Injection Mould Design for Toy Helicopter
Jun Rong School of Mechanical Engineering Beijing Institute of Technology Beijing,P.R.China
Email:Rongjun123@126.com
Ruijun Wang、Yichun Wang、Zhanchun Duan and Yingjun Yang School of Mechanical Engineering Beijing Institute of Technology Beijing,P.R.China Email:   wych@bit.edu.cn
Abstract--Through particular analysis of manufacturability for toy helicopter, this paper presents the main technical process of injection mould design and manufacturing. The best  kind and position of gate is discussed, the Harvard structure and side parting core-pulling structure guided by angle pins are adopted, and the problem of plastic parts demoulding after  moulding is perfectly solved. The whole mould structure is designed and the working  procedure of the injection mould is stated.
Keywords--toy helicopter; plastic injection mould; pin gate; Harvard structure
I.INTRODUCTION
 In the modern plastic processing industry, injection mould becomes more and more important, which mainly used for molding thermoplastic plastic products. Injection mold has a  lot of advantages, such as large range of molding precision, high production efficiency, wide  rang of application, and so on[1]. In the course of moulding design, the forming technique of  plastic parts should be fully analyzed and the forming scheme should be reasonablely designed.  As an example, the main technical process of the toy helicopter injection mold design is described in the following.
II.THE STRUCTURE AND FORMING TECHNOLOGY OF TOY HELICOPTER
 The toy helicopter is showed in figure 1. This plastic parts belongs to shell plastic products, the whole structure is symmetrical about the center, which to some extent can  simplify the processing technology of the molding parts. In addition, the side convex structures of the helicopter’s wings are too complex, so the mould must set side parting core-pulling  structure, only in this way can the plastic parts be parting from the mould cavity successfully.  Due to the symmetry structure of helicopter and the difficulties in molding the wings, Harvard  structure is used to form the plastic parts. The use of two pieces of identical Harvard block  structure can not only greatly simplify the processing technology of mold cavity, but also can set side parting core-pulling structure guided by angle pins to apart the two pieces of Harvard block structure to emerge the plastic parts from the mold[2]. The toy helicopter at the bottom of the hollow parts is relatively simple to mold, it can be directly formed by a core insert. But structure of the support frame part is special, the connecting part with the helicopter fuselage is less, and the cross section is circular, just shown in figure 1. It is quite obvious that the support frame structure can not be simply mold by a core insert. So a half cylindrical cavity in the  Harvard piece and the other cylindrical cavity in the main core insert is designed to compose a complete circle cavity to mold the support frame part structure.

     The plastic is made from the material of hard PVC, he shrinkage is 0.6%~1.5%, the draft angle is 1º[3]. This material has good molding characteristics and great mechanical properties,  but it`s thermal stability is poor, this shortage can be overcame by adding stabilizer to the melt  plastic [2]. This plastic parts is just an ordinary children`s toy, rather than the industrial product.  So the precision of the size is not high, just to meet with the grade for MT6.The appearance of the plastic parts should maintain smooth, the disfigurement of injection such as weld lines, sink  mark, flashing, splay, etc. is not allowable to appear either.
III.STRUCTURE DESIGN AND WORKING PROCESS OF THE MOLD
  A.Determining the parting surface and cavity quantity. 
  The plane`s structure is symmetry about the center, and the center is the largest  section place, so it`s easy to determine that the center is the parting surface of the Harvard block structure, as shown in figure 2. The wings structure is convex and perpendicular to the parting surface of the Harvard block structure, this characteristic will bring difficulties to the release of  the plastic parts after molding. So it`s a must to design side parting core-pulling structure to  open the Harvard block, or else the plastic parts can not be emerged normally.
 The production scale requirement of this plastic parts is batch production, if the mold  designs just one cavity, the production is too low to reach the requirement, so the mold designs  two cavities to improve the production efficiency.
B.Confirming the feed system
The maximum wall thickness of the helicopter is 2mm, the minimum wall thickness is  1mm, so the helicopter belongs to thin wall plastic parts. The size of the length is much lager than the width, which will lead a long process of the melt plastic while filling the cavity. Therefore, the melt hard PVC belongs to high viscosity of melt plastic, its mobility is very poor. So this mold  uses pin gate to fill the cavity, which locates in the middle of the helicopter. In this way, the melt plastic can have high pressure and high speed while filling, and the pin gate can close automatically after fully filling, so that the mold can effectively prevent the uncooled melt plastic reflowing.
C.Designing the demoulding structure
The mold adopts pin gate while filling, so it is necessary to design a sequence parting  mechanism to strip off the feed system material. The fist parting will pullout the cementitious  material from the sprue and separate the pin gate with plastic pates in the gate location. The  second parting will disengage the material of feed system completely, the accurate parting  distance of the mold was determined by the stop pins. While ejecting the plastic parts from the cavity, it is necessary to design side parting core-pulling mechanism guided by angle pin to open  the Harvard structure, or else the product can not eject from the cavity normally. Specifically,  the side parting core-pulling structure includes angle pin, cavity insert slide, heel block and slide
retainer[6-8]. In addition, core insert is used to form the internal of the helicopter, where is  hollow, so it is necessary to set stripper pins to get off the plastic parts from the core insert. Since the thickness of the core insert is small, this mold choses the size of 2.5mm diameter stripper pin.   The length size of the core insert is large, if just set one stripper pin, the strip force will too uneven to lead to deformation of the plastic parts while prolapsing, so multiple stripper pins  are used for each cavity. These stripper pins are arranged in the force greater part of the helicopter, so that the plastic parts can be stripped off from the core insert effectively.
D.Structure and the whole working process of the mold.
 

1-Limit screw   2-Main core insert   3-Core panel board  4-Cavity plate  5-Front plate    6-Sprue bush 7-Small core insert  8-Ball head sucker pin  9-Fixed pitch screw  10-stripper  pin1   11-stripper pin2 12-Support frame  13-Ejector guide pin   14-Ejector guide bush  15-Angle pin  16-Core retaining plate   17-Core plate  18-Guide bush   19-Cavity insert  plate   20-Lead guide sleeve   21-The lead guider pillar  22-Location ring  23-Heel block   24-Cavity slide block  25-Return pin  26-Stripper retaining plate   27-Stripper plate  28-Moving mould plate
  The melt plastic flows into the cavity of the mold through the feed system and the mold is opened after full packing and cooling. While opening, the mold injection machine nozzle is  returned, the injection pressure is removed, the mold get apart firstly between the front plate and the cavity insert plate. In this process of parting, gate breaks away the plastic parts by the  driving force of material in the inclined holes, which is in the end of runner. At the same time,  ball head sucker pin pulls out the mainstream material from sprue bush. When parting to a  certain distance, cavity insert plate reaches the opening stroke and stops moving by resistance of distance screw,and feed system material emerge from cavity insert plate automatically. The  moving half of the mould continues to move back. The angle pins drive cavity slide block to  open until reach the opening stroke, which is precisely determined by heel block. When hit  with the heel block, the cavity insert plate stops the slide parting movement and completes the  opening action of the Harvard structure, the plastic parts will eject from the cavity insert plate  entirely. The mold continues to open, when reaches the opening stroke, the injection ejector rod drives stripper plate to push stripper pins move forward, plastic parts will emerge from the main core insert under the force of stripper pins and drop off from the mold automatically. Then the  mold components return to their original position by the driving force of the return pins until the mold is closed. The second working cycle begins after the mold is closed.
IV.CONCLUSION
 Through rational analysis of the mold designing process, considering the performance  and technical requirements of the plastic pares, pin gate, Harvard structure and side parting core-pilling structure guided by angle pin are designed and adopted in this injection mould. The  structure of the mould is rational and compact, the operation is reliable, and the production of plastic parts is completely meeting with technical requirements.
V.REFERENCES
[1]Tang.Z.Y:Plastic Mould Designer Guide[M],edited by Mechanical Industry Publishing House,Beijing(2008)
[2]Rong.J:Toy Helicopter Injection Mould Design of Undergraduate Graduation Design(2012)
[3]Wu.Z.Z,Wang.Q,Pang.Y.X,Zhang.H.A:Plastic Mold Design Guide,edited by National Defense Industry Press,Beijing(2006).
[4]Luo.Z.G,Chen.J.Z:Plastic Molding Process and Mold Design,edited by Machinery   Industry Press,Beijing(2009)
[5]Feng.A.X:Plastic Engineer Manual[M],edited by Mechanical Industry Publishing  House,Beijing(2009)                                                      

玩具飞机的注塑模设计
荣俊  中华人民共和国 北京 北京理工学院机械工程学院
王瑞军、王一淳、段占春和杨英俊  中华人民共和国 北京 北京理工学院机械工程学院

摘要：通过特定的玩具直升机的工艺性分析,介绍了注塑模具设计和制造的主要工艺流程。讨论最好的浇口类型和位置,采用角针的指导下哈佛结构和分离侧抽芯的压铸模结构,并且完美解决塑料零件成型后脱模的问题。开始注塑模整个模具结构设计和工作过程。
关键字：玩具直升机、塑料注塑模具、点浇口、哈佛结构
一、概述
在现代塑料加工行业、注塑模具变得越来越重要,它主要用于成型热塑性塑料制品。注塑模具有很多优势,如大范围的成型精度,生产效率高,应用广泛等[1]。在造型设计的过程中,应充分分析和塑料件的成型工艺形成方案应合理设计。作为一个例子,将在下面描述玩具直升机注塑模具主要技术设计的过程。
二、玩具直升机的结构和成形工艺
玩具直升机是显示在图1。这个塑料部件属于塑料壳体制品，整体结构是中心对称的,这在某种程度上可以简化成型零件的加工工艺。此外,直升飞机机翼的侧凸结构过于复杂,所以模具必须设置侧分型抽芯结构,只有这样才能成功从模具型腔分型塑料部分。由于直升机的对称结构和成型困难的翅膀,哈佛结构是用来成型塑料部件。使用两块相同的哈佛块结构不仅可以大大简化模具型腔的加工技术,还可以角针指导下设置分侧抽芯的压铸模结构分开两块哈佛块结构出现的塑料件模具[2]。玩具直升机底部的空心部分相对简单的模具,它可以直接由一个核心插入。但支撑架的结构部分是特别,直升机机身的连接部分是少,横截面是圆形,如图1所示。很明显的支持框架结构不能简单模具的核心插入。所以半圆柱形谐振腔在哈佛块和其他圆柱形谐振腔的主要核心插入的目的是组成一个完整的圆腔模具支撑架的部分结构。
图1、玩具直升机
塑料是由硬聚氯乙烯材料,他收缩0.6% ~ 1.5%,拔模角度是1º[3]。这种材料具有良好的成型特性和力学性能,但热稳定性差,通过添加稳定剂的熔融塑料可以克服这种缺点[2]。这个塑料件只是一个普通的孩子的玩具,而不是工业产品。所以尺寸的精度不高,只要求MT6等级。塑料部件的外观应保持平稳,注入的缺陷如焊线,缩痕,闪烁,斜面等，也是不允许出现的。
三、模具的结构设计和工作过程
A、确定分型面和型腔数量
飞机的结构是对称中心,中心是最大的部分,所以很容易确定中心是哈佛块的分型面结构,如图2所示。机翼结构是凸状的和垂直的哈佛块分型面结构,这一特点将带来的困难是成型后的塑料零件的释放问题。这是一个必须设计分侧抽芯的压铸模结构打开哈佛块,否则其他塑料部件不能正常脱模了。

图2：最佳分型面位置的分析结果
这个塑料部件的生产规模要求批量生产,如果模具设计只是一个空腔,产量太低达到要求,所以模具设计两个型腔来提高生产效率。
B、确认进料系统
直升机的最大壁厚是2毫米,最小壁厚1毫米,所以直升机属于薄壁塑料部件。尺寸的大小长度比宽度大得多,这将导致一个长时间的熔融塑料的过程中，同时填补型腔的问题。因此,融化硬聚氯乙烯塑料熔体属于高粘度,其流动性很差。这模具使用点浇口来填补位于中间的直升机的空腔。在这种方式下，塑料熔体具有高压力、高速度的同时填充和浇口可自动关闭后完全填充，使模具能有效防止非致冷塑料熔体回流。
C、脱模结构设计
该模具在灌装时采用了针式浇口，因此有必要设计一个序列分型机构来剥离出料系统的材料。第一部分将拔出的胶凝材料从浇口与塑板在浇口浇口位置分离。第二部分将从完全从材料的进给系统,由止动销确定模具的准确分型距离。而将塑件从型腔中脱模，需要设计、侧向分型机构的角针引导打开哈佛结构抽芯，否则产品不能正常脱模。具体来说,侧向分型抽芯结构包括角销、腔镶块、滑块和滑块保持器[6-8]。此外,核心插入用于直升机的内部形式,是中空的,所以有必要的脱模针芯插入的塑料零件。因为核心插入的厚度很小,这个模具的2.5毫米直径的大小冲孔模板销。核心插入的长度尺寸很大,如果只设置一个冲孔模板销,剥离力太不均匀会导致塑料零件的变形而下垂,所以多个脱模针用于每个腔。
D、结构和模具的整个工作过程
模具结构如图3所示
图3、模具结构
1、限制螺丝2、主要核心插入3、核心板4、板模5、前板6、浇道套7、小芯插入8、球头抽油杆9、固定螺距螺旋桨10、冲孔模板销1 11、冲孔模板销2 12、支撑架13、喷射器导料销14、喷射器导套15、角销16、核心固定板17、核心板18、导套19、腔插入板20、导向导套21、铅导向器支柱22、位置环23、垫块24、腔滑块25、返回销26、汽提塔固定板27、卸料板28、移动模板
模具的塑料熔体流入腔模具浇注系统和完整的包装和冷却后打开。虽然开放,但返回模具注射机喷嘴,喷射压力移除,模具分开首先在前板和空腔插入板。在这个过程中,在这一过程中，门在倾斜的孔中，通过驱动力的驱动力把塑料件分开，这是流道的末端。同时，球头杆销拉出主流材料从浇口套。在一定距离的距离处，空腔插入板到达开孔时，通过距离螺钉的阻力，并停止移动，并从空腔插入物中出现进给系统材料自动盘。移动模具的一半继续向后移动。角针驱动腔滑块开直到达到开放中风,这正是由垫块。用垫块时,腔插入板停止滑动分离运动开幕式并完成动作的哈佛结构,塑料部件将驱逐出腔完全插入板。模具继续开放,当到达中风,注射顶出杆驱动卸料板将脱模针前进,塑料零件会出现从主核心的力量下插入自动脱模别针放从模具。然后返回初始位置的模具组件的驱动力返回针,直到模具关闭。第二个工作循环开始后,模具是关闭的。
四、结论
通过对设计过程的理性分析，考虑模具，塑料的技术要求和性能比较，浇口，哈佛结构和侧分核心起球结构的角销引导设计该模具采用。该模具结构合理，结构紧凑，运行可靠，塑料件的生产与技术完全满足要求。
五、参考文献
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