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Special injection molding machine for disposable PP fast-food boxes

Number of visits: Date:2013-2-26 11:30:47





    At present, more than 15 billions disposable fast-food boxes were used each year in China. The main of them are made by foaming and some by vacuum forming, only less than 10% by PP.

    As everybody knows the harm of foaming tableware, the Government has specifically forbidden the usage of it. But some undesirable businessmen use some recycle materials in the production process of vacuum forming tableware in order to reduce the cost. It has great harm to the human body. At the same time, the hardness of vacuum forming tableware is insufficient, which can not fully meet the market requirements.

    Since 2003, some manufacturer began to produce disposable tableware by PP injection . Most of them used the imported machine & mould. In the early time the products were exported to the Europe and the United States and other developed markets. Along with the raising quality of domestic production and more attention of the government to food safety, the domestic market prospers gradually in recent years.

    The prosperity of the market brings the machine manufacturer’s attention. The injection molding machine brand, such as Haitian, Powerjet, Chen hsong and other enterprises have developed their own high speed injection molding machine to meet the market demand.

    After several years of development, many brands of the high speed machines came into the market. Most people including some factory technicians believe that all brands of high speed machines almost have the same mature quality. Because the understanding to the market from each big brand is not identical technically, their high speed machine project has big difference.

    Someone who wants to know the stand or fall of PP fast-food machine project, he must have a certain understanding to something about the fast-food injection molding materials, products and use, production process . Below is concerned with these aspects.

    Firstly, Materials

    While injecting such thin wall products as PP fast-food boxes, the material should has high liquidity, enough strength, hardness and compared single molecular weight distribution. Material features as followed:

    Melt index(MFI) for 35 - 50g / 10 min (230ºC/2.16kg) is preferred. In the early time the imported Samsung 828 thin wall special material was mostly used . Now it was replaced by LanGang Petrochemical H9018. When we do the injection,these materials have something in common:

    1. The temperature of injection can’t be too high, as the product will be crisp while higher than 300 degrees.

    2. The injection pressure can not be too high. The increased product internal stress leads to deformation if higher than 120 hPa. The increase of density will increase the weight, and the increase weight will not add much hardness, but only add to the cost of materials.

    Secondly, the nature of charging PP fast-food box

    The PP fast food box is low value product. Generally, only the fast production and short cycle can make more cost-efficient. Calculated a cycle with normal 5 ~ 6 seconds , basically five to ten degrees cold water can drop mould temperature. In the filling process, the runner wall is very cold. So the runner wall can form solidified layer and reduce the thickness of the runner when the melt filling cavity.

    The situation is more serious while the product’s wall is more thin. Usually, the thickness of PP fast-food box can not exceed over 0.5 mm. Because if there is 0.2 mm or so curing layer in the wall thickness of 0.5mm, there is only 0.1 mm thickness for runner, it is very difficult to charge. When the charging is not finished completely, the runner is disappeared for thick curing layer, so the product will not finished completely. This is also why pp fast-food box need high speed injection molding machine.

    Below is a simple diagram for injection:

    Thirdly, the requirements of the injection

    Combined the characteristics of pp material with the essence of thin-walled injection, we can understand the requirements to injection molding machine of PP fast-food box. In brief, it goes to the following three fields:

    1.Fast injection with high response:Thin-wall product is very light, and therefore the charging stroke is very short. When the machine achieves higher injection speed in 0.3 ~ 0.5 seconds, it can finish injection before runner forms a thick layer of curing. If the response is slow, the cavity resistance is too large, and it cannot make the absolute charging rate (general injection speed is calculated with empty injection) achieve the best. The solidified layer is too thick to fill the cavity, only if increase the injection pressure.

    2.Injection with low pressure:High pressure injection only can increase the required clamping force, can make the product have inner stress and deformation when the finished product in demoulding. At the same time it increases the density of the products, make the product weight and aggravate the wearing and tearing of the machine and mould.

    3.Injection with low temperature: Fast-food box with PP material will be crisp when temperature achieves more than 300 degrees. It will be damaged in the process of transportation or use.

product comes from high temperature material --- high brittle , easy to damage!

     Product comes from Low speed and high pressure---obvious distortion, meanwhile has radial striae.

    Fourthly, brief introduction to the fast injection in the market

    Here are three ways to make PP fast -food box

    Increase system:Increase the pump size will improve injection speed by 25%. At the same time, increase the oil valve, oil pipe, oil throat, etc. The mature technology has durability and good quality. It’s the market mainstream application solutions.

    Regenerative injection:Increase the injection pressure instead of that of injection speed. At the beginning period of injection it doesn’t need high injection pressure. Injection speed can be increased by more than 100%. This technique is not mature, regenerated flow is difficult to control. The following is a schematic diagram.

    Accumulator injection:  Nitrogen cylinder can store pump energy in the form of pressure and release during injection. It is a good way to improve the speed of injection by a large margin. Not only it is hard to control speed, but it also has high cost to servo valve and high failure rate. Only some manufacturers put into use in the early time, nowadays few manufacturers use it.

    Through the above simple introduction, everyone can understand the basic situation of injection molding machine for the PP fast-food box. Hope it is helpful to those friends who have the opportunity to choose the type of machine!

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