Open a chocolate bar wrapper and you will notice something unusual: the seal that kept the chocolate fresh peeled apart cleanly, without tearing the film or leaving residue. That is the signature of a cold seal — a packaging technology that bonds film to film using nothing but pressure, no heat required.
Cold seal BOPP film is coated on one side with a specially formulated natural or synthetic latex emulsion. This latex layer is pressure-sensitive: when two coated surfaces are pressed together, they bond instantly. No heating element, no dwell time, no warm-up cycle. The seal forms at room temperature in milliseconds.
The reason this matters becomes obvious the moment you consider what goes inside the package. Chocolate melts at body temperature — around 34°C for milk chocolate, slightly higher for dark. A standard heat-sealable film requires sealing temperatures of 130°C to 160°C at the jaw. Even though the contact time is brief, the radiant heat from a hot sealing jaw is enough to soften the surface of a chocolate bar, leaving bloom marks or deforming the product. Biscuits and wafers face a similar problem: they are fragile and heat-sensitive, and a hot sealing jaw can crack or scorch them.
Cold seal eliminates this entirely. The sealing mechanism is two coated surfaces pressed together by a cold metal jaw or a pressure roller. Since no heat is applied, the product inside never experiences a temperature spike. This is why virtually every major chocolate manufacturer — from mass-market bars to premium assortments — uses cold seal packaging.
The latex coating itself is the heart of the technology. Most cold seal coatings are based on natural rubber latex, which has an inherent self-tack property: two surfaces coated with natural rubber latex will bond to each other on contact. This is chemically different from the heat-activated adhesives used in standard heat-seal films. A heat-seal adhesive melts and flows under temperature; a cold seal latex merely needs contact pressure to form a bond. The bond strength of a properly formulated cold seal can reach 200 to 400 grams per 15 mm strip width — more than enough to survive the rigors of distribution while remaining peelable by the consumer.
As a manufacturer that produces cold seal BOPP film for confectionery and snack customers across multiple regions, we have learned that the coating quality is everything. The latex must be applied uniformly at a coat weight of 1.5 to 3.0 grams per square meter. Too little, and the seal strength is inconsistent. Too much, and the film becomes blocking-prone — the roll self-adheres under the pressure of winding, and you cannot unwind it without tearing. Blocking is the single most common quality complaint in cold seal film, and it is almost always a coating weight or storage temperature problem.
The BOPP film substrate itself is chosen for specific properties. For cold seal applications, the film is typically 20 to 30 microns thick, with a reverse-printed design on the outside and the cold seal coating on the inside. The film must have excellent optical clarity so the printed design shows through vividly, good stiffness to maintain package shape on high-speed wrapping machines, and low-temperature toughness so it does not become brittle in refrigerated transport.
A common question from packaging engineers is whether cold seal can replace heat seal entirely. The answer is no — cold seal excels in specific applications but has limitations. The peelable nature of the bond means the package is not hermetically sealed in the same way a heat-sealed package is. Cold seal packages have higher moisture vapor transmission rates at the seal interface. For products with very low moisture tolerance — such as dry powdered milk or certain crackers — a heat seal or a hybrid approach (cold seal for the main fin seal, heat seal for the end seals) may be necessary.
Another consideration is machine speed. Cold seal packaging machines can run significantly faster than heat seal machines because there is no thermal cycle to manage. A typical heat seal wrapper operates at 40 to 80 packages per minute. A cold seal wrapper can reach 120 to 300 packages per minute, with some high-speed biscuit lines exceeding 500 ppm. The absence of a heating element means the machine does not need to wait for temperature recovery between cycles. This speed advantage is why large-volume snack manufacturers overwhelmingly prefer cold seal.
Storage and handling of cold seal film requires attention. Because the latex coating is pressure-sensitive, the film rolls must be stored under controlled conditions. Storage temperature should be 15°C to 25°C, and the rolls should not be stacked on top of each other in a way that applies pressure to the coated surface. In hot climates — above 35°C — the latex can soften and begin to block even without external pressure. Shelf life of cold seal film is typically 6 to 12 months from the date of manufacture, after which the latex may begin to lose tack strength.
The cold seal coating also interacts with printing inks. Since the design is reverse-printed on the outside of the film and the cold seal is on the inside, there is no direct contact. However, some converters apply a white ink layer on the inside surface before the cold seal coating. The ink layer must be fully cured and compatible with the latex emulsion — certain ink formulations can interfere with the latex bonding chemistry and reduce seal strength.
For packaging buyers, the key specification to check is the seal strength value, usually expressed in grams per 15 mm. A typical specification calls for a minimum of 150 g/15 mm at room temperature, with some premium applications requiring 250 g/15 mm or higher. The test is performed by sealing two film samples under a standard pressure (typically 0.3 MPa for 0.5 seconds), then pulling them apart in a tensile tester at 180 degrees. The peak force is recorded as the seal strength.
Cold seal BOPP film is a specialized product that solves a specific problem — how to seal packaging quickly without damaging heat-sensitive contents. Understanding how it works, what can go wrong, and how to specify it correctly is essential for anyone involved in confectionery or snack packaging.
Frequently Asked Questions
Can cold seal film be used for all types of snack packaging?
Cold seal is ideal for chocolate, biscuits, wafers, and other heat-sensitive snacks. However, for products requiring a hermetic moisture barrier (such as dry milk powder or moisture-sensitive crackers), heat seal is typically necessary because cold seal does not provide the same level of moisture barrier at the seal interface. Some packages use a hybrid approach with cold seal for the fin seal and heat seal for the end seals.
What causes cold seal film to block during storage?
Blocking occurs when the latex-coated surfaces adhere to each other under pressure or heat. The most common causes are: storage temperature above 30°C, excessive coating weight, rolls stacked under heavy load, or film that is past its shelf life. Prevent blocking by storing rolls at 15-25°C in a single-layer orientation, and use within 6-12 months of manufacture.
How fast can a cold seal packaging machine run compared to heat seal?
Cold seal machines typically run 50-100% faster than equivalent heat seal machines. A standard heat seal wrapper operates at 40-80 packages per minute, while cold seal wrappers commonly reach 120-300 ppm, with some high-speed biscuit lines exceeding 500 ppm. The speed advantage comes from eliminating the thermal cycle — no heating, no cooling, no temperature recovery time.
Is cold seal film more expensive than standard heat sealable BOPP?
Yes, cold seal film typically costs 15-30% more per kilogram than standard heat-sealable BOPP due to the latex coating step. However, the total packaging cost can be lower because faster machine speeds reduce labor and overhead per package, and reduced product damage from heat exposure lowers waste rates.
How do I test cold seal strength?
Seal two film samples face-to-face under standard pressure (0.3 MPa for 0.5 seconds at room temperature). Cut a 15 mm wide strip from the sealed area. Pull the strip apart in a tensile tester at 180 degrees at 300 mm/min crosshead speed. Record the peak force. Typical specification: minimum 150 g/15 mm, with premium applications requiring 250+ g/15 mm.
Can cold seal coating be applied to PET film as well as BOPP?
Yes, cold seal latex can be applied to PET film, though BOPP is far more common for confectionery packaging due to its lower cost, excellent printability, and adequate barrier properties. PET cold seal film is used in applications requiring higher stiffness or temperature resistance, such as retort-ready or dual-ovenable packaging.
This article draws on the hands-on production experience of the technical team at Shandong Shunzhan New Materials Co., Ltd., where we have been manufacturing BOPP and PET functional films for over two decades. Our films are used by packaging converters, printers, and label manufacturers in more than 50 countries — from the Middle East to Southeast Asia to Australia.
If you found this article helpful and have a specific film selection or processing question, we are happy to share what we know. Reach out for a technical consultation or sample evaluation.
冷封BOPP膜：巧克力和饼干包装如何在不加热的情况下封口
撕开一条巧克力棒的包装，你会注意到一个不寻常的现象：保持巧克力新鲜的封口干净地剥离，没有撕裂薄膜也没有留下残胶。这就是冷封的标志 — 一种仅靠压力就能粘合薄膜的包装技术，完全不需要加热。
冷封BOPP膜的一面涂有特殊配方的天然或合成乳胶乳液。这层乳胶具有压敏特性：当两个涂层表面被压在一起时，它们瞬间粘合。不需要加热元件，不需要保压时间，不需要预热周期。封口在室温下毫秒级形成。
这项技术为何重要，当你考虑包装里装的是什么时就一目了然了。巧克力在体温温度下融化 — 牛奶巧克力约34°C，黑巧克力略高。标准热封膜需要130°C到160°C的封口温度。即使接触时间很短，热封夹具的辐射热也足以使巧克力棒表面软化，留下起霜痕迹或使产品变形。饼干和威化也面临类似问题：它们脆弱且对热敏感，热封夹具可能导致裂纹或焦化。
冷封完全消除了这个问题。封口机制是两个涂层表面被冷金属夹具或压力辊压在一起。由于不施加热量，内部产品从不经历温度升高。这就是为什么几乎所有主要巧克力制造商 — 从大众市场到高端礼盒 — 都使用冷封包装。
乳胶涂层本身是这项技术的核心。大多数冷封涂层基于天然橡胶乳胶，具有内在的自粘特性：两个涂有天然橡胶乳胶的表面在接触时会互相粘合。这与标准热封膜中使用的热敏胶水在化学上完全不同。热敏胶在温度下熔化流动；冷封乳胶只需要接触压力就能形成粘合。正确配方的冷封粘合强度可达每15毫米条宽200到400克 — 足以经受配送过程中的各种考验，同时消费者可以轻松撕开。
作为一家为多个地区的糖果和零食客户生产冷封BOPP膜的企业，我们深知涂布质量就是一切。乳胶必须以每平方米1.5到3.0克的涂布量均匀施加。太少，封口强度不稳定。太多，薄膜容易粘连 — 卷在卷绕压力下自粘，展开时一撕就破。粘连是冷封膜最常见的质量投诉，几乎总是涂布量或储存温度的问题。
BOPP薄膜基材本身因其特定性能而被选用。对于冷封应用，薄膜通常厚20到30微米，外侧反面印刷图案，内侧涂冷封胶。薄膜必须具有优异的光学透明度，使印刷设计鲜艳透出；良好的挺度，以在高速包装机上保持包装形状；以及低温韧性，在冷藏运输中不变脆。
包装工程师常问的一个问题是冷封能否完全替代热封。答案是否定的 — 冷封在特定应用中表现出色，但也有局限性。可剥离的粘合特性意味着包装的密封性不如热封包装。冷封包装在封口界面处的水蒸气透过率更高。对于对水分极度敏感的产品 — 如干燥奶粉或某些饼干 — 可能需要热封或混合方式（主纵封用冷封，端封用热封）。
另一个考虑因素是机器速度。冷封包装机的运行速度可以显著高于热封机，因为没有热循环需要管理。典型热封包装机以每分钟40到80包的速度运行。冷封包装机可达每分钟120到300包，一些高速饼干线超过500包/分钟。没有加热元件意味着机器不需要在周期之间等待温度恢复。这种速度优势是大批量零食制造商普遍偏好冷封的原因。
冷封膜的储存和搬运需要注意。由于乳胶涂层是压敏的，薄膜卷必须在受控条件下储存。储存温度应为15°C到25°C，卷不应以对涂层表面施加压力的方式叠放。在炎热气候中 — 高于35°C — 乳胶可能软化并开始粘连，即使没有外部压力。冷封膜的保质期通常为制造日起6到12个月，之后乳胶可能开始失去粘性。
冷封涂层还与印刷油墨互动。由于设计印在薄膜外侧（反面印刷），冷封在内侧，两者不直接接触。然而，一些加工商在冷封涂层之前在内侧施加一层白墨。油墨层必须完全固化并与乳胶乳液兼容 — 某些油墨配方可能干扰乳胶粘合化学，降低封口强度。
对于包装采购人员来说，需要检查的关键规格是封口强度值，通常以每15毫米克数表示。典型规格要求室温下最低150 g/15 mm，一些高端应用要求250 g/15 mm或更高。测试方法是将两个薄膜样品在标准压力（通常0.3 MPa，0.5秒）下封合，然后在拉力试验机中以180度拉开。峰值力记录为封口强度。
冷封BOPP膜是一种解决特定问题的专用产品 — 如何在不损坏热敏感内容物的情况下快速封口包装。理解它的工作原理、可能出现什么问题以及如何正确选择规格，对于任何参与糖果或零食包装的人来说都是必不可少的。