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Polyethylene (PE) is a plastic material known for its low cost and high processability. It is processed in various ways including injection molding, blow molding, and inflation molding, and is used widely from everyday items to industrial products, holding the number one spot in terms of usage among all plastics.
*Reference: Vinyl Industry and Environment Association - "Production Volume by Type of Plastic"
Commonly used PE is broadly classified into two types:
| Properties / Characteristics | Remarks | |
|---|---|---|
| Molecular Structure | Crystalline Resin | |
| Shrinkage Rate | High | 2.0~3.0% |
| Color | Generally, white and translucent | LDPE has higher transparency. |
| Glass Transition Temperature | Low | -120°C: Excellent at low temperatures, remains soft |
| Melting Point | Low | 110~130°C |
| Impact Resistance | Fair | Low |
| Heat Resistance | Fair | Low heat resistance, vulnerable to heat and fire |
| Weather Resistance | Fair | Degrades under UV light, can be improved with additives |
| Electrical Properties | Excellent | Good electrical insulation |
| Chemical Resistance | Good | Stable against acids and alkalis except for fuming sulfuric and nitric acids |
| Dimensional Stability | Fair | High shrinkage due to crystallinity |
Mechanical PropertiesTypical Uses.png) Processed into sheet forms such as films and wrap sheets, accounting for about 30% to 50% of uses such as supermarket and convenience store shopping bags, vinyl bags for horticultural fertilizers, and wrap packaging agents for construction materials. It is also commonly used for potable water and gas piping components. In blow molding, it is used for gasoline tanks, and in injection molding, it is generally used for containers and daily necessities. In the medical device industry, due to its harmlessness and non-toxicity, it is also used in kits for in vitro diagnostics. Uses Leveraging its Low-Cost BenefitBeing the least expensive of all plastic materials, it is suitable for mass production. It is ideal for everyday items such as buckets and containers that do not require high impact resistance or mechanical properties. Uses Leveraging the Characteristics of PEDue to its excellent water resistance, oil resistance, and chemical resistance, it is used for containers for in vitro diagnostic drugs in the medical field. During setup, it is used as a purging agent. Because of its good thermal stability, when changing raw materials that have significant temperature differences, PE is temporarily substituted to adjust the barrel temperature. Purging with inexpensive PE reduces the purge loss rate of expensive raw materials. Points to Consider During the Molding Process of PE ResinThe following are points to consider and common methods for addressing appearance defects during the molding process of PE resin: .png) During Raw Material PreparationPE does not absorb moisture, so pre-drying with hopper dryers or box-type dryers is not necessary. During Heating Cylinder / Mold Temperature Setting
Common Appearance DefectsPE is an easy-to-mold resin, but there are critical points to consider. Below are the typical issues and their countermeasures: Silver StreaksSilver streaks may appear on the product surface. The following two factors greatly influence this:
Additionally, silver streaks can occur when the filling front wraps gas as it passes ribs or bosses. Adjusting the filling speed in multiple stages can improve this. Weld LinesWeld lines are unavoidable at flow convergence points. Adjust the injection speed in multiple stages to prevent poor gas escape from dead ends. Jetting and DroolingJetting and drooling occur in contrast. They can be improved by adjusting the suck-back amount and back pressure. Specific measures for each are:
Short Shots and Gas BurnsAfter tens of thousands of shots, short shots and gas burns can occur due to poor gas escape. Routine inspection and regular mold overhauls are necessary, including cleaning the parting line and gas vent areas. WarpingPE is often processed into containers due to its functionality. Box-type containers may strongly exhibit warping, which is caused by the inherent shrinking nature of the resin. Insufficient cooling time in the mold or inappropriate settings of mold temperature controllers can cause warping. Adjusting the cooling time can improve warping. Additionally, separating the mold cooling water circuits for the surfaces where warping occurs and operating them at different temperatures can improve the situation. This method aims to change the shrinkage by creating a temperature differential. Dimensional DeviationsDuring molding, if the mold cooling pipes are clogged and the flow is poor, the mold is not sufficiently cooled, and the product is removed while still hot. This results in strong shrinkage and smaller dimensions. Also, due to its crystalline nature, PE has a high shrinkage rate. Especially in molds for multiple medical parts, tens of thousands of units are stored in a single product receptacle after molding. If a defect occurs in one cavity, a full inspection becomes necessary, and if sorting from the product receptacle is challenging, the entire box may need to be discarded. Especially when all cavities must be collected in one product receptacle due to space or budget constraints, it is crucial to take time samples and ensure that all cavities are problem-free. SummaryPE, the most inexpensive of all plastic materials, is the most widely used raw material. It has very high moldability and is easy to handle. As a crystalline resin, it has a high shrinkage rate, so it is important to take type samples and monitor management to ensure that the dimensions after shrinkage fall within the standard values. Consider the factors mentioned above to aim for more productive molding processing. 関連のあるコラム記事 | ||
