PBT composite, and a plate cavity with a rib across center was used. Various processing variables were examined in terms of their in?uence on the length of water penetration in molded parts, which included melt temperature, mold temperature, melt ?lling speed, short-shot size, water pressure, water temperature, water hold and water injection delay time. Mechanical property tests were also performed on these molded parts, and XRD was used to identify the material and structural parameters. Finally, a comparison was made between water-assisted and gas-assisted injection molded parts.
2. Experimental procedure
2.1. Materials
The materials used included a virgin PBT (Grade 1111FB, Nan-Ya Plastic, Taiwan) and a 15% glass ?ber ?lled PBT composite (Grade 1210G3, Nan-Ya Plastic, Taiwan). Table 3 lists the characteristics of the composite materials.
2.2. Water injection unit
A lab scale water injection unit, which included a water pump, a pressure accumulator, a water injection pin, a water tank equipped with a temperature regulator, and a control circuit, was used for all experiments [3]. An ori- ?ce-type water injection pin with two ori?ces (0.3 mm in diameter) on the sides was used to mold the parts. During the experiments, the control circuit of the water injection unit received a signal from the molding machine and controlled the time and pressure of the injected water. Before injection into the mold cavity, the water was stored in a tank with a temperature regulator for 30 min to sustain an isothermal water temperature.
2.3. Molding machine and molds
Water-assisted injection molding experiments were conducted on an 80-ton conventional injection-molding machine with a highest injection rate of 109 cm3/s. A plate cavity with a trapezoidal water channel across the center was used in this study.Fig. 2 shows the dimensions of the cavity. The temperature of the mold was regulated by a water-circulating mold temperature control unit. Various processing variables were examined in terms of their in?uence on the length of water penetration in water channels of molded parts: melt temperature, mold temperature, melt ?ll pressure, water temperature and pressure, water injection delay time and hold time, and short shot size of the polymer melt. Table 4 lists these processing variables as well as the values used in the experiments
2.4. Gas injection unit
In order to make a comparison of water and gas-assisted injection molded parts, a commercially available gas injection unit (Gas Injection PPC-1000) was used for the gasassisted injection molding experiments. Details of the gas injection unit setup can be found in the Refs. [11–15]. The processing conditions used for gas-assisted injection molding were the same as that of water-assisted injection molding (terms in bold in Table 4), with the exception of gas temperature which was set at 20℃.
2.5. XRD
In order to analyze the crystal structure within the water-assisted injection-molded parts, wide-angle X-ray di?raction (XRD) with 2D detector analyses in transmis-XRD samples, the excess was removed by polishing the 40 kV and 40 mA. More speci?cally, the measurements were performed on the mold-side and water-side layers of the water-assisted injection-molded parts, with the 2h angle ranging from 7 to 40 . The samples required for these analyses were taken from the center portion of these molded parts. To obtain the desired thickness for the sion mode were performed with Cu Ka radiation at samples on a rotating wheel on a rotating wheel, ?rst with wet silicon carbide papers, then with 300-grade silicon carbide paper, followed by 600- and 1200-grade paper for a better surface smoothness.
2.6. Mechanical properties
Tensile strength and bending strength were measured on a tensile tester. Tensile tests were performed on specimens obtained from the water-assisted injection molded parts (see Fig. 3) to evaluate the e?ect of water temperature on the tensile properties. The dimensions of specimens for the experiments were 30 mm · 10 mm · 1 mm. Tensile tests were performed in a LLOYD tensiometer according to the ASTM D638M test. A 2.5 kN load cell was used and the crosshead speed was 50 mm/min. Bending tests were also performed at room temperature on water-assisted injection molded parts. The bending specimens were obtained with a die cutter from parts subjected to various water temperatures. The dimensions of the specimens were 20 mm · 10 mm · 1 mm. Bending tests were performed in a micro tensile tester according to the ASTM D256 test. A 200 N load cell was used and the crosshead speed was 50 mm/min.
3. Conclusions
This report was made to experimentally study the waterassisted injection molding process of poly-butylene-tere-phthalate (PBT) composites. The following conclusions can be drawn based on the current study.
1. Water-assisted injection molded PBT parts exhibit the ?ngering phenomenon at the channel to plate transition areas. In addition, glass ?ber ?lled composites exhibit more severe water ?ngerings than those of non-?lled materials.
2. The experimental results in this study suggest that the length of water penetration in PBT composite materials increases with water pressure and temperature, and decreases with melt ?ll pressure, melt temperature, and short shot size.
3. Part warpage of molded materials decreases with the length of water penetration.
4. The level of crystallinity of molded parts increases with the water temperature. Parts molded by water show a lower level of crystallinity than those molded by gas.
5. The glass ?bers near the surface of molded PBT composite parts were found to be oriented mostly in the ?ow direction, and oriented substantially perpendicular to the ?ow direction with increasing distance from the skin surface.
8
參考文獻(xiàn)
[1] Miguel S′anchez-Soto . Optimising the gas-injection moulding of an automobile plastic cover using an experimental design procedure. Spain.2006
[2] Integrated microfluidic systems for automatic glucose sensing and insulin injection
PBT玻璃纖維增強(qiáng)復(fù)合材料水輔注塑成型的實(shí)驗(yàn)研究
摘要：本報(bào)告的目的是通過實(shí)驗(yàn)研究聚對(duì)苯二甲酸丁二醇復(fù)合材料水輔注塑的成型工藝。實(shí)驗(yàn)在一個(gè)配備了水輔注塑統(tǒng)的80噸注塑機(jī)上進(jìn)行，包括一個(gè)水泵，一個(gè)壓力檢測(cè)器，一個(gè)注水裝置。實(shí)驗(yàn)材料包括PBT和15%玻璃纖維填充PBT的混合物以及一個(gè)中間有一個(gè)肋板的空心盤。實(shí)驗(yàn)根據(jù)水注入制品的長(zhǎng)度的影響測(cè)得了各種工藝參數(shù)以及它們的機(jī)械性能。XRD也被用來(lái)分別材料和結(jié)構(gòu)參數(shù)。最后,作了水輔助和氣體輔助注塑件的比較。實(shí)驗(yàn)發(fā)現(xiàn)熔體壓力,熔融溫度，及短射類型是影響水注塑行為的決定性參數(shù)。材料在模具一面比在水一面展示了較高的結(jié)晶度。氣輔成型制品也要比水輔成型制品結(jié)晶度高。另外，制品表面的玻璃纖維大部分取向與流動(dòng)方向一致，而隨著離制品表面距離的增加，越來(lái)越多的垂直與流動(dòng)方向。
關(guān)鍵詞：水輔注塑成型，玻璃纖維增強(qiáng)PBT，工藝參數(shù)，機(jī)械性能，結(jié)晶，
1．前言
依靠重量輕，成型周期短，消耗低，水輔注塑成型技術(shù)在塑料制品制造方面已經(jīng)取得了突破。在水輔注塑成型中，模具行腔被部分注入聚合物熔體，而后向這些聚合物中心注入水。水輔注塑成型的原理如圖1
圖1 水輔注塑成型的原理如圖
水輔注塑成型能夠在更短的循環(huán)時(shí)間內(nèi)生產(chǎn)出收縮小，翹曲小，表面質(zhì)量好的各種薄厚的制品。水輔注塑成型工藝也可根據(jù)工具及設(shè)備的承受壓力在設(shè)計(jì)，節(jié)省材料，減輕重量，減少成本方面取得更大的自由。典型的應(yīng)用有棒，管材，水路管網(wǎng)建設(shè)用的大型復(fù)合結(jié)構(gòu)管。另一方面，盡管有很多優(yōu)勢(shì)，由于加入了額外的工藝參數(shù)，模具和工藝控制變的更加嚴(yán)峻和困難。水也可能腐蝕模具鋼，同時(shí)一些材料包括熱塑性塑料難以成型。成型后水的清除也是對(duì)這個(gè)新技術(shù)的一個(gè)挑戰(zhàn)。表1列出了水輔注塑成型技術(shù)的優(yōu)勢(shì)和局限性。
優(yōu)勢(shì)
局限性
1，成型周期短
2，成本低（水更便宜而且可方便地循環(huán)利用）
3，制品內(nèi)部不產(chǎn)生泡沫現(xiàn)象。
1，水腐蝕模具
2，需要較大的注塑元件。（容易陷入聚合物熔體）
3，一些材料難以成型（尤其是非晶態(tài)熱塑性材料）
4，成型后需要清除水
表1
水輔注塑成型有優(yōu)勢(shì)超過它更有名的競(jìng)爭(zhēng)對(duì)手，氣輔注塑成型，因?yàn)橐揽克诔尚瓦^程中更好的冷卻能力，水輔注塑成型獲得了更短的成型周期。它的不可壓縮性，低成本以及易循環(huán)利用，水成為這一過程的理想媒介。既然水不會(huì)溶解和擴(kuò)散到聚合物熔體中，那么經(jīng)常在氣輔成型工藝出現(xiàn)的氣泡現(xiàn)象也便消除了。另外，水輔注塑成型能更好的用小剩余壁厚成型大型制件。表2是對(duì)水輔和氣輔成型工藝的一個(gè)比較。
表2水輔和氣輔成型工藝比較。
水輔
氣輔
1成型周期
2介質(zhì)成本
3氣泡現(xiàn)象
5殘余壁厚
6表面粗糙度
7表面光澤
8指形效應(yīng)
9非均勻穿透
10制品透明度
11內(nèi)表面（熱塑性半晶）
12內(nèi)表面（熱固性）
短
低
無(wú)
小
小
高
大
穩(wěn)定
高
平滑
粗糙
長(zhǎng)
高
有
大
高
低
小
不穩(wěn)定
低
粗糙
平滑
隨著對(duì)密度小，強(qiáng)度高，價(jià)格便宜，成型周期短的優(yōu)良性能材料需求的增加，塑料工程是一個(gè)不可忽視的工藝。這些塑料包括熱塑性和熱固性塑料。一般來(lái)說，熱塑性塑料以其更高的沖擊強(qiáng)度，斷裂阻力，疲勞強(qiáng)度而更有優(yōu)勢(shì)。這使得熱塑性塑料在工程建設(shè)中廣泛使用。
PBT是廣泛使用的熱塑性工程塑料之一，它有1，4—丁烯乙2醇和DMT聚合而成。玻纖增強(qiáng)混合材料適用于提高原材料的機(jī)械性能。今天，短玻璃纖維增強(qiáng)PBT已被廣泛應(yīng)用與電子，通信，汽車領(lǐng)域。所以，對(duì)玻璃纖維增強(qiáng)PBT的研究更加重要了。本文是通過實(shí)驗(yàn)研究聚對(duì)苯二甲酸丁二醇水輔注塑的成型工藝，實(shí)驗(yàn)在一個(gè)配備了水輔注塑統(tǒng)的80噸注塑機(jī)上進(jìn)行，包括一個(gè)水泵一個(gè)壓力檢測(cè)器，一個(gè)注水裝置。實(shí)驗(yàn)材料包括PBT和15%玻璃纖維填充PBT的混合物以及一個(gè)中間有一個(gè)肋板的空心盤。實(shí)驗(yàn)根據(jù)水注入制品的長(zhǎng)度的影響測(cè)得了各種工藝參數(shù)以及它們的機(jī)械性能。XRD也被用來(lái)分別材料和結(jié)構(gòu)參數(shù)。最后,作了水輔助和氣體輔助注塑件的比較。
2．實(shí)驗(yàn)步驟
2.1 材料
實(shí)驗(yàn)材料包括PBT（牌號(hào)1111FB，南亞塑料，臺(tái)灣）和15%玻璃纖維填充PBT的混合物（牌號(hào)1210G3，南亞塑料，臺(tái)灣）。表3列出了此混合材料的特征。
表3 纖維增強(qiáng)PBT復(fù)合材料特征
性質(zhì)
ASTM
PBT
15%G.F.PBT
屈服應(yīng)力(kg/cm2)
彎曲應(yīng)力(kg/cm2)
硬度
熱變形溫度（℃）
MFI
沖擊強(qiáng)度
熔點(diǎn)（℃）
D-638
D-570
D-785
D-648
D-1238
D-256
DSC
600
900
119
60
40
5
224
1000
1500
120
200
25
5
224
2.2 水輔注塑元件
一個(gè)實(shí)驗(yàn)室注水元件，包括一個(gè)水泵，一個(gè)壓力檢測(cè)器，一個(gè)注水閥，一個(gè)配備了溫度調(diào)節(jié)裝置的水箱，以及一個(gè)控制電路。這個(gè)孔板型注水閥每邊有兩個(gè)孔，用來(lái)成型制件。實(shí)驗(yàn)過程中，注水閥的控制電路收到由注塑機(jī)產(chǎn)生的信號(hào)實(shí)現(xiàn)對(duì)時(shí)間和注水壓力的控制。在注入模具行腔之前，水在有溫控裝置的水箱里加熱30分鐘。
2.3注塑機(jī)和模具
水輔注塑成型實(shí)驗(yàn)在一個(gè)最高注塑速率109cm3/s的80噸注塑機(jī)上進(jìn)行。研究使用了一個(gè)中間有一個(gè)肋板的空心盤。圖2顯示了這個(gè)行腔的尺寸。模具溫度由一個(gè)水循環(huán)模溫控制元件調(diào)節(jié)。實(shí)驗(yàn)根據(jù)水注入制品的長(zhǎng)度的影響測(cè)得了各種工藝參數(shù)，包括熔體溫度，模具溫度，熔體充模壓力，水溫和水壓，注水延遲時(shí)間和保持時(shí)間，以及熔體短射類型。表4列出這些工藝參數(shù)及在實(shí)驗(yàn)中的數(shù)值。
A
B
C
D
E
F
熔體壓力
熔體溫度
短射類型
水 壓
水 溫
模具溫度
140
126
114
98
84
280
275
270
265
260
76
77
78
80
81
8
9
10
11
12
80
75
70
65
60
80
75
70
65
60
表 4
2.4氣輔注塑元件
為了對(duì)水輔和氣輔注塑成型制件進(jìn)行比較，氣輔注塑成型實(shí)驗(yàn)使用了一個(gè)商用氣輔注塑成型元件，其具體配置可參考RCFS。氣輔注塑成型工藝控制和水輔注塑成型一樣，除了氣體溫度設(shè)置為25外。
圖2 模具行腔的尺寸和外形
2.5 XRD
為了分析水輔注塑成型制品的晶體結(jié)構(gòu)，實(shí)驗(yàn)使用了具有二維探測(cè)分析傳輸模式的廣角X射線衍射儀。更特別的是實(shí)驗(yàn)對(duì)水輔注塑成型制品模具一邊和水一邊的樣品在7到40的范圍內(nèi)進(jìn)行測(cè)量。分析所用的樣品來(lái)自制品中心。為了獲得XRD樣品要求的厚度，多余的部分在一個(gè)旋轉(zhuǎn)輪上打磨掉。首先用濕的碳硅紗布，而后用粒度300的，再用粒度600和1200的，以獲得更好的表面質(zhì)量。
2.6機(jī)械性能
拉伸強(qiáng)度和彎曲強(qiáng)度測(cè)試在一個(gè)拉力測(cè)試機(jī)上進(jìn)行。實(shí)驗(yàn)對(duì)水輔注塑成型制件樣本進(jìn)行拉力測(cè)試以評(píng)估水溫對(duì)拉伸性能的影響。樣本的尺寸為30mm*10mm*1mm.
水輔注塑成型制件的彎曲實(shí)驗(yàn)也在室溫下進(jìn)行。彎曲樣本的尺寸為20mm*10mm*1mm。
3 結(jié)論
本報(bào)告的目的是通過實(shí)驗(yàn)研究聚對(duì)苯二甲酸丁二醇復(fù)合材料水輔注塑的成型工藝?；诋?dāng)前實(shí)驗(yàn)可得出以下結(jié)論
1. 水輔注塑成型制品在水道的過度區(qū)域出現(xiàn)了指形效應(yīng)。并且，玻璃纖維增強(qiáng)復(fù)合材料的指形效應(yīng)比不增強(qiáng)的更嚴(yán)重
2. 研究的實(shí)驗(yàn)結(jié)果顯示PBT復(fù)合材料的水穿透長(zhǎng)度隨著水溫和水壓的增加而增加。隨著熔體充模壓力，熔體溫度，模具溫度，短射量的增加而降低。，
3. 制品的翹曲隨著水穿透的程度而降低了。
4. 注塑制品的結(jié)晶度隨著水溫的升高而提高。水輔成型制品的結(jié)晶度比氣輔的要低。
5. 模具一邊的制品表面的玻璃纖維取向大部分與流動(dòng)方向一致，而隨著離這一表面距離的增加，纖維取向逐漸的垂直與流動(dòng)方向。
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